Photons at less than speed of light?

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Discussion Overview

The discussion centers around the behavior of photons when traveling through different media and whether this behavior contradicts the principles of Special Relativity. Participants explore the implications of photons moving at speeds less than the speed of light in a vacuum, addressing theoretical and conceptual aspects of light propagation in various contexts.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that photons, which are massless particles, can only exist at the speed of light, leading to questions about their behavior in media where light appears to travel slower.
  • Others argue that the speed of light in a medium is an average speed due to interactions with atoms, where photons are absorbed and re-emitted, maintaining that their instantaneous speed remains at c.
  • A participant mentions that the term "speed of light" typically refers to the speed in a vacuum, suggesting that the behavior of light in media should be considered separately.
  • Some contributions discuss the concept of group and phase velocities in specially prepared nonlinear media, where waveforms can appear to travel faster than the speed of light without violating relativity.
  • Questions are raised about the implications of "lucky" and "unlucky" photons in terms of their travel times through a medium and the nature of coherent light.
  • There is a reference to discrepancies between different educational resources, highlighting that some textbooks may oversimplify the concept of light speed in media.

Areas of Agreement / Disagreement

Participants express differing views on the nature of photons and their speed in media, with no consensus reached on whether the behavior of light in a medium contradicts the principles of Special Relativity. The discussion remains unresolved regarding the implications of these interactions.

Contextual Notes

Participants note that the average speed of light in a medium is influenced by the interactions with atoms, which may lead to confusion about the instantaneous speed of photons. There is also mention of the need for careful definitions when discussing light speed in various contexts.

rushil
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According to Special Relativity, A particle with rest mass = 0 can exist if and only if it is moving at the speed of light. A photon is a particle which comes into this category. However, when light travels in a medium it's speed is always reduced.
Does this contradict relativity? What I want to ask is that when light travels at a lower speed in some medium. do photons exist and why? Why do photons 'seem' to exist at speeds less than that of the speed of light.
 
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Photons can't go slower then the speed of light because photons ARE light. It's like asking can a truck go slower then the speed of a truck.
 
So what's happening when light is going through a medium where its speed is less than 3 * 10^8 m/s ? Doesn't relativity say ( without making any reference to 'light'!) that photons can exist ONLY at 3 *10^8 m/s ? What;s happening here? That's my question!
 
No, it says that photons can only exist at the speed of light. 300,000,000m/s is simply the speed of light in a certain medium (a vacuum).
 
Light never travels with a speed that is anything less than c.

When light travels through a medium, it is constantly absorbed and then re-emitted by the atoms of that medium. The photons may excite the atoms into a metastable state which has a finite duration. The average speed of the light is decreased from c by an amount that depends on how long the photons are "held captive" by the atoms, but the instantaneous value of their speed is always c.
 
That was exactly what I thought! It was collaborated by what I read in Hecht's Optics... and now by you, Tom! Well, all this is in contradiction to what Indian books say! You can understand the true phenomenon only by reading more! :-p
 
"Speed of light" almost always means "speed in a vacuum", to a sufficient approximation speed between astronomical bodies. Density between local stars is around 1 proton per cubic kilometer, and it's much lower between galaxies.

So travel in media is excluded from the definition. What happens in media? There are two kinds of media; normal ones and specially prepared nonlinear ones.

In the normal media the photons travel until they are captured by an atom, then they are absorbed and the atom is driven by this to a higher energy state ("quantum leap"). After a moment it "relaxes" down to its prior state by emitting a new photon. Conservation of momentum means that the new photon has the same momentum as the old one, and it continues along the original path,... until it meets another atom and the process repeats. In this way the photons always travel at c (which, since they are massless, relativity requires them to do), but the time of passage, due to the successive atomic interactions, is longer and the effective speed from point to point is lower.

In the specially prepared nonlinear media, the group velocity and phase velocity are jiggered deliberately to make some wave form travel faster than the photons (held up by those interactions) can travel. That a wave form (but not a signal) can travel faster can be illustrated by swinging a flashlight while pointing it at a wall. Obviously the photons aren't traveling any faster, but if the wall was a light-year away, you could make the spot from the flashlight move faster than light. The clever experimenters can set up special media to make this happen in the laboratory, and reprters are always delighted to hear and report that "Einstein has been refuted".
 
rushil said:
That was exactly what I thought! It was collaborated by what I read in Hecht's Optics... and now by you, Tom! Well, all this is in contradiction to what Indian books say! You can understand the true phenomenon only by reading more! :-p
Just an editorial comment: that's a common oversimplification made by American high school textbooks as well. Because they never tell you that that speed through a medium is only an average or apparent speed due to the interaction with particles, the first time just about anyone hears about Relativity, they get confused.

It annoys me.
 
  • #10
Tom Mattson said:
When light travels through a medium, it is constantly absorbed and then re-emitted by the atoms of that medium.
Then we should see 'lucky' photons squirt through more rapidly than the bulk of their cohort and 'unlucky' ones that arrive late. Do we? Also, how do you explain coherent light in this picture?
 
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