Do photons have mass experiment?

[RobertsMrtn]

I have heard that photons do not have any mass which is why they travel at the speed of light.
However consider the following thought experiment.
You are box in space accelerating at 1 G.
From your perspective, you are in a gravitational field of 1G.
If you shine a light from one wall of the box to the other (across the gravitational field), it will hit the wall at a slightly lower point that it would if the box were traveling at constant velocity.
This is because the wall will have moved compared to where it would have been had the box been traveling at constant velocity.
From the perspective of the observer in the box, the light beam has been bent by the gravitational field.
The same will apply with a gravitational field created by the existence of an object with mass.
We can therefore say that the photon is attracted by an object with mass.
Does it not follow that the photon must itself have mass?

 

[jtbell]

See the following FAQ in our relativity forum:

If photons don't have mass, why do their paths "bend" in a gravitational field?

 

[pgardn]

See the following FAQ in our relativity forum:

If photons don't have mass, why do their paths "bend" in a gravitational field?

So do e fields and b fields get bent as well?

Sorry if the question was already answered, I only saw your post.

 

[HallsofIvy]

"Fields" are not objects. They do not have paths to be bent.

 

[wasi-uz-zaman]

photons rest mass is zero , but what would be there mass at speed of light and different material light speed decreases does the mass of photon get decrease too?

 

[pgardn]

"Fields" are not objects. They do not have paths to be bent.

So if a charged object moves in a gravitational field...

By our definition of field, the fields produced by the moving charge (that does move through some curved path in space time) are not affected?

I know I should probably read something, but if the answer is painfully obvious like your first one, we enlighten a lowly classical poster quickly.

 

[ZapperZ]

photons rest mass is zero , but what would be there mass at speed of light and different material light speed decreases does the mass of photon get decrease too?

This makes no sense. Even if you want to content that there is something called "relativistic mass" for a photon, just look at the relativistic mass expression, which is

$$m = \gamma m_0$$

If $m_0 = 0$, what is m at ANY speed?

Zz.

 

[RobertsMrtn]

If the mass of a photon is zero, the proton drive idea for rocket propulsion would not work. (Newton's third law).

 

[jtbell]

Newton's Third Law is basically about conservation of momentum. Even though photons don't have mass, they have momentum. If a photon reflects off something and changes direction, then the reflecting object must also change its momentum correspondingly.

 

[DrewD]

If the mass of a photon is zero, the proton drive idea for rocket propulsion would not work. (Newton's third law).

Newton's Laws were written down 300 years ago. While Newton thought that light was made of particles, the modern conception of a photon did not exist until a little over 100 years ago. Until then, the explanation of light was consistent with waves. Waves can carry momentum without mass.

Be careful when trying to combine the logic of two theories that may not be compatible.

 

[RobertsMrtn]

momentum

Momentum is mass times velocity. No mass, no momentum.

 

[ZapperZ]

Momentum is mass times velocity. No mass, no momentum.

This is incorrect. Momentum can also be defined as hbar k. Look it up!

Zz.

 

[DrewD]

Momentum is mass times velocity. No mass, no momentum.

nope. I'm not interested in the conversation, so others will hopefully take over, but if you do not have a physics background (which this statement makes clear) you should ask questions instead of making statements like that. I will translate for you.

"In high school I was taught that momentum was mass times velocity. I don't understand how a photon can have momentum without having a rest mass. If it has no mass, shouldn't it have no momentum?"

 

[RobertsMrtn]

Reply to DrewD

I do have a physics background. I do not however claim to know everything. I made the post to stimulate conversation on existing theories which may or may not be correct. One person behaving as if he has all the answers and disrespecting everyone else does not help.

 

[No-where-man]

"Fields" are not objects. They do not have paths to be bent.

But what fields are than, is there any definition of fields?
I mean you have energy fields and you force fields what's the key difference?

