July 27, 2021 • 46 mins
Daniel and Jorge talk about whether its possible to outrun the fastest thing in the Universe
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Speaker 1 (00:08):
Hey, Daniel, what's the fastest a person has ever run? Well,
Hussain Bolt, the fastest person in history, actually only runs
about twenty miles per hour. But what's the fastest anything
has ever gone on Earth? That's about seven hundred and
sixty three miles per hour in a crazy rocket powered car.
That sounds like a good idea. But what about in space?
(00:31):
What's the fastest a spaceship has ever gone? There's the
Parker Solar Probe, which is whizzed around the Sun, and
it reached about five percent of the speed of light.
All right, so then technically nothing humans have ever made
has ever approached the speed of flight except for actual
light from flashlights. Oh well, but technically we made the flashlight,
but then the flashlight made the light. We want our
(00:52):
names on the pack as well? Yeah, can we label
each photon made by Jorge hi am or? Hey, I'm
(01:13):
a cartoonist and the creator of PhD comics. Hi, I'm Daniel.
I'm a particle physicist and the creator of many photons.
Oh really, you have a certain glow about you. Is
that what you're saying to all physicists? Glow? I think
usually it's not a good idea. If you're working with
radioactive things, especially if you come from Los Almos, then
you do tend to glow in the dark. But now
every single one of us gives up a unique pattern
(01:34):
of photons that we personally have crafted. My photons are artisanal.
Oh yeah, like you indivigitally craft each one. Yeah, they're
not very good. Isn't that what artisanal means? Yeah, I
guess what you mean is that you glow like in
the infrared, like from our body heat. Yeah, I definitely
do give off photons like every object in the universe,
if you have a temperature and then you glow. Sometimes
(01:55):
it's not in the visible light, but you definitely are
pumping out photons into the universe. Even if you are
a black hole, well then you are a shining example
I think for all of us. Welcome for our podcast,
Daniel and Jorge Explained the Universe, a production of I
Heart Radio in which we try to shine the light
of knowledge into your mind, pushing back on the forefront
of ignorance and talking about all of the biggest questions
(02:16):
in the universe, questions about the universe, questions around the universe,
questions in the universe, and questions from the universe, and
we talk about all of it and try to make
jokes to entertain you. Yeah, because there is a lot
in the universe out there that is bright and interesting
and powerful and curious and exciting, and there are also
dark things out there that are just escaped our site.
(02:37):
That's right. We can only see a little fraction of
the universe, an unknown fraction of the universe, because information
comes to us usually via light, and that light travels
at a very very fast but not infinite speed, and
we have learned all sorts of crazy things about how
information travels through the universe. Yeah, because the universe is
not just filled with mysterious things, it's also sort of
(02:58):
filled with mysterious rules. The rules of the universe are
not quite what we experience in an everyday basis, and
some of them kind of go against our intuition about
how things actually work. And thank God for that, or
my job would be boring. I think about science is
like a grand mystery novel. We're trying to figure out
how things work and deduce what the rules are from
(03:19):
the little clues left here and there. And you're absolutely
right that along the way we have figured out that
the universe is not the one we thought we were
living in, that the rules are weird, that the rules
are strange, that they require real creativity to on earth
how things actually work. Are you saying the universe is
not cliche? Thankfully, It's got it's twists and turns. I've
seen this universe so many times before. Oh my gosh,
(03:42):
bang crunch, bang, crunch, bang crunch. Hey do you think
every time there's a big bang and a big crunch
and we come back with the same universe that would
be boring? Yeah, obviously it was made by committee the
big bang. Let's just reboot the last universe that's sold
pretty well. Yeah right. Audiences don't want anything new. People
don't want to exist in the in a new universe.
This is the new J. J Abrams Big Bang theory.
(04:04):
Oh a dig on J. J Abrams. I feel like
you're always looking for an excuse there. Hey man, he
created the whole universe. You know, I got notes, but
it's pretty impressive. Wait. JJ Abrams created the whole universe
this theory. Yes, there's a J. J Abrams equivalent, you know,
writing the simulation or guiding the direction of the universe
that would explain all the lens flares, and I see
(04:24):
all the time when I look at the universe. But yeah,
it is a weird universe with weird rules, and there
is none weirder than special relativity, which is kind of
one of our main theories about the universe. That's right.
It turns out that when you start going really really quickly,
the rules that work at slow speeds down here on
Earth no longer apply. The rules are actually quite different,
(04:45):
and for hundreds and thousands of years we have been
learning rules that only work under special conditions, very very
slow movement. If you actually push the boundaries and start
going fast, the universe reveals that things we thought were
true are not actually true, leadings all sorts of weird
consequences that really violate our intuition. They break our notion
of like simultaneity and the idea of time being universal. Right. Yeah,
(05:09):
do you think like if we were all moving faster,
we would be used to these strange special rules, and
like our current rules would team we would have to
be moving really really fast. But I think in that
scenario our current rules would just be a natural extension
of what we already understood. But it's hard to imagine
growing up in a universe where special relativity feels intuitive,
(05:30):
where it makes sense to you, where our conclusions about
the universe would seem strange. But I'm sure somebody's written
that science fiction story, or maybe not. It's all there
for you, Daniel, somebody out there right it. But it's
called special relativity because it sort of applies to special situations.
Why does it have that special name? Special relativity is
called special because it's not general relativity. It applies to
(05:51):
scenarios where space is flat. We don't have to think
about space actually being curved and like changing the path
of photons or changing the motion of Earth around the star.
It has to do only with a sort of simpler
scenario where space is mostly empty and you're like shooting
laser pulses back and forth, or you have light bulbs
on trains. It has to do with relative velocities of
(06:11):
things and how information moves through the universe. It avoids
all complications from curved space. Oh really, wow, I never
knew that. I always thought that it was called special
relativity because it was special, But actually you're sort of
sitting the opposite. It's special because it only applies to
a boring universe. Yeah, it's a specialized condition. Right. He
came up with this first to understand this, and he thought, well,
you know, it's much trickier if space is actually curved,
(06:33):
and then he generalized it. He made it much more broad.
Special relativity is a subset of general relativity under the
conditions that basically the universe is mostly empty. You hadn't
figured that part out yet. So it's not like special
because people think it's like it ess special. It's just
special because actually it's like specially boring relativity that would
be technically more accurate. Right, Yeah, it's a special case.
(06:56):
It's not a better case. It's a simple case. But
we often do this in physics. We think about simple
scenarios to help us like distill what's going on and
get the clearest picture, so we can separate these ideas
because even in the scenario where you have no big
masses destroying the shape of space like black holes or
even just the sun, there are lots of weird things
that happened in special relativity. You know, clocks that don't
(07:17):
agree because you're moving at high speed. It's pretty weird
and hard to get your mind around. Even if there
is no curvature of space. So I think it was
sort of a good intellectual exercise. But even though it
applies to boring situations, it's still true, right, Like it's
still applies to the whole universe. It still applies to
the whole universe. The rules of special relativity do assume
(07:37):
that there are no heavy masses, and so if you
have big masses around curving space, you can't use calculations
from special relativity. You've got to use general relativity. But
I guess then the irony is that special relativity is
actually especially boring relativity. But it gives rise to these
really strange situations about the universe that don't seem to
make sense. And a lot of that has to do
(07:59):
with light experiments involving light. You know, there was this
famous experiment by Michaelson and Morley that showed that the
speed of light doesn't change no matter how fast the
Earth is moving through space. For example, that no matter
who measures a photon, they always measure it moving at
the speed of light, no matter how fast that person
is actually moving relative to anything else. It's pretty weird stuff. Yeah,
(08:20):
And so this weirdness is sort of a maybe especially
illustrated by asking a very simple question that you can
ask about the speed of light. So to be on
the podcast, we'll be asking the question is it possible
to outrun a flashlight? First of all, the flashlights run?
(08:41):
Do they have little legs that I don't know about?
Or that reminds me the old joke about refrigerators. Is
your refrigerator running? No, you better check the power. Your
flashlights don't actually run, of course, but the photons from
them come out at a blistering speed right of three
times tend the meters per second, So it's pretty hard
to imagine our pacing a flashlight. So it's more like,
is it possible to outrun a flash of light from
(09:04):
a flashlight? Yeah? I think the scenere I'm imagining is
you take off as your highest speed and I'm standing
behind you with a flashlight. Is it possible that once
I send off a flash of light, that you could
outpace it, that that flash of light would not catch you,
like it would not shine on me, like the photons
would never hit me. Yes, exactly. Or from your point
of view, is it possible you could run fast enough
(09:25):
that you could look backwards and you could not see
me right, that I would be passed some sort of
horizon beyond which you could not see. I see like
I would never know that you turned on the flashlight
because those photons would never reach me. Like I'll be
running and be like Daniel still hasn't but you did.
But it's the light would never reach me. And why
(09:46):
are you running away from me so quickly? What did
I don't know? Why are you shining a light on me?
What are you trying to do blind me? I'm just
playing flashlight tag? Man, chill out. So this applies to
that nineties toy laser tag, actually exactly. This helps you
strategize for laser tag. This is special relativity laser tag. Yeah,
physics is useful for all kinds of situations, even ones
(10:09):
that required to travel to the nineties and play laser tag.
I knew this degree would come in handy some day. Well,
at first, you have to solve time travel. I'm working
on it. I'm working on it, all right. So then
that's the scenario we're asking is if I take off
running and you shine a flashlight at me, is it
possible for me to outrun those photons or will they
inevitably hit me at some point in the future. So
(10:29):
that's a pretty interesting question, and so we thought we'd
post it to people on the internet and see what happens.
So usually Daniel went out there and ask listeners if
they thought that one could outrun a flash of light.
And so, as usual, I'm immensely grateful to all of
you who wanted to participate in the podcast and answer
these weird and random questions. If you're out there and
(10:49):
you're a listener to the podcast and you've been itching
to participate but you haven't quite yet, please send me
a message to questions at Daniel and Jorge dot com.
Think about it for a second. Do you think it
is possible to outrun a flashlight? Here's what people had
to say. Um, I don't think that I can. I'm
not faster than lights, so I don't think that I
(11:10):
could do that. I'd like to believe that you can
outrund a flashlight, but for that you have to be faster,
traveling faster than the speed of light, and as far
as I know, nothing in the universe can travel faster
than the speed of light. Yes, definitely, just don't throw
it too hard so you can, you know, run faster.
