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As you are watching this video right now on your screen, billions upon billions of tiny
quantum objects called photons are entering your eyes.
They are the universe’s smallest fundamental carriers of measurable energy.
They carry not only energy but also information.
It’s what makes up light.
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 many decades, so we seem to know a lot about them.
Yet, if I ask the simple question, “Do photons have mass?”
What would be your answer?
This might seem like a 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 every actually confirmed this in a measurement?
Hmm, not really.
So what’s the answer?
It’s a bit nuanced, and I’m going to explain that coming up right now…
We have measured the speed of light incredibly accurately.
It’s 299,792,458 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 is no rest 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 equation for velocity from special relativity, and insert it into
the 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 not c, the maximum velocity,
but something less than that.
But since we have 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 could 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 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, 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 way 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 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.
Why is this the case?
Well, to this day we have not been able to devise an experiment to truly test this.
Experiments on earth tell us that the mass of photons, if it has mass, can’t be larger
than 10^-18 eV, because we would have been able to detect that mass.
In other words, if it has mass, that mass is smaller than our experiments can detect.
I have a link in the description, if you want to know where this number comes from.
Our theories tells us that it should be massless, but there is nothing else we can measure which
could be faster.
Imagine if there were some other particle which moved at the speed of information.
If we measured the velocity of this particle, then we could compare this to the speed of
light and check whether it is faster or not?
If it’s slower, it must have mass because special relativity tells us that truly massless
particles move at the speed of causality.
There is only one other known particle that is thought to be massless, the gluon.
This is the elementary particle which carries the strong force.
It is responsible for keeping the quarks which make up neutrons and protons in the nuclei
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 if 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 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 they should have
mass, or how they gain that mass.
According the standard model, they do not 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 masses precisely.
We just know they must not have a mass higher than 0.8 eV, 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 are practically
moving as fast as photons.
So, you would be hard-pressed to experimentally detect the difference between a truly massless
particle and a photon with such a tiny mass.
But what would we notice if photons had a mass 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 will see the irons being
affected by the magnet via 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 affect 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 deBroglie 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 could 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 speculative descriptions are not what we observe.
We CAN detect the CMB, and we don’t see any prism or smudging effects of light.
So we can be quite sure that any photon mass, if it is not truly massless, is very small.
So, basically, there is no big problem with continuing the 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 have some mass, at this time,
cannot be excluded.
I suspect a lot of you will probably want to explore this subject in more depth.
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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.