Speed of light in a vacuum... not constant?

This was also reported on PhysicsWorld, see Structured photons slow down in a vacuum. Miles Padgett et al from Glasgow pops up a lot when it comes to optics, see this website:

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I'm not quite sure about this work, but stuff from the Glasgow optics group is usually pretty good. And I do know that the speed of light varies with gravitational potential. The speed of light is lower near the floor than up near the ceiling. Unfortunately a lot of people are taught that the speed of light is absolutely constant, and have great difficulty in accepting that it isn't:

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Of course we get Farsight's pseudoscience. At least this time he posted a link with some content that has nothing to do with his beliefs but that is informative.

This once again turns out to be a case of group velocity, so it is an expected change in wave propagation. It's not something people have done on individual photons before, so good for these researchers. It is not a change in the speed of causation as laid out in GR (and maligned by Farsight's misunderstanding).

 

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figures, farsight contaminating another topic with his ridiculous shenanigans.
it's wise just to ignore farsight. this is obviously far advance for him, you can tell by his posting of that same einstein" ideas and method supplement".

 

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farsight,
explain what this means,
except, there is no true vacuum. outside of dark matter and energy, in the cosmos, is considered a void.

 

post deleted - already answered by others

 

The local spacetime curvature, gravity, determines the path not the speed.

 

Bingo!!! Something Frasight has yet to accept.

In other words, photons/light follow geodesics in curved spacetime.

 

Thatt's how GR works. For light the natural path is the null geodesic. For objects with mass the natural path is the timelike geodesic. From the metric we can derive equations of motion to describe these natural paths. The details, derivations, about these paths is truly amazing. It would be counterproductive 'to the troll' for Farsight to accept. It would be like Leopold accepting the science of evolution

 

It's peculiar how they sent light through a mask to give it more wavelike properties, but there was no act of observation that destroyed those properties. Then how would they have known exactly when the light left the mask? They wouldn't have. The rules for quantum uncertainty would prove that they didn't even know if the photons even left to start the race at the same time. If they did check for this, the photon that went through the mask would have "changed shape" (like they say) so that it would be no different than the other photon. Therefore, it would be impossible to prove that they even started the race at the same time!

 

I dont know...I gonna have to turn my vacuum on.

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But, seriously. You sure light has a speed?

 

Whats to argue really ?

 

I suggest you read the paper: http://arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf This has the details of the experimental setup. These people are not morons.

Here's a link to the other thread where we have been discussing this, started by Magic Realist: http://www.sciforums.com/threads/what-if-the-speed-of-light-is-not-constant.144417/

Discussion starts with Paddoboy's post No. 8. I made several replies to this, as I read more about it, but as you will see I'm a bit hobbled by not really understanding how a Bessel beam works, which seems to be key to this phenomenon.

 

Conceptually, it's a bit like a shaped charge used in anti-tank shells.

See this article: http://www.st-andrews.ac.uk/seeinglife/science/research/bessel/bessel.html

This picture is the key:

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The blue slanted lines are light waves, and so are the red slanted lines. They interfere, such that the horizontal green line is a peak of light which is the core of the Bessel beam.

 

No, the mathematics is the key.

 

Surely what you need to think about is how the "shape" of a wave can be altered, and how changing it alters its velocity. What's the relation between the speed of a wave and the shape of a wave? Those italicised words are both vague kinds of terms, aren't they? Just the kind of thing you'd expect to see in a newspaper report.

Waves, as any undergrad knows (who has studied some physics) have a phase velocity and a group velocity, and these are distinct properties of waves in general. It would seem that understanding the Herriot-Watt experiment under discussion requires an understanding of Bessel waves (maybe Bessel functions too).

 

I think that basically the same as I more specifically described here: http://www.sciforums.com/threads/what-if-the-speed-of-light-is-not-constant.144417/#post-3269367

 

--http://en.wikipedia.org/wiki/Group_velocity

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In the above animation, the group velocity is in the opposite direction to the phase velocity (for those who don't know).

 

Exactly. What everybody is trying to do is transfer information > the constant c. Did you ever read about the claims of Gunter Nimtz to have achieved this by quantum tunneling. The experiments are interesting even though the claims were not well received by his peers.
http://en.m.wikipedia.org/wiki/Günter_Nimtz#Experiments_related_to_superluminal_quantum_tunneling
I remember when the phase velocity experiments sent the cranks to trumpet the downfall of relativity theory. LOL.

 

I didn't say they where morons. I just said it would be impossible to know exactly when the wavelike structure of any group of photons where at a certain location and a specific time. They would only be able to calculate an approximation of when around that time would be.

Looking at the paper, I noticed that it mentions it travels through a waveguide, and it also says that the material it is made out of doesn't matter. The thing is that I think it does matter. When waves travel through a waveguide they actually just travel on the skin of that waveguide. Then that would explain why the light is so slow if it is slower that what could be accounted for by just from quantum uncertainty. From personal experience (specifically from reading people talking about it here), everyone is a moron when it comes to light traveling through a medium.

The way people treat electrons moving through circuits is like saying that your mule can move your things at the speed of sound, because when he doesn't want to move you can hear him hee-haw that quickly afterwards.

 

No, it's not in a waveguide, the whole point is its travel in free space is delayed. This is section "C" of the experimental setup shown in Figure 2 of the paper. The "SLM" is the device that produces the Bessel beam structure of the light. Have another look……….