I think light from a flashlight travels its speeds much
(11:32):
less than the speed of lights and a vacuum that's
slowed down by the materials in its way as it
moves from inside the flashlight outside of it. And then
there are many different materials in the way, so because
of each material's index of refraction, the speed will be
reduced by a number of factors. As for whether I
can outrun it, I don't think so. I think it
(11:55):
will still be faster than my running speed. I guess
it depends on how fast it's the out No, if
if you actually mean the light itself, then nothing can
trovel a fast light, so you cannot let it. Sure, Yeah,
the flashlight itself is just like the box that that
sends out the light, right, So you could just set
(12:16):
the flashlight on the table and then run past it.
Would would outrun a flashlight? No, because it would really
piss s Einstein and also added additivity, I think maybe
the only way to kind of out run a flash
the light coming out of a flashlight would be maybe
(12:36):
if you could go through a worm hole. So if
the light from a flashlight, let's say, is headed towards
Jupiter and there's a wormhole between Earth and Jupiter, and
you took that shortcut in your spaceship m you could
probably hopefully get to Jupiter faster than the light from
(12:59):
your flashlight quid Rich Jupiter. No, you cannot outrun a
flashlight because the light traveling from the torch you're holding
will always be traveling at the speed of light relative
to you, so will always be traveling away from you
speed a light. All right, some pretty good answers here.
Somebody said you can't outrun the flashlight itself. I feel
(13:21):
like that's a different philosophical question, like can I throw
something at you that will always hit you? Yeah? Or
is the light part of the flashlight? After all? Will
always be the flashlight of infinite extent because of the
photons that come out of it. It's like the photon
of theseus kind of maybe the flashlight of theseus. Yeah,
Like is the photon from a flashlight part of the flashlight?
(13:44):
Bump bump bump. That's if somebody's thesis right there in
philosophy of science done? All right, I'll take that degree.
But yeah, lots of interesting answers here. Most people that
say that maybe not because you can't go faster than
the speed of light. So if light always goes as
fast as anything can go into universe, it will eventually
catch you, right, Yeah, and it's a very reasonable answer
given most people's understanding of special relativity. So yeah, I
(14:07):
see some clever answers here too, like what if there's
a wormhole? Did you think about that one? Yeah? But
why can't the light go through the wormhole? Also? Right,
if you run through photons right behind, you can't go
through the wormhole. So I guess if you open and
close that stargate really quickly, right, Yeah, Yeah, a lot
of clever answers here, but let's jump into answering this question,
(14:28):
can you out run a flash of light? And so
I guess maybe we should talk about a little bit
about this idea of the speed of light and special
relativity and why exactly it is kind of weird or
why weird things might happen if you try to this
experiment in space. Yeah, And the basic thing that we
need to understand is how different people measure velocity. Like
(14:50):
the way to think about it intuitively is imagine like
somebody throwing a natural object like a ball, if they
are in a car and they throw a ball forward
at ten miles an hour and then it's being a
ten miles an hour relative to them. No big deal.
But if the car is also driving at ten miles
an hour relative to the ground, then you might ask, well,
how fast is the ball moving relative to the ground. Well,
(15:11):
it's ten miles an hour relative to the car, and
the car is moving ten miles an hour relative to
the ground, So obviously twenty miles per hour, right, And
you think that's obvious and it's intuitive. And what you're
doing there is you're applying a rule which you sort
of intuitely we have figured out and applied, and it's
a Galilean transformation. It says, the speed of the ball
relative to the ground is the speed of the ball
(15:33):
relative to the car plus the speed of the car
relative to the ground. And that mostly works. But what
we discovered is that it's not true at very very
high speeds, and most specifically, it's not true for light.
So if I'm standing in a car and I shine
a flashlight, how fast is the light leaving my flashlight? Well,
the speed of light right now, the car is moving
(15:54):
in ten miles per hour. How fast is the light
traveling relative to the ground. Well, you're old rule would say, well,
if the speed of light plus ten miles per hour, right,
like faster than the speed of light. But that can't happen.
And so the light always travels at the speed of light,
no matter who's measuring it and how fast they are
going relative to the thing that made the light. So
(16:14):
the person in the car measures the photon is going
at the speed of light, and the person on the
ground measures the photon and going at the speed of light. Right, Yeah,
you had me on galilee and transformation. I think what
you're saying is that, you know, we're used in our
everyday lives this idea that velocities like add like they
add with simple like arithmetic, like you know one plus
one equals too, but that things get weird with the
(16:36):
speed of light because nothing can go faster than the
speed of light. So like you can't keep adding velocities
because that would eventually make them go faster than the
speed of light. Yeah, they add nonlinearly, right, so they
get closer and closer to the speed of light, but
they don't just stack up on top of each other
in a simple way. And so as you say things
with masking get faster and faster and approached the speed
(16:56):
of light. But nothing can go faster than the speed
of light, and so you have to have an new
addition rule. It doesn't just like A plus B. It's
some weird combination of A and B that helps you
approach the speed of light but never gets you past it.
And for light itself, it's always at the speed of light,
never slower, never faster, right, It's kind of this weird thing.
Like if you're a photon that's shooting out of a flashlight,
(17:18):
and so you're going at the speed of light, and
then somehow you as that photon shoot off another photon
in front of you, that photon is not gonna go
like a twice the speed of light. It's going to
go still at the speed of light. That's exactly right.
Although a photon itself can't have a frame of reference
because photon can't be at rest, so you can't measure
the speed of one photon relative to another one, which
(17:40):
is another for the tricky little wrinkle there. But exactly
if somebody's flying in a spaceship near the speed of
light relative to Earth and they turn on a flashlight,
that photon is not going at like one times the
speed of light relative to Earth. It's only moving at
the speed of light. And that's pretty weird, right, because
these things no longer add up. It's like the people
in the spaceship tell a different story about what happened
(18:02):
than the people on Earth. Because the people on Earth
see that floton as moving at near the speed of light,
it's relative speed to the ship is actually quite small,
whereas on the ship that people see the photon is
moving away from them at the speed of light. And
so you get like a different story about what happened.
And that's the mind bending thing about special relativities, that
different observers tell different stories about what happened, and they're
(18:25):
both accurate. They're both like honest observers telling conflicting stories
and both being correct. But I guess it's not about
different things happening. It's more about our perception of these
things happening, maybe because it's all related to how it
affects time, right, like time is sort of flexible, time
is not universal, and exactly we don't have a consistent
(18:45):
clock through the whole universe that says like what happened
at every moment, and then what happened at the next moment.
What happens depends on where you are and how fast
you are going relative to the events, and sometimes this
flexibility there. If there's no like causal connection where one
thing has to happen before another, then different people can
give different orders of events for what happened and both
(19:06):
be correct. It's not just an issue of perception. Although
you can, you know, use special relativity to say, well,
I understand why you are seeing the opposite thing that
I'm seeing. That's because of special relativity, but neither of
you can say like A happen before b or be
happen for a It depends on who you are and
how fast you're going. Right, It's like nobody's clock is official.
(19:27):
Everyone has a different clock, and so that you can't
sort of say, like who's right or who's wrong, because
in the end, everything is relative. Right, velocity itself is relative.
You can't have a velocity just on its own. You
can't say my spaceship is going at the speed of
light and say well relative to what right, As we
talked about once recently, like velocity doesn't even have a
(19:47):
meaning if you are in a universe all by yourself, like,
you can't have a speed if there's nothing else in
the universe. So it's a whole new way to think
about the nature of the universe and all these weird consequences,
a lot of which come from this idea that light
always travels at the speed of light. Yeah, So while
it may seem intuitive that if you shine a flashlight
at me, no matter how fast I'm going, that light
(20:08):
is going to catch up to me, that may not
actually be true depending on some of these weird consequences
of special relativity. That's right in a simple case where
you're like in a normal universe and space is flat
and it's obeying these rules, then if somebody shines a
flashlight in your direction, it doesn't matter how far away
they are. Eventually that light will reach you. There's no
(20:29):
like effective horizon, give an infinite time, that photon will
catch you. There's nothing you can do, right, and that's
because it's moving at the speed of light relative to you.
Because light always moves at the speed of light. So
in this sort of simple universe where space is not curved,
nothing weird is going on. That flash of light will
always catch you, right. But in our universe, the answer
might be different. But first, let's take a quick break.
(21:04):
All right, Daniel, you're trying to outrun a flashlight. I
don't know why. I guess you don't want people to
see you, or you want to remain sort of in
the dark and mysterious. But you're trying to outrun a
flashlight and somebody's shying a flashlight on you, and so
it seems like it would inevitably the light would reach
me because it's going faster than anything can go in
the universe. But you're saying there might be some instances
(21:24):
where I can actually outrun light. Mm hmmmm. And you
know this is not just a silly physics thought experiment.
This is really important. If you were playing laser tag
and you're cornered and they shoot that thing right at you,
you gotta find a way to escape. And so that's
where we come in, right. This is totally serious and
totally relevant to everyone's everyday lighte No, there are some
scenarios where somebody points a flashlight at you and it
(21:47):
doesn't ever hit you even an infinite time, even if
it's pointed right at you, and you don't actually even
have to be moving to avoid it. In some scenarios.
So you might have heard my sort of legalistic maneuver
there to avoid saying that the flashlight will always catch you.
The condition is that if you are operating in flat space,
in space, that's just where general relativity applies, and nothing
(22:08):
is growing or shrinking. Right, So that gives us one
that maybe the first instance in which you could maybe
outrun a flashlight. Right. It has to do with this
idea that space is not actually flat or constant or
kind of boring. Yeah, it turns out that our universe
is a lot more exciting than the special relativity universe, right.
Most importantly, our universe is expanding. Now, A lot of
(22:30):
people think that that means that galaxies are flying out
from some sort of central dot at a high speed,
that they're moving through space. But it's actually much weirder
and more amazing than that. It's the expansion of space itself. Right.
