# Thread: Mass of Photons

1. Imagine a box that is lined with a material that very effectively reflects EM waves of a certain frequency. I then manage to somehow trap a very sizable amount of radiation with that frequency inside that box. Let's say the relativistic mass of all the photons inside the box is equal to 2kg. Would that box then be 2kg heavier than an empty box? What if the box was in deep space, far away from any massive objects, would it require more force to accelerate it to a certain speed than an empty box?

2.

3. Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc

4. Originally Posted by Bad Wolf
Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc
Photons don't have REST mass. Then do have energy, determined by the frequency, and that energy is equivalent to mass precisely according to , NOT "a lot less". It is also possible, when one has photons of sufficient energy, to have photon collisions that produce particles that do have rest mass, and that mass is also consistent with those equations.

5. Originally Posted by ScienceWizard
Would that box then be 2kg heavier than an empty box? What if the box was in deep space, far away from any massive objects, would it require more force to accelerate it to a certain speed than an empty box?
Yes, I believe so.

6. Originally Posted by DrRocket
Originally Posted by Bad Wolf
Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc
Photons don't have REST mass. Then do have energy, determined by the frequency, and that energy is equivalent to mass precisely according to , NOT "a lot less". It is also possible, when one has photons of sufficient energy, to have photon collisions that produce particles that do have rest mass, and that mass is also consistent with those equations.
OK but the photons do not have mass at all in normal circumstances do they? By photons becoming mass I meant exactly what you said, I just neglected to say as I though I would be deterring from the OP. I also thought something with a lot of energy will make a lot less mass? I learned that from experiments at CERN attempting to combine energy to make mass, although I am not aware of the specifics...

Finally, am I right in saying that in E=hv

E= energy
h = planck's constant
v = velocity

PS. How do you work the tex, is there a list of input anywhere you know of?

7. Photons are little packets of pure energy, not mass a bit like sound, sound has no mass only energy (although unlike photons it requires medium to travel through)

Funny little buggars, we keep trying to categorise them as things we understand but there is no equivalent in our natural experience, they exist as energy only.

8. The Stress-energy tensor is the source term in Einstein's field equations. So this means that your box of "light" would bend the local space-time (i.e. produce some gravity) more than an identical box without any "light" in it.

Here is a pretty decent tutorial on using TeX:

http://en.wikipedia.org/wiki/Wikipedia:Editing_Math

Only difference is that Wikipedia uses $E=mc^2$ while these forums use
Code:
Oh, and in E=hv the v stands for frequency, not velocity. The "v" is actually lower case Greek letter Nu which looks like a v. If I'm not mistaken Nu is used by physicists while engineers and mathematicians use f. Just like mathematicians and physicists use i for the imaginary unit and engineers use j. It would be nice if we all could just get along :-)

10. Thanks for that .

11. Originally Posted by Bad Wolf
Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc
What is the experimental proof that photons don't have mass? Since we can detect a photon it must consist of something; but I guess the view would be that that something is pure energy? Is it possible that energy is a form of matter with unmeasurable (infinitesimal) mass?

12. Originally Posted by jonathancollins225
Originally Posted by Bad Wolf
Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc
What is the experimental proof that photons don't have mass? Since we can detect a photon it must consist of something; but I guess the view would be that that something is pure energy? Is it possible that energy is a form of matter with unmeasurable (infinitesimal) mass?

Photons are wierd,
Look up the Double Slit Experiment to see why

13. what about the double slit experiment? Do you mean the absolutely asinine conclusions we've drawn from it regarding a weird sense of choice in a photon based on it's being observed? or do you mean the odd way photons distribute themselves across a wave pattern?

14. Originally Posted by Bad Wolf
Originally Posted by DrRocket
Originally Posted by Bad Wolf
Nope, because the photons don't have mass. Remember the mass of the photons if they could become mass is going to be A LOT less than the energy, as of course as ever dictated by E=mc
Photons don't have REST mass. Then do have energy, determined by the frequency, and that energy is equivalent to mass precisely according to , NOT "a lot less". It is also possible, when one has photons of sufficient energy, to have photon collisions that produce particles that do have rest mass, and that mass is also consistent with those equations.
OK but the photons do not have mass at all in normal circumstances do they? By photons becoming mass I meant exactly what you said, I just neglected to say as I though I would be deterring from the OP. I also thought something with a lot of energy will make a lot less mass? I learned that from experiments at CERN attempting to combine energy to make mass, although I am not aware of the specifics...

Finally, am I right in saying that in E=hv

E= energy
h = planck's constant
v = velocity

PS. How do you work the tex, is there a list of input anywhere you know of?
Mass and energy are equivalent, as per relativity. Photons always have energy. They don't have rest mass, but then again they are never at rest. Photons ALWAYS travel at the speed of light.

Yes

Under the right circumstances particles with enough energy, say two very energetic photons can interact to produce a particle that does have rest mass.

You can find a tutorial, started by Jane Bennet who is a whiz, on Tex in the mathematics forum. It is a sticky and it at the top of the list of threads.

15. Originally Posted by Arcane_Mathamatition
what about the double slit experiment? Do you mean the absolutely asinine conclusions we've drawn from it regarding a weird sense of choice in a photon based on it's being observed? or do you mean the odd way photons distribute themselves across a wave pattern?
Spedifically what "assinine conclusions" are you talking about? And who is drawing such conclusions ?

