Why No One Knows If Photons Really Are Massless: What if they Aren't?

REFERENCES

CHAPTERS
0:00 Do photons have mass?
1:34 Why we presume speed of light is the maximum speed
3:13 If light is not the maximum speed, then what is?
4:51 Why don't we know whether photons are massless?
6:58 Wouldn't the universe collapse if photons had mass?
8:10 What would we see if photons had a significant mass?
10:04 So do photons have mass or not?
11:13 How to learn advanced math to learn physics in depth

SUMMARY
“Do photons have mass?” in most textbooks, the answer is no. But is it proven that light does not have any mass? Has anyone every actually confirmed this in a measurement? No.

Einstein’s theory of relativity tells us that massless objects always move at the same velocity in a vacuum, which is the speed of light or 299,792,458 m/s. This assumes that the photon is massless. But it doesn’t have to be. We presume that the photon is massless because we have not measured anything faster.

If a photon has a slight mass, its velocity is not the maximum. Bit since we have not measured anything faster, we presume that the speed of light is the maximum velocity possible.

Conceptually, it doesn’t matter what the speed of light is in special relativity. What matters is the speed of information flow. This is necessary to ensure that causality is not violated. In the case where the speed of light is less than c, then c would represent the speed of information flow, not the speed of light. This speed would be the maximum speed allowed in the universe. So the true limit of special relativity is the speed limit of information flow, not the speed of light.

This would not be a problem in science because since photons are the fastest way we know to send information, it would just mean that we don’t have a way to communicate at the maximum speed, but only at the speed of light, which would be slower than that maximum.

This would not invalidate Relativity theory either. It's just for practical purposes we state relativity in terms of speed of light, but we could have just as well say speed of causality to be more specific.

The problem is that we actually don’t know if photons really are massless because we have not been able to devise an experiment to truly test this. Experiments tell us that its mass can’t be larger than 10^-18 eV, because we would have been able to detect that mass.

Our theories tells us that it should be massless, but there is nothing else we can measure which could be faster.
There is only one other known particle that is thought to be massless, the gluon, but due to the laws of the strong force, these gluons are not free and are always bound with quarks. So we can’t measure their speed.

The lightest particles we know of neutrinos, would still be around one quintillion times heavier than photons.

If photons have a very slight mass, the universe would not look much different but if it had a significant mass, then we would see some differences. One example would be that the electromagnetic force would become finite.

Another change you would notice with a significantly massive photon is that higher energy photons, that is photons with a shorter wavelength, would travel faster than longer wavelength photons. This probably would not make much of a difference if you were looking at still objects, but a moving object like a fast white car at a distance might look like a blur of colors, with the blues being further away than the red. This is because the blue color within the white of the car would reach your eyes before the red color. Another effect that would come with a massive photon is gravity bending light like a prism.
#photonmass
#photon
The observable universe would appear to be much smaller because light from very far distances would not have reached us yet. The cosmic microwave background, or CMB, might not be visible because its light may not have reached us yet.

Since we don’t have any observations that support a truly massless photon, the possibility that it really does have some mass, at this time, cannot be excluded.

[Mark-efpi]

"I don't know" - For the countless physicists and armchair physicists that will give an answer, then passionately defend that answer, those three words are profound. Thank you Arvin.

[barryon]

There's a hypothetical phenomenon called the Scharnhorst effect, where a tiny gap in which vacuum fluctuations are suppressed (like with the Casimir effect), light should be moving a little faster than C. The idea being that photons are slowed by interaction with vacuum fluctuations, so if there are fewer vacuum fluctuations then you get faster photons. It's a tiny effect, too small to be detectable with current technology (like 1x10-36), but it suggests that even if photons are massless that they travel a tiny bit less than the maximum speed for the universe.

[jimmyzhao]

Fun Fact: Lamps in video games emit photons and consume electricity just like in real life.

[effectingcause]

2:18 “If there’s no mass, that is, if M is zero” … This wording should actually be “if there’s no REST mass, that is, if rest-M is zero” … bcus rest mass is what you’ve implied in this equation. For it is the rest mass plus the momentum energy which equals the total energy. So, the question is wether photons have “rest mass.” We’re not wondering about relativistic mass. BUT, we know that a photon has no rest mass bcus a photon can never be at rest, since there is no frame of reference from which to view the photon at rest. Therefore the rest mass of a photon is definitely zero. It shouldn’t matter that light is just the fastest particle we know of and there could be faster speeds possible. It is only that the light travels with invariable speeds from the perspective of any observer which inherently causes light to be the fastest speed possible, period. If the rest mass equals zero, it means all of the energy in the photon must be “momentum energy.” There can be no rest mass. If there could be rest mass for a photon, then the speed of light would not be invariant, for that is the essence of the special theory just there- the fact that light traverses across space invariably from the perspective of all observers. The Michelson Morley experiment proves that light is 100% invariable against any motion of any observer, so we can deduce here from Einstein’s answer to the Michelson Morley experiment that light must be 100% rest-massless. Light does not stand at rest from any perspective, ever. This is why Einstein said the photon is massless. It wasn’t just an assumption he made. Rather, it was the only possibility.

