# Is gravity an infinite source of energy?

Borek
Mentor
Simplest explanation - in GR photon is massless, best measurements confirm its mass is below 10-18 eV.

If we measure and can confirm its mass is under 10^-18 eV, then where did the notion that it's massless come from if we can only confirm that it is under a specific mass? Er more specifically, what in GR measured/estimated the photon to be massless?

russ_watters
Mentor
"massless" is not something you can really measure since no measurement is exact so the best a measurement can get is close to zero.

You're really reaching here. If you really read the wiki, you'd see that it states in crystal clear terms that the photon is massless.

It also discusses the historical development - where the idea that it is massless came from. (it is a consequense of Maxwell's theory)

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russ_watters
Mentor
Ocean waves do involve some mass transport IIRC (e.g. Stokes drift).

CS
I'm not clear on if that is a function of the wave motion itself or if it is a biproduct of wind friction on the water surface or even a combination of both. Point taken, though.

The wiki says the photon is currently believed to be massless.

Then it also says

If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c.

Isn't the entire point of the speed of light, being the speed of a photon. Wasn't the speed of light derived from the potential notion that the photon was massless, if the photon was massless then the speed of light would be stop speed. Meaning if it was not truly massless, then another particle could exist that travels faster than the speed of the photon... also I have read the speed of light wiki page and am aware of the precision we can measure to, though measurements are simply certain units of size that we have quantified in order to measure with. Meaning there can very easily be units smaller than what we can currently measure.

This is also because the speed of light can not be the perfect speed. Saying math is absolutely true is unscientific; Math just corresponds very close to exact objects it nature, which is why it is so reliable on the scale which we use it. How can precision ever be ensured when you start getting to such smaller measurements. Because in fact, if the photons mass is so small, and there does exist a lighter particle traveling faster than the photon, then how would we know because it could very easily be to fast for us to measure it. Just as, several decades ago, quarks and other particles existence were to small to be measured.

How do we know this is not the case?

Basically I think what I'm trying to ask is, if we derived the speed of light from measuring the photon, how do we not know if there is particles out there, hypothetically 1000x smaller than a photon, meaning they would travel much faster than the measured speed of the photon, we just can't measure them and currently have trouble detecting them.

How else would we come up with the speed of light besides measuring a particle which we thought was massless, if the photon truly had some excruciatingly small mass that we just say to be massless, what is stopping another particle from existing that is smaller which we can't measure yet?

Borek
Mentor
Iwhere did the notion that it's massless come from if we can only confirm that it is under a specific mass?

Theory.

When you are checking things experimentally you can only do the experiment with a finite accuracy. That means you can never say "exactly zero" but "zero within the accuracy of the experiment". Best experimental accuracy so far proved mass is below 10-18 eV.

Similarly - when talking about mass, we in fact talk about two different properties of matter, inertial mass and gravitational mass. Seems like they are identical - but experimentally we were so far able to prove it "only" with accuracy around 10-12 (that was result of Braginski & Panow from 1972, could be there are better results available now).

Theory.

When you are checking things experimentally you can only do the experiment with a finite accuracy. That means you can never say "exactly zero" but "zero within the accuracy of the experiment". Best experimental accuracy so far proved mass is below 10-18 eV.

Similarly - when talking about mass, we in fact talk about two different properties of matter, inertial mass and gravitational mass. Seems like they are identical - but experimentally we were so far able to prove it "only" with accuracy around 10-12 (that was result of Braginski & Panow from 1972, could be there are better results available now).

Yes I understand that, but I don't understand what's preventing smaller particles from existing. If the speed of light is the measured speed of a photon, to as much accuracy as we can measure, would lighter particles not travel faster? If not, why would they not travel faster. Btw please don't interpret this as me trying to "debunk" science, because that's what people have said that's what it appears I'm doing, but I am not, I am trying to get a clear picture, when thoughts/questions pop in my head on various subjects.

Borek
Mentor
Yes I understand that, but I don't understand what's preventing smaller particles from existing.

You want them to have mass lower than zero?

