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

this video is brought to you by brilliant click the link in the description to sign up for free and support this channel as you're watching this video right now on your screen billions upon billions of tiny Quantum objects called photons are entering your eyes they're the universe's smallest fundamental carriers of measurable energy they carry not only energy but also information it's what makes up life it's what allows you to see and gather information about your surroundings but photons are much more important than even that they are the carriers of the electromagnetic force which keeps atoms

together and is the foundation for electricity and modern Electronics if photons did not exist Not only would the universe be dark but Life as we know it would not exist given the importance of photons as you might imagine scientists have studied them tirelessly for decades so we seem to know a lot about them yet if I ask the fundamental question do photons have mass what would be your answer this might seem like a really silly question and in every textbook you've probably ever read the answer is clearly no but is that really the case is

it proven that light does not have any Mass has anyone ever actually confirmed this in a measurement hmm not really so what's the answer it's a bit more nuanced and I'm going to explain that coming up right now [Music] we've measured the speed of light incredibly accurately it's 299 million 792 458 meters per second 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 this assumes that the photon is massless but it conceptually doesn't have to be we presume that the photon

is massless and if it's massless it must travel at the maximum speed allowed in the universe and this speed is finite because we measure it to be so why is it the case that a massless object must travel at the maximum speed this can best be seen by looking at this energy momentum relation where total energy is equal to the rest energy plus the momentum energy if there's no mass that is if m is zero then since the object must have some energy to exist the velocity can only be C the maximum velocity now if

we combine the equations for velocity from special relativity and insert it into this energy momentum equation we can get an equation that shows velocity in terms of mass and momentum you'll notice that the velocity of a photon has to be C the maximum velocity if its mass m is zero but if a photon has a slight Mass its velocity is not the maximum it would be less than C now it could be that the velocity of the photon is in fact Nazi the maximum velocity but something less than that but since we've not measured anything

faster we presume that the speed of light is the maximum velocity but is that really the case well we don't really know 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 I made a detailed video on how causality could be violated if you go faster than the speed of information flow you can check it out here if you want to know how that works in the case where the speed of light is less than

C then C would represent the speed of information 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 limit of information flow the speed of causality and not the speed of light if photons which make up light are truly massless then the speed limit of information is equal to the speed of light but if photons have a slight mass then that would mean that information could travel faster than light now you might think this would be a problem but it really

wouldn't be since photons are the fastest way we know to send information it would just mean that we don't have a means to communicate at the maximum speed but only at the speed of light which would be slower than that this would not invalidate relativity Theory the speed of light being the definition of c in the theory is because we assume photons are massless with a massive Photon we would just need to decouple the speed of light from the speed of causality it's just for practical purposes that we State relativity in terms of the speed

of light but we could 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 why is this the case well to this day we've not been able to devise an experiment to truly test this experiments on Earth tell us the mass of photons if it has mass can't be larger than 10 to the negative 18 electron volts because we would have been able to detect that mass in other words if it has mass that mass is smaller then our experiments can detect

I have a link in the description if you want to know where this number comes from our theories tell us that it should be massless but there's nothing else we can measure which could be faster imagine if there were some particle which moved at the speed of information if we measure the velocity of this particle then we could compare this to the speed of light and check whether it's faster or not if it's lower it must have mass because special relativity tells us that truly massless particles move at the speed of information or the speed

of causality there's only one other known particle that is thought to be massless the gluon this is the elementary particle which carries the strong force it's responsible for keeping the quarks which make up neutrons and protons in the nuclear of atoms glued together without it the nuclei of atoms would fall apart there would be no atoms no sun no Earth and no Life as we know it but the problem is that due to the laws of the strong force these gluons are not free and are always bound with quarks and possibly with themselves in glue

balls within the nuclei of atoms so we can't really measure the speed of these gluons as they cannot propagate freely like photons can because we have no experiment which can conclusively test that the photon is really massless the real answer to the question do photons have mass is actually we don't know we only know that if photons do have mass that mass must be incredibly low but this would mean that the speed of light is not the speed of causality you might say but I thought the whole universe would be completely different if photons had

a mass this is true if the photon had a significant Mass while we don't know if it's truly massless we know that it is practically massless so the universe would not be too different the lightest particles we know of are neutrinos and at one time we actually thought they were massless too because we don't have a good theory to explain why this should have a mass or how they gain that mass according to the standard model they don't interact with the Higgs field so they should not have a rest Mass but experiments seem to show

that they are slightly massive but neutrinos are so light that we don't know their Mass precisely we just know that they must not have a mass higher than about 0.8 electron volts otherwise we would have detected that Mass to give you an idea of the relative mass of photons compared to neutrinos which seem to be almost massless photons would still be around one quintillion times lighter than neutrinos remember that until recently we thought neutrinos were massless because they're practically moving as fast as photons so you'd be hard-pressed to experimentally detect the difference between a truly

massless particle and a photon with such a tiny Mass but what would be notice if photons did have a mass that we could detect in other words what would happen if they had a significant Mass well there would be some differences that could be observed one example would be that the electromagnetic force would become finite if you consider a magnet and sprinkle iron dust around it you'll see the irons being affected by the magnet by the electromagnetic force the strength of this Force decreases as an inverse of distance this is why you see a slow

decrease in the effect of the iron dust as you go further away from the magnet but if the photon was a lot more massive then you would see a different pattern depending on the exact details it might be that the range becomes so short that the magnet only affects the most nearby iron dust and has no effect on the dust further away 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 can be seen by rearranging

this equation showing the debris wavelength for massive particles now 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 this is because the energy corresponding to different wavelengths would

bend differently in a gravitational well because faster colors like blue would be more likely to escape the gravitational well and bend less whereas slower colors like red would bend more because they move slower so any light behind distant heavy Stars would resolve into different colors of the spectrum like a prism but if such effects were noticeable then at this point the photon would be so massive that there would be other weird effects as well 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 but these space-time descriptions are not what we observe we can detect the CMB and we don't see any prism or smudging effects of life so we can be pretty sure that any Photon Mass if it is not truly massless is very small so basically there's no big problem with continuing to use our current equations describing the universe even with a massive Photon because from our experiments and observations its mass is predicted to be so small that it's practically

massless but the argument can be made that we don't have any observations that support a truly massless Photon so the possibility that it really does in fact have some mass at this time cannot be excluded now I suspect a lot of you will probably want to explore this subject in much more depth but one of the hurdles many people encounter in getting super deep into this is understanding the complicated math the Hands-On math courses available the brilliant today's sponsor can really help you get a better understanding the great thing about the brilliant advanced math learning

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animations I really appreciate it and if you like our videos please subscribe so that you can be informed when we post new videos but tell me what you think is the photon massive or not let me know in the comments I'll see you in the next video my friend [Music] foreign [Music]