Photon: Mass or Mass-less ?

[TillICollapse]

Do you accept that a photon is massless ... if so, can you explain why ? Do you believe this is definitive or tentative and debatable ?

Similarly ... do you assert that a photon has mass ? If so, can you explain why ? Do you believe this is debatable or a given that a photon has mass ?

Does it matter to your argument if we're discussing relativistic mass, the resting mass, the total energy, or the invariant mass ?

I've taken for granted what I learned years ago ... I once understood that a photon was mass-less and that speaking of it's "resting mass" was somewhat of a misnomer. But I can no longer explain all the reasoning why to great length, I'm too rusty, so to refresh me and see what has or hasn't changed ... please feel free to respond with your thoughts and knowledge. Thanks

 

[Loudmouth]

I accept that no experiment has detected mass for photons. It is a bit absolutist to say that they are massless since their mass could be less than the lower limit of sensitivity of our instruments.

If photons did have mass, then they couldn't travel at c, at least that is my understanding of relativity.

 

[Michael]

Do you accept that a photon is massless ... if so, can you explain why ? Do you believe this is definitive or tentative and debatable ?

Similarly ... do you assert that a photon has mass ? If so, can you explain why ? Do you believe this is debatable or a given that a photon has mass ?

Does it matter to your argument if we're discussing relativistic mass, the resting mass, the total energy, or the invariant mass ?

I've taken for granted what I learned years ago ... I once understood that a photon was mass-less and that speaking of it's "resting mass" was somewhat of a misnomer. But I can no longer explain all the reasoning why to great length, I'm too rusty, so to refresh me and see what has or hasn't changed ... please feel free to respond with your thoughts and knowledge. Thanks

Oddly enough I tend to agree with LM in terms of lower limit of detection, speed limits and such. I'd also point out that it *does* matter (to me at least) what type of "mass' you're talking about. I'd buy the idea that photons have no 'rest mass' because they are never at rest. On the other hand, they do carry *momentum*, a trick that is typically reserved for things with *some type* of mass. They do contain different amounts of energy depending on the wavelength, and therefore *some* type of variable mass/energy equivalence.

 

[KEBO12345]

From a great site....Ask a mathmatician

physicist: Classically (according to Newton) kinetic energy is given by E=\frac{1}{2}mv^2 and the momentum is given by P=mv, where m is the mass and v is the velocity. But if you plug in the mass and velocity for light you get E=\frac{1}{2}0c^2=0. But that’s no good. If light didn’t carry energy, it wouldn’t be able to heat stuff up.

The difficulty comes from the fact that Newton’s laws paint an incomplete (and ultimately incorrect) picture. When relativity came along it was revealed that there’s a fundamental difference in the physics of the massive and the massless. Relativity makes the (experimentally backed) assumptions that: #1) it doesn’t matter whether, or how fast, you’re moving (all physical laws stay the same) and #2) the speed of light is invariant (always the same to everyone).

Any object with mass travels slower than light and so may as well be stationary (#1).

Anything with zero mass always travels at the speed of light. But since the speed-of-light is always the speed-of-light to everyone (#2) there’s no way for these objects to ever be stationary (unlike massive stuff). Vive la différence des lois! It’s not important here, but things (like light) that travel at the speed of light never experience the passage of time. Isn’t that awesome?

The point is: light and ordinary matter are very different, and the laws that govern them are just as different.


Light and Matter: different

That being said, in 1905 Einstein managed to write a law that works whenever: E^2=P^2c^2+m^2c^4. The same year (the same freaking year) he figured out that light is both a particle and a wave and that the energy of a photon isn’t governed by it’s mass or it’s velocity (like matter), but instead is governed entirely by f, it’s frequency: E=hf, where h is Planck’s constant.

For light m=0, so E=Pc (energy and momentum are proportional). Notice that you can never have zero momentum, since something with zero mass and zero energy isn’t something, it’s nothing. This is just another way of saying that light can never be stationary.

Also! Say you have an object with mass m, that isn’t moving (P=0). Then you get: E=mc2 (awesome)!

 

[TillICollapse]

Thanks for the responses thus far ... perhaps someone who would assert that a photon DOES definitely have mass will chime in as well.

 

[essentialsaltes]

Photons are massless particles, in the vernacular of particle physics.

What this means is that they have no rest mass.

Photons are seldom at rest.

As Einstein demonstrated, everything that has mass has energy (and vice versa). In fact, they are the same thing.

Photons have energy, therefore they have mass.

Nevertheless, we physicists speak of them as massless particles. So sue us.

