Speed of Light Not Constant?

[Argylas]

I was wondering what the more knowledgeable people on this forum have to say about this - http://www.businessinsider.com/speed-of-light-isnt-constant-2015-1

Is this paper credible? And if we really can change the speed photons travel at permanently what are the implications of this?

[sgt_flyer]

C will still remain a set constant of speed as in, the speed of a non altered photon in vacuum, and anything other than photons will still require infinite energy to reach C.

Now, it is well known that photons can travel at speeds lower than C, when in mediums other than vacuum.

What they discovered is that 'structured' light photons (laser / lense beams) are slower in vacuum than normal photons.

There already is instances where something can exceed the speed of the photons (though are still slower than C) in a given medium (like water) here's the result of such thing happening :

http://en.m.wikipedia.org/wiki/Cherenkov_radiation

So, i can guess this won't change anything

[YNM]

It's sounded a bit silly at first, but then I saw they said group velocity... Phase velocity might remain constant (like say, the animation here)

Or I'd be curious about what kind of glasses they use and what they did to the light beam.

[Argylas]

But wouldn't the fact that structured photons are slower than normal ones in vacuum change the way we look at the universe? If there is a way in nature to produce coherent light (that is structured photons), then we may have to rethink what we are seeing in the universe, or more exactly, how far back in time are we seeing things. Imagine a star 100 000 light years away can emit coherent light and we only see these structured photons. We presume they have traveled to us with a speed of c, but they may have been slower, making us see the star as 101 000 light years away. I don't have much knowledge in the field of lasers and optics, so I have no idea if anything like this is even remotely possible, but if it is, this may turn out to be a significant discovery.

[YNM]

But wouldn't the fact that structured photons are slower than normal ones in vacuum change the way we look at the universe? If there is a way in nature to produce coherent light (that is structured photons), then we may have to rethink what we are seeing in the universe, or more exactly, how far back in time are we seeing things. Imagine a star 100 000 light years away can emit coherent light and we only see these structured photons. We presume they have traveled to us with a speed of c, but they may have been slower, making us see the star as 101 000 light years away. I don't have much knowledge in the field of lasers and optics, so I have no idea if anything like this is even remotely possible, but if it is, this may turn out to be a significant discovery.

We get distances not from the time light takes to travel. There are a few methods : annual parallax, secular parallax (yeah, useful within one's own galaxy), HR diagram main sequence fitting, standard candles, redshift (actually converted into scale factor). Granted that standard candles and redshifts are fail-prone if lights emitted from a star in some phase behaving as a coherent light; but then, space is big, and even a few nanoarcsec of different direction would ends up a lot (guess a few of photons each square cm isn't really a coherent light anymore). Also, the problems within these methods itself (luminosity variability for standard candles, variable hubble parameter for redshifts) would yield a larger error than problem arises from a slower coherent light.

Edited January 23, 2015 by YNM

[pincushionman]

The speed of light through a medium is slower than that in a vacuum because the light is interacting with the medium, no? They're "structuring" the light, then "unstructuring" it again at the end. Which sounds suspiciously like "we're forcing the light to interact with a more significant medium." And they're surprised the light travels slower…why?

[-Velocity-]

The paper is not incorrect, but it's another example of the popular science news media completely overblowing a completely mundane topic, and also of people who somehow still cannot grasp that light is both a wave and a particle. When a wave propagates, it can diverge in different manners. In 3 dimensions, a wave that is NOT diverging is a plane wave. Its wavefront is planar. The wavefront progresses at the propagation velocity of that particular kind of wave. However, really you're never going to observe any true plane waves in nature, though many wavefronts can be approximated as planar. For sources far away, wavefronts you observe are usually going to be diverging spherically (but over short distances can still be approximated as planar- for example, if you climb a ladder at night, you should not expect the stars to become measurably brighter because you're closer to them). The magnitude of the wave will decrease by 1/r, thus meaning the energy in the wave will decrease by (1/r)^2, leading to the "law" that the intensity of distant sources of light will decrease by a factor of 4 if you double the distance.

However, there are other ways that waves can diverge too, such as if the medium is bounded in some way. Imagine you're a whale. The sound waves you produce are bounded by the ocean surface (a pressure release surface with a reflection coefficient of -1) and the ocean bottom (a pressure doubling surface with a reflection coefficient of 1)- as long as there is not a thermal layer (which can also reflect sound). Relatively close to the whale, the sound the whale makes will diverge spherically, but at distances much beyond the distance between the sea surface and sea floor, you actually get into a cylindrical divergence regime. The sea surface and the sea floor acts as a crude waveguide, preventing the sound from escaping in three dimensions, and reducing the rate at which it decreases with distance. Instead of 1/r for the sound intensity, you get 1/r^0.5. Whales use this waveguiding effect to help communicate long distances. (In reality, I think I've simplified this a bit and that whales actually frequently use two thermal layers or a thermal layer and the surface as the bounds for their planar waveguide, since the seafloor and sea surface are far apart and there is usually a thermal layer anyway.)

Why I bring this up is that because of this wave guide effect, the wavefront of the sound is NOT aligned with the sea surface- the wavefronts are bouncing between the top and bottom surfaces. This means that, transverse to the surface of the ocean, the sound is NOT propagating at the speed of sound in water. It's instead propagating at the "group velocity". The group velocity is determined by the angle between the wavefronts and the ocean's surface. Only at 90 degrees is the group velocity equal to the speed of sound.

Sorry for the long, drawn-out examples, but the end message is that light is a wave, and the same wave theory rules apply. If the wavefront is not aligned with the group propagation direction, then the speed of the light-MEASURED IN THE GROUP PROPAGATION DIRECTION- will not be c, even in a vacuum. I never considered that this applied to gaussian beams too (lasers emit gaussian beams), but of course it makes sense that it does. From what I'm reading all this research group did was reduce the intensity and/or duration of the beam till it was a "single photon", and noted that the group velocity remained unchanged. Whooptie-freaking-do. However, while the science is unimpressive (well, there is no real science), the usefulness of this paper is to help point out to people working on precisely pulsed laser systems that they should not blindly assume that the laser beam propagates exactly at c.

To be honest though, what I am still confused about is why the velocity would not be exactly c at the center of the beam. Maybe I'll have to think some more on this.

Edited January 23, 2015 by |Velocity|

[Jovus]

The flip answer here is that DeBroglie pilot waves are weird.

The less flip answer is that this is just experimentally confirming theory that we've known for a long time.