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Einstein said that nothing can travel faster than the speed of light.
You have probably heard something like that.
But is this really correct?
This is what we will talk about today.
But first, a quick YouTube announcement.
This channel has seen a lot of new subscribers in the past year.
And I have noticed that the newcomers are really confused each time I upload a music
video.
They’re like oh my god she sings, what’s that?
So, to make it easier for you, I will no longer post my music videos here, but I have set
up a separate channel for those.
This means if you want to continue seeing my music videos, please go and subscribe to
my other channel.
Now about faster than light travel.
To get the obvious out of the way, no one currently knows how to travel faster than
light, so in this sense it’s not possible.
But you already knew that and it’s not what I want to talk about.
Instead, I want to talk about whether it is possible in principle.
Like, is there anything actually preventing us from ever developing a technology for faster
than light travel?
To find out let us first have a look at what Einstein really said.
Einstein’s theory of Special Relativity contains a speed that all observers will measure
to be the same.
One can show that this is the speed of massless particles.
And since the particles of light are, for all we currently know, massless, we usually
identify this invariant speed with the speed of light.
But if it turned out one day that the particles of light have a small, nonzero mass, then
we would still have this invariant speed in Einstein’s theory, it just would not be
the speed of light any more.
Next, Einstein also showed that if you have any particle which moves slower than the speed
of light, then you cannot accelerate it to faster than the speed of light.
You cannot do that because it would take an infinite amount of energy.
And this is why you often hear that the speed of light is an upper limit.
However, there is nothing in Einstein’s theory that forbids a particle to move faster
than light.
You just don’t know how to accelerate anything to such a speed.
So really Einstein did not rule out faster than light motion, he just said, no idea how
to get there.
However, there is a problem with particles that go faster than light, which is that for
some observers they look like they go backwards in time.
Really, that’s what the mathematics says.
And that, so the argument goes, is a big problem because once you can travel back in time,
you can create causal paradoxes, the so-called “grandfather paradoxes”.
The idea is, that you could go back in time, kill your own grandfather – accidentally,
we hope – so that you would never be born and could not have travelled back in time
to kill him, which does not make any sense whatsoever.
So, faster than light travel is a problem because it can lead to causal inconsistencies.
At least that’s what most physicists will tell you or maybe have already told you.
I will now explain why this is complete nonsense.
It’s not even hard to see what’s wrong with this argument.
Imagine you have a particle that goes right to left backwards in time, what would it look
like?
It would look like a particle going left to right forward in time.
These two descriptions are mathematically just entirely identical.
A particle does not know which direction of time is forward.
*Our observation that forward in time is different than backward in time comes from entropy increase.
It arises from the behavior of large numbers of particles together.
If you have many particles together, you can still in principle reverse any particular
process in time, but the reversed process will usually be extremely unlikely.
Take the example of mixing dough.
It’s very easy to get it mixed up and very difficult to unmix, though that is in principle
possible.
In any case, you probably don’t need convincing that we *do have an arrow of time and that
arrow of time points towards more wrinkles.
One direction is forward, the other one is not.
That’s pretty obvious.
Now the reason for the grandfather paradox is *not faster than light travel, but it’s
that these stories screw up the direction of the arrow of time.
You are going back in time, yet you are getting older.
*That is the inconsistency.
But as long as you have a consistent arrow of time, there is nothing physically wrong
with faster-than-light travel.
So, really, the argument from causal paradoxes is rubbish, they are easy to prevent, you
just have to demand a consistent arrow of time.
But there is another problem with faster-than-light travel and that comes from quantum mechanics.
If you take into account quantum mechanics, then a particle that travels faster than light
will destroy the universe, basically, which would be unfortunate.
Also, it should already have happened, so the existence of faster-than-light particles
seems to not agree with what we observe.
The reason is that particles which move faster than light can have negative energy.
And in quantum mechanics you can create pairs of new particles provided you conserve the
total energy.
Now, if you have particles with negative energy, you can pair them with particles of positive
energy, and then you can create arbitrarily many of these pairs from nothing.
Physicists then say that the vacuum is unstable.
Btw, since it is a common confusion, let me mention that anti-particles do NOT have negative
energy.
But faster than light particles can have negative energy.
This is clearly something to worry about.
However, the conclusion depends on how seriously you take quantum theory.
Personally I think quantum theory is not itself fundamental, but it is only an approximation
to a better theory that has not yet been developed.
The best evidence for this is the measurement problem which I talked about in an earlier
video.
So I think that this supposed problem with the vacuum instability comes from taking quantum
mechanics too seriously.
Also, you can circumvent the problem with the negative energies if you travel faster
than light by using wormholes because in this case you can use entirely normal particles.
Wormholes are basically shortcuts in space.
Instead of taking the long way from here to Andromeda, you could hop into one end of a
wormhole and just reappear at the other end.
Unfortunately, there are good reasons to think that wormholes don’t exist which I talked
about in an earlier video.
In summary, there is no reason in principle why faster than light travel or faster than
light communication is impossible.
Maybe we just have not figured out how to do it.
Thanks for watching, see you next week.