Neutrons emitted under water (Nuclear Submarine) exceed the speed of light in water. (Hence Cerenkov Radiation).

Can these Neutrons be consider to time travel ?

Neutrons emitted under water (Nuclear Submarine) exceed the speed of light in water. (Hence Cerenkov Radiation).

Can these Neutrons be consider to time travel ?

No. The Cerenkov radiation is emitted by the Neutrons decelerating to less than the speed of light in water.

But the neutrons exceed the speed of light. So do the neutrons time travel and why not ?

Andy_fl, the neutrons aren’t exceeding c, the speed of light in a vacuum, they’re exceeding the speed of light in water. There is no relativistic significance to anything but c.

In any given medium the speed of light in that medium is the maximum speed with which one can communicate. So anything exceeding this speed should be able to time travel. Correct me why I am wrong here ?

Say, given Points A and Point B. Any event happening at point A, point B can come to know about it only at the speed of light in that medium. So if the neutron starts from point A and reaches the point B then it can observe a photon which left at the same time as itself from point A and observe it again at point B.

Is’nt the neutron than seeing its past ? WHy is this not time travel ?

As **Nametag** has already correctly put it, you have to distinguish between C, the universal constant that is equivalent to the speed of light in a vacuum, and c, the speed of light in its current local medium. The universal constant C is the only number that is meaningful when people talk about faster than the speed of light. You can safely exceed c without involving infinite energy or setting up paradoxes or involving time travel. Particles that exceed c and therefore give off Cerenkov radiation never exceed C. So no time travel.

I understand that the speed of light in **vacuum** is what matters in relativity.

Relativity exposes limits on time travel in vacuum but can you prove that it poses the same limits in water ? Or for that matter infinite energy to with time travel ?

As I illustrated in the example, the neutrons are time travelling. Explain to me why not, please?

No, it doesn’t.

Seeing a photon you emit is not the same thing as time travel. Traveling faster than light is not the same thing as time travel. Maybe you should explain where you got this idea from in the first place. Traveling faster than light in a vacuum is the same thing as time travel.

This is why c should be renamed Einstein’s Constant instead of the unwieldy (and, apparently, confusing) circumlocution `speed of light in a vacuum.’

**andy_fl**, ignore the fact that light in a vacuum happens to travel at c. A lot of other things do, too, as c is one of the basic constants of the Universe. The only thing you need to know is this: Traveling at a speed beyond c is the equivalent of time travel.

**andy**, you have made a number of false statements so far in your posts.

This is incorrect. Neutrons are far too heavy to be accelerated even to exceed the local speed of light. Beta particles (electrons) are the ones involved.

This is obviously incorrect. If beta particles are moving faster than photons then one can communicate faster with electrons. The limiting factor in any medium is C, the universal constant, not c, the local speed of light.

Relativity certainly applies to speeds lower than C, the universal constant. Can you supply a single cite that it does not?

I’m not exactly sure what this is supposed to mean, but if you are suggesting that by exceeding c, the electrons use infinite energy, this is purely ridiculous.

You have illustrated nothing of the kind. As you have been told repeatedly, time travel is only possible given theoretical particles that travel faster than C, the universal constant. These electrons (not neutrons) are not traveling faster than C. Therefore there is no time travel.

Perhaps you should think of it this way. When a car execeeds the speed limit on a highway, it does not turn into an airplane.

But! What if the *car* where to drive around the planet really, really, really, REALLY fast and make the Earth spin *backwards*…

Only if some guy named Kent is driving it.

There was a comment made in some science magazine that I read which stated, “There’s some evidence that anti-matter is simply ordinary matter traveling backwards in time.” That’s all it said, nothing more, nothing less. No explaination as to how it might work, who thought that up or what their evidence was.

The speed of sound in water is faster than it is in air therefore Concorde would get from London to New York much quicker if it flew underwater.

**Mangetout**.

Ok heres how I see it.

c is Einsteins Constant (the speed of light travelling in a vaccum).

You are asking that since Cerenkov Radiation travels faster than light in water can Cerenkov Radiation be considers to be time traveling?

The answer is No.

The speed of light in a vaccum is the constant. The speed of light in water isn’t. It doesn’t matter if Cerenkov Radiation is faster than light underwater, its still not faster then light in a vaccum. And according to **Exapno Mapcase** Cerenkov Radiation isn’t actually faster than light underwater either.

Did I get that right?

It’s not that Cherenkov radiation itself travels faster than light, but it is caused by beta particles which travel faster than light.

Nobel-Prize-winning physicist Richard Feynman came up with this theory.

It works, theoretically and mathematically, but I don’t believe many physicists consider it a true description of our universe.

Yes, as long as you keep in mind that we keep talking about two things both of which are commonly referred to as “the speed of light.”

Cerenkov radiation is never faster than the speed of light in a vacuum, better referred to as C or Einstein’s Constant.

It is by definition faster than c - the local speed of light: in your example, the speed of light in water.

It’s this difference between the local speed of light and the speed of light in a vacuum that causes so much confusion.

I apologize that this post doesn’t add anything to the primary discussion (which seems to be be well in hand), but I thought I’d nitpick a bit.

In the case of nuclear power, yes, electrons (“beta” radiation) happen to be the source of Cerenkov light.

In general, one can certainly get neutrons moving fast enough. However, they will never produce Cerenkov light because they are neutral. (Only charged particles produce Cerenkov light.)

-P

Agreed and thanks for correcting me.

Not so fast. Yes, beta particles are exceeding the speed of light so I assume they are exceeding the speed of gravitrons too (correct me If I am wrong on this one). So the maximum speed of communication by EM or any other means is limited to the speed of light. The limiting factor is surely C as you say and I am not disputing that.

My understanding is that the time cone - as I understand from Hawkins - is steeper in water, and the electron is going outside the limits of the time cone in water. Explain why not.

I never said relativity does not apply in water; or for that matter anything. My question was as I understood the relativity problem - the derivation of length contraction and time dilation were from examples like a guy moving on train or something. So, if you consider all the examples given in the derivation of the lorentz transformation in water C should be replaced by c. So tell me why not ?

**I’m not exactly sure what this is supposed to mean, but if you are suggesting that by exceeding c, the electrons use infinite energy, this is purely ridiculous.**

I am not suggesting anything - just trying to understand. I am not an expert on this.

**You have illustrated nothing of the kind. As you have been told repeatedly, time travel is only possible given theoretical particles that travel faster than C, the universal constant. **

See post above.

** Perhaps you should think of it this way. When a car execeeds the speed limit on a highway, it does not turn into an airplane.**

Of course not. But when a airplane exceeds the speed of a rocket, you do wonder why.