It's not the motion of things through space. I mean
that's also happening, but dark energy. There's accelerating expansion of
the universe. This is creating new space between our galaxy
(22:54):
and other galaxies. It's not just moving the galaxies through
the existing space. It's like stre watching the fabric of
the universe itself. Right, I think that you're saying sort
of like maybe I could outrun Hussein Bold if maybe
I play around with the track, or like how the
track is moving. Yeah, exactly. Put Usain Bolt on a
treadmill and stand in front of the treadmill. It doesn't
(23:15):
matter how fast he runs, That's what I mean. Yeah. Yeah,
And in the same way, the universe is laying new track,
new space between us and some really really distant photons,
photons which left their galaxy or their star billions of
years ago and they've been straining to reach us. Some
of them will never get to us, even though they're
(23:35):
pointed right at us, because the space between us and
them is expanding faster than light can go through it,
faster than the speed of light. Right. Well, it's it's
not that the space itself at any given point is
expanding faster than the speed of light. But it's more
like there's so much space in between, and it's growing
a little bit at each spot, so much that it
(23:56):
overall is expanding faster than the speed of light. Yes,
it's a very gentle effect on a local scale. Right,
It's not like the distance between you and your partner,
or between the Earth and the Sun is growing very
very rapidly. Right, It's a very gentle effect over small distances.
But as you say, larger distances, it adds up, right,
And so between our star and the next star, the
(24:18):
acceleration is a larger number. Between our galaxy and the
next galaxy is an even bigger number between really really
distant objects. Then that speed is faster than the speed
of light. The space is being created faster than the
light can go through it. Right, So I guess maybe
paint the scenario out for us, Like if I wanted
to outrun a flash of light from a flashlight, how
would I do this? Like, we can't start in the
(24:40):
same spot. I would have to go away from you
for a while, right, or a certain amount of distance.
That's right, If you start from the same spot as
the photon, it's going to catch you instantly, Like you
know by definition. If I'm holding the flashlight to your
back and I press the button as soon as you
start running, I caught you before you've even gone anywhere, Right,
A T equal zero? Right if we're both like standing
next to each other. Yeah, But if I give you
(25:01):
a head start, and you start running, right, and you
get a little distance, you get ten ms or so,
or ten seconds before I shine the flashlight. Then if
space is expanding between the flashlight and how far you
got before I turned the flashlight on, then it might
be that that photon never makes it through that space
to catch you. Right. Well, let's take it one step
(25:22):
at a time. Let's say I fly to Jupiter, or
you let me run to Jupiter before you turn on
the flashlight. So now I mean, but in Jupiter and
I'm running somehow in space, and then you shine a
flashlight on me. It's going to take a while, but
it will catch me at that point, right, Eventually it
will catch you at that point, Yes, because the expansion
of space between here and Jupiter is not that impressive,
not enough to overcome the speed of the photon. Right,
(25:45):
It's like the space is growing a little bit, maybe
like what like a millimeter or something per year or something. Yeah,
And a lot of people ask this question, They say,
why can't we see the expansion of space and our
solar system? Why isn't it tearing things apart? Well, the
reason is that gravity is pretty strong on a local scale, right,
Remember it goes like one who were distant squared, and
so the distances are pretty small, like Earth to Jupiter.
(26:08):
Then gravity is more powerful than dark energy, so the
Solar system holds itself together. So the distance between the
Sun and Jupiter is not actually growing at all due
to the expansion of the universe, because they're holding tight
onto each other, the same way the Earth and the
Moon are, or the same way the atoms in your
body are holding on to each other. So that's why
we only really see this thing between galaxies or even
(26:28):
between galaxy clusters, because smaller than that, gravity sort of wins.
It's like tying everybody together. Imagine like a gentle breeze
is blowing out everywhere, but people are holding on to
each other and so they're able to resist it. But
over large distances, this breeze adds up and it becomes
a really powerful force. It's a breezy universe. Alright. Well,
(26:48):
let's say then that you give me a really big
head start and I start running at the nearest galaxy,
which I think is Andromeda. Andromeda. Yeah, I was about
to say that. So let's say you let me get
as far as in Drameda and then I start running,
and then you shine a flash light on me, It's
still gonna catch me. It's still gonna catch you because
the expansion of space between here and Andromeda is not
(27:09):
that impressive, right, So, like, I'll start running, and how
far is Andromeda? Like eight millions of light years? Will
take millions of light years for the light to sort
of get to where I'm running, And in those millions
of years, I will also have run a good bit.
But eventually those two things will catch up, right, Like
the light will eventually. It might take millions of years,
(27:29):
but it'll light will eventually hit me in the back.
That's right. If you are running a constant speed, like
let's say you were running at half the speed of
light because you're super impressive relative to the Earth, and
I shine that flashlight at the speed of light, then
it will eventually catch you. And you'll look back at
the flashlight and you'll say, oh, that light is moving
at the speed of light relative to me, and it
will catch up to you, you know, in several million years,
(27:50):
if you are several million light years ahead when it begins.
But I see, if you're moving at constant speed and
there's not that much expansion of the space between us
and between us and Drama. H it's not enough for
you to outrun the flashlight, right, Like the space between
here and Drama is is expanding, but it's maybe expanding
at I don't know, ten meters per second or something.
(28:10):
That's right, And you know there's a little wrinkled there
because in Drama actually happens to be moving towards us
even though the space is expanding between us, gravity there
is winning and it's pulling and Drameda towards us, and
you know their local deviations, Like space itself is expanding,
but things are still moving through that space as we
talked about, like driven by gravity and other forces. So
Andrameda is getting closer to us, even though the space
(28:33):
is expanding, sort of sort of like swimming upstream against
that expansion, right, I guess I'm just saying, like Andrama
is where I start running, not that I stay with
and Drameda, right, Right, even if you start running from
Andrameda and go past it, then that photon will still
catch you. The expansion of the universe not enough to
overcome that, right, Like it's expanding but only a little bit,
so light can still rip through it. So then kind
(28:53):
of like, at what point does the expansion of the
universe start to approach the speed of light so that
like can't rip through it. So it's something like sixty
billion light years away. If I gave you a head
start of sixty billion light years then and shine a
flashlight at you, that light would never catch you sixty
(29:13):
billion light years. Is there enough universe for me to
get that much of a head start. We just don't know, right,
We have no idea what's out beyond the edge of
the observable universe, And that's actually the threshold of the
observable universe. Like photons that were created at T equal zero,
the beginning of the universe sixty two billion light years
away will never reach us. Those photons, even if they're
(29:36):
pointed right at us, we'll never get here because the
expansion of the universe will create new space faster than
they can move it. So nothing that's beyond that we
will ever ever see. Right. It's kind of like the
resverse problem, right, Like there might be somebody sixty billion
light years away that shines a flashlight at us to
try to tag us, but that light will never catch
(29:57):
up to us because the space is expanding too fast, exactly,
and in the same way a flashlight we send from
here to there. If you start running there, then that
photon is never going to catch those people. No matter
how fast they're going or slow. They could just sit
on their butts and they will never be hit by
that photon. Right, And that's what you call the Hubble's law. Right,
Like the velocity of how space is growing is getting
(30:19):
bigger with distance, That's right. Hubble's law tells us about
the recession velocity, how fast something is moving away from us,
and how that's getting faster and faster as you go
further and further. So as you get further away from us,
things are moving away from us faster and faster. At
some point that speed exceeds the speed of light. And
that's called the Hubble volume. And the Hubble volume is
this spear that surrounds us. Right, And because the universe
(30:41):
is expanding and that expansion is accelerating, then that volume
is actually shrinking. Right. We can see a smaller and
smaller fraction of the universe as time goes on. As
time goes on, you can be closer and closer to us.
Shoot a flashlight at us, and it will never get
to us because this expansion is accelerating. Right, Yeah, that
(31:02):
whole part of the universe, maybe the rest of the
entire universe is basically dark to us, right, it's invisible,
like we can never see it. Yeah, and things that
we used to be able to see are disappearing. Right.
It might be that if you shine a flashlight at
us an early part of the universe, it would get here.
But then later on if you waited too long, if
you waited ten billion years and then showing your flashlight
(31:22):
at us, it wouldn't ever get here because the expansion
has increased and accelerated beyond that threshold. Well, if it
wants to move away that fast, away from us, you know,
maybe we don't want to see it. What have they
got to hide anyway? Yeah, what's wrong with us? What
do you mean? Why are they running away from us
that fast? So then what does that mean? What that
means that the furthest anything we'll ever see is about
(31:42):
sixty two billion years away, Yes, sixty two billion light
years away. If the expansion of the universe continues accelerating
the way that it has, then we will never see
anything further away. Than sixty two billion light years. There's
a caveat. There's always a caveat, a fine print, meaning
like of the space will stop expanding, right, Like, we
don't know, well, we don't understand why that expansion is
(32:05):
accelerating and what's doing it. All we see is that
it's happening. And since we don't understand why, we have
no idea what the mechanism is, we can't predict its future.
We don't know what's driving it. We know that it
turned on a few billion years ago, so we don't
understand any of that. And so we could stop, and
it could turn around. It could shrink the universe so
that the things that we're always invisible to us now
become visible. The simplest thing to do is to extrapolate
(32:26):
that nothing's going to change. But you know, we've been
wrong before, right And like you said, like you have
been sort of wrong before, Like this expansion of the
universe wasn't always there, like it seems to have turned
on at some point. Yeah, exactly. It's not something we
understand very well. We talked about this on the podcast
actually once about the history of dark energy. We think
that maybe the amount of dark energy is constant, but
(32:47):
dark energy doesn't get deluded as the universe grows, like
as more spaces created, every unit of space also has
more dark energy, and so over time it comes to
sort of dominate what's happening in the universe. And that's
why we think maybe it sort of took over about
five billion years ago and became the thing that drove
the whole universe. All right, So that's one way to
(33:07):
answer the question can you outrun a flashlight? And the
answer is yes, you know, because of expanding space. If
the space between you and the person shooting the flashlight
at you is big enough and that space is expanding
fast enough, then you can't outrun light. Amazingly, I guess
you can avoid it. What if I just moved sideways,
(33:28):
Daniel would save us all a lot of trouble. Wow
podcast simplified. So right now, the way to outrun a
flashlight is just travel sixty two billion light years away
and then nobody will ever be able to laser tag
you or invent a dark energy machine that can stretch
space between you and that photon arbitrarily quickly, and then hey,
you can just do it yourself. Wow. That would be
(33:49):
like dark energy tag that would be a totally different product.
All right, Well, it turns out that that's not the
only way that you can outrun a flashlight. There is
another way that you can do it within the rule
of the universe, and it doesn't require you to go
out that far. So let's get into it. But first,
let's take another quick break, right, Daniel, Can you outrun
(34:21):
a flashlight? And we figured out one way to do
that that if you go out far enough, the space
is expanding fast enough that light will never catch up
to you. But apparently there is another way that you
can outrun light without having to depend on the you know,
the diet of space. That's right, And this is a
real mind bender. This one took me a while to
(34:42):
figure out. It feels like it really contradicts everything you
know about special relativity. If you spent a lot of
time thinking about special relativity and adjusting to the idea
that light always moves at the speed of light relative
to everybody else, this one's gonna feel like it breaks
that rule. But it actually is a natural consequence of
special relativity, and it breaks that rule because it breaks
(35:04):
one of the assumptions of special relativity, which is about acceleration.