Massless photon is part of a theory that is already over 100 years old, during this time it was successfully tested over and over. It doesn't mean theory is 100% sure - single, reproducible experiment that will give results differing from predicted is enough to show the theory is limited. But there was no such experiment, quite the opposite - countless experiments proved theory is valid. So it is not only best we have, we are also trying it for over 100 years - and so far we fail to invalidate it.

Note that experiments that try to measure photon mass are in fact trying to invalidate the theory - IF the results of some experiment will be 7+/-2 it will be obvious we were wrong But so far it seems like photon is massless with a very high accuracy.

There's key words here though. Very high accuracy, "seems like". Of course with such its fragile size that we can not measure it perfectly, as the probability of that is very low. Though I am confused in the area of, if we are so sure that it is massless, then why is there that area of uncertainty?

I realize that the point of experiments is to try to disprove theories, meaning if these attempts to disprove keep failing then the area of certainty increases. When you simply look at the mass range we have confirmed that it IS under a certain mark, but how much lower can it go? Is there a scientific point that separates 0 mass from "> 0" mass? Numbers are an invention to quantify things in reality which has lead us to development of higher mathematics

Massless, means without mass, meaning 0. I very well see the point you are making but the speed of light is based off the photon which we have tested over and over again. How are we supposed to confirm 0 mass. We can't. All we can do is keep experimenting and testing with smaller and smaller numbers, into higher and higher decimal places. There's no doubt that the mass of a photon is under a certain mark, but I fail to see where such a level of certainty comes from that it is completely massless. Surely it would be unscientific to say that it is entirely massless? Of course we can agree that it is under a certain mark, which we can confirm and test, but saying it is completely massless, is a very big claim considering how small numbers can get....infinitely small... which almost represents a logical paradox in itself.

Think of it this way. As technology advances, we are able to measure to higher degrees of uncertainty as the numbers get smaller and smaller. Recently in the past 38 years the level of certainty about Light Speed had increased by 100x. Meaning, what is to prevent technology from continually advancing. We can keep estimating it smaller and smaller, but how small can we go? Do we just assume that it is massless because the range which we can measure it to be under is so small?

How can something that is massless be measured? The only thing we know with a very very high accuracy is that it is under a specific very small range, we can confirm this with test.

Countless experiments only prove that the theory is extremely accurate. For it to be proven 100% valid(that the photon is massless), we would have to be able to measure to infinitely small numbers, can we agree on this? That there IS a level of uncertainty. Meaning that there IS a chance that the speed of light is not top speed, but only the speed of particles with the mass of a photon. Meaning if there is a chance that the photon is not massless, then there could be particles which have less mass, meaning they travel faster. Can we agree on this part? You just said yourself, it doesn't mean the theory is 100% true and accurate, but it is extremely accurate. You must keep in mind how small values can actually go and how small we can currently quantify.

With this all being said, 'is' there a possibility that there could be a particle lighter than a photon, if the photon was not massless. We can't confirm masslessness, we can only confirm mass under a very small range.

Or if what I have said is all wrong, how have we jumped from "range that its beneath" to masslessness... and this can not be based off comparing measurements to light speed as the max speed, because the notion of light speed being the universal speed limit is founded on the notion that the photon is massless, which would in turn be circular reasoning. Also before you misunderstand me, I am agreeing with you on the fact that we can't measure to 100% precision, but we CAN measure it to a very small range... I'm just confused as to why we assume its massless(besides aforementioned speed of light reason)

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Photon has energy. E = mc^2. Therefore photon has mass.

Photon has energy. E = mc^2. Therefore photon has mass.

See this is where I get confused to a degree. That formula is based on a constant which is derived from a measured speed of a photon.

If c is just the speed of the photon, and the photon has a mass.. If there exist a particle lighter than a photon, then why would we still use C as what it currently is? Or am I misunderstanding something?

Edit: Would this insinuate that we do have a very accurate measure for energy, but the discovery of a lighter particle would also lead to discoveries of a higher speed, which would inturn lead to more accurate predictions of energy content(to higher decimal places).... Since the formula is the energy more or less as mass approaches the limit of 0?

