# Why are photons massless? **URL:** \ **Category:** Factual Questions\ **Created:** [January 30, 2020, 8:21pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244 "2020-01-30T20:21:14Z")\ **Posts on this page:** 18\ **Page:** 2
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [January 31, 2020, 10:46pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/21 "2020-01-31T22:46:41Z")
> [@Pardel-Lux](#): > > That is interesting. Reminds me of something I read ages ago on a popular science book (might be by Feynman?): when light passes through more than one medium, the bending of the path taken by the light we measure is because the photon “goes” from A to B through the “fastest” combined path in both media\*. Does that mean that when passing though a prism (like the Pink Floyd cover) red light travels slower than violet light? And if so, how come there is a difference in light speed in a medium like glass but not in a vacuum? Yes, the speed of light through a medium is different for different wavelengths. (Read about [refraction](https://en.wikipedia.org/wiki/Refraction).) The simplest explanation is the electric-magnetic waves interact with the charged particles in the medium. If you read about [Maxwell’s equations](https://en.wikipedia.org/wiki/Maxwell%27s_equations), you’ll see that the speed of propagation through a medium depends on its permittivity and permeability (constants describing the “stretchiness” of the medium to electric and magnetic fields, respectively). That’s all based on a wave description of light. If you want to use a particle-based description, you can assign an effective “mass” to photons in a medium, which causes them to move slower than _c_. Just as Higgs bosons cause particles to have mass, virtual photons from the medium give mass to the photons propagating through it. ---
**Author:** ![Spectralist](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Spectralist](https://boards.straightdope.com/u/Spectralist)\ **Post date:** [February 1, 2020, 5:55am UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/22 "2020-02-01T05:55:11Z")
> [@Lucas\_Jackson](#): > > Given that explanation, is there anything in existence that doesn’t get affected by gravity? Gravity, I would think, is unaffected by gravity. ---
**Author:** ![Charlie\_Wayne](https://avatars.discourse-cdn.com/v4/letter/c/4bbf92/32.png) [@Charlie\_Wayne](https://boards.straightdope.com/u/Charlie_Wayne)\ **Post date:** [February 1, 2020, 10:22am UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/23 "2020-02-01T10:22:47Z")
> [@Spectralist](#): > > Gravity, I would think, is unaffected by gravity. Good answer! ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 1, 2020, 11:04am UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/24 "2020-02-01T11:04:17Z")
> [@Spectralist](#): > > Gravity, I would think, is unaffected by gravity. Gravity is affected by gravity in that general relativity is non-linear. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 1, 2020, 1:03pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/25 "2020-02-01T13:03:21Z")
The reason why we can describe gravity as a distortion of spacetime is that it affects everything equally. If there was something not affected by gravity, gravity-as-spacetime-warping wouldn’t work as a description of reality. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [February 1, 2020, 3:58pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/26 "2020-02-01T15:58:35Z")
> [@Spectralist](#): > > Gravity, I would think, is unaffected by gravity. If gravity is waves then would it be affected by gravity via interference? ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [February 1, 2020, 4:08pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/27 "2020-02-01T16:08:33Z")
> [@Spectralist](#): > > Gravity, I would think, is unaffected by gravity. If gravity is waves then would it be affected by gravity via interference? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [February 1, 2020, 4:22pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/28 "2020-02-01T16:22:58Z")
> [@Chronos](#): > > **Keeve** , we also think that gravitons (the particles of gravity) are massless, and in fact our best bound for the mass of the graviton is several orders of magnitude stronger than our best bound for photons. So even if anyone found evidence that the photon does have a tiny nonzero mass, we’d still expect the graviton to be massless, and thus travel at the Fundamental Speed. Claudia de Rham, Gregory Gabadadze, and Andrew J. Tolley published “Resummation of Massive Gravity” in [Phys. Rev. Lett](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.106.231101) in 2011. > [@](#): > > We construct four-dimensional covariant nonlinear theories of massive gravity which are ghost-free in the decoupling limit to all orders. These theories resum explicitly all the nonlinear terms of an effective field theory of massive gravity. We show that away from the decoupling limit the Hamiltonian constraint is maintained at least up to and including quartic order in nonlinearities, hence excluding the possibility of the Boulware-Deser ghost up to this order. We also show that the same remains true to all orders in a similar toy model. And that, fellow non-scientists, is why these jargon-free discussions seldom satisfy. They apparently got massive, pun deliberate, pushback on this. But an article in [The Guardian](https://www.theguardian.com/science/2020/jan/25/has-physicists-gravity-theory-solved-impossible-dark-energy-riddle) this year says it’s gaining attention because the math works, although there is yet no evidence. I’m mentioning this solely because it was so recent I remembered the article. You physics people can do all the commenting on it. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 1, 2020, 5:10pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/29 "2020-02-01T17:10:33Z")
