Faster than Light?

Victoria Silverwolf

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This is the important thing:

The finding doesn't go against Einstein's theory because this effect happens in the jet medium not a vacuum.
It's vital to remember that nothing can exceed the speed of light in a vacuum, but that there is nothing that prevents certain things from going faster than light in a medium other than a vacuum. (That's because the light is slowed down by the medium.) This is what causes Cherenkov radiation.

Cherenkov radiation - Wikipedia

While electrodynamics holds that the speed of light in a vacuum is a universal constant (c), the speed at which light propagates in a material may be significantly less than c. For example, the speed of the propagation of light in water is only 0.75c. Matter can be accelerated beyond this speed (although still to less than c) during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light propagates in the same medium.
The article is interesting, but doesn't violate anything already known about physics.
 

Serendipity

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Okay - the reason this is giving me a headache is that I can visualise the possibility of cosmic equivalent of ley-lines along which gamma rays travel that allow the superluminal effect for something to go faster than light - and therefore effectively travel through time... aagghhhh!
 

Star-child

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Is it an effect that is observed to be faster than light, but not an actual thing that moves that fast? The universe is expanding faster than light, for instance.
 

Venusian Broon

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Is it an effect that is observed to be faster than light, but not an actual thing that moves that fast? The universe is expanding faster than light, for instance.
No, as @Victoria Silverwolf stated the hard limit of the speed of light in a vacuum has not been breached.

And no the universe is not expanding faster than light. It is true that some parts of our universe, depending on their geometry are effectively travelling faster than light and are disappearing from our view, as the expansion of the universe has nothing to do with Relativity.

But, if the universe was expanding faster than light then we wouldn't see anything as any light emitted wouldn't be able to travel to hit our eyeballs.
 

Star-child

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No, as @Victoria Silverwolf stated the hard limit of the speed of light in a vacuum has not been breached.

And no the universe is not expanding faster than light. It is true that some parts of our universe, depending on their geometry are effectively travelling faster than light and are disappearing from our view, as the expansion of the universe has nothing to do with Relativity.

But, if the universe was expanding faster than light then we wouldn't see anything as any light emitted wouldn't be able to travel to hit our eyeballs.
I was trying to use to illustrate two points traveling apart from each other at a total speed exceeding C as not violating relativity, using the edges of the universe as an example. I missed the local medium part, which is why I was asking if that applied or not. Thank you for the correction.
 

mosaix

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I seem to remember that the limit is a particle breaking through the barrier of the speed of light but that it’s ok for a particle to exceed the speed of light if it is already doing so. (a tachyon?).

I may be talking nonsense here and dreamt it all. Apologies if so.

Also my post may have nothing at all to do with the thread. Apologies again.
 

Robert Zwilling

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These particle faster than light articles should have the reminder in the headline that it is still traveling slower than the vacuum speed of light. The Cherenkov articles I see are all talking about charged particles going straight through the medium. When high speed charged particles travel in a circular orbit in a vacuum in an particle accelerator, the charged particles give off synchrotron radiation, which are basically high energy x-rays.

The article did make me wonder why a sychrotron needs a vacuum to operate. Fill the circular path with no vacuum and a gas or liquid that slows down light. Then accelerate the particles through that medium over and over again so they go faster than the light will travel in it, which is going slower than the vacuum speed of light. Would you get amplified Cherenkov radiation?

Make it real simple, replace all the fancy synchrotron hardware with a battery and a wire connecting the terminals, where the speed of light is zero, the battery voltage provides the acceleration of the electrons and the wire replaces the electromagnetic field that keeps the electrons flowing in a tightly controlled area and you generate a magnetic field with no radiation. The mechanics of the device is still working. Maybe it would be possible to make a time machine for the past by making the speed of light less than zero in a suitable medium where light was going backwards (whatever that is), and the energy flow was going in the opposite direction the light was trying to go in, then you would be traveling slower than time, which could put you into the past.
 

Venusian Broon

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I seem to remember that the limit is a particle breaking through the barrier of the speed of light but that it’s ok for a particle to exceed the speed of light if it is already doing so. (a tachyon?).

I may be talking nonsense here and dreamt it all. Apologies if so.

Also my post may have nothing at all to do with the thread. Apologies again.
okay, this is purely without looking at any other sources and just using the grey matter in my noggin (hence I am probably about to write down something wrong...:))

I think a tachyon is a conceptual particle that is 'allowed' by the equations of General Relativity, because it fits the mathematics of the theory. However if such a particle exists, it must have some experimental evidence of its existence. And I am not aware of such evidence.

I think also one must be careful of extrapolating mathematics with reality. It is bizarre that mathematics actually does describe reality as well as it does, but that does not mean that a certain theory can explain everything, or that you should trust it.

Best example I know. Quantum mechanics, probably one of the most tested theories ever (and generally correct, and my fav!) manages to estimate that the vacuum energy of space is 10^114 J/m^3 but...the real answer is closer to 10^-9 J/m^3. That's a pretty wrong, wrong, wrong answer.

Add to the fact that QM and GR must be fundamentally 'wrong' as we know that both can't explain each others strong points, so best to stick to experimentally observed facts. :)
 

Daysman

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J Riff

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All good theory, but not applicable to spaceships in the void, where dust particles and teeny little rocks lurk. Light effectively reflects its way along, you can stop it with your hand. Don't try this with a spaceship going 9/10 of C.
 
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