So the idea is that you can out run a
flash of light if you're moving in a rocket ship
with constant acceleration, if you're always always speeding up, then
that photon will never catch you. And the way that
it avoids the rules of special relativity is that one
(35:24):
word acceleration. Most of the rules of special relativity require
you to not be accelerating to have constant relative velocity. Interesting,
I see, so like, if I'm trying to outrun a
flashlight and I start running, it's not just about never
stopping or always running or always you know, moving, It's
about like each second you have to go a little
(35:45):
bit faster than you were before. That's what acceleration means.
That's right, when Usain Bolt comes out of the blocks,
he goes from zero meters per second to ten per
second pretty quickly. He's accelerating, he's changing his speed, but
then at some point he stays at ten ms or second,
right Like, even losing Bold can't accelerate forever on the ground,
right Like, somehow there's something about his body and air
(36:08):
resistance and his muscles that just prevent them from going
faster and faster and faster and faster. That's right. So
you need constant acceleration to make this work, and you
don't need to be able to go faster than the
speed of light. Is not some trick where you're like
always adding the same amount of speed per second. You
can be constantly accelerating and asthemptotically approaching the speed of light,
(36:29):
never even actually reaching the speed of light. But if
you're getting faster and faster every second, then you can
avoid that photon ever hitting your back. Right, Because even
you are always accelerating, always gaining speed, you're still never
going to go faster than the speed of light. Like somehow,
the way special relativity works, it's like you're just gonna
be trying and trying, but you'll never actually go faster
(36:50):
than the speed of light. You can never reach the
speed of light because you have mass things with mask
and never reach the speed of light. What you can
do is add more energy, right, You can get more energy.
You're energy can increase the arbitrary infinite amounts, but your
velocity doesn't track it approaches the speed of light, never
actually gets there, right, And this is where it gets
kind of counterintuitive, because you know, I mean, I'm accelerating,
(37:13):
going faster and faster, but I'll never actually reach the
speed of light. And so you're saying that even if
somebody shoots his light at me that is going at
the speed of light, even though it will always be
going faster than me, you're saying, there's a possibility that
it might not catch me. Yeah, it will not catch you.
You know. The scenario I'm imagining is that you're like,
you start out ahead of me, maybe I'll give you
(37:34):
a ten meter head start, and you start accelerating, and
then I turned on this flash of light. Then you
know you are going to be going a certain speed,
going faster and faster and faster, approaching the speed of light.
I'm shooting this flashlight that's moving at the speed of light,
and so you might think, well, I'm pretty good special relativity.
If Jorge looks back and asks how fast is that
photon traveling relative to me, he should give the answer
(37:56):
of the speed of light, because, as we said before,
photons travel at the speed of light no matter who's
measuring them, right, And so then it feels pretty simple.
You're like, well, if the photon is moving relative to
me at the speed of light, it will eventually catch me.
And that seems pretty solid. But all those calculations you
just did in your head assume no acceleration. Those calculations
(38:17):
are only true in inertial frames, where there's no acceleration,
there's only relative constant velocity. It's a little bit different.
When you add acceleration. You have to go to general
relativity because acceleration, it turns out, is equivalent to gravity
a right. That's one of Einstein's big discoveries is that
you can't tell the difference, for example, between gravity and acceleration.
(38:39):
If you're like in an elevator in space, you can't
tell is that elevator accelerating or am I near some
planet that's creating gravity. It's the same thing, and so
you have to account for the effective curvature of space
that you're creating for yourself when you're accelerating. Right. But
I guess, maybe let's be clear, like, there are situations
where it will catch you. Right. If I'm standing next
(39:01):
to using bold and he starts running and he's accelerating,
he might be accelerating getting up to his ten per second,
But if I shine a flashlight on him right away,
it's going to catch him, right, if you shine a
flashlight on him soon enough. Absolutely, But if his acceleration
is high enough, right, and he started out with enough
of a head start, then it will never catch him.
If he has constant acceleration. So those are two special things, right,
(39:24):
Like he needs a head start, yes, and I also
have to wait a certain amount of time before I
turn on my flashlight. You can turn on your flashlight
at the same moment as long as he has a
physical head start. He either needs a head start in
time so he can get some distance gap, or he
needs to start like ten ms away from you. But
he can start running at the same moment as you
turn on that flashlight, and the flashlight will never catch
(39:44):
him if he continues to accelerate forever till the end
of which is pretty tough. Yeah, I don't know if
he signed up for that. He might have other things
he wants to do. I mean, somebody give that guy
a power bar or something. I mean in a number
of power bars, right, Because you can look at it
the other way and say, well, the photon will catch
you at time equals infinity because your speed is approaching
(40:05):
the speed of life but never catching it. So at
time equals infinity, the photon will get to you. But
time equals infinity isn't a real time, right, the universe
could go on forever, will never get there. I see.
But there are two ingredients of this, right, Like you
need a head start, and you need to be always
accelerating at a certain rate, right, And I imagine that
if I'm only accelerating a little bit at a time,
(40:27):
that I need a really big head start. But if
I accelerate really fast, if I have like a constant
rocket pushing me, then I don't need that big of
a head start exactly. And you can calculate it by
looking at it from the other point of view, like
from us sain Bolt point of view, What is this
Like if you're riding on his shoulder, for example, and
you're looking back as he's accelerating, then you might think, well,
then what this means is that there's a distance beyond
(40:49):
which you cannot see, Like somebody who shines a flashlight
at you from there tries to send you a message
that information will never reach you. So there's like this horizon,
this wall beyond which it's just lack and the distance
to that wall depends on his acceleration. So if he's
accelerating really really fast, then that horizon is closer. If
he's accelerating really really slowly, the horizon needs to be
(41:10):
further further away, So as the acceleration goes to zero,
that horizon is infinitely far away. But the distance to
that horizon is the speed of light squared divided by
your acceleration. But I guess it still feels counterintuitive, right
because the light is going at the speed of light,
but Hussein Bulb will never reach the speed of light.
So how is it the light will never reach him? Right, Like,
(41:30):
wouldn't it eventually make up ground? Yeah, you would think so,
and that would be true if there was no acceleration.
But remember, acceleration means all these rules get bent a
little bit, the same way like space gets bent. And
so the way that sort of incorporated into your brain
is to think about how acceleration is like an effective
bending of space, and when space gets bent, all of
your intuition about who catches what go out the window.
(41:53):
You know, for example, like if somebody's inside a black
hole and they shine a flashlight at you, that photon
is never getting to you. It doesn't matter how much
time it takes. Why because space is bent, right, and
those photons follow that bent space. So if you are
doing constant acceleration, then it's sort of equivalent to gravity.
It's sort of like bending space. And so that's what
(42:14):
creates this horizon. It's not an event horizon. It's not
like a real physical horizon, but for you, it creates
like an information horizon. Accelerating objects have an information horizon,
which is pretty weird interesting, sort of like a content acceleration.
It's almost like the space between us is expanding, that's
what you're saying. It's like somehow if I accelerate fast
enough and I'm further enough away, then that expansion of space,
(42:37):
which is really just my acceleration, is going to prevent
the light from reaching me. Yeah, exactly. But it's not
like a real physical horizon, right, It's only it's from
your point of view. And again, different people have different
points of view, and those things can conflict. If I'm
watching this whole series of events from a spaceship floating nearby,
I might see the photon catching Usain bolt, right, and
(42:57):
so I can see a different series of events. It's
just like with a black hole, right, if I'm watching
you fall into a black hole from the outside, I
never see you following a black hole. You never get in.
You smeared across the event horizon forever. Oh wait a minute,
for you, you fall right through that event horizon. You're
inside the black hole. In the same way. These different
(43:18):
accounts can conflict. Uh Now I feel a little cheated, Daniel.
I think what you're saying is that if I try
to Lusain Bold, who's always accelerating, Hussein Bold will think
that he ut ran the flash of light, but we
are going to see him lose. The person sending the
light won't see it hit him, because he's accelerating relative
to them, he won't see it hitting them. But another
(43:38):
observer moving in a different direction it's possible for them
to see the light hitting him. The story depends on
exactly the location and velocity of everybody involved. It gets
pretty hairy with general relativity. Man, right, I guess that
you're saying that to us Saint's Bold kind of frame
of reference, his experience of things, that light will reach
him but only in an infinity, like when time ends
(44:00):
for him. But for someone who's moving in another frame
or speed, there is a time at which the light
will hit him. Yes, exactly. There's almost always another velocity
or location you can get to to tell a different
story about the same series of events. That's the lesson
of the universe is that there is no universal history.
There's no true single account of what happens in this universe.
Well there is. I mean, the light does catch up
(44:22):
to him, it's just that for him it happens at infinity,
and for us it happens non infinity. Yeah, if happening
in infinity accounts is happening, then you know, I'll pay
you that twenty infinity. Yeah, let's keep talking here until
infinity and see if that's the same thing for everyone.
But that's kind of what you mean, is like, it
does happen, but at infinity. But that's only from his
(44:42):
point of view. That's right, because relativity is weird in
that way. It's pretty weird stuff, all right. Well, then
that's the answer to the question is can you running flashlight?
The answer is yes if you got far enough and
space is expanding fast enough. And also yes if you
are bold, I guess and you you can't wait till
an infinity or you only consider it infinity as never,
(45:04):
or if you have an infinite number of power bars
and you can accelerate forever into the future. Right to you,
you will always win. But maybe to somebody else the
answer will be different. But you'll never have to talk
to them because you're accelerating away from them forever. That's
the true benefit here is never haven't you talked to
or see anyone? Ever? Again? All right, that's a pretty
(45:24):
mind bending question, and again just a reminder of how
weird this universe is and how weird the rules of
it are. Right, it's not just that there are weird
things in it. It's just that it is a weird
universe in itself. It certainly is. But we love it.
