Unraveling the Mystery of Photon Mass and Its Interplay with Gravity

[Ambitwistor]

Originally posted by Eepl
Less Mass=Less Force. No Mass=No Force.

But that is simply wrong: there is no such law of physics.

 

[GijXiXj]

Can I join in?

Would things not be a whole lot easier if we just forgot this whole concept of mass thing altogether?

I mean to regard mass as just another manifestation of energy, something like heat is (or other vibration types, perhaps mass is just another vibration type: QM, anybody?).

So in regard to behaviour of photons, it's not their mass or lack of that's important, but that they also too are a manifestation of energy - perhaps the fundamental manifestation.

Further in regard to GR, is there anything wrong with saying that it's not mass, per se, that warps space-time, but the energy (E = mc2, and all that, roughly) that that mass is a manifestation of.

So energy (whatever that is!) warps space-time, causing the appearance that energy attracts energy. Hence the Sun appears to bend light towards it (like refraction), 1919 Eddington experiment, and all that.

Thanks.

Dennis Revell

 

[Ambitwistor]

Originally posted by GijXiXj
Would things not be a whole lot easier if we just forgot this whole concept of mass thing altogether? [...] I mean to regard mass as just another manifestation of energy,

Unlike energy, mass is an intrinsic, invariant property of a particle, and as such, is a useful physical property.

So in regard to behaviour of photons, it's not their mass or lack of that's important, [...]

Their masslessness has important consequences; for instance, it's the reason why they can travel at the speed of light.

Further in regard to GR, is there anything wrong with saying that it's not mass, per se, that warps space-time, but the energy (E = mc2, and all that, roughly) that that mass is a manifestation of.

No, there's nothing wrong with that (although mass-energy density is only one of several physical quantities that contribute to spacetime curvature).

 

[GijXiXj]

Originally posted by Ambitwistor
"Unlike energy, mass is an intrinsic, invariant property of a particle, and as such, is a useful physical property."

Is it? What about E = mc2? Mass, as commonly understood certainly isn't invariant when it's changing velocity. What about the spontaneous destruction of mass to create photons?

The viewpoint I proposed is more shocking than may first appear. It also covers the idea that there is no such thing as particles. That "particle" interactions are no more than the interactions between different ("particle") forms of energy. Another way of looking at it is that this or that particular particle name is just convenient code to describe how that particular "particle" manifestation of energy behaves. But I think too much emphasis on a particle as a particle, implying some form of solidity is confusing.

If a billiard ball hit me on the head it would certainly hurt; but as you know, a billiard ball is far from solid (whatever that means), comprising mostly empty space (whatever that means). As you try to look at the billiard ball under higher and higher magnification it looks less and less solid, and the "electrons" around its atoms disappear into diaphanous clouds of ... something ...; but you certainly never get to "see" them.

"Their masslessness has important consequences; for instance, it's the reason why they can travel at the speed of light."

Yes, I realize that photon masslessness has important consequences; they probably also all enjoy a "time-free" existence ('til they hit something that's non reflective, that is). That said there's something a little tautological in what you say: " ... the reason they can travel at the speed of light"? They ARE light.

Unlike energy, mass is an intrinsic, invariant property of a particle, and as such, is a useful physical property.

Is it a physical property? Or just useful shorthand that summarises a wide spectrum of behaviours? When you say here's a particle mass (rest or otherwise) of such-and-such, aren't you just saying here's a really tight little bundle of energy = mc2?

 

[selfAdjoint]

The "no particles, only fields" idea isn't shocking, it's been widely circulated among the folks who write about interpretation because they can't do (or are burnt out on doing) theory.

The counter to all-field theories is Haag's theorem that a pure interacting quantum field theory can't hang together at the Hilbert space level.

 

[Ambitwistor]

Originally posted by GijXiXj
Is it? What about E = mc2? Mass, as commonly understood certainly isn't invariant when it's changing velocity.

When we speak of photons being massless, which is what this thread is about, the mass involved is the invariant mass.

If you want to use relativistic mass, which is not invariant, then that's more or less the same thing as energy, so as I said before, it is a redundant concept that has fallen out of favor among modern physicists.

What about the spontaneous destruction of mass to create photons?

There is a difference between invariant and conserved.

Energy is conserved, so it's an interesting quantity, but it's not invariant. Mass is invariant, so it's also an interesting quantity, but it's not conserved.
"Their masslessness has important consequences; for instance, it's the reason why they can travel at the speed of light."

That said there's something a little tautological in what you say: " ... the reason they can travel at the speed of light"? They ARE light.

That's missing the point, which is that anything which is massless can travel at the speed of light. As such, "the speed of light" is something of a misnomer; there is a maximum, invariant speed that things can travel at, but only if they're massless.

Is [mass] a physical property?

Yes. We can measure it.

 

[Ambitwistor]

The counter to all-field theories is Haag's theorem that a pure interacting quantum field theory can't hang together at the Hilbert space level.

Are you implying that a particle interpretation of interacting quantum field theory is less susceptible to the consequences of Haag's theorem than a field interpretation??

I think you're rather overstating the theorem, too. What Haag's theorem attacks is the validity of doing perturbation theory (in interacting QFT). If anything, I'd say that it presents more problems for a particle interpretation of QFT than a field interpretation.

 

[GijXiXj]

selfAdjoint & Ambitwistor:

The "no particles, only fields" idea isn't shocking, it's been widely circulated among the folks who write about interpretation because they can't do (or are burnt out on doing) theory.