> [@Saint\_Cad](#): > > If gravity is waves then would it be affected by gravity via interference? The total influence on a how an object/particle/etc behaves will always depend on all the influences, but when the total influence is just the sum total of the influences then the situation can be described linearly, but when those influences are also influencing each other then it is non-linear. Waves and interference are linear phenomena, but can be used to describe gravity in certain situations when the self-interaction can effectively be ignored. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 2, 2020, 1:04am UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/30 "2020-02-02T01:04:40Z")
Since we’re mentioning the weak gauge bosons, it’s worth noting that the Z particle behaves very much like a photon, except that it has mass. But any interaction that a photon can mediate, a Z can also mediate (and in fact, a complete description of either electromagnetism or the weak force is impossible without including the other). **Exapno Mapcase** , I haven’t read that article, but from the abstract, it sounds like it’s addressing a toy model: A model which does not correspond to any known real phenomenon in nature, but which is in some way simpler than the real phenomena, and which has some interesting properties to be examined which might be somewhat analogous to real properties. In other words, it’s not saying that the graviton really is massive; it’s saying that thinking about what gravity would be like if it were can lead to interesting results. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [February 2, 2020, 3:18pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/31 "2020-02-02T15:18:45Z")
Duplicate post. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [February 2, 2020, 3:16pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/32 "2020-02-02T15:16:45Z")
> [@Saint\_Cad](#): > > If gravity is waves then would it be affected by gravity via interference? Like separate sets of ripples on a pond intersecting? Not being a scientist, a large part of my misunderstanding of gravity comes from too many dumbed-down science shows that demonstrated gravity with the ‘bowling ball on a trampoline’ method. So when I think of situations of things (like light) not being affected by gravity I think of the above example with a yardstick laid across the depression as a possible direction of travel. I know that’s not right (now) but it’s hard to get those early ‘lessons’ out of my head. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 2, 2020, 8:26pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/33 "2020-02-02T20:26:11Z")
Gravitational waves do interfere in (almost) the same way as any other waves. In fact, a large part of the difficulty in detecting them is that your detector is always detecting _all_ the gravitational waves that are passing through it, and you have to break that down into all of the separate waves. Though admittedly this isn’t as big a problem for the LIGO-type detectors (the only ones that have actually made definite detections), because in those frequency ranges, you’ll usually have one source at a time that’s much stronger than the others. The “almost” there, incidentally, is because gravity is non-linear (that is to say, gravity itself gravitates). But for gravitational waves that might realistically be passing by Earth, the wave amplitude is so small that the nonlinearity can safely be completely neglected (the wave itself only shows up when you look at changes as small as one part in around 10^20, so the nonlinearities would be way down at 1 part in 10^40). ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 2, 2020, 8:41pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/34 "2020-02-02T20:41:24Z")
People don’t have much experience with non-linear waves - but this video of oobleck might demonstrate how weird those waves can get [Non-Newtonian Fluid on a Speaker Cone - YouTube](https://www.youtube.com/watch?v=3zoTKXXNQIU) - for a much smaller driving force, the waves would look like typical water waves, but the driving force of the speaker cone is evidently large enough for the nonlinearity to be obvious. ---
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [February 6, 2020, 3:00pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/35 "2020-02-06T15:00:49Z")
> [@Saint\_Cad](#): > > If gravity is waves then would it be affected by gravity via interference? The possibility that gravitational waves should interact with light makes [some people](https://www.space.com/graviton-photon-interaction-hunting-machine.html) believe that photons may reveal ‘massive gravity’. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [February 6, 2020, 6:22pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/36 "2020-02-06T18:22:33Z")
Why don’t photons have mass? Because they’re really light. ---
**Author:** ![Atamasama](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atamasama/32/12961_2.png) [@Atamasama](https://boards.straightdope.com/u/Atamasama)\ **Post date:** [February 6, 2020, 7:06pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/37 "2020-02-06T19:06:30Z")
> [@Colophon](#): > > Why don’t photons have mass? > > Because they’re really light. They can seem heavy but those are faux tons. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [February 6, 2020, 7:16pm UTC](https://boards.straightdope.com/t/why-are-photons-massless/847244/38 "2020-02-06T19:16:22Z")
> [@Chronos](#): > > Since we’re mentioning the weak gauge bosons, it’s worth noting that the Z particle behaves very much like a photon, except that it has mass. But any interaction that a photon can mediate, a Z can also mediate (and in fact, a complete description of either electromagnetism or the weak force is impossible without including the other). As I understand it, the electroweak force has 4 bosons, all of which are massless. Two of them are mixed to form the W+ and W- bosons, and the other two are mixed to form the Z and photon. The W and Z have mass because of the Higgs boson, but I don’t really understand why the Higgs affects them and not the photon. [Previous page](https://boards.straightdope.com/t/why-are-photons-massless/847244.md?page=1)