We love the weirdness. Stay weird universe. All right, Well,
we hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel
(45:52):
and Jorge explain the Universe is a production of I
Heart Radio or more podcast from my Heart Radio. Visit
the I Heart Radio, Apple Apple Podcasts, or wherever you
listen to your favorite shows. H
Hey, Daniel, what's the fastest a person has ever run? Well,
Hussain Bolt, the fastest person in history, actually only runs
about twenty miles per hour. But what's the fastest anything
has ever gone on Earth? That's about seven hundred and
sixty three miles per hour in a crazy rocket powered car.
That sounds like a good idea. But what about in space?
(00:31):
What's the fastest a spaceship has ever gone? There's the
Parker Solar Probe, which is whizzed around the Sun, and
it reached about five percent of the speed of light.
All right, so then technically nothing humans have ever made
has ever approached the speed of flight except for actual
light from flashlights. Oh well, but technically we made the flashlight,
but then the flashlight made the light. We want our
(00:52):
names on the pack as well? Yeah, can we label
each photon made by Jorge hi am or? Hey, I'm
(01:13):
a cartoonist and the creator of PhD comics. Hi, I'm Daniel.
I'm a particle physicist and the creator of many photons.
Oh really, you have a certain glow about you. Is
that what you're saying to all physicists? Glow? I think
usually it's not a good idea. If you're working with
radioactive things, especially if you come from Los Almos, then
you do tend to glow in the dark. But now
every single one of us gives up a unique pattern
(01:34):
of photons that we personally have crafted. My photons are artisanal.
Oh yeah, like you indivigitally craft each one. Yeah, they're
not very good. Isn't that what artisanal means? Yeah, I
guess what you mean is that you glow like in
the infrared, like from our body heat. Yeah, I definitely
do give off photons like every object in the universe,
if you have a temperature and then you glow. Sometimes
(01:55):
it's not in the visible light, but you definitely are
pumping out photons into the universe. Even if you are
a black hole, well then you are a shining example
I think for all of us. Welcome for our podcast,
Daniel and Jorge Explained the Universe, a production of I
Heart Radio in which we try to shine the light
of knowledge into your mind, pushing back on the forefront
of ignorance and talking about all of the biggest questions
(02:16):
in the universe, questions about the universe, questions around the universe,
questions in the universe, and questions from the universe, and
we talk about all of it and try to make
jokes to entertain you. Yeah, because there is a lot
in the universe out there that is bright and interesting
and powerful and curious and exciting, and there are also
dark things out there that are just escaped our site.
(02:37):
That's right. We can only see a little fraction of
the universe, an unknown fraction of the universe, because information
comes to us usually via light, and that light travels
at a very very fast but not infinite speed, and
we have learned all sorts of crazy things about how
information travels through the universe. Yeah, because the universe is
not just filled with mysterious things, it's also sort of
(02:58):
filled with mysterious rules. The rules of the universe are
not quite what we experience in an everyday basis, and
some of them kind of go against our intuition about
how things actually work. And thank God for that, or
my job would be boring. I think about science is
like a grand mystery novel. We're trying to figure out
how things work and deduce what the rules are from
(03:19):
the little clues left here and there. And you're absolutely
right that along the way we have figured out that
the universe is not the one we thought we were
living in, that the rules are weird, that the rules
are strange, that they require real creativity to on earth
how things actually work. Are you saying the universe is
not cliche? Thankfully, It's got it's twists and turns. I've
seen this universe so many times before. Oh my gosh,
(03:42):
bang crunch, bang, crunch, bang crunch. Hey do you think
every time there's a big bang and a big crunch
and we come back with the same universe that would
be boring? Yeah, obviously it was made by committee the
big bang. Let's just reboot the last universe that's sold
pretty well. Yeah right. Audiences don't want anything new. People
don't want to exist in the in a new universe.
This is the new J. J Abrams Big Bang theory.
(04:04):
Oh a dig on J. J Abrams. I feel like
you're always looking for an excuse there. Hey man, he
created the whole universe. You know, I got notes, but
it's pretty impressive. Wait. JJ Abrams created the whole universe
this theory. Yes, there's a J. J Abrams equivalent, you know,
writing the simulation or guiding the direction of the universe
that would explain all the lens flares, and I see
(04:24):
all the time when I look at the universe. But yeah,
it is a weird universe with weird rules, and there
is none weirder than special relativity, which is kind of
one of our main theories about the universe. That's right.
It turns out that when you start going really really quickly,
the rules that work at slow speeds down here on
Earth no longer apply. The rules are actually quite different,
(04:45):
and for hundreds and thousands of years we have been
learning rules that only work under special conditions, very very
slow movement. If you actually push the boundaries and start
going fast, the universe reveals that things we thought were
true are not actually true, leadings all sorts of weird
consequences that really violate our intuition. They break our notion
of like simultaneity and the idea of time being universal. Right. Yeah,
(05:09):
do you think like if we were all moving faster,
we would be used to these strange special rules, and
like our current rules would team we would have to
be moving really really fast. But I think in that
scenario our current rules would just be a natural extension
of what we already understood. But it's hard to imagine
growing up in a universe where special relativity feels intuitive,
(05:30):
where it makes sense to you, where our conclusions about
the universe would seem strange. But I'm sure somebody's written
that science fiction story, or maybe not. It's all there
for you, Daniel, somebody out there right it. But it's
called special relativity because it sort of applies to special situations.
Why does it have that special name? Special relativity is
called special because it's not general relativity. It applies to
(05:51):
scenarios where space is flat. We don't have to think
about space actually being curved and like changing the path
of photons or changing the motion of Earth around the star.
It has to do only with a sort of simpler
scenario where space is mostly empty and you're like shooting
laser pulses back and forth, or you have light bulbs
on trains. It has to do with relative velocities of
(06:11):
things and how information moves through the universe. It avoids
all complications from curved space. Oh really, wow, I never
knew that. I always thought that it was called special
relativity because it was special, But actually you're sort of
sitting the opposite. It's special because it only applies to
a boring universe. Yeah, it's a specialized condition. Right. He
came up with this first to understand this, and he thought, well,
you know, it's much trickier if space is actually curved,
(06:33):
and then he generalized it. He made it much more broad.
Special relativity is a subset of general relativity under the
conditions that basically the universe is mostly empty. You hadn't
figured that part out yet. So it's not like special
because people think it's like it ess special. It's just
special because actually it's like specially boring relativity that would
be technically more accurate. Right, Yeah, it's a special case.
(06:56):
It's not a better case. It's a simple case. But
we often do this in physics. We think about simple
scenarios to help us like distill what's going on and
get the clearest picture, so we can separate these ideas
because even in the scenario where you have no big
masses destroying the shape of space like black holes or
even just the sun, there are lots of weird things
that happened in special relativity. You know, clocks that don't
(07:17):
agree because you're moving at high speed. It's pretty weird
and hard to get your mind around. Even if there
is no curvature of space. So I think it was
sort of a good intellectual exercise. But even though it
applies to boring situations, it's still true, right, Like it's
still applies to the whole universe. It still applies to
the whole universe. The rules of special relativity do assume
(07:37):
that there are no heavy masses, and so if you
have big masses around curving space, you can't use calculations
from special relativity. You've got to use general relativity. But
I guess then the irony is that special relativity is
actually especially boring relativity. But it gives rise to these
really strange situations about the universe that don't seem to
make sense. And a lot of that has to do
(07:59):
with light experiments involving light. You know, there was this
famous experiment by Michaelson and Morley that showed that the
speed of light doesn't change no matter how fast the
Earth is moving through space. For example, that no matter
who measures a photon, they always measure it moving at
the speed of light, no matter how fast that person
is actually moving relative to anything else. It's pretty weird stuff. Yeah,
(08:20):
And so this weirdness is sort of a maybe especially
illustrated by asking a very simple question that you can
ask about the speed of light. So to be on
the podcast, we'll be asking the question is it possible
to outrun a flashlight? First of all, the flashlights run?
(08:41):
Do they have little legs that I don't know about?
Or that reminds me the old joke about refrigerators. Is
your refrigerator running? No, you better check the power. Your
flashlights don't actually run, of course, but the photons from
them come out at a blistering speed right of three
times tend the meters per second, So it's pretty hard
to imagine our pacing a flashlight. So it's more like,
is it possible to outrun a flash of light from
(09:04):
a flashlight? Yeah? I think the scenere I'm imagining is
you take off as your highest speed and I'm standing
behind you with a flashlight. Is it possible that once
I send off a flash of light, that you could
outpace it, that that flash of light would not catch you,
like it would not shine on me, like the photons
would never hit me. Yes, exactly. Or from your point
of view, is it possible you could run fast enough
(09:25):
that you could look backwards and you could not see
me right, that I would be passed some sort of
horizon beyond which you could not see. I see like
I would never know that you turned on the flashlight
because those photons would never reach me. Like I'll be
running and be like Daniel still hasn't but you did.
But it's the light would never reach me. And why
(09:46):
are you running away from me so quickly? What did
I don't know? Why are you shining a light on me?
What are you trying to do blind me? I'm just
playing flashlight tag? Man, chill out. So this applies to
that nineties toy laser tag, actually exactly. This helps you
strategize for laser tag. This is special relativity laser tag. Yeah,
physics is useful for all kinds of situations, even ones
(10:09):
that required to travel to the nineties and play laser tag.
I knew this degree would come in handy some day. Well,
at first, you have to solve time travel. I'm working
on it. I'm working on it, all right. So then
that's the scenario we're asking is if I take off
running and you shine a flashlight at me, is it
possible for me to outrun those photons or will they
inevitably hit me at some point in the future. So
(10:29):
that's a pretty interesting question, and so we thought we'd
post it to people on the internet and see what happens.
So usually Daniel went out there and ask listeners if
they thought that one could outrun a flash of light.
And so, as usual, I'm immensely grateful to all of
you who wanted to participate in the podcast and answer
these weird and random questions. If you're out there and
(10:49):
you're a listener to the podcast and you've been itching
to participate but you haven't quite yet, please send me
a message to questions at Daniel and Jorge dot com.
Think about it for a second. Do you think it
is possible to outrun a flashlight? Here's what people had
to say. Um, I don't think that I can. I'm
not faster than lights, so I don't think that I
(11:10):
could do that. I'd like to believe that you can
outrund a flashlight, but for that you have to be faster,
traveling faster than the speed of light, and as far
as I know, nothing in the universe can travel faster
than the speed of light. Yes, definitely, just don't throw
it too hard so you can, you know, run faster.