Herer is a pretty good explanation, with some interesting pictures of the interference patterns being formed by individual particles.

http://en.wikipedia.org/wiki/Double-slit_experiment

16. If you like cartoons for a double slit watch:
Double slit

17. Mass and energy are equivalent, as per relativity. Photons always have energy. They don't have rest mass, but then again they are never at rest. Photons ALWAYS travel at the speed of light.
I know that thank you, please don't insult my intelligence or ego

I'll check out Jane but she's not been on in a while...

18. Originally Posted by ScienceWizard
Imagine a box that is lined with a material that very effectively reflects EM waves of a certain frequency. I then manage to somehow trap a very sizable amount of radiation with that frequency inside that box. Let's say the relativistic mass of all the photons inside the box is equal to 2kg. Would that box then be 2kg heavier than an empty box? What if the box was in deep space, far away from any massive objects, would it require more force to accelerate it to a certain speed than an empty box?

Let's consider what happens in that box. First imagine that is floating in free space with no gravity.

The light will bounce off all the sides, and light exerts a pressure due to its momentum. Since it bounces off all faces equally, there is no net force acting on the box in any one direction.

Now rest it on a surface in a gravity field. Consider what happens to the light as it bounces up and down in the box. Light traveling upwards against gravity red shifts to a longer wavelength, while light traveling down blue shifts. Since the momentum of light is inversely proportional to its wavelength, light hitting the upper side of the box has less momentum and exerts less pressure then the light hitting the bottom of the box. So, yes, the box weighs more.

The same thing happens if you try to accelerate the box in a given direction. Light hitting the wall in the direction of the acceleration is red-shifted and light hitting the opposite side is blue shifted, so the the light pressure acting against the acceleration is greater than that acting with the acceleration and the box resists movement more than if it didn't contain the light.

19. Originally Posted by DrRocket
Originally Posted by Arcane_Mathamatition
what about the double slit experiment? Do you mean the absolutely asinine conclusions we've drawn from it regarding a weird sense of choice in a photon based on it's being observed? or do you mean the odd way photons distribute themselves across a wave pattern?
Spedifically what "assinine conclusions" are you talking about? And who is drawing such conclusions ?

Herer is a pretty good explanation, with some interesting pictures of the interference patterns being formed by individual particles.

http://en.wikipedia.org/wiki/Double-slit_experiment
He's talking about the heisenberg uncertainty principle.

And I meant the wave intersection patterns, not the principle.

But it isn't asinine, its not saying it chooses, its saying that by being there and watching we are effecting the outcome, as much as experimental human error

20. Originally Posted by okkaoboy
Originally Posted by DrRocket
Originally Posted by Arcane_Mathamatition
what about the double slit experiment? Do you mean the absolutely asinine conclusions we've drawn from it regarding a weird sense of choice in a photon based on it's being observed? or do you mean the odd way photons distribute themselves across a wave pattern?
Spedifically what "assinine conclusions" are you talking about? And who is drawing such conclusions ?

Herer is a pretty good explanation, with some interesting pictures of the interference patterns being formed by individual particles.

http://en.wikipedia.org/wiki/Double-slit_experiment
He's talking about the heisenberg uncertainty principle.

And I meant the wave intersection patterns, not the principle.

But it isn't asinine, its not saying it chooses, its saying that by being there and watching we are effecting the outcome, as much as experimental human error
I don't think so. There is nothing whatever assinine about the Heisenberg uncertainty principle. Nor is there anything assinine about the interference patterns.

21. Originally Posted by Bad Wolf
Mass and energy are equivalent, as per relativity. Photons always have energy. They don't have rest mass, but then again they are never at rest. Photons ALWAYS travel at the speed of light.
I know that thank you, please don't insult my intelligence or ego

I'll check out Jane but she's not been on in a while...
If you know that then you answered your own question. Your ego is your problem.

22. On balance I suppose the answer to the original question depends on what you mean by mass. Photons, as far as I am aware, do not interact with the gravitational force, and so this box of yours would not weight any more, despite having inside it the potential for 2kg extra.

23. Originally Posted by drowsy turtle
On balance I suppose the answer to the original question depends on what you mean by mass. Photons, as far as I am aware, do not interact with the gravitational force, and so this box of yours would not weight any more, despite having inside it the potential for 2kg extra.
But they do interact with it. They red-shift while climbing against it, blue shift when falling with it, and have their path changed when traveling perpendicular to it.

24. Originally Posted by drowsy turtle
On balance I suppose the answer to the original question depends on what you mean by mass. Photons, as far as I am aware, do not interact with the gravitational force, and so this box of yours would not weight any more, despite having inside it the potential for 2kg extra.
Photons DO interact with gravity. That is one of the more striking results of general relativity and the measurement of that interaction experimentally was one of the early triumphs of the theory.

Moreover the producton of large gravitational field by light itself has been studied. It was studied by John Archibald Wheeler and he called such a structure a "geon".

That box of light WOULD in fact weigh more with the photons inside it. It also true that if you take a box of air and heat it the heat energy causes the box to weigh more. However, if you look at the equation or equivalently what you find is that it takes a tremendous amount of energy, far greater than is normally available, to make a measureable difference in mass.

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