 

[TillICollapse]

Photons are massless particles, in the vernacular of particle physics.

What this means is that they have no rest mass.

Photons are seldom at rest.

As Einstein demonstrated, everything that has mass has energy (and vice versa). In fact, they are the same thing.

Photons have energy, therefore they have mass.

Nevertheless, we physicists speak of them as massless particles. So sue us.

Are you a physicist in some capacity ? If so ... cool, time for questions

The statement: "Everything that has mass has energy," I thought was a misnomer in certain contexts. That is, that energy is not dependent on mass once we get out of classical Newtonian application, rather energy relies more upon momentum (and with photons, the energy is proportional to the frequency and wavelength, etc). If a photon were to have actual mass in the way we think of "mass" ... all photons would have the same mass. And we can already show that all photons do not have the same energy (because their energy is not dependent on mass, but the aforementioned frequency and wavelength, etc), which would mean they have different mass depending on energy, all the way down to a zero value with no momentum, which is impossible for a photon to have no momentum, thus reaffirming their "true" mass-less-ness even beyond a resting mass, showing their energy is not dependent on an increase or decrease in mass, rather it reaffirms a zero value resting mass. It's energy is proportional to it's momentum, which is why it's expressed as E=pc.

Yes ? No ? Did I bastardize something lol ? I'm trying to refresh here ... I've gotten a bit lazy, so feel free to educate if you're inclined

 

[essentialsaltes]

Are you a physicist in some capacity ? If so ... cool, time for questions

The statement: "Everything that has mass has energy," I thought was a misnomer in certain contexts. That is, that energy is not dependent on mass once we get out of classical Newtonian application, rather energy relies more upon momentum...

The relativistic formula for energy E = mc^2 shows that the relativistic mass (m) is directly related to the energy of the particle.

This equation can be unpacked a little bit, as seen at the end of this.

This relates the energy to the rest mass (m0) and the momentum (p).

[In a weird way, the mathematics is like you had a right triangle that had m0c^2 as one side and pc as the other side, then the relativistic energy is the length of the hypotenuse.]

For a photon, of course, m0 = 0, and we just get E=pc.

[Thinking more about light as a wave, the momentum of a photon is directly proportional to its frequency. Thus so is the energy.]

If a photon were to have actual mass in the way we think of "mass" ...

Well, it depends on what way 'we' think of 'mass'. If we think of it as rest mass, then photons do not have mass. If we think of it as relativistic mass, then like everything else, mass is the same as energy except for a factor of c^2.

all photons would have the same mass.

If you mean rest mass, then yes all photons do have the same rest mass (i.e. 0). If you changed the rest mass to something else, it would presumably be that same fixed value you chose.

And we can already show that all photons do not have the same energy ... which would mean they have different mass depending on energy

Things do have different relativistic masses depending on their energy. Photons, electrons, spaceships.

rather it reaffirms a zero value resting mass

.

Yes, so if we're only talking about rest mass, that's zero. The question is what we mean by mass. Rest mass or relativistic mass.

Now on the whole, when we talk about the mass of a particle, we physicists mean the rest mass. Because it is invariant. It doesn't change. And this is why we do in fact talk about photons as being massless.

But if someone wants to insist on using the relativistic mass, then photons have a relativistic mass, as given by Einstein's formula.

Whether that's the reeeaallll mass is not an easy question to answer. That's philosophy, not physics.

 

[TillICollapse]

The relativistic formula for energy E = mc^2 shows that the relativistic mass (m) is directly related to the energy of the particle.

This equation can be unpacked a little bit, as seen at the end of this.

This relates the energy to the rest mass (m0) and the momentum (p).

[In a weird way, the mathematics is like you had a right triangle that had m0c^2 as one side and pc as the other side, then the relativistic energy is the length of the hypotenuse.]

For a photon, of course, m0 = 0, and we just get E=pc.

[Thinking more about light as a wave, the momentum of a photon is directly proportional to its frequency. Thus so is the energy.]



Well, it depends on what way 'we' think of 'mass'. If we think of it as rest mass, then photons do not have mass. If we think of it as relativistic mass, then like everything else, mass is the same as energy except for a factor of c^2.



If you mean rest mass, then yes all photons do have the same rest mass (i.e. 0). If you changed the rest mass to something else, it would presumably be that same fixed value you chose.



Things do have different relativistic masses depending on their energy. Photons, electrons, spaceships.

.

Yes, so if we're only talking about rest mass, that's zero. The question is what we mean by mass. Rest mass or relativistic mass.