I think light from a flashlight travels its speeds much
(11:32):
less than the speed of lights and a vacuum that's
slowed down by the materials in its way as it
moves from inside the flashlight outside of it. And then
there are many different materials in the way, so because
of each material's index of refraction, the speed will be
reduced by a number of factors. As for whether I
can outrun it, I don't think so. I think it
(11:55):
will still be faster than my running speed. I guess
it depends on how fast it's the out No, if
if you actually mean the light itself, then nothing can
trovel a fast light, so you cannot let it. Sure, Yeah,
the flashlight itself is just like the box that that
sends out the light, right, So you could just set
(12:16):
the flashlight on the table and then run past it.
Would would outrun a flashlight? No, because it would really
piss s Einstein and also added additivity, I think maybe
the only way to kind of out run a flash
the light coming out of a flashlight would be maybe
(12:36):
if you could go through a worm hole. So if
the light from a flashlight, let's say, is headed towards
Jupiter and there's a wormhole between Earth and Jupiter, and
you took that shortcut in your spaceship m you could
probably hopefully get to Jupiter faster than the light from
(12:59):
your flashlight quid Rich Jupiter. No, you cannot outrun a
flashlight because the light traveling from the torch you're holding
will always be traveling at the speed of light relative
to you, so will always be traveling away from you
speed a light. All right, some pretty good answers here.
Somebody said you can't outrun the flashlight itself. I feel
(13:21):
like that's a different philosophical question, like can I throw
something at you that will always hit you? Yeah? Or
is the light part of the flashlight? After all? Will
always be the flashlight of infinite extent because of the
photons that come out of it. It's like the photon
of theseus kind of maybe the flashlight of theseus. Yeah,
Like is the photon from a flashlight part of the flashlight?
(13:44):
Bump bump bump. That's if somebody's thesis right there in
philosophy of science done? All right, I'll take that degree.
But yeah, lots of interesting answers here. Most people that
say that maybe not because you can't go faster than
the speed of light. So if light always goes as
fast as anything can go into universe, it will eventually
catch you, right, Yeah, and it's a very reasonable answer
given most people's understanding of special relativity. So yeah, I
(14:07):
see some clever answers here too, like what if there's
a wormhole? Did you think about that one? Yeah? But
why can't the light go through the wormhole? Also? Right,
if you run through photons right behind, you can't go
through the wormhole. So I guess if you open and
close that stargate really quickly, right, Yeah, Yeah, a lot
of clever answers here, but let's jump into answering this question,
(14:28):
can you out run a flash of light? And so
I guess maybe we should talk about a little bit
about this idea of the speed of light and special
relativity and why exactly it is kind of weird or
why weird things might happen if you try to this
experiment in space. Yeah, And the basic thing that we
need to understand is how different people measure velocity. Like
(14:50):
the way to think about it intuitively is imagine like
somebody throwing a natural object like a ball, if they
are in a car and they throw a ball forward
at ten miles an hour and then it's being a
ten miles an hour relative to them. No big deal.
But if the car is also driving at ten miles
an hour relative to the ground, then you might ask, well,
how fast is the ball moving relative to the ground. Well,
(15:11):
it's ten miles an hour relative to the car, and
the car is moving ten miles an hour relative to
the ground, So obviously twenty miles per hour, right, And
you think that's obvious and it's intuitive. And what you're
doing there is you're applying a rule which you sort
of intuitely we have figured out and applied, and it's
a Galilean transformation. It says, the speed of the ball
relative to the ground is the speed of the ball
(15:33):
relative to the car plus the speed of the car
relative to the ground. And that mostly works. But what
we discovered is that it's not true at very very
high speeds, and most specifically, it's not true for light.
So if I'm standing in a car and I shine
a flashlight, how fast is the light leaving my flashlight? Well,
the speed of light right now, the car is moving
(15:54):
in ten miles per hour. How fast is the light
traveling relative to the ground. Well, you're old rule would say, well,
if the speed of light plus ten miles per hour, right,
like faster than the speed of light. But that can't happen.
And so the light always travels at the speed of light,
no matter who's measuring it and how fast they are
going relative to the thing that made the light. So
(16:14):
the person in the car measures the photon is going
at the speed of light, and the person on the
ground measures the photon and going at the speed of light. Right, Yeah,
you had me on galilee and transformation. I think what
you're saying is that, you know, we're used in our
everyday lives this idea that velocities like add like they
add with simple like arithmetic, like you know one plus
one equals too, but that things get weird with the
(16:36):
speed of light because nothing can go faster than the
speed of light. So like you can't keep adding velocities
because that would eventually make them go faster than the
speed of light. Yeah, they add nonlinearly, right, so they
get closer and closer to the speed of light, but
they don't just stack up on top of each other
in a simple way. And so as you say things
with masking get faster and faster and approached the speed
(16:56):
of light. But nothing can go faster than the speed
of light, and so you have to have an new
addition rule. It doesn't just like A plus B. It's
some weird combination of A and B that helps you
approach the speed of light but never gets you past it.
And for light itself, it's always at the speed of light,
never slower, never faster, right, It's kind of this weird thing.
Like if you're a photon that's shooting out of a flashlight,
(17:18):
and so you're going at the speed of light, and
then somehow you as that photon shoot off another photon
in front of you, that photon is not gonna go
like a twice the speed of light. It's going to
go still at the speed of light. That's exactly right.
Although a photon itself can't have a frame of reference
because photon can't be at rest, so you can't measure
the speed of one photon relative to another one, which
(17:40):
is another for the tricky little wrinkle there. But exactly
if somebody's flying in a spaceship near the speed of
light relative to Earth and they turn on a flashlight,
that photon is not going at like one times the
speed of light relative to Earth. It's only moving at
the speed of light. And that's pretty weird, right, because
these things no longer add up. It's like the people
in the spaceship tell a different story about what happened
(18:02):
than the people on Earth. Because the people on Earth
see that floton as moving at near the speed of light,
it's relative speed to the ship is actually quite small,
whereas on the ship that people see the photon is
moving away from them at the speed of light. And
so you get like a different story about what happened.
And that's the mind bending thing about special relativities, that
different observers tell different stories about what happened, and they're
(18:25):
both accurate. They're both like honest observers telling conflicting stories
and both being correct. But I guess it's not about
different things happening. It's more about our perception of these
things happening, maybe because it's all related to how it
affects time, right, like time is sort of flexible, time
is not universal, and exactly we don't have a consistent
(18:45):
clock through the whole universe that says like what happened
at every moment, and then what happened at the next moment.
What happens depends on where you are and how fast
you are going relative to the events, and sometimes this
flexibility there. If there's no like causal connection where one
thing has to happen before another, then different people can
give different orders of events for what happened and both
(19:06):
be correct. It's not just an issue of perception. Although
you can, you know, use special relativity to say, well,
I understand why you are seeing the opposite thing that
I'm seeing. That's because of special relativity, but neither of
you can say like A happen before b or be
happen for a It depends on who you are and
how fast you're going. Right, It's like nobody's clock is official.
(19:27):
Everyone has a different clock, and so that you can't
sort of say, like who's right or who's wrong, because
in the end, everything is relative. Right, velocity itself is relative.
You can't have a velocity just on its own. You
can't say my spaceship is going at the speed of
light and say well relative to what right, As we
talked about once recently, like velocity doesn't even have a
(19:47):
meaning if you are in a universe all by yourself, like,
you can't have a speed if there's nothing else in
the universe. So it's a whole new way to think
about the nature of the universe and all these weird consequences,
a lot of which come from this idea that light
always travels at the speed of light. Yeah, So while
it may seem intuitive that if you shine a flashlight
at me, no matter how fast I'm going, that light
(20:08):
is going to catch up to me, that may not
actually be true depending on some of these weird consequences
of special relativity. That's right in a simple case where
you're like in a normal universe and space is flat
and it's obeying these rules, then if somebody shines a
flashlight in your direction, it doesn't matter how far away
they are. Eventually that light will reach you. There's no
(20:29):
like effective horizon, give an infinite time, that photon will
catch you. There's nothing you can do, right, and that's
because it's moving at the speed of light relative to you.
Because light always moves at the speed of light. So
in this sort of simple universe where space is not curved,
nothing weird is going on. That flash of light will
always catch you, right. But in our universe, the answer
might be different. But first, let's take a quick break.
(21:04):
All right, Daniel, you're trying to outrun a flashlight. I
don't know why. I guess you don't want people to
see you, or you want to remain sort of in
the dark and mysterious. But you're trying to outrun a
flashlight and somebody's shying a flashlight on you, and so
it seems like it would inevitably the light would reach
me because it's going faster than anything can go in
the universe. But you're saying there might be some instances
(21:24):
where I can actually outrun light. Mm hmmmm. And you
know this is not just a silly physics thought experiment.
This is really important. If you were playing laser tag
and you're cornered and they shoot that thing right at you,
you gotta find a way to escape. And so that's
where we come in, right. This is totally serious and
totally relevant to everyone's everyday lighte No, there are some
scenarios where somebody points a flashlight at you and it
(21:47):
doesn't ever hit you even an infinite time, even if
it's pointed right at you, and you don't actually even
have to be moving to avoid it. In some scenarios.
So you might have heard my sort of legalistic maneuver
there to avoid saying that the flashlight will always catch you.
The condition is that if you are operating in flat space,
in space, that's just where general relativity applies, and nothing
(22:08):
is growing or shrinking. Right, So that gives us one
that maybe the first instance in which you could maybe
outrun a flashlight. Right. It has to do with this
idea that space is not actually flat or constant or
kind of boring. Yeah, it turns out that our universe
is a lot more exciting than the special relativity universe, right.
Most importantly, our universe is expanding. Now, A lot of
(22:30):
people think that that means that galaxies are flying out
from some sort of central dot at a high speed,
that they're moving through space. But it's actually much weirder
and more amazing than that. It's the expansion of space itself. Right.
It's not the motion of things through space. I mean
that's also happening, but dark energy. There's accelerating expansion of
the universe. This is creating new space between our galaxy
(22:54):
and other galaxies. It's not just moving the galaxies through
the existing space. It's like stre watching the fabric of
the universe itself. Right, I think that you're saying sort
of like maybe I could outrun Hussein Bold if maybe
I play around with the track, or like how the
track is moving. Yeah, exactly. Put Usain Bolt on a
treadmill and stand in front of the treadmill. It doesn't
(23:15):
matter how fast he runs, That's what I mean. Yeah. Yeah,
And in the same way, the universe is laying new track,
new space between us and some really really distant photons,
photons which left their galaxy or their star billions of
years ago and they've been straining to reach us. Some
of them will never get to us, even though they're
(23:35):
pointed right at us, because the space between us and
them is expanding faster than light can go through it,
faster than the speed of light. Right. Well, it's it's
not that the space itself at any given point is
expanding faster than the speed of light. But it's more
like there's so much space in between, and it's growing
a little bit at each spot, so much that it
(23:56):
overall is expanding faster than the speed of light. Yes,
it's a very gentle effect on a local scale. Right,
It's not like the distance between you and your partner,
or between the Earth and the Sun is growing very
very rapidly. Right, It's a very gentle effect over small distances.
