Hey, Vsauce. Michael here. If you were driving in a car
at the speed of light and you turned on your headlights
what would happen? Would light be able to come out
or would your headlights just stay dark?
Or maybe, light would come out but it just pull up
inside the headlight like a overflowing sink.
Or maybe the light would come out but at twice the speed of light - at its normal
speed plus the speed of your car.
No, that doesn't sound right. None of these answers sound right because,
of course, there is no answer.
A car cannot travel at the speed
of light. Nothing that has mass can.
But come on, you say, there are things that can travel at the speed of light.
Things that have no mass, like photons. Let's build a car
out of light. Okay.
Let's take that suggestion because imagining what it is like
to be light, brings up a lot of questions about
light, about why things are the way they are
and about whether or not we are real.
In order to see what I mean we need to talk about "C".
The speed of light.
It is Constant. As long as you aren't accelarating, you can move as fast or
slow or
in whatever direction you want and you will still
always see light moving at light
speed. Even when light appears to slow down, as it passes through different
materials,
the actual photons themselves are still always traveling at "C".
They're just taking a longer path, which takes
more time. This law always holds true because whenever you move,
relative to other things, your measurements of them and their
measurements of you
change. When you move, everything not moving with you appears to you to be
shortened
in length and to be experiencing time more slowly.
Whereas to everything else, it is you that these changes happen to.
These changes really happen - just not noticeably or measurably at the speeds
we usually travel at.
For example, while walking to your refrigerator for a snack,
you will measure your fridge to be a 100 quadrillionth
of a meter nearer and thinner than you would while at rest with it.
You'll also notice that it experiences time more slowly
than you. Each one of its seconds will be a 100 quadrillionth
of a second longer than yours. But your fridge
will measure that these transformations are happening to you.
These are small numbers, but at speeds nearer the speed of light
these changes can be dramatic. As I mentioned in an earlier video,
to a particle racing toward Earth at 99.9999999999991%
the entire Earth would only appear to be
70 meters deep. Our whole planet.
Now with all of this in mind, let's take a look at headlights
being used by a stationary ship and one that is moving
at 99 percent the speed of light.
The Near Light Speed Ship would be flattened in the direction of its travel
to us,
while a stationary one would not. As their headlights come on
the light coming from both ships travels at the same rate.
The speed of a lights' source doesn't push it faster.
The moving ships velocity does, however, give its headlight energy
in the direction it's headed, causing a blue shift from the front and a red shift
from behind.
What's really cool though is that this is just what
we would see, floating in space at rest with the stationary ship.
The crew of the fast-moving ship wouldn't see their beams, only gradually
gaining on them
like we do, they would see it exactly what the crew the stationary ships sees.
Light beams rapidly fleeing their headlights at the speed of light
just as if they were moving at all. And here's why. Let's say this distance
is a light second. The distance light travels in one second. Which is
299,792,458 meters.
Now, after one of our seconds, sure enough,
both beams will have traveled a light second. But the crew of the moving ship
measures light speed to be the same as us.
But how does that make sense? I mean, light hasn't traveled a light second from
them yet. No problem. You see, what they measure a light second to be
is shorter than what we measure. And
they wouldn't agree that a second has passed yet either. Because, of course, to
us
time is slower for them. They count one second finishing later then we do,
at which point their light has, indeed, traveled one of their light seconds
from them. Observers will often disagree
about time and space, but those disagreements with always conspire
to make sure they agree on one thing. The speed of light.
It is always the same for everyone.
But what if you were travelling
at the speed of light? Well, the only thing that could do that would be a
massless vehicle. So, fine, let's assume that we can build one.
Such a vehicle would travel with
light. Light would never even pull
a tiny tiny bit ahead of it. No matter how short
it thought a meter was. No matter how long it thought a second was.
It would never register light ever moving ahead of it.
It would say that light was stationary. It would not agree that light travelled
at the speed of light. Right? Not really.
You see, at the speed of light there is nothing
to see. Not because there's nothing to see but because there is nothing
to do any seeing.
