How do Photons Reach the Speed of Light?

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Discussion Overview

The discussion centers on the nature of photons and their ability to travel at the speed of light, particularly in the context of their creation during atomic transitions. Participants explore various analogies and theoretical frameworks to understand why photons do not accelerate and how they achieve their speed without mass.

Discussion Character

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

Main Points Raised

  • One participant questions what propels photons to the speed of light upon their creation, noting that photons do not accelerate due to their lack of mass.
  • Another suggests that it is the nature of photons that allows them to travel at nearly the speed of light.
  • A different viewpoint posits that the question should focus on why other matter moves slower than light, referencing historical philosophical perspectives.
  • Several analogies are presented, including comparing photons to disturbances in an electromagnetic field or ripples in water, suggesting that these disturbances can propagate at light speed without needing to accelerate.
  • Some participants discuss the concept that all particles may be viewed as traveling at the speed of light, with massive particles appearing to move slower due to their zigzagging paths.
  • One participant introduces the idea from quantum field theory (QFT) that particles may take all possible paths, including those exceeding the speed of light, but most paths interfere destructively.
  • Another participant raises the question of what allows massless particles to exist only at the speed of light, suggesting it is an inherent property.
  • There is a discussion about the concept of terminal velocity and its applicability to the motion of particles in different contexts, with some skepticism about the relevance of classical analogies to modern physics.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of photons and their speed, with no consensus reached. Some analogies and interpretations are challenged, and there is ongoing debate about the implications of mass and motion in relation to light speed.

Contextual Notes

Participants highlight the complexity of the topic, noting that certain analogies may not fully capture the nuances of modern physics. There are references to historical concepts that may not align with current understanding, and discussions about the implications of mass and energy conservation remain unresolved.

Who May Find This Useful

This discussion may be of interest to students and enthusiasts of physics, particularly those exploring the fundamental properties of light and the behavior of particles in quantum mechanics.

  • #31
turin said:
You are presuming that energy is used to detect the particles.
If it's not energy, it's momentum and it would be zero the same (as you rightly say)

The issue is stronger than this (alluded in your parenthetical). Even if detection does not depend on energy, there is another problem. A (classical) massless particle traveling at a speed less than c must be at rest (have zero momentum as well as zero energy).
Don't understand this. Zero momentum doesn't mean "at rest".

In other words, a (classical) massless particle cannot exist at any speed other than c (as you suggest in your parenthetical):
Agreed.
 
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  • #32
Galap said:
Is time dilation proportional to acceleration?
Not according to the assumptions of relativity theory. Perhaps you're thinking of gravitational time dilation. This is proportional to the difference in gravitational potential w.r.t. some reference, but not the local gradient. That is Δφ rather than gradφ.

lightarrow said:
If it's not energy, it's momentum ...
Not so. Charge can be used, for instance.

lightarrow said:
Zero momentum doesn't mean "at rest".
I guess we'll have to agree to disagree on this one. Would you care to explain?

My explanation is that momentum is the quantity of motion. When the quantity of motion is zero, that means "at rest". Of course, steady-state wave mechanics combined with deBroglie's relation (i.e. quantum mechanics) confounds the notion of rest because it describes (free) particles in terms of plane waves. Perhaps that is your complaint?
 
  • #33
turin said:
Not so. Charge can be used, for instance.
Ok.
Zero momentum doesn't mean "at rest".
I guess we'll have to agree to disagree on this one. Would you care to explain?
To me "at rest" means velocity = 0. Momentum can be zero even with v not zero.
 
  • #34
As some have noted there is no perfect answer to the posters question. right.

Post 24 is an interesting framework to think about speed and distance versus time...That post implies that everything (EVERYTHING) moves through spacetime at c...
massless particles move entirely in space and experience no passage of time; massive particles move in spacetime and so some of their speed is diverted from space to time..

If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, and massive particles move diagonally and are restricted from ever reaching the horizontal...

As a personal observation, I think the ambiguity results from our lack of understanding of exactly what space and time are...how can they transform, rotate, "become" one another as in Lorentz transforms...We accept Lorentz-Fitzgerald's and Einstein's mathematical formulations, because they work so well and have been observationally verified, but we do REALLY understand it completely??...
 
  • #35
Naty1 said:
As some have noted there is no perfect answer to the posters question. right.

Post 24 is an interesting framework to think about speed and distance versus time...That post implies that everything (EVERYTHING) moves through spacetime at c...
massless particles move entirely in space and experience no passage of time; massive particles move in spacetime and so some of their speed is diverted from space to time..

If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, and massive particles move diagonally and are restricted from ever reaching the horizontal...

As a personal observation, I think the ambiguity results from our lack of understanding of exactly what space and time are...how can they transform, rotate, "become" one another as in Lorentz transforms...We accept Lorentz-Fitzgerald's and Einstein's mathematical formulations, because they work so well and have been observationally verified, but we do REALLY understand it completely??...

Yes, that is exactly what I was getting at. Personally I think it's a good way to look at it. IMO it makes more sense to use that interpretation. You did a much better job explaining it than me. Hopefully more people will understand it then.
 
  • #36
Naty1 said:
If you think of one dimension of space as horizontal,x, and time as vertical, in the y direction, then massless photons move horizontally, ...
This is only correct from an interpretive QFT standpoint, but not if you consider photons as classical particles that follow a single, definite trajectory. I have personally done experiments that demonstrate a nonzero time of flight for light. Also, the design of the particle detectors used in the LHC (e.g. CMS) would not work (i.e. would not give meaningful results) if photons (and any other ultrarelativistic particles) had zero time of flight, because the readout of the detector excitationis based on a very accurate timing between the proton bunch collision and the different radii of the detector materials.

To put it in your terms, photons move diagonally with a slope of dy/dx=1/c; dy/dx=0 has been ruled out by countless experiments.
 
  • #37
I read Post 24, and it offers no explanation, no cause. It only offers a math relation.
One can find at page 1506 de l' ACADEMIE DES SCIENCES (5 JUIN 1905) le doc. from M. H. Poincaré : ELECTRICITÉ . - Sur la dynamic de l'electron:
(en francais)
"en supposant que l'electron, déformable et compressible, est soumis à une sorte de pression constante extérieure don't le travail est proportionnel aux variation du volume"
in eng.:
"assuming that the electron, deformable and compressible, is subjected to a kind of constant external pressure whose work is proportional to the volume change"

It seems a 'cause', and one explanation is better than none.
 
  • #38
I thought that usually the light cone is defined by the 45 degree line...(setting appropriate units where c=1). Space like separation is to the left and right of this cone, and time-like separation is top and bottom. In this way, events with space-like separations could not affect each other. I never read that the light cone is defined by the horizontal line...? Wouldn't that imply light moving at infinite speed?

Reference:
http://en.wikipedia.org/wiki/Light_cone
 
  • #39
lightarrow said:
Momentum can be zero even with v not zero.

Example?
 
  • #40
jtbell said:
Example?
mass = 0 (we were discussing hypotetical massless particles traveling at speeds less than c).
 

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