Massless photons? Is everything light?

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Discussion Overview

The discussion centers around the nature of photons, specifically whether they are massless and the implications of this on their behavior in relation to gravity and the speed of light. Participants explore theoretical and experimental perspectives on the mass of photons, as well as the conceptual challenges of understanding reference frames at the speed of light.

Discussion Character

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

Main Points Raised

  • Some participants question whether photons are truly massless, suggesting that their energy and gravitational effects imply they might have mass.
  • Others clarify that, within current theories and experimental precision, photons are treated as massless, though they can still be affected by gravity due to their energy-momentum tensor.
  • A participant notes that it is nonsensical to consider a reference frame for photons, as time does not pass for them, complicating the idea of everything being "light" relative to photons.
  • There is a suggestion that while photons appear massless in theory and experiments, one could hypothetically assign a mass to them, though this is not supported by current understanding.
  • Some participants emphasize that gravity interacts with energy, not just mass, which is relevant to understanding the behavior of photons.

Areas of Agreement / Disagreement

Participants express differing views on the implications of photons being massless and the conceptual understanding of reference frames at light speed. There is no consensus on whether the mass of photons can be considered negligible or if they could theoretically possess mass.

Contextual Notes

The discussion highlights limitations in understanding the nature of photons, particularly regarding the definitions of mass and energy, as well as the challenges in conceptualizing reference frames at the speed of light.

Bendelson
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Are photons massless? They have energy and can be affected by gravity, so don't they have mass? I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass?
Also, if only light/photons can move at the speed of light, then wouldn't everything be light considering that, relative to the photons, they aren't moving while everything else is moving at the speed of light? I'm not super knowledgeable about physics so I expect I have errors in a few of my assumptions so please correct me, I'm just curious and have been thinking about this.
 
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Bendelson said:
Are photons massless?
Within the precision of all measurements, and within theories: yes.
Bendelson said:
They have energy and can be affected by gravity, so don't they have mass?
Those are two independent things.
Bendelson said:
I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass?
Experimental results are never exact, but in theory they are treated as exactly massless. And the experimental limits are really good. It would be odd to have a particle with a mass of just 0.00000000000000000000000000000000000000001 the mass of other particles.
Bendelson said:
Also, if only light/photons can move at the speed of light
And gluons, and gravity.
Bendelson said:
then wouldn't everything be light considering that, relative to the photons, they aren't moving while everything else is moving at the speed of light?
That does not make sense.
 
Bendelson said:
...wouldn't everything be light considering that, relative to the photons, they aren't moving...
Photons do not have a valid reference frame; time does not pass. It is nonsensical to ask what a photon (or anything moving at c) 'sees' or 'experiences'.
 
You'll find several relevant threads in the relativity FAQ: https://www.physicsforums.com/forums/relativity-faq.210/
 
Last edited by a moderator:
Bendelson said:
Are photons massless?

Nothing stops you from adding a mass to the photon. In theory however as well as in experiments, the photons appear to be massless.

They have energy and can be affected by gravity, so don't they have mass?

Yes they have energy which corresponds to their momenta, not mass... Gravity doesn't couple only to masses but to the energy momentum tensor. For the photons the last is non-vanishing.

I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass?

Up to very high precision photons appear to be very light and experiments also verify that they can be massless. Note: experiments will never tell that the photon has zero mass, but they can verify it within some precision...

Also, if only light/photons can move at the speed of light, then wouldn't everything be light considering that, relative to the photons, they aren't moving while everything else is moving at the speed of light? I'm not super knowledgeable about physics so I expect I have errors in a few of my assumptions so please correct me, I'm just curious and have been thinking about this.

There is no well-defined reference frame where the photon is at rest. That's exactly because they are considered massless...
 

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