[seanspartan]

I thought that one of the reasons we discovered Neutrinos have mass is because we have observed them changing flavors. Since they "experience" time, they do not travel at c, hence they cannot be massless.

[rob-hgss]

Interesting video! Just a minor point: at 10:20 you say that the CMB might not be visible because its light may not have reached us yet. This doesn't seem correct to me because the CMB started everywhere in the universe, including here. It's just that we would receive now CMB light which started closer to us and the CMB would still be visible, as it is everywhere in the universe.

[DemonetisedZone]

This channel gets to the inconsistencies and gaps in our understanding every time

Great video Arvin👍😉

[agmuntianu]

gravitational waves also propagate at the speed of causality, so we could compare the time it takes for the light from a neutron star merger to reach us, compared with the time it takes to detect it using LIGO :)

[FlirtUniversity]

Arvin, I just love the way how you explain super complicated topics in an easy to understand way!

[barefootalien]

I wish you'd made a much bigger deal of just how "almost sure" we are that photons are massless. This is the same "we don't really know" that lies at the bottom of _everything_ in science, simply because science, as a philosophical construct, can never have 100% certainty about _anything._

The chance that photons are not massless is extremely small. At least, real photons. Virtual photons can and do have mass, sometimes quite high, but those are only a mental construct to help us make sense of a universe that doesn't actually seem to have particles at all.

That lower bound on photon mass is _absurdly_ small. I'm guessing it comes from something like the limit at which the prism effect would be seen, or possibly the timing of the arrival of light and gravitational waves of that supernova a few years back, which also puts any potential difference between the speed of light and the speed of causality as exceedingly small as well.

[PLARice]

If photons were found to have some miniscule mass, would that mean they do get to experience time after all? Also what happens if it is the information they carry that provides the mass?

[itsawonderfullife]

Also if photons were massive then the QED interactive Lagrangian wouldn't be symmetric (e.g. under Lorentz transformations or gauge transformations) and conservation of electric charge would be violated. A photon field as a gauge field (with 2 DoFs only instead of 3) is necessary to conserve charge.

[jonathanreynolds]

Thank you, thank you, thank you so much for including your work with the math equations. You did it so seamlessly and it was beautiful.

[JJ-FOXTROT]

Finally something that is worth watching on youtube, that a 6 yr old or a 60 year old can learn something worthwhile.

I'd think that gravitational wave experiments would be a great candidate for figuring this out. Since GWs don't have mass, we can observe ripples and correlate their time of arrival with a flash. Biggest challenge is probably to figure out whether the GW and photons were indeed emitted in the same instant (or close enough to be a significant indication). Would also have to be corrected for things like gravitational lensing, but AFAIK this is something we're already quite good at.

[IIJOSEPHXII]

Great video again. I studied Ecology and in behavioural ecology and specifically "signalling theory, " information is seen as the change that takes place in the receivers of signals. Signalling theory also states that for a signal to create change in the recipient it must be new. For example if I were to say to you E = mc² it wouldn't be information to you.

[OMGanger]

I was waiting for you to quote a physicist as I’m quite certain from grad school that integer spin bosons (such as photons) are always massless (and this is necessary for all of quantum theory to work).

[det]

Thank you Arvin for diving into this, i’ve been wondering about this for years and I have never found any experiment to prove it’s massless. Yet every textbook says its massless like a fact. I always thought this was strange. Thanks for this video showing my observation is not insane.
Reason why I wondered about this, is if the photon had a slight mass, it would be just below the speed of information, which means time does not stop for it. But the inverse I always thought was strange with a massless particle. Its perspective would be instantaneous travel since as you approach the speed of information time slows down. At the speed feels like it should be undefined, yet slightly below seems more realistic.

[davidklang]

This is a profoundly arcane topic of which even an excellent video can barely scratch the surface. It has implications for nearly everything in modern physics and cosmology.

[jthepickle]

I say photons don't move. They stand up and sit down - like a 'stadium wave' and only appear to move. They also have no mass while 'sitting down'.