Slow Light: Can Light Gain Mass and Accelerate?

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Discussion Overview

The discussion centers around the concept of light slowing down in various mediums and whether this implies that light gains mass or potential energy when it travels slower than its maximum speed in a vacuum. Participants explore the implications of light's behavior in different materials and the nature of its speed and momentum.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if light gains mass when it slows down, suggesting it builds potential energy for when it exits a medium and accelerates back to maximum speed.
  • Another participant asserts that light moves at speed c between electrons in a material, with the observed slowdown attributed to absorption and re-emission processes.
  • A different viewpoint suggests that light can be described as an effective slow particle in a medium, and that a force is only required if momentum changes, not velocity.
  • One participant argues that a photon cannot exist at less than speed c, stating that it "ages" when its velocity drops below c, and that the appearance of slower speed is due to constant deflection and absorption in materials.
  • Several participants emphasize that light never travels below c, attributing the slower apparent speed to delays caused by absorption and re-emission in the medium.
  • There is a moment of confusion regarding the similarity of responses, with one participant acknowledging the overlap and apologizing for the perceived plagiarism.

Areas of Agreement / Disagreement

Participants generally agree that light does not travel below speed c and that the observed slowing is due to interactions with the medium. However, there is disagreement regarding the implications of this behavior, particularly concerning the concepts of mass and potential energy.

Contextual Notes

Some assumptions about the nature of light and its interactions with materials remain unresolved, particularly regarding the definitions of mass and momentum in this context.

sirchick
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Hey

I was reading about an experiment that slows light down to 1 mile an hour:

Source:
http://news.bbc.co.uk/1/hi/sci/tech/655518.stm

What i wanted to ask was, if light slows down, does it gain mass whilst it is moving slower than its maximum speed limit in a vacuum, in effect building "potential energy" for when it exits an object that slows it down, then shoots off to maximum speed in a vacuum again ? ?

If light escapes an object that slowed it down, where is the force that speeds it back up once its left that object, instead of just staying at the speed it was slowed down to.

Shouldn't there be some kind of force that accelerates when it leaves the object, perhaps by expelling mass or something ?
 
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light is moving at c when it is in between electrons in the material. There is just a lot of absorption and re emission that's causes this time lag.
 
cragar said:
light is moving at c when it is in between electrons in the material. There is just a lot of absorption and re emission that's causes this time lag.
You can describe light in a medium as an effective slow particle, too.

A force is required only if the momentum changes, not the velocity. As far as I know, the momentum of slow light is an ongoing discussion (for example, see this pdf). In any case, light does interact with a medium, therefore it can exchange momentum.
 
A photo cannot exist at less than C. it is emitted at C and only exists because at C it doesn't experience time as passing. as soon as its velocity drops below C it "ages" and is gone. When traveling through some materials it is constantly deflected or absorbed and re-emitted so it seems to be traveling slower but it always travels at C.
Paul
 
The speed of light is a constant in a vacuum, as you have said. As for the slowing down of light, light appears to travel slower within different mediums as the photon is constantly being absorbed and remitted from the atoms of that medium. Once the photon leaves one of the atoms of the medium, it travels at a constant speed until it is reabsorbed by another atom. Light never travels below c, it is only that the absorption and reemission of the photons causes a slight delay which gives the appearance of a slower traveling speed.
 
AbsoluteZer0 said:
The speed of light is a constant in a vacuum, as you have said. As for the slowing down of light, light appears to travel slower within different mediums as the photon is constantly being absorbed and remitted from the atoms of that medium. Once the photon leaves one of the atoms of the medium, it travels at a constant speed until it is reabsorbed by another atom. Light never travels below c, it is only that the absorption and reemission of the photons causes a slight delay which gives the appearance of a slower traveling speed.

I believe I just said that...?!
Paul
 
PaulS1950 said:
I believe I just said that...?!
Paul

I didn't read the previous responses. After reading your response, I now see how similar our two responses are. Despite their similarities, it is purely coincidence that they sound so similar. I apologize for the apparent plagiarism.
 
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