Now on the whole, when we talk about the mass of a particle, we physicists mean the rest mass. Because it is invariant. It doesn't change. And this is why we do in fact talk about photons as being massless.

But if someone wants to insist on using the relativistic mass, then photons have a relativistic mass, as given by Einstein's formula.

Whether that's the reeeaallll mass is not an easy question to answer. That's philosophy, not physics.

Okay I follow you, but it's the bold which I want to focus on (unfortunately lol ...

It seems that this is often what is appealed to by those who want to assert that a photon DOES have mass (at least from online Googling it appears this way).

So what is the trip up point that throws the "relativistic mass of a photon" into philosophy and not an easy question to answer ? I get that the intrinsic mass of a particle is it's rest/invariant mass, but why with a photon does it specifically get tricky ? Because it's either at rest, or c, nothing in-between ?

 

[essentialsaltes]

So what is the trip up point that throws the "relativistic mass of a photon" into philosophy and not an easy question to answer ?

'The mass' of the photon is ambiguous.

If you mean the rest mass, it's zero.

If you mean the relativistic mass, it's nonzero.

So maybe it's just a matter of definitions. When you ask about what the mass of the photon is, which mass do you mean, the rest mass or the relativistic mass?

Quoting what you said earlier: "If a photon were to have actual mass in the way we think of "mass""

There's no 'actual mass'. I don't know what way 'we think' of mass.

We have a rest mass and a relativistic mass.

If you ask me 'which one is the actual mass?' I don't have an answer for you. It's whichever one you want to define as the actual mass, I guess.

'But I don't want to define the actual mass, I want to know which one is reaallllly the mass?'

I can't answer that, because if I can't define one of them to be 'real', then I will have to defer to the philosophers to tell us what Reality is so they can decide which is really the mass.

Again, since the rest mass is invariant, and actually tells us something interesting about the particle, whether it's a photon or a neutron, the rest mass would appear to be a more fundamental description of the mass. That makes it zero.

But maybe 'the way we think about mass' is that you put your tomatoes on a scale and measure their weight, which is directly related to mass. When you try to put a photon on a scale, they keep flying away at the speed of light, but imagine....

We build a box with perfect mirrors coating the inside. We put it on a super-duper sensitive scale. We turn out the lights so we know there are no photons inside the box. Close it. And weigh it. Then we open the box and shine a flashlight inside and close the box again before the photons bounce out. Now there's a bunch of photons bouncing around inside the box. If we weigh the box again, it will weigh just a tiny bit more, because there's more energy inside. The only thing we added to the box was photons, so they must weigh something, right? That would mean they have mass.

Both answers are meaningful, and I don't see any way to select one as being the 'real' answer. The rest mass is a more fundamental and invariant property, and that's why physicists usually mean this when they say 'the mass' of any given particle. But maybe the relativistic mass may be closer to what we naively(?) think of as mass. Gravity only affects things that have mass. Photons are affected by gravity. Ergo, photons have mass.

Massless supporter: No no no. Gravity only affects things that have relativistic mass. That's just the same as energy. Clearly a non-naive way to think of gravity is that gravity only acts on things with energy. And photons have energy, so that's fine. But photons don't have mass.

Mass supporter: No no no. Photons don't have rest mass. But they do have relativistic mass.

Massless supporter: Of course, I agree with you.

Mass supporter: Well when I say mass, I mean relativistic mass.

Massless supporter: Well when *I* say mass, I mean rest mass.

Mass supporter: So photons have nonzero mass.

Massless supporter: No, they don't have any mass.

Mass supporter: But we just agreed that photons don't have rest mass. But they do have relativistic mass.

Massless supporter: Yes, we agreed that photons do have relativistic mass. But they don't have rest mass.

Confused observer: But which one is the real mass?

Mass supporter: Based on my definition, it's the relativistic mass.

Massless supporter: Based on my definition, it's the rest mass.

In reality, an 'argument' like this doesn't occur, because as long as everyone is clear about the terminology, there are no disagreements in the predictions and equations. There is no need to decide which one is 'real'. That's for the philosophers.

 

[TillICollapse]

'The mass' of the photon is ambiguous.

If you mean the rest mass, it's zero.

If you mean the relativistic mass, it's nonzero.

So maybe it's just a matter of definitions. When you ask about what the mass of the photon is, which mass do you mean, the rest mass or the relativistic mass?

Quoting what you said earlier: "If a photon were to have actual mass in the way we think of "mass""

There's no 'actual mass'. I don't know what way 'we think' of mass.

We have a rest mass and a relativistic mass.