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russ_watters
Mentor
There's key words here though. Very high accuracy, "seems like". Of course with such its fragile size that we can not measure it perfectly, as the probability of that is very low. Though I am confused in the area of, if we are so sure that it is massless, then why is there that area of uncertainty?
That's just how science works: no measurement can ever be 100% accurate and no theory can ever be 100% proven. So the best we can do is 99.9999999999999% certain.

That's just how science works: no measurement can ever be 100% accurate and no theory can ever be 100% proven. So the best we can do is 99.9999999999999% certain.

I realize exactly that, which is why we can only measure to a certain level of mass accuracy.

Does that not defy the Mass–energy equivalence formula to say that the photon is massless, when the formula says otherwise? We know the photon has energy.

Isn't it unscientific to say the speed of light is the top speed? Wouldn't it be more accurate to say it's the highest speed we can measure, but there very easily could be higher speeds if lighter particles existed that we don't have the current technology to measure? I'm sure everyone can agree it will always be unscientific to say we've reached the peak of universal discoveries.

I'm just confused as to why we assume its massless(besides aforementioned speed of light reason)

Reading a physics book and understanding the theory first before jumping in to doubts/conclusions can solve all your confusions.

What is discussed till now is 100 years old theory and if you comprehend it, you will be in a better position to ask right questions. All the best.

russ_watters
Mentor
Does that not defy the Mass–energy equivalence formula to say that the photon is massless, when the formula says otherwise? We know the photon has energy.
That formula is not saying the photon has mass. That's a common misuse of the formula.
Isn't it unscientific to say the speed of light is the top speed?
Of course not! A theory predicts it, all evidence we have to date validates the theory, and no other theory exists that can explain the evidence. That's perfectly scientific.
Wouldn't it be more accurate to say it's the highest speed we can measure, but there very easily could be higher speeds if lighter particles existed that we don't have the current technology to measure?
Certainly not. There is no theory that predicts a non-zero mass for the photon that doesn't contradict the evidence we already have.
I'm sure everyone can agree it will always be unscientific to say we've reached the peak of universal discoveries.
Of course. That's not what's being claimed. What you're claiming is no less profound: That a 100 year-old, exquisitely well proven theory is wrong. That would be earthshattering.

Russ I understand what you're saying, but why is there that area of uncertainty.

Yes there are countless test reverifying that the mass of a photon is very small. Just because no theory exist predicting otherwise does not mean that 'otherwise' isn't true. If a smaller mass, under the already known range, were predicted, that wouldn't contradict the current test done on the speed of light.

I'm baffled at the level of certainty that exist for something so small. I mean, I can completely understand the it being measured under <1×10−18 eV, because we can confirm this.

But we can not confirm masslessness, we can only predict really close to it, which is why it baffles me that we can just say, there is no mass. We can't verify no mass, it's logically impossible, we can only verify extremely small masses.

Edit: you also say

Of course. That's not what's being claimed. What you're claiming is no less profound: That a 100 year-old, exquisitely well proven theory is wrong. That would be earthshattering.

I'm not saying the proven theory is wrong. In fact, I wouldn't use the word proven. All the test that have been done only reassure us that it lies under a certain range. It does not prove massless, it only proves that it is under a certain range.

Or unless you know of a way to measure, the lack of mass. Which would have to go on the assumption that we can measure things infinitely small, which we can't, there's limits to our technology, this fact can not be argued, the method can though. So if you have some method I am unaware of for measuring masslessness or infinitely small numbers, please do post it.

russ_watters
Mentor
Russ I understand what you're saying, but why is there that area of uncertainty.
We're starting to go in circles here: It is a combination of logic and measurement imperfection. The uncertainty in the measurement exists because it is impossible to have 100% measurement accuracy. At the very least, measurement devices are man-made and therefore have flaws that can never be completely eliminated.

And logically, the uncertainty in the measurement plus the fact that you can't ever measure the speed of every photon means that you can't ever prove that every photon moves at C.

Do not make the mistake of believing that this little opening is a place where scientists should be looking to build a theory of a massive photon. It's simply a demonstration of the level of scientific certainty and its limits.
Yes there are countless test reverifying that the mass of a photon is very small. Just because no theory exist predicting otherwise does not mean that 'otherwise' isn't true.
Of course: Being 99.999999999% sure of one thing means there is a 0.0000000000001% chance that there is something being missed.
But we can not confirm masslessness, we can only predict really close to it, which is why it baffles me that we can just say, there is no mass. We can't verify no mass, it's logically impossible, we can only verify extremely small masses.
That's not quite right. First, the prediction is masslessness. I think you meant measurement: A measurement would say something like 0.01 +-.02, which verifies that the mass is somewhere between -.01 and +.03. .03 is an upper bound in the range. So the experiment verifies no mass (because 0 is inside that range) to within a certain accuracy.

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We both agree there is that level of uncertainty. Can we agree that the possibility for a very small mass of a photon does exist, we just can't measure it? The theory of a "massive photon" would not contradict any theories, other than the notion of it being massless(which can't be 100% accurate obviously).

I'm not trying to establish any scientific theories here, just trying to establish that the possibility 'does' exist. It's talked about as if it's nonexistent, when it very well may be measurable in the future.

(also if you want to take the time, could you clarify why "That's a common misuse of the formula.", so I can understand for future references)

Isn't mass just energy in a different form? And a photon is just energy? Which would make it impossible for a photon to have mass(per definition that is)?

Isn't mass just energy in a different form? And a photon is just energy? Which would make it impossible for a photon to have mass(per definition that is)?

That would clarify a lot if true, but if that's true, then what is the entire point of attempting the measure the mass of a photon?

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I think this shouldn't really be discussed here, but well. No theory can be 100% proven. The entire point of measuring the mass of a photon is in my opinnion that it would disprove the theory "directly".
And I think this is the mistake:

"The theory of a "massive photon" would not contradict any theories, other than the notion of it being massless"

The theory is not just one notion. Measuring the mass of a photon would disprove the entire theory of relativity, and probably some other things and concepts too.

I think this shouldn't really be discussed here, but well. No theory can be 100% proven. The entire point of measuring the mass of a photon is in my opinnion that it would disprove the theory "directly".
And I think this is the mistake:

"The theory of a "massive photon" would not contradict any theories, other than the notion of it being massless"

The theory is not just one notion. Measuring the mass of a photon would disprove the entire theory of relativity, and probably some other things and concepts too.

It wouldn't disprove the theory of relativity because we have already tested it and can verify a large portion of it. It may alter the theory, but that's it.

If a photon mass was discovered then it could lead to the discovery of other particles, which would make it so there would be better numbers to use than c in the mass equivalence formula.... but the overall measure of the amount of energy would only change slightly. It would expand the formula or open up another formula for use.

The discovery of a potential massive photon would not destroy all of the current theory of relativity, I don't understand how you can say that when a majority of it has already been proven. All it would do is redefine it and make it more accurate.

Edit: If you haven't realized, Massless is not an undefinable number, massless means 0. Think of it this way, if a mass was discovered for a photon(smaller than we can currently measure obviously), then it would just move the scale up slightly. I don't see how you can say it would destroy the entire theory.

I am not an expert on R.T., but am pretty sure it would destroy it, someone might back me on it. But again, it really shouldn't be discussed under this topic.
Also you should understand that no theory can be 100% proven, that's how physics works.

I am not an expert on R.T., but am pretty sure it would destroy it, someone might back me on it. But again, it really shouldn't be discussed under this topic.
Also you should understand that no theory can be 100% proven, that's how physics works.

I understand no theory can be 100%.

If it would destroy it, I am curious as to why, because from my PoV that doesn't seem like it would happen, but I'm probably wrong..

Of course it's possible that photons have mass. It's also possible that I'll quantum-tunnel my way into your room after I make this post to tell you that in person.

I am not aware of any coherent theories that predict a massive photon. When you come up with one, let us know. Otherwise it makes no more sense to talk about a massive photon than it does to talk about a 5-headed unicorn that lives underneath the Gobi desert.