Speed of Light: Constant or Variable?

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Discussion Overview

The discussion centers around the nature of the speed of light, specifically whether it is a constant value or varies depending on the medium through which it travels. Participants explore theoretical implications, experimental observations, and the definitions surrounding the speed of light in different contexts, including vacuum and various materials.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the speed of light is constant only in a vacuum, while it appears to slow down in materials due to interactions with matter.
  • Others argue that the speed of light is fundamentally constant everywhere, emphasizing that it is termed a "constant" because it does not change.
  • A participant mentions that light can be measured at much lower speeds in Bose-Einstein condensates, suggesting that this observation complicates the notion of a universal constant.
  • There is a discussion about the distinction between apparent speed and actual speed, with some noting that light travels at speed c between interactions in a medium.
  • One participant references a phenomenon where a photon does not reappear until another photon is sent in, raising questions about the implications of light traveling at c in a condensed medium.
  • Another participant highlights that the speed of light can vary based on the medium, which is evident in phenomena like refraction.
  • Some participants suggest that the understanding of light's speed may depend on whether one is discussing it at a macro or microscopic scale.

Areas of Agreement / Disagreement

Participants express differing views on whether the speed of light is a constant in all contexts or if it varies based on the medium. There is no consensus reached, as multiple competing perspectives remain present throughout the discussion.

Contextual Notes

Participants note the importance of distinguishing between the speed of light in a vacuum and its apparent speed in various materials. The discussion also touches on the complexity of defining light's speed in different physical contexts, such as quantum mechanics versus relativity.

wolram
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everywhere i look the statement "the speed of light is constant" is sited, yet in experiments with BOSS EINSTEIN condensates the speed of light has been measured at mere meters per second, this from WIKIPEDIA, so am i missing somthing?
 
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Originally posted by wolram
everywhere i look the statement "the speed of light is constant" is sited, yet in experiments with BOSS EINSTEIN condensates the speed of light has been measured at mere meters per second, this from WIKIPEDIA, so am i missing somthing?
Yes. What you are missing is the other half of the statement.

"The speed of light is constant...IN A VACUUM"
 
Actually what your missin is that the speed of light is constant everywhere. What happens in material is that light interacts with matter, i.e. is absorbed and emitted, making it appear to have slown down, but between those interactions it is still moving at c.
 


Originally posted by russ_watters
Yes. What you are missing is the other half of the statement.

"The speed of light is constant...IN A VACUUM"


uhhh... no. it's the constant everywhere. that's why it's called a "constant".

and it is called the constant (i suppose) because nothing known can travel faster than it.

oh, and "mere meters a second" is technically true, but may be a bit of an understatement.
 


[quotee]Actually what your missin is that the speed of light is constant everywhere. What happens in material is that light interacts with matter, i.e. is absorbed and emitted, making it appear to have slown down, but between those interactions it is still moving at c.[/quote]
Originally posted by maximus
uhhh... no. it's the constant everywhere. that's why it's called a "constant".
Ok, maybe what I used is the high school definition, but its a very common one. Here is a college physics class website that words it that way.

I understand the distinction though.
and it is called the constant (i suppose) because nothing known can travel faster than it.
Well its *A* constant because it never changes. Its constant.

I guess which you use depends on if you are talking macro or microscopically. Relativity or Quantum Mechanics. Thats why while talking about the speed of light in a medium, even though there really is no such thing as "the speed of light through a medium" there is an APPARANT speed of light through a medium - people generally don't make the distinction.
 
off to the T.P. forum this goes!
 
yes it is an understatement maximus, i have seen a report that said,"light stopped in its tracks" aphoton is directed into the condensate and dosent reappear until a second photon is sent in then the firt photon reappears traveling at c, i didnt quote this because i canot recall where i found it, its hard to imagine a photon traveling at c in what is only a tiny volume.
 
The speed of light does depend on what it is traveling through. Thats how you can separate light into its different freqencies.

I've read about them slowing the speed of light to somsething like 38Miles per hour, which is a lot slower than the 3x10^8 m/s that it travels in a vacuum, but I don't know how that fits into everything.
 
Why does refraction occur when light travels from air to a glass block? It is because the speed of light in air and in the glass block is different.
 
  • #10
Kl and Armed, again, it depends on your frame of reference. These other guys are right when in the micro scale - light DOES always travel at C in a vacuum. Light travels from one molecule to the next at C, then is either absorbed and re-emitted or reflected. The effect is that it APPEARS the light has been slowed down.

In the macro scale, it is simply easier to say it has slowed down. The math is much simpler that way.
 

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