If you ask me 'which one is the actual mass?' I don't have an answer for you. It's whichever one you want to define as the actual mass, I guess.

'But I don't want to define the actual mass, I want to know which one is reaallllly the mass?'

I can't answer that, because if I can't define one of them to be 'real', then I will have to defer to the philosophers to tell us what Reality is so they can decide which is really the mass.

Again, since the rest mass is invariant, and actually tells us something interesting about the particle, whether it's a photon or a neutron, the rest mass would appear to be a more fundamental description of the mass. That makes it zero.

But maybe 'the way we think about mass' is that you put your tomatoes on a scale and measure their weight, which is directly related to mass. When you try to put a photon on a scale, they keep flying away at the speed of light, but imagine....

We build a box with perfect mirrors coating the inside. We put it on a super-duper sensitive scale. We turn out the lights so we know there are no photons inside the box. Close it. And weigh it. Then we open the box and shine a flashlight inside and close the box again before the photons bounce out. Now there's a bunch of photons bouncing around inside the box. If we weigh the box again, it will weigh just a tiny bit more, because there's more energy inside. The only thing we added to the box was photons, so they must weigh something, right? That would mean they have mass.

Both answers are meaningful, and I don't see any way to select one as being the 'real' answer. The rest mass is a more fundamental and invariant property, and that's why physicists usually mean this when they say 'the mass' of any given particle. But maybe the relativistic mass may be closer to what we naively(?) think of as mass. Gravity only affects things that have mass. Photons are affected by gravity. Ergo, photons have mass.

Massless supporter: No no no. Gravity only affects things that have relativistic mass. That's just the same as energy. Clearly a non-naive way to think of gravity is that gravity only acts on things with energy. And photons have energy, so that's fine. But photons don't have mass.

Mass supporter: No no no. Photons don't have rest mass. But they do have relativistic mass.

Massless supporter: Of course, I agree with you.

Mass supporter: Well when I say mass, I mean relativistic mass.

Massless supporter: Well when *I* say mass, I mean rest mass.

Mass supporter: So photons have nonzero mass.

Massless supporter: No, they don't have any mass.

Mass supporter: But we just agreed that photons don't have rest mass. But they do have relativistic mass.

Massless supporter: Yes, we agreed that photons do have relativistic mass. But they don't have rest mass.

Confused observer: But which one is the real mass?

Mass supporter: Based on my definition, it's the relativistic mass.

Massless supporter: Based on my definition, it's the rest mass.

In reality, an 'argument' like this doesn't occur, because as long as everyone is clear about the terminology, there are no disagreements in the predictions and equations. There is no need to decide which one is 'real'. That's for the philosophers.

Okay ... a couple of more questions and then I'll chill and see if anyone else responds. Thanks for the time

it's the ambiguity between a nonzero value for relativistic mass, and a zero value for rest mass, that is the tripping point, yes ?

Yeah I wasn't really thinking "weight" though ... I've heard the photons in a box of mirrors thing before. As far as what is happening in the box, it's a matter of energy, not "mass" in the way we would consider "weight of the structure" so to speak. IOW, the photon may not actually have "physical mass that can be weighed" IOW, it's the energy from the change in momentum they undergo while in the box. This would be thinking of "mass" differently, yes ? Concerning gravity ... the fact that photons are effected by gravity ... isn't it just that they are following the curvature of space-time along a geodesic ?

I was sort of trying to find the actual line which causes the trip, so to speak, between the massless proponents and the mass-ers (or m[bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse]s maybe lol ) ... and so it's merely the fact it can have an intrinsic zero mass, but a non zero relativistic mass. Basically. Yes ?

 

[essentialsaltes]

Okay ... a couple of more questions and then I'll chill and see if anyone else responds. Thanks for the time

it's the ambiguity between a nonzero value for relativistic mass, and a zero value for rest mass, that is the tripping point, yes ?

No, that's not ambiguous at all. Two different things have two different values. Nothing wrong or ambiguous about that. The ambiguity is when you try to ask about the mass of the photon. There is no unambiguous meaning for that. It could be the zero one, or it could be the nonzero one.

Concerning gravity ... the fact that photons are effected by gravity ... isn't it just that they are following the curvature of space-time along a geodesic ?

Yes.

 

[agua]

I suggest that the photon has mass; but I can't recall why atm

Crooke's radiometer brought several explanations that don't require photons to have mass though.

 

[TillICollapse]

No, that's not ambiguous at all. Two different things have two different values. Nothing wrong or ambiguous about that. The ambiguity is when you try to ask about the mass of the photon. There is no unambiguous meaning for that. It could be the zero one, or it could be the nonzero one..

