Kind: captions
Language: en
What would it look like to ride
alongside a beam of light? This is a
question Einstein asked over a hundred
years ago. Now, the trouble was he
couldn't actually do the experiment, so
he had to use his imagination. He had to
do a thought [music] experiment to
figure out what a reasonable outcome
would be.
I could imagine there'd be some element
of nausea.
I'd imagine everything would get more
intense.
Maybe it would confuse the brain. I
don't really know if you'd be seeing
very much.
I was on a water slide just a couple of
days ago and and I found that really
difficult.
Einstein's intuition told him that the
world should feel the same in all frames
of reference moving with constant
velocity. These are called inertial
frames of reference. So it doesn't
matter if you're in a train or on a
plane or in a car or in a room. As long
as you're moving with constant velocity,
the laws of physics should apply in
exactly the same way. And this is what's
called the principle of relativity.
Is there a speed limit in the universe?
I don't think so.
You can go as fast as you want.
Probably.
Could we go as fast as light?
I'm sure eventually. Absolutely.
Can you compare the speed of sound and
the speed of light?
Yeah. It's however many times quicker,
isn't it? Because you got the
the lightning lightning and thunder
roughly. How much quicker is is light?
10 times.
Roughly 10 times.
10 times. I think is it 10 or 100? I
don't know. Something like that. The
answer for you is a million. It's light
is a million times faster than sound.
Perhaps thought or
thought is faster than light.
Yeah,
it's immediate. It's there.
And let's do a little thought
experiment. Let's say I'm on a train
traveling the speed of light that way.
Who am I kidding? This is city rail.
We're lucky to be moving. But let's use
our imagination. So, let's say I'm
traveling at the speed of light this
way. If I hold up a mirror in front of
my face, will I be able to see my
reflection?
Do you see the problem? In order for me
to see my reflection, light has to
bounce off my face and travel to the
mirror. But if the mirror is going
forwards at the speed of light, how is
the light ever going to catch up?
So, a logical answer would be that no,
you can't see your face in a mirror when
you're on a train traveling at the speed
of light. But that violates the
principle of relativity because it means
that I can tell I'm in a frame of
reference that's going at the speed of
light that way. Oh crap, not again. So
let's consider the other option. Maybe I
do see my reflection, which means the
light must travel from my face to the
mirror at its regular speed, the speed
of light.
But now the problem is someone outside
the train would see light traveling at
twice the speed of light because I'm
already traveling at the speed of light
and the light is traveling at the speed
of light relative to me if that makes
any sense. So that leads to a really
inconsistent answer because how can the
speed of light be twice the speed of
light in that frame of reference?
So Einstein's conclusion was that I must
see my reflection because otherwise it
violates the principle of relativity.
But in order for me inside the train to
measure the same speed of light as
someone outside the train, the train
must actually contract in the direction
that it's moving. And my time must tick
by slower relative to someone outside.
So Einstein changed what we thought were
absolutes, space and time, and made
something that seemed like it should be
relative, the speed of light, into the
true universal constant. So, what would
it look like to travel alongside a beam
of light? Well, according to Einstein,
we'll never know because we can't go
that fast. At that speed, our length
would become zero. Time would stop
[snorts] relative to an outside
observer. And perhaps worst of all, our
mass would become infinite. But a recent
experimental result has suggested that
nutrinos can go faster than light.
Still, most physicists think that, well,
this result probably won't hold up to
scrutiny. because the principle of
relativity is just uh so well
established nowadays. But who knows,
perhaps one day we'll find a way to
break what we thought was the universal
speed limit, the speed of light.