 

[No-where-man]

photons rest mass is zero , but what would be there mass at speed of light and different material light speed decreases does the mass of photon get decrease too?

But do photons have some other mass? I forgot the term how you say it in physics.
I also read that Photons inside superconductors do develop a nonzero effective rest mass; as a result, electromagnetic forces become short-range inside superconductors.

 

[ZapperZ]

But do photons have some other mass? I forgot the term how you say it in physics.
I also read that Photons inside superconductors do develop a nonzero effective rest mass; as a result, electromagnetic forces become short-range inside superconductors.

This is now a completely different phenomena and such a discussion will confuse and derail this thread. If you wish to understand why photons have that effect in a superconductor, open another thread.

Zz.

 

[ModusPwnd]

But do photons have some other mass? I forgot the term how you say it in physics.

Photons (plural) can have an invariant mass if their momentum sums to zero in your frame of reference. This system's invariant mass would not be equal to the sum of the photon's individual rest masses (which is zero).

There are lots of ways to look at mass, a "simple" concept that can get quite confusing.

 

[jtbell]

Photons (plural) can have an invariant mass if their momentum sums to zero in your frame of reference.

Actually, the total momentum doesn't have to be zero. You can always calculate the invariant mass of a collection of particles using

$$mc^2 = \sqrt{E_{total}^2 - (|\vec p_{total}|c)^2}$$

For a given collection of particles (photons or otherwise) this gives the same result in any inertial reference frame. It's simply easier to calculate in the frame in which $\vec p_{total} = 0$.

 

[ModusPwnd]

Then why do we say that a single photon has zero mass? I thought it was because there is no frame of reference with a photon having zero momentum. A collection of photons who's momentum cannot be zero in any frame of reference would still have an invariant mass? But a single photon would who's momentum cannot be zero would not? ugg, no wonder I failed out of physics... This stuff confuses me still, even having studied it for a bit. It seems to me if your collection of photons cannot be boosted to some frame with zero momentum then you will have a zero value for mc^2.

Sorry to cause any confusion, Ill shut up now since I don't really get it. :tongue:

 

[jtbell]

Then why do we say that a single photon has zero mass?

Because for a single photon $E = |\vec p| c$ so

$$mc^2 = \sqrt{E^2 - (|\vec p|c)^2} = 0$$

I thought it was because there is no frame of reference with a photon having zero momentum.

Indeed, for a single photon, there is no reference frame in which $\vec p = 0$.

A collection of photons who's momentum cannot be zero in any frame of reference would still have an invariant mass?

It makes a difference whether the photons are moving in the same direction or in different directions.

If the photons are all traveling in the same direction (like in an ideal beam or plane wave), then their momentum vectors are all in the same direction, the magnitude of the total momentum is just the sum of the magnitudes of the individual momenta, and the equation I gave in my previous post gives $mc^2 = 0$ for the collection. In this case, there is no frame in which $\vec p_{total} = 0$.

If the photons are traveling in different directions (like in a "gas" of photons confined inside a mirrored box), then their momentum vectors are in different directions, their magnitudes don't simply "add", and $mc^2 \ne 0$ for the collection. If the box is stationary, then it's likely that $\vec p_{total}$ for the photons is zero (or very very very nearly so). If the box is moving, and carrying the photons along with it, then $\vec p_{total} \ne 0$.

 

[ModusPwnd]

Aww man, isn't that what I said in my post and you corrected me as though it was wrong? They can only have a invariant mass if their momentum sums to zero.

I said: "Photons (plural) can have an invariant mass if their momentum sums to zero in your frame of reference."
You corrected me: "Actually, the total momentum doesn't have to be zero."

But it does have to be zero, as you just now explained. Otherwise your invariant mass is zero. So I still think my original claim is correct, photons can only have a non-zero invariant mass if their momentum sums to zero.

 

[jtbell]

You posted while I was editing my post to expand on it. Consider a moving box with mirrored walls, containing a "gas" of photons. The total momentum of the "photon gas" is nonzero, and it has a nonzero invariant mass, the same invariant mass that you would calculate if you were moving along with the box so that it is at rest with respect to you.

