# The possibility of time travel?

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1. Feb 24, 2015

### Auliter

I've recently read a report about a group of Scottish scientists have managed to slow down the speed of light, not in a certain condition but through a special mask and the particles of light can be forever slowed down. Does that means the speed we need to achieve to make time travel is slower down and make it possible? Or the thing is the light speed we asked to do the time travel is just a certain number and don't really have anything to do with the actual speed of light?

2. Feb 24, 2015

### Staff: Mentor

The $c$ that appears in all the equations of relativity is the speed of light in vacuum. Light in a medium generally travels at less than that speed, and it is possible to travel faster than light (but still faster than $c$) in such a medium - google for "Cerenkov radiation".

However, none of this has much of anything to do with time travel.

3. Feb 24, 2015

### Auliter

Thanks for explanation. But the thing is that the speed of particles of light is slowed down forever which means even in vacuum(not any medium) the speed of light is slower than the original one. You may take a look at the link: http://www.bbc.com/news/uk-scotland-glasgow-west-30944584.

4. Feb 24, 2015

Staff Emeritus
Did you read the scientific publication? Of course you didn't. How do I know? Because it says that your claim is wrong in the abstract!

Before you tell us we are all wrong, perhaps you should so a little homework yourself.

5. Feb 24, 2015

### Auliter

As you said, I didn't. Just get a glance at the BBC news. Thanks for reminding, I may do some search next time beforehand.

6. Feb 24, 2015

### PAllen

The c that is relevant for spacetime is the invariant speed that figures in the laws of EM (and all other laws of physics). That a particular photon is put in a state where its detected speed after emerging from a mask into vacuum is slightly slower than c is completely irrelevant to time dilation or any other spacetime phenomena. It is very interesting, but it is really in the category of light in matter going less than c. The researcher's remarkable achievement is to show, without a wavegide, that a non-plane wave reduced to a single quantum has the propagation characteristics of the non-plane wave: that it travels less than c.

Here is the primary reference on this (rather than the pop sci version):

http://arxiv.org/abs/1411.3987

7. Feb 24, 2015

### Ibix

They haven't slowed the speed of light in the sense you mean. They've managed to construct a beam profile whose wavefronts travel slower than plane waves do. This isn't particularly exciting - the Gaussian profile they use is a common laser beam profile. What's pretty cool is that they stepped their light source down to emit one photon at a time and still observed the effect.

It doesn't change anything about relativity, or allow time travel. The c used in relativity is the speed of plane waves in vacuum. It might be less misleading to say that these guys made a photon that travels slower than c, rather than that they changed the speed of light.

8. Feb 24, 2015

### Staff: Mentor

That BBC article is a <delicate>less than completely accurate</delicate> description of the actual research to which PAllen has supplied a link.

9. Feb 24, 2015

### Staff: Mentor

This thread can be closed now.