Do Photons Have Mass and Why Does it Matter?

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Discussion Overview

The discussion revolves around the question of whether photons have mass and the implications of this for understanding light and its behavior in physics. Participants explore concepts from special relativity, empirical evidence, and theoretical frameworks related to mass and energy.

Discussion Character

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

Main Points Raised

  • Some participants assert that the rest mass of a photon is zero due to the implications of special relativity, particularly the relationship between mass and the speed of light.
  • Others argue that the concept of mass in relation to photons is complex, suggesting that if photons had mass, it would imply they could not travel at the speed of light.
  • A few participants discuss the empirical evidence regarding the upper limit of photon mass, noting that it is very small but not definitively zero.
  • There are questions about whether a non-zero photon mass could vary with energy, leading to further speculation about gravitational effects and the behavior of light.
  • Some contributions highlight the role of electromagnetic radiation as a form of energy that can influence gravitational fields, despite photons not having mass in the traditional sense.
  • Participants mention theoretical frameworks, such as the Higgs mechanism and gauge theories, to explain the absence of mass in photons, though these explanations are noted to be complex.
  • One participant emphasizes the challenge of providing a simpler explanation for a younger audience while maintaining scientific accuracy.

Areas of Agreement / Disagreement

There is no consensus on the nature of photon mass, with multiple competing views and ongoing debate about the implications of massless photons versus the possibility of a very small, non-zero mass.

Contextual Notes

Participants acknowledge limitations in understanding and explaining the concepts involved, particularly regarding the definitions of mass and energy, as well as the complexities of theoretical physics that may not be easily conveyed.

Who May Find This Useful

This discussion may be of interest to students and enthusiasts of physics, particularly those exploring the nature of light, mass, and the principles of special relativity.

  • #31
We think the mass of a photon at rest is zero because that matches the mathematics of the equations we have that model the experimental and experiential data we have.

I don't know if there are other equations that can accurately describe the relationship between mass, energy, and velocity that yield resting photons with an actual mass. I certainly wasn't exposed to any rumor of such in Physics 101 or 102 in college.
 
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  • #32
Mass is a measure of the inertia of an object, that is the resistance of it against a change of motion. Since light always travels with the same speed for any observer it has no mass, per definition.
 
  • #33
I've had trouble with this concept, too. After all, light waves are bent by the gravitational curvature of space and they impart momentum to objects on which they impinge. How can they be affected by gravity and have momentum, if they have no mass? Chemistry students I tutor have trouble with this as well. The key is the distinction between rest mass and inertial/gravitational mass. This is how I think of it, physically:

To say that photons have a nonzero rest mass is to imply that they can exist at rest. However, QM assures us that photons are the particles that are equivalent to light waves. Like like all waves, light and photons of light, must be in motion. There's no such thing as a wave at rest, so photons at rest cannot exist. Their energy and mass derive solely from their motion. In other words, the notion that photons have a nonzero rest mass is as fictional as light that doesn't move.
 
  • #34
Mark Harder said:
To say that photons have a nonzero rest mass is to imply that they can exist at rest.

Come to think of it, that's not a logical statement by itself, is it? Hypothetically, you could have a particle, system or whatever that does not exist at rest, but might have mass at rest. Also, must anything that exists, at rest or otherwise, have mass?
Sorry 'bout that.
 
  • #35
Mark Harder said:
Also, must anything that exists, at rest or otherwise, have mass?
There are obviously many cases where gravity can be ignored, but everything I know anything about has an attraction to ANYTHING else, especially energy which most of baryonic mass consists of.
 

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