As we have seen, as an object's velocity approaches
"C", time for everything around it approaches
a standstill. An object
traveling very very very very very very near the speed of light
could travel for billions of our years
before a single second past for it.
But a massless vehicle, travelling at the speed of light,
could travel forever
before an instant even began for it.
It experiences no time.
And it would have no time to do anything.
Certainly no time to turn on its
headlights. The concept of time doesn't even really
apply. A massless vehicle couldn't even have its headlights on
before it hits "C", because object with no mass
must always travel at "C". Such an object
would never have a before. Its origin,
its journey and its destination are simultaneous for it.
A billion light year trip from a distant quasar
might take a photon a billion years
to us. But it literally takes no time
for the photon. And to the photon, the distance to us from that quasar
is literally nothing. Photons
are how we see. How we know anything about our place in the universe,
and the energy they bring our planet from the Sun is responsible for nearly
all life on Earth. But despite that list of accomplishments,
they don't think much of themselves.
In fact, as far as they're concerned, they
don't even exist. Outside observers, of course,
see them and would measure them to be moving along at the speed of light
with nothing passing or gaining on them. So if you were moving
at the speed of light, you'd have to be massless,
like a photon,
and you wouldn't notice anything, because to you there would be nothing,
and no time to notice it. Other people would see you doing nothing
and you would be unaware of yourself. You could never turn on headlights
because you would feel exactly like you felt before you were conceived.
But why is the speed of light what it is?
I mean, this is how far light travels
in a second. Could it also travel
this far, or this far, or this far? Could we conceive of a universe where light
traveled
that quickly? Yeah, easily.
So then, why is it that the universe we live in
is only one of these ways? Why are the laws
of physics what they are? Instead of
some other way they could be.
Well, there are a lot of theories, but so far we're not exactly sure how to
test any of them, because we are stuck in this
universe and don't yet know how to create universes of our own
for experiments. But perhaps there is something we have yet to discover that
compels
reality to be this way, and only this way.
Or, perhaps, nothing compels it to be this way but instead
there's a multiverse. Every single possible universe
exists. Some of them collapsed immediately, some of them are empty and dark,
some of them involve you watching this very video but
Earth has two Suns.
For those of you, in that universe watching, that was probably very confusing
what I just say
because your Earth does have two Suns. But the point is, the multiverse
would be a pretty elegant explanation. Or maybe,
universes are born inside black holes.
Universes whose physical laws are only slightly different from the universe
that gave birth to it.
In this way, universes could replicate according to natural selection.
Universes that are more fit for creating black holes
would then have more baby universes,
making universes that support black holes more common,
like maybe the one that we're in. And that's pretty lucky because universes that
support black holes
support Suns, and us.
Or - and here's my favorite theory -
maybe we're just living in a simulation. And someone or something else
programmed it up.
That sounds pretty sci-fi.
Sure. But I like how Julian Baggini puts it in
"The Pig That Wants to Be Eaten". If you assume, that at some point in time,
virtual reality could become good enough that even its own inhabitants
didn't know they were in a simulation, and if you assume
creating such simulations would be way
easier than creating real flesh-and-blood creatures
that require an entire universe and billions of years.
Well, then it's pretty easy to conclude that many many many many more simulated,
faked beings could be created than real ones.
Say, 999 fake ones for every one
real one.
What's more likely? That you're part of the 0.1%
that are real, or that you are part the 99.9%
who think they're real but
aren't. Perhaps, we live in a simulation created by some other
intelligent species. John Gribbin,
however, points out one possible way to dismiss this idea:
irrational numbers. Like the square root of 2
or Pi. These numbers don't
end. I mean, they contain a never-ending,
never repeating sequence of digits, which would mean that whoever programmed this
universe could
fit in all of, say, Pi.
So, if in our tireless pursuit of calculating more and more and more
digits of Pi,
we ever run into an endlessly repeating series
or the end, that could be a pretty good sign that we live
in a simulation. A universe that is not real.
But so far we haven't found that and we don't think that we will.
So, thank you irrational numbers, for keeping
it real.
And as always,
thanks for watching.