Well, I hope that's not meant to imply that those who are "burnt out" shouldn't post here. ;-)

I didn't really mean to imply that the alternative to particles is fields (although it might be, and although I know I mentioned vibrations). Who knows what it might be? May be a scalar "Energy" that needs to stuff itself somewhere for uncertainty principle or similar or perhaps I mean analagous reasons, and so the four space-time dimensions being insufficient for this stuffing, the "energy" (whatever that is) goes (went) right ahead and created extra dimensions over and above the "standard" four into which to stuff itself: which particular quite transmutable form of energy comprises what we know as matter?

______________________

When we speak of photons being massless, which is what this thread is about, the mass involved is the invariant mass.

Actually I understood the thread to be about Eepl's confusion/misinterpretation of the consequences of the "masslessness" of photons. I tried to address this by effectively saying "forget mass, think energy", which viewpoint really should make Eepl's problem go away: of course no one has yet disputed that it is energy, per se, and not mass that warps space-time. So, if the Sun has "mass" energy (I might want to call it, burnt out old windbag that I am: the "mass manifestation of energy"), the photon has EM - pure? energy, that should go some way to helping Eepl.

Energy is conserved, so it's an interesting quantity, but it's not invariant. Mass is invariant, so it's also an interesting quantity, but it's not conserved.

I'm sorry, I'm out of touch. Could you explain more rigorously the difference between "invariant" and "conserved"?

There is a difference between invariant and conserved.

As above. My rough understanding is that "invariant" applies to a process, "conserved" to a quantity (although I know that dS^2 = GijdXidXj - aka Pythagoras, and other quantities are regarded as invariants). Mass is obviously not "invariant" if we agree to speak in plain English: it varies (changes). Energy, as commonly accepted, doesn't change in overall amount (well, may be ... unless that turns out to be relative too - in which case we're screwed ;-).

That's missing the point, which is that anything which is massless can travel at the speed of light. As such, "the speed of light" is something of a misnomer; there is a maximum, invariant speed that things can travel at, but only if they're massless.

And do you happen to know, I mean for sure, of any such other "things"?

Is [mass] a physical property?

Yes. We can measure it.

Hmmm. How do you know you're just not measuring the property that mass is "merely" ;-) another manifestation of energy, one so "dense" in energy that it exhibits measurable inertia (resistance to change)? If I heat an iron bar up, does its gravitational "pull" not increase, or more correctly, does it not "tighten up" the space-time around it, and does it not become "stiffer" with regard to trying to change its state of motion? In other words, have I not increased both its gravitational and inertial mass - which have varied no matter how little I heat the bar? Or are all those just stories told to school children to impress them with Relativity?

______________________

I guess as I'm arguing against too much emphasis on the validity of the concept of mass and particles, that I should also do so against the concept of photons. Aren't "they" just a manifestation that energy exchanges with radiation occur in discrete amounts, and that is equally validly (or more validly) regarded as a property of the object interacted with, or may be even space-time itself, than as a property of the radiation? In other words a photon comprises the lowest sub-multiple of an amount of energy that is allowed to be exchanged whenever radiation is involved? The photoelectric effect seems to argue for this viewpoint: radiation of a certain frequency won't cause electron emission from one substance, whereas it will from another.

______________________

Just thinking aloud. ...

 

[Eepl]

AND E EQUAL MC SQUARED.

I think that we are far beyond accomplishing anything here. It's very simple really. Does a photon have mass? Yes or No. Use the right math and you'll get the right answer. No names, no theorums, just good ol' fashion math and common sense will get you the answer. This is no longer trade of knowledge, but a battle of technicalities. What we are doing here is what has destroyed the new discovery of science so many times.

May I recommend that a Administrator close this thread.

 

[Ambitwistor]

Originally posted by GijXiXj
I'm sorry, I'm out of touch. Could you explain more rigorously the difference between "invariant" and "conserved"?

"Conserved" means "has the same value at all times". "Invariant" means "has the same value in all reference frames" (assuming we're talking about Lorentz invariance).

Energy is a conserved quantity, so a given observer will measure the total energy of a system to be constant at all times. But energy isn't (Lorentz) invariant: different observers will measure different energies. Mass is not conserved; you can create or destroy it. But it is invariant: all observers will agree on the mass of a particle. (At least, if the mass we're talking about is the invariant mass, and not the relativistic mass, which is the same as energy.)

And do you happen to know, I mean for sure, of any such other "things"?

Gluons are massless. People used to think neutrinos were massless too, but now they don't...

If I heat an iron bar up [...] have I not increased both its gravitational and inertial mass

Yes, but you're talking about relativistic mass (which most people today just call "energy" or "mass-energy").

The rest of your post was too incoherent for me to respond to.

 

[Ambitwistor]

Originally posted by Eepl
Does a photon have mass? Yes or No.

The answer is the same as it's been throughout this whole thread: if you're talking about invariant mass, no. If you're talking about relativistic mass-energy, yes. These are both experimental facts.

(Well, we can't actually ever prove that something's mass is exactly zero, but we can set stringent upper bounds on it. But the masslessness of the photon is, contrary to your argument, not incompatible with what we know about photons, either).

 

[GijXiXj]

Eepl

Your signature reads:

"Imagination is more important than knowledge."

-Albert Einstein

And then you say:

"This is no longer trade of knowledge, but a battle of technicalities. What we are doing here is what has destroyed the new discovery of science so many times."

Now, I agree that my imagination might be a bit wild, as evidenced perhaps by Ambