# Thread: The mass of a photon of light.

1. ## The mass of a photon of light.

How does one prove that a photon of light has 0 mass?

2. Originally Posted by tommac
How does one prove that a photon of light has 0 mass?
What are the allowed assumptions in the proof? Special relativity?

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It has 0 rest mass because it doesn't rest.

While moving, which is the only thing it does, it does have momentum based on the wavelength of light.

I'm sure someone else can go into more detail if needed.

4. Originally Posted by alainprice
It has 0 rest mass because it doesn't rest.
It's hard to hog-tie the restless feet of a photon.

5. Originally Posted by hhEb09'1
What are the allowed assumptions in the proof? Special relativity?
whatever ... I understand that this is taken as fact but I just want to know what the evidence is.

6. OK so it has 0 rest mass but it does have momentum.

Now follow up question ( should I create a new thread or just ask here ). Kind of ATM ... OK will question it in ATM

Originally Posted by alainprice
It has 0 rest mass because it doesn't rest.

While moving, which is the only thing it does, it does have momentum based on the wavelength of light.

I'm sure someone else can go into more detail if needed.

7. If light always has momentum, then what happens when it enters a black hole?

8. Originally Posted by RalofTyr
If light always has momentum, then what happens when it enters a black hole?

relative to? I think the light still has momentum but just doesnt escape the receeding "fabric of space-time". From what I understand this is different slightly than when a galaxy receeds past the line of the visible universe.

When entering a black hole light becomes blue shifted and actually gains momentum. But then past the event horizon it disappears.

Now I guess a good question here. As light recesses on the edge of the visible universe. its wavelegnth goes to infinity and its energy goes to 0 ( period goes to 0 ?? ) whatever it does but something goes to 0.

For blue shifting stuff. What happens to the light just as it passes the event horizon? Is there a max blueshift value? Does light just as it approaches a black hole do anything strange?

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That's easy. The lorentz transformation factor stipulates that as something approaches the speed of light, it's mass approaches infinity.

If just one photon had any mass at all, at c it would weigh more than the entire universe.

Photons do have momentum, however, and it's directly proportional to its frequency (alternatively it's inversely proportional to it's wavelength).

10. Originally Posted by mugaliens
If just one photon had any mass at all, at c it would weigh more than the entire universe.
What if the universe was infinite and contained an infinite number of these mass-photons?
This is being asked with regards to the math of your statement. Please ignore any consequences in the physical impossibility.

11. Originally Posted by tommac
What if the universe was infinite and contained an infinite number of these mass-photons?
In an infinite universe, pretty much everything will appear in infinite numbers. What is then important is density, which can be finite, even in an infinite universe.

12. Originally Posted by tommac
How does one prove that a photon of light has 0 mass?
As hhEb09'1 alluded to, one does not prove that. One constructs a model for a photon, and finds that the model works great, within the axioms of relativity, which also work great. Are you asking "what is the observational upper limit established for the photon mass in any model that would work as well as the zero-rest-mass model"? That's a way you could phrase it in a scientifically meaningful way-- unfortunately I have no idea what the answer is other than "very small indeed".

13. Originally Posted by Amber Robot
In an infinite universe, pretty much everything will appear in infinite numbers. What is then important is density, which can be finite, even in an infinite universe.
exactly ... so a photon of light even if it was of infinite mass would not be greater or even equal to the mass of the universe.

my infinity is stronger than your infinity.

14. One of these days I will learn to be eloquent.

Thanks for the clarification.

Originally Posted by Ken G
As hhEb09'1 alluded to, one does not prove that. One constructs a model for a photon, and finds that the model works great, within the axioms of relativity, which also work great. Are you asking "what is the observational upper limit established for the photon mass in any model that would work as well as the zero-rest-mass model"? That's a way you could phrase it in a scientifically meaningful way-- unfortunately I have no idea what the answer is other than "very small indeed".

15. ## Photon Rest Mass

The rest mass of a photon must be zero, if special relativity is correct. Since special relativity is well established theory, it is normally assumed that the rest mass of a photon is in fact zero. Laboratory experiments cannot prove that the rest mass is in fact zero, but they can establish upper limits on the rest mass. Luo, et al., 2003 established an upper value of the rest mass for a photon at 1.2×10-51 grams in a torsion balance experiment. That's pretty close to zero. Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 established an upper limit of 4.2×10-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.

16. what does rest mass of a photon mean?

Originally Posted by Tim Thompson
The rest mass of a photon must be zero, if special relativity is correct. Since special relativity is well established theory, it is normally assumed that the rest mass of a photon is in fact zero. Laboratory experiments cannot prove that the rest mass is in fact zero, but they can establish upper limits on the rest mass. Luo, et al., 2003 established an upper value of the rest mass for a photon at 1.2×10-51 grams in a torsion balance experiment. That's pretty close to zero. Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 established an upper limit of 4.2×10-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.

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Rest mass is what you measure on a scale, or better yet, a balance.

edit: It's like saying it is the mass you would feel if you held it in your hand. Unfortunately, you can't hold a photon; you can only be hit by one.

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Originally Posted by Tim Thompson
Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 established an upper limit of 4.2×10-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.
We could always put a nuke on the next deep-space probe, install a 50-year timer, then observe the resulting flash, compare it to known models, and check to see if any frequencies arrived sooner than others...

Nah. The greenies would never go for it. Er, unless we didn't tell them...

From what I understand about upper limits, it's simply due to the statistics of the errors inherent in the measuring equipment, ergo, "It measures zero, but with a 95% confidence interval, the measurement could be +/- 4.2x10-44 grams off." Naturally, since it can't have negative mass, the limit is simply 4.2x10-44 grams.

19. i Just had a strange thought ... I think I need to head to ATM ... but to give you a preview ...

What if light moved at infinite speeds .... but the speed we detect as a constant is based on the local warping of space time.

20. One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass.

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Originally Posted by tommac
i Just had a strange thought ... I think I need to head to ATM ... but to give you a preview ...

What if light moved at infinite speeds .... but the speed we detect as a constant is based on the local warping of space time.
Er... <brakes screeching, wildly>

Observational experiments with differing forms of electromagnetic radiation (of which visible light is a small subset) have universally concluded that no electromagnetic radiation (including light) moves at "infinate speeds."

So please hold you ATM post and save others some grief trying refute yet another ATM theory with no basis in fact (and a heap of evidence to the contrary), and take the time to research your ideas before wasting our time refuting them.

Thanks.

22. Originally Posted by mugaliens
Er... <brakes screeching, wildly>

Observational experiments with differing forms of electromagnetic radiation (of which visible light is a small subset) have universally concluded that no electromagnetic radiation (including light) moves at "infinate speeds."

So please hold you ATM post and save others some grief trying refute yet another ATM theory with no basis in fact (and a heap of evidence to the contrary), and take the time to research your ideas before wasting our time refuting them.

Thanks.
Yes I know that relative to us the speed is not infinite, but what I was going to post to ATM is that what we see is only the curvature of space-time as a photon of light moves through it. Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum. The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself. The state of light would desire to go to an infinite wavelength and 0 energy.

I will hold the question ... and not post to ATM until I have done more research on the matter.

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Explain how such a model can allow light to behave as a particle in one instance and then a wave in another and you will be making ground. Feel free to do so in ATM.

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Originally Posted by tommac
Yes I know that relative to us the speed is not infinite, but what I was going to post to ATM is that what we see is only the curvature of space-time as a photon of light moves through it. Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum. The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself. The state of light would desire to go to an infinite wavelength and 0 energy.

I will hold the question ... and not post to ATM until I have done more research on the matter.

25. Originally Posted by alainprice
Explain how such a model can allow light to behave as a particle in one instance and then a wave in another and you will be making ground. Feel free to do so in ATM.
The wave is the bending of space ... The particle ( still need to think if it is a really a particle or a quasi particle ) is its natural state.

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Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass. However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving. The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.

27. doesnt e=mc^2 mean that matter is made of energy?
I am not sure if energy needs to be in a state of matter though. But who knows ... I am mostly just ATM

Originally Posted by dcl
Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass. However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving. The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.

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It means that matter 'could' be made of energy. They are equivalent, but not necessarily the same.

Different arrangements of matter even have difference masses. For example, a helium nucleus is lighter than 2 protons and 2 neutrons.

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Originally Posted by tommac
Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum.
But space-time isn't distorted, except on a very local level. It's largely flat.

Only near matter (the mean density of which, throughout the observable universe, black holes and all, is a whopping 6 hydrogen atoms per cubic meter), is space-time curved to any appreciable degree.

The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself.
All Young proved is that light has wave-like properties such that depending upon the photon's wave-like state when it went through the slit, it might have been refracted this way, or that way, or a little bit that way, or not at all.

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Originally Posted by dcl
Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass.
Exactly! Which is why it's said that a photon's rest mass is zero. But photons are hardly at rest, and at speed (c), they do impart a momentum that's equavalent to their energy/mass equivalence.

However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving.
Actually, I believe it depends on it's wavelength! All photons have the same "amplitude" (I dilike using that term, as the wave theory only stretches so far). Photons with shorter wavelengths have more energy, thus more momentum, thus more energy/mass equivalence.

Still no actual mass, however, as no mass can move at c.

The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.
Yes, rest mass zero. Yet no mass can move at c. The equations governing this are clear that it's apparent mass (still exerts a gravitational attraction) is proportional to it's E=Mc^2 (energy-mass) equivalence. The shorter the wavelength, the more energetic the photon, and thus the more mass-equivalence it has, therefore the more gravitational attraction that photon exerts, which, amazingly enough, is all tied together via Planck's equations, as well as Einstein's and Lorentz's!

Historians will probably record that they were secretly collaborating with one another behind closed doors...

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