But as you say, larger distances, it adds up, right,
And so between our star and the next star, the
(24:18):
acceleration is a larger number. Between our galaxy and the
next galaxy is an even bigger number between really really
distant objects. Then that speed is faster than the speed
of light. The space is being created faster than the
light can go through it. Right, So I guess maybe
paint the scenario out for us, Like if I wanted
to outrun a flash of light from a flashlight, how
would I do this? Like, we can't start in the
(24:40):
same spot. I would have to go away from you
for a while, right, or a certain amount of distance.
That's right, If you start from the same spot as
the photon, it's going to catch you instantly, Like you
know by definition. If I'm holding the flashlight to your
back and I press the button as soon as you
start running, I caught you before you've even gone anywhere, Right,
A T equal zero? Right if we're both like standing
next to each other. Yeah, But if I give you
(25:01):
a head start, and you start running, right, and you
get a little distance, you get ten ms or so,
or ten seconds before I shine the flashlight. Then if
space is expanding between the flashlight and how far you
got before I turned the flashlight on, then it might
be that that photon never makes it through that space
to catch you. Right. Well, let's take it one step
(25:22):
at a time. Let's say I fly to Jupiter, or
you let me run to Jupiter before you turn on
the flashlight. So now I mean, but in Jupiter and
I'm running somehow in space, and then you shine a
flashlight on me. It's going to take a while, but
it will catch me at that point, right, Eventually it
will catch you at that point, Yes, because the expansion
of space between here and Jupiter is not that impressive,
not enough to overcome the speed of the photon. Right,
(25:45):
It's like the space is growing a little bit, maybe
like what like a millimeter or something per year or something. Yeah,
And a lot of people ask this question, They say,
why can't we see the expansion of space and our
solar system? Why isn't it tearing things apart? Well, the
reason is that gravity is pretty strong on a local scale, right,
Remember it goes like one who were distant squared, and
so the distances are pretty small, like Earth to Jupiter.
(26:08):
Then gravity is more powerful than dark energy, so the
Solar system holds itself together. So the distance between the
Sun and Jupiter is not actually growing at all due
to the expansion of the universe, because they're holding tight
onto each other, the same way the Earth and the
Moon are, or the same way the atoms in your
body are holding on to each other. So that's why
we only really see this thing between galaxies or even
(26:28):
between galaxy clusters, because smaller than that, gravity sort of wins.
It's like tying everybody together. Imagine like a gentle breeze
is blowing out everywhere, but people are holding on to
each other and so they're able to resist it. But
over large distances, this breeze adds up and it becomes
a really powerful force. It's a breezy universe. Alright. Well,
(26:48):
let's say then that you give me a really big
head start and I start running at the nearest galaxy,
which I think is Andromeda. Andromeda. Yeah, I was about
to say that. So let's say you let me get
as far as in Drameda and then I start running,
and then you shine a flash light on me, It's
still gonna catch me. It's still gonna catch you because
the expansion of space between here and Andromeda is not
(27:09):
that impressive, right, So, like, I'll start running, and how
far is Andromeda? Like eight millions of light years? Will
take millions of light years for the light to sort
of get to where I'm running, And in those millions
of years, I will also have run a good bit.
But eventually those two things will catch up, right, Like
the light will eventually. It might take millions of years,
(27:29):
but it'll light will eventually hit me in the back.
That's right. If you are running a constant speed, like
let's say you were running at half the speed of
light because you're super impressive relative to the Earth, and
I shine that flashlight at the speed of light, then
it will eventually catch you. And you'll look back at
the flashlight and you'll say, oh, that light is moving
at the speed of light relative to me, and it
will catch up to you, you know, in several million years,
(27:50):
if you are several million light years ahead when it begins.
But I see, if you're moving at constant speed and
there's not that much expansion of the space between us
and between us and Drama. H it's not enough for
you to outrun the flashlight, right, Like the space between
here and Drama is is expanding, but it's maybe expanding
at I don't know, ten meters per second or something.
(28:10):
That's right, And you know there's a little wrinkled there
because in Drama actually happens to be moving towards us
even though the space is expanding between us, gravity there
is winning and it's pulling and Drameda towards us, and
you know their local deviations, Like space itself is expanding,
but things are still moving through that space as we
talked about, like driven by gravity and other forces. So
Andrameda is getting closer to us, even though the space
(28:33):
is expanding, sort of sort of like swimming upstream against
that expansion, right, I guess I'm just saying, like Andrama
is where I start running, not that I stay with
and Drameda, right, Right, even if you start running from
Andrameda and go past it, then that photon will still
catch you. The expansion of the universe not enough to
overcome that, right, Like it's expanding but only a little bit,
so light can still rip through it. So then kind
(28:53):
of like, at what point does the expansion of the
universe start to approach the speed of light so that
like can't rip through it. So it's something like sixty
billion light years away. If I gave you a head
start of sixty billion light years then and shine a
flashlight at you, that light would never catch you sixty
(29:13):
billion light years. Is there enough universe for me to
get that much of a head start. We just don't know, right,
We have no idea what's out beyond the edge of
the observable universe, And that's actually the threshold of the
observable universe. Like photons that were created at T equal zero,
the beginning of the universe sixty two billion light years
away will never reach us. Those photons, even if they're
(29:36):
pointed right at us, we'll never get here because the
expansion of the universe will create new space faster than
they can move it. So nothing that's beyond that we
will ever ever see. Right. It's kind of like the
resverse problem, right, Like there might be somebody sixty billion
light years away that shines a flashlight at us to
try to tag us, but that light will never catch
(29:57):
up to us because the space is expanding too fast, exactly,
and in the same way a flashlight we send from
here to there. If you start running there, then that
photon is never going to catch those people. No matter
how fast they're going or slow. They could just sit
on their butts and they will never be hit by
that photon. Right, And that's what you call the Hubble's law. Right,
Like the velocity of how space is growing is getting
(30:19):
bigger with distance, That's right. Hubble's law tells us about
the recession velocity, how fast something is moving away from us,
and how that's getting faster and faster as you go
further and further. So as you get further away from us,
things are moving away from us faster and faster. At
some point that speed exceeds the speed of light. And
that's called the Hubble volume. And the Hubble volume is
this spear that surrounds us. Right, And because the universe
(30:41):
is expanding and that expansion is accelerating, then that volume
is actually shrinking. Right. We can see a smaller and
smaller fraction of the universe as time goes on. As
time goes on, you can be closer and closer to us.
Shoot a flashlight at us, and it will never get
to us because this expansion is accelerating. Right, Yeah, that
(31:02):
whole part of the universe, maybe the rest of the
entire universe is basically dark to us, right, it's invisible,
like we can never see it. Yeah, and things that
we used to be able to see are disappearing. Right.
It might be that if you shine a flashlight at
us an early part of the universe, it would get here.
But then later on if you waited too long, if
you waited ten billion years and then showing your flashlight
(31:22):
at us, it wouldn't ever get here because the expansion
has increased and accelerated beyond that threshold. Well, if it
wants to move away that fast, away from us, you know,
maybe we don't want to see it. What have they
got to hide anyway? Yeah, what's wrong with us? What
do you mean? Why are they running away from us
that fast? So then what does that mean? What that
means that the furthest anything we'll ever see is about
(31:42):
sixty two billion years away, Yes, sixty two billion light
years away. If the expansion of the universe continues accelerating
the way that it has, then we will never see
anything further away. Than sixty two billion light years. There's
a caveat. There's always a caveat, a fine print, meaning
like of the space will stop expanding, right, Like, we
don't know, well, we don't understand why that expansion is
(32:05):
accelerating and what's doing it. All we see is that
it's happening. And since we don't understand why, we have
no idea what the mechanism is, we can't predict its future.
We don't know what's driving it. We know that it
turned on a few billion years ago, so we don't
understand any of that. And so we could stop, and
it could turn around. It could shrink the universe so
that the things that we're always invisible to us now
become visible. The simplest thing to do is to extrapolate
(32:26):
that nothing's going to change. But you know, we've been
wrong before, right And like you said, like you have
been sort of wrong before, Like this expansion of the
universe wasn't always there, like it seems to have turned
on at some point. Yeah, exactly. It's not something we
understand very well. We talked about this on the podcast
actually once about the history of dark energy. We think
that maybe the amount of dark energy is constant, but
(32:47):
dark energy doesn't get deluded as the universe grows, like
as more spaces created, every unit of space also has
more dark energy, and so over time it comes to
sort of dominate what's happening in the universe. And that's
why we think maybe it sort of took over about
five billion years ago and became the thing that drove
the whole universe. All right, So that's one way to
(33:07):
answer the question can you outrun a flashlight? And the
answer is yes, you know, because of expanding space. If
the space between you and the person shooting the flashlight
at you is big enough and that space is expanding
fast enough, then you can't outrun light. Amazingly, I guess
you can avoid it. What if I just moved sideways,
(33:28):
Daniel would save us all a lot of trouble. Wow
podcast simplified. So right now, the way to outrun a
flashlight is just travel sixty two billion light years away
and then nobody will ever be able to laser tag
you or invent a dark energy machine that can stretch
space between you and that photon arbitrarily quickly, and then hey,
you can just do it yourself. Wow. That would be
(33:49):
like dark energy tag that would be a totally different product.
All right, Well, it turns out that that's not the
only way that you can outrun a flashlight. There is
another way that you can do it within the rule
of the universe, and it doesn't require you to go
out that far. So let's get into it. But first,
let's take another quick break, right, Daniel, Can you outrun
(34:21):
a flashlight? And we figured out one way to do
that that if you go out far enough, the space
is expanding fast enough that light will never catch up
to you. But apparently there is another way that you
can outrun light without having to depend on the you know,
the diet of space. That's right, And this is a
real mind bender. This one took me a while to
(34:42):
figure out. It feels like it really contradicts everything you
know about special relativity. If you spent a lot of
time thinking about special relativity and adjusting to the idea
that light always moves at the speed of light relative
to everybody else, this one's gonna feel like it breaks
that rule. But it actually is a natural consequence of
special relativity, and it breaks that rule because it breaks
(35:04):
one of the assumptions of special relativity, which is about acceleration.
