Time travel: Light speed results cast fresh doubts (Sorry Doc, Time Travel Not Likely)

BBC ^ | 07/27/2011

[SeekAndFind]

Physicists have confirmed the ultimate speed limit for the packets of light called photons - making time travel even less likely than thought.

The speed of light in vacuum is the Universe's ultimate speed limit, but experiments in recent years suggested that single photons might beat it.

If they could, theory allows for the prospect of time travel.

Now, a paper in Physical Review Letters shows that individual photons too are limited to the vacuum speed limit.

That means that photons maintain the principle of causality laid out in Einstein's theory of special relativity - that is, an event's effect cannot precede its cause by travelling faster than light. It is violation of this causality that would, in principle, permit time travel.

While the limit in vacuum is a fixed number - some 300,000km per second - the speed of light can vary widely in different materials.

These differences explain everything from why a straw looks bent in a glass of water to experiments in cold gases of atoms in which light's speed is actively manipulated.

Some of those experiments showed "superluminal" behaviour, in which photons travelled faster than the speed of light in a given medium.

It remained, however, to determine whether or not individual photons could exceed the vacuum limit.

[ZX12R]

Yup, time passage = 0 and distance traveled = 0, from their point of view

So, from their point of view (if a photon can have a point of view) they are not moving, but space is rushing by them at 186,000 mps. Weird.

[The Cajun]

(if a photon can have a point of view) they are not moving, but space is rushing by them at 186,000 mps. Weird.

Even weirder from the *off the wall* theory (forget its' real name and by who).

Since time doesn't *run* for it in its' reference frame (its' reality) and since a photon shows no acceleration to the speed of light (when created it is at the speed of light), it is both created and destroyed (when it finally slams into something) in the same instant (from its' point of view).

If I half way understand the theory (probably not even half way), in a photon's reality time, length and distance (since time dilation and length contraction are concurrent at the speed of light) do not exist.

Read about that theory years and years ago, it was an attempt to explain how entangled photons can communicate their spin info to each other faster than the speed of light.

It's probably wrong and my interpretation is probably *wronger*, but makes more sense than *spooky action at a distance*, to me anyway :^)