 

[ModusPwnd]

Thanks for explaining, but I am still confused. I don't see how the box moving makes any difference on the photons contained within. When you say the box is at rest with respect to you, are you saying that the total momentum of the photon gas is zero? And because it was originally nonzero, you are saying when you boost from one frame to another the total momentum of the photons changes? I thought that relativistic doppler shift would undue that and keep the total momentum constant.

I can boost a two photon zero momentum system into a nonzero momentum system? And since my zero momentum two photon system has invariant mass, so does my non-zero momentum system. ??

So now I think my statement should be amended as follows;
Photons (plural) [STRIKE]can[/STRIKE] do have an invariant mass if their momentum sums to zero in [STRIKE]your[/STRIKE] any frame of reference.

Is this right?

A single photon system can never have zero momentum, thus it has no invariant mass. Similarly for a many photon system with parallel momentum. Otherwise, if your photon system's momentum can be zero in any frame, it has an invariant mass.

Thanks for your help and patience. I always wanted to understand this better.

 

[infinite.curve]

I remember asking myself the same question proposed in this topic.

Well, I satisfied my question by a google search.

There is different proportionality in different 'languages'.

I think this link will give you am intuitive understanding of a photon being massless or not.

math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

I apologize for my bad English.

 

[infinite.curve]

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

 

[WannabeNewton]

I apologize for my bad English.

Your English is fine; there's never a need to apologize for something like that anyways . Cheers!

 

[infinite.curve]

Your English is fine; there's never a need to apologize for something like that anyways . Cheers!

Thank you for your generosity. I just want others to be aware that I am prone to grammatical errors since English is not my first language.

 

[RobertsMrtn]

Rest mass

Correct me if I am wrong but I don't think that there is such a thing as a photon which is at rest. The instant a photon is created it reaches the speed of light. Incidentally, from the photons perspective, the instant it is created, it is destroyed (according to special relativity).
But if a photon does have a rest mass, according to the Lorentz transformation, its moving mass would have to be infinite which is of course impossible. So it seems to me that either photons do not have mass or they are somehow exempt from the Lorentz mass transformation.
Another question is - can they be said to have kinetic energy? The standard formula for kinetic energy being 1/2mv^2 which of course becomes 1/2mc^2

 

[ehild]

I have heard that photons do not have any mass which is why they travel at the speed of light.

Photons do not have "rest mass" as they can not be in rest. The speed of light is the same as the speed of the photons - the quanta of light.

An electromagnetic wave of frequency f , wavelength λ, and wave vector k can be imagined as a beam of photons, particles with energy E=hf, and momentum p pointing in the direction of k and magnitude p=h/λ.

From mass-energy equivalence E=mc², mass is attributed to an object having energy E : so the mass of the photon is hf/c². The relativistic momentum is p=mv. it is p=mc for the photon, and with its relativistic mass, p=hf/c=h/λ.
As the photons have mass they are subject to gravity. The light bends in the gravitational field of sun.

ehild

 

[ehild]

Another question is - can they be said to have kinetic energy? The standard formula for kinetic energy being 1/2mv^2 which of course becomes 1/2mc^2

In Special Relativity, the kinetic energy of a particle is defined as KE=mc²-m₀c² where m₀ is the rest mass. That is zero for a photon, so its KE equal to its energy, KE=mc²=hf.

ehild

 

[WannabeNewton]

Correct me if I am wrong but I don't think that there is such a thing as a photon which is at rest.

Indeed. In order to define an object "at rest" in special relativity, we must first construct an inertial frame and then define an object "at rest" simply as one whose velocity relative to this frame vanishes. As a consequence of one of the fundamental postulates of special relativity, there exists no inertial frame in which light has vanishing speed hence the notion of "at rest" in the above sense does not apply to light.