So the idea is that you can out run a
flash of light if you're moving in a rocket ship
with constant acceleration, if you're always always speeding up, then
that photon will never catch you. And the way that
it avoids the rules of special relativity is that one
(35:24):
word acceleration. Most of the rules of special relativity require
you to not be accelerating to have constant relative velocity. Interesting,
I see, so like, if I'm trying to outrun a
flashlight and I start running, it's not just about never
stopping or always running or always you know, moving, It's
about like each second you have to go a little
(35:45):
bit faster than you were before. That's what acceleration means.
That's right, when Usain Bolt comes out of the blocks,
he goes from zero meters per second to ten per
second pretty quickly. He's accelerating, he's changing his speed, but
then at some point he stays at ten ms or second,
right Like, even losing Bold can't accelerate forever on the ground,
right Like, somehow there's something about his body and air
(36:08):
resistance and his muscles that just prevent them from going
faster and faster and faster and faster. That's right. So
you need constant acceleration to make this work, and you
don't need to be able to go faster than the
speed of light. Is not some trick where you're like
always adding the same amount of speed per second. You
can be constantly accelerating and asthemptotically approaching the speed of light,
(36:29):
never even actually reaching the speed of light. But if
you're getting faster and faster every second, then you can
avoid that photon ever hitting your back. Right, Because even
you are always accelerating, always gaining speed, you're still never
going to go faster than the speed of light. Like somehow,
the way special relativity works, it's like you're just gonna
be trying and trying, but you'll never actually go faster
(36:50):
than the speed of light. You can never reach the
speed of light because you have mass things with mask
and never reach the speed of light. What you can
do is add more energy, right, You can get more energy.
You're energy can increase the arbitrary infinite amounts, but your
velocity doesn't track it approaches the speed of light, never
actually gets there, right, And this is where it gets
kind of counterintuitive, because you know, I mean, I'm accelerating,
(37:13):
going faster and faster, but I'll never actually reach the
speed of light. And so you're saying that even if
somebody shoots his light at me that is going at
the speed of light, even though it will always be
going faster than me, you're saying, there's a possibility that
it might not catch me. Yeah, it will not catch you.
You know. The scenario I'm imagining is that you're like,
you start out ahead of me, maybe I'll give you
(37:34):
a ten meter head start, and you start accelerating, and
then I turned on this flash of light. Then you
know you are going to be going a certain speed,
going faster and faster and faster, approaching the speed of light.
I'm shooting this flashlight that's moving at the speed of light,
and so you might think, well, I'm pretty good special relativity.
If Jorge looks back and asks how fast is that
photon traveling relative to me, he should give the answer
(37:56):
of the speed of light, because, as we said before,
photons travel at the speed of light no matter who's
measuring them, right, And so then it feels pretty simple.
You're like, well, if the photon is moving relative to
me at the speed of light, it will eventually catch me.
And that seems pretty solid. But all those calculations you
just did in your head assume no acceleration. Those calculations
(38:17):
are only true in inertial frames, where there's no acceleration,
there's only relative constant velocity. It's a little bit different.
When you add acceleration. You have to go to general
relativity because acceleration, it turns out, is equivalent to gravity
a right. That's one of Einstein's big discoveries is that
you can't tell the difference, for example, between gravity and acceleration.
(38:39):
If you're like in an elevator in space, you can't
tell is that elevator accelerating or am I near some
planet that's creating gravity. It's the same thing, and so
you have to account for the effective curvature of space
that you're creating for yourself when you're accelerating. Right. But
I guess, maybe let's be clear, like, there are situations
where it will catch you. Right. If I'm standing next
(39:01):
to using bold and he starts running and he's accelerating,
he might be accelerating getting up to his ten per second,
But if I shine a flashlight on him right away,
it's going to catch him, right, if you shine a
flashlight on him soon enough. Absolutely, But if his acceleration
is high enough, right, and he started out with enough
of a head start, then it will never catch him.
If he has constant acceleration. So those are two special things, right,
(39:24):
Like he needs a head start, yes, and I also
have to wait a certain amount of time before I
turn on my flashlight. You can turn on your flashlight
at the same moment as long as he has a
physical head start. He either needs a head start in
time so he can get some distance gap, or he
needs to start like ten ms away from you. But
he can start running at the same moment as you
turn on that flashlight, and the flashlight will never catch
(39:44):
him if he continues to accelerate forever till the end
of which is pretty tough. Yeah, I don't know if
he signed up for that. He might have other things
he wants to do. I mean, somebody give that guy
a power bar or something. I mean in a number
of power bars, right, Because you can look at it
the other way and say, well, the photon will catch
you at time equals infinity because your speed is approaching
(40:05):
the speed of life but never catching it. So at
time equals infinity, the photon will get to you. But
time equals infinity isn't a real time, right, the universe
could go on forever, will never get there. I see.
But there are two ingredients of this, right, Like you
need a head start, and you need to be always
accelerating at a certain rate, right, And I imagine that
if I'm only accelerating a little bit at a time,
(40:27):
that I need a really big head start. But if
I accelerate really fast, if I have like a constant
rocket pushing me, then I don't need that big of
a head start exactly. And you can calculate it by
looking at it from the other point of view, like
from us sain Bolt point of view, What is this
Like if you're riding on his shoulder, for example, and
you're looking back as he's accelerating, then you might think, well,
then what this means is that there's a distance beyond
(40:49):
which you cannot see, Like somebody who shines a flashlight
at you from there tries to send you a message
that information will never reach you. So there's like this horizon,
this wall beyond which it's just lack and the distance
to that wall depends on his acceleration. So if he's
accelerating really really fast, then that horizon is closer. If
he's accelerating really really slowly, the horizon needs to be
(41:10):
further further away, So as the acceleration goes to zero,
that horizon is infinitely far away. But the distance to
that horizon is the speed of light squared divided by
your acceleration. But I guess it still feels counterintuitive, right
because the light is going at the speed of light,
but Hussein Bulb will never reach the speed of light.
So how is it the light will never reach him? Right, Like,
(41:30):
wouldn't it eventually make up ground? Yeah, you would think so,
and that would be true if there was no acceleration.
But remember, acceleration means all these rules get bent a
little bit, the same way like space gets bent. And
so the way that sort of incorporated into your brain
is to think about how acceleration is like an effective
bending of space, and when space gets bent, all of
your intuition about who catches what go out the window.
(41:53):
You know, for example, like if somebody's inside a black
hole and they shine a flashlight at you, that photon
is never getting to you. It doesn't matter how much
time it takes. Why because space is bent, right, and
those photons follow that bent space. So if you are
doing constant acceleration, then it's sort of equivalent to gravity.
It's sort of like bending space. And so that's what
(42:14):
creates this horizon. It's not an event horizon. It's not
like a real physical horizon, but for you, it creates
like an information horizon. Accelerating objects have an information horizon,
which is pretty weird interesting, sort of like a content acceleration.
It's almost like the space between us is expanding, that's
what you're saying. It's like somehow if I accelerate fast
enough and I'm further enough away, then that expansion of space,
(42:37):
which is really just my acceleration, is going to prevent
the light from reaching me. Yeah, exactly. But it's not
like a real physical horizon, right, It's only it's from
your point of view. And again, different people have different
points of view, and those things can conflict. If I'm
watching this whole series of events from a spaceship floating nearby,
I might see the photon catching Usain bolt, right, and
(42:57):
so I can see a different series of events. It's
just like with a black hole, right, if I'm watching
you fall into a black hole from the outside, I
never see you following a black hole. You never get in.
You smeared across the event horizon forever. Oh wait a minute,
for you, you fall right through that event horizon. You're
inside the black hole. In the same way. These different
(43:18):
accounts can conflict. Uh Now I feel a little cheated, Daniel.
I think what you're saying is that if I try
to Lusain Bold, who's always accelerating, Hussein Bold will think
that he ut ran the flash of light, but we
are going to see him lose. The person sending the
light won't see it hit him, because he's accelerating relative
to them, he won't see it hitting them. But another
(43:38):
observer moving in a different direction it's possible for them
to see the light hitting him. The story depends on
exactly the location and velocity of everybody involved. It gets
pretty hairy with general relativity. Man, right, I guess that
you're saying that to us Saint's Bold kind of frame
of reference, his experience of things, that light will reach
him but only in an infinity, like when time ends
(44:00):
for him. But for someone who's moving in another frame
or speed, there is a time at which the light
will hit him. Yes, exactly. There's almost always another velocity
or location you can get to to tell a different
story about the same series of events. That's the lesson
of the universe is that there is no universal history.
There's no true single account of what happens in this universe.
Well there is. I mean, the light does catch up
(44:22):
to him, it's just that for him it happens at infinity,
and for us it happens non infinity. Yeah, if happening
in infinity accounts is happening, then you know, I'll pay
you that twenty infinity. Yeah, let's keep talking here until
infinity and see if that's the same thing for everyone.
But that's kind of what you mean, is like, it
does happen, but at infinity. But that's only from his
(44:42):
point of view. That's right, because relativity is weird in
that way. It's pretty weird stuff, all right. Well, then
that's the answer to the question is can you running flashlight?
The answer is yes if you got far enough and
space is expanding fast enough. And also yes if you
are bold, I guess and you you can't wait till
an infinity or you only consider it infinity as never,
(45:04):
or if you have an infinite number of power bars
and you can accelerate forever into the future. Right to you,
you will always win. But maybe to somebody else the
answer will be different. But you'll never have to talk
to them because you're accelerating away from them forever. That's
the true benefit here is never haven't you talked to
or see anyone? Ever? Again? All right, that's a pretty
(45:24):
mind bending question, and again just a reminder of how
weird this universe is and how weird the rules of
it are. Right, it's not just that there are weird
things in it. It's just that it is a weird
universe in itself. It certainly is. But we love it.
We love the weirdness. Stay weird universe. All right, Well,
we hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel
(45:52):
and Jorge explain the Universe is a production of I
Heart Radio or more podcast from my Heart Radio. Visit
the I Heart Radio, Apple Apple Podcasts, or wherever you
listen to your favorite shows. H
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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