Light and Gravity

[foodchain]

Would light speed up or slow down in the presence of gravity?

[Sisyphus]

Neither.

[foodchain]

Neither.

 

I read this article and it had me a bit confused as light or the speed of such is a constant, so I don’t know if its more of an issue of quantum mechanics or not, or if I am even applying physics wording correctly.

 

"Black hole recipe: Slow light, swirl atoms

Peter Weiss

 

Physicists may soon create artificial black holes in the laboratory, analogous to the ones expected to lurk in distant space. A new study by a pair of theorists in Sweden describes how swirling clouds of atoms could slug down all nearby light, making them as black as their astronomical cousins.

 

 

Computer-generated plot shows paths of light rays sucked into an optical black hole.

Leonhardt and Piwnicki/Physical Review A

 

 

Called optical black holes, these eddies could provide an extraordinary test-bench for the theory of general relativity, which gave rise to the concept of gravitational black holes, the researchers say. Ulf Leonhardt and Paul Piwnicki of the Royal Institute of Technology in Stockholm find that the same mathematics describes both the terrible tug of an astronomical black hole on light and the gentle corralling of rays by an atom vortex.

 

"We were quite surprised that it worked that well," Piwnicki says. "We're still working on it to understand it more deeply," he adds. The researchers report their findings in the Jan. 31 Physical Review Letters and the December 1999 Physical Review A.

 

The laboratory analogy goes only so far, however. Black holes out in space are massive remnants of collapsed stars that pull in not just light but everything else in their vicinity. By contrast, the proposed atomic whirlpools would have too little gravity to swallow any matter. Tiny tornadoes within wispy clouds of gas, they would snag photons through their remarkable ability to slow light pulses.

 

The proposed mechanism by which such a vortex would capture light rests on principles discovered in the 1800s. Many substances, such as water or glass, retard light as it passes through them. Consequently, a fluid flow can drag light along with it.

 

Leonhardt and Piwnicki show that, theoretically, an eddy can trap a beam if it circulates faster than the speed of light in the liquid, just as fish can be trapped in a whirlpool that's rotating faster than they can swim. So far, however, no material slows down light enough for a vortex's velocity to exceed the radiation's reduced pace.

 

That may change soon. Last year, Lene V. Hau, now of Harvard University, and her colleagues used a stationary, laser-manipulated atom cloud to limit light to an astoundingly sluggish 17 meters per second—roughly bicycle speed (SN: 3/27/99, p. 207).

 

To make an optical black hole work, Leonhardt and Piwnicki estimate that light would need to crawl along still more slowly, at a mere 1 centimeter per second.

 

"We're actually aiming for that, and I think it will be possible," Hau says. Her experiments have already achieved a slowdown to 50 cm/s, she told Science News. She calls the Stockholm proposal "a very exciting idea" but cautions that aspects of the proposed vortex might prevent the black-hole effect. "There are things one must look into," she says.

 

The Stockholm researchers discovered that German physicist Walter Gordon, back in the 1920s, found the same mathematical equivalence between light in a moving fluid and in a gravitational field but didn't investigate specific patterns of flow, Piwnicki says.

 

Modeling swirling flows in their new study, he and Leonhardt found that photons passing near the fast-spinning optical black hole, but outside a critical radius, follow a bent course. Those straying close spiral inexorably into the center.

 

Pursuing similar studies, theorist James Anglin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his coworkers are exploring links between sound waves in a moving fluid and light waves in a gravitational field. Atom clouds may also trap acoustic vibrations, creating sonic or so-called dumb, black holes, Anglin says."

 

http://www.sciencenews.org/articles/20000205/fob4.asp

 

I got interested in the idea simply from wondering what effect it might have in trying to gauge dark matter, that is if any such thing existed, that is gravity effecting the behavior of light in terms of speed.

[m4rc]

Interesting article.

 

I can see where you may have been confused by the article. Both gravity and matter can affect the propagation of light.

 

The effect of gravity on light:

A black hole can change the direction of light (deflection of any angle) but this will be without changing its speed. The photon will gain energy as it moves towards a black hole (blue shift) or it will lose energy if it is moving away from a black hole (red shift). However for light near a black hole the speed of light is constant.

 

Slowing down light in a medium

When people say that the speed of light is lower in matter, the mean that the distance traveled over the time interval is lower.

 

What is happening is that as the light passes though matter, it gets absorbed then re-emitted after a short delay. This occurs multiple times. As it travels from one atom to the next it is traveling at the speed of light, but because of all the delays, it has a lower average speed .

 

The subject of the article involves strange effect that occur when a liquid is flowing faster than the (average) speed of light that liquid.

[CriticZan.np]

According to relativity, light bends in presence of gravity. it implies that light experiences gravitational force of attraction.

Also wavelength=h/mc

=> m=h/(c*wavelength)

SO retardation on light must be given by F/m => F*c*wavelength/h. But force of gravitation varies in space. It acts as accelerating force if light approaches a mass and retards it if it tends to get away. So, i think due to varying force of gravitation and it's nature of acc and retardation. It can't be exactly quoted if light is accelerating or retardingin universe.

[swansont]

You can get the appearance that light changes speed in a gravitational field if you measure it from a region with a different potential. (As I understand it, you aren't properly accounting for the change in curvature, so your distance is incorrect). If you measure it locally it will be the same, as m4rc has said.