Ah

Okay thanks, I appreciate it

 

[TillICollapse]

I suggest that the photon has mass; but I can't recall why atm

Crooke's radiometer brought several explanations that don't require photons to have mass though.

Huh, interesting rabbit hole to go down. Thx

 

[Loudmouth]

So maybe it's just a matter of definitions. When you ask about what the mass of the photon is, which mass do you mean, the rest mass or the relativistic mass?

I have always considered "mass" to be the rest mass by default, unless specific elsewhere. In biology, no one asks for the relative velocity of the sodium chloride when they are weighing it out for a buffer. When there is a weight loss contest on TV, the loser doesn't challenge the results because the winner would gain weight if they were traveling at 0.9c.

However, with photons this gets a bit messy since they are never at rest. They are always moving at c. As far as QM and relativity go, they need a photon with no rest mass, so finding a photon with no detectable rest mass is a test of those theories, within the sensitivity of our equipment.

 

[DogmaHunter]

Photons are affected by gravity, yes? What else keeps them from escaping a black hole?
Don't they need mass to be affected by gravity?

I'm probably looking at it to simplisticly...
I'll enjoy reading this thread.

 

[essentialsaltes]

Photons are affected by gravity, yes? What else keeps them from escaping a black hole?
Don't they need mass to be affected by gravity?

I'm probably looking at it to simplisticly...

Not simplistic at all. But again, when you say just plain vanilla 'mass', it's not clear what you mean.

If you mean rest mass, then no, because photons don't have rest mass, and obviously as you say, photons are affected by gravity. So you don't need (rest) mass to be affected by gravity.
If you mean relativistic mass, then yes, because that's the same as energy.

Or maybe we want to define vanilla 'mass' to be 'anything that's affected by gravity'. Well, that would mean photons have mass by definition, but I'm not sure we've gained anything by that. And even then... allow me to muse...

In Newton's day it was clearer, his law of universal gravitation showed that mass was the source of the gravity field.

But Einstein, as usual, complicates things. In general relativity, gravity is not a force, but the effect of the curvature of space time. And what causes that curvature? In its simplest form, the equation for GR is:

G= 8pi*T

The Einstein curvature tensor is 8pi times the 'stress-energy' tensor, which "describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is an attribute of matter, radiation, and non-gravitational force fields. The stress–energy tensor is the source of the gravitational field in the Einstein field equations of general relativity, just as mass density is the source of such a field in Newtonian gravity."

So for Newton gravity comes from mass. In GR, gravity comes from energy and momentum. Now mass is obviously buried in both energy and momentum, but energy and momentum is more key. Photons have energy (and momentum), so gravity affects them.

Now what's really going to bake your noodle is if we ask again about whether gravity 'affects' photons. Now that we understand that gravity is actually the curvature of spacetime, photons actually travel in straight lines. Well, geodesics, like the great circle routes that planes take that are the 'shortest distance' between two points in a curved space like the surface of the earth. And this is a real separation that is of use in physics. Photons, and other massless particles, follow geodesic paths. They follow 'straight lines' as best they can in a curved space. Massive particles do not follow geodesics. Because they are 'affected by gravity', they do not follow 'straight line' geodesics. So if we think about things in terms of Newton's laws, which state that an object without any force acting on it will move in a straight line (at constant velocity)... that's exactly what photons do. Looked at in that way, they're not affected by the force of gravity. They're moving in straight lines (as best they can in a curved space).

So your explanation was not simplistic. It's just that Einstein's is way too complicated. But in this sense we can see a way that photons are actually not affected by gravity.

 

[TillICollapse]

I have always considered "mass" to be the rest mass by default, unless specific elsewhere. In biology, no one asks for the relative velocity of the sodium chloride when they are weighing it out for a buffer. When there is a weight loss contest on TV, the loser doesn't challenge the results because the winner would gain weight if they were traveling at 0.9c.

"The Biggest Relativistic Loser"

 

[Loudmouth]

Or maybe we want to define vanilla 'mass' to be 'anything that's affected by gravity'. Well, that would mean photons have mass by definition, but I'm not sure we've gained anything by that. And even then... allow me to muse...

If we wanted to differentiate between things with mass and photons, perhaps it would be better to say that things with mass decelerate as they try to move out of a gravity well. Photons don't. In a vacuum, photons will still move at velocity c as they move away from the center of the gravity well. While photons may curve around a massive object, it is not a ballistic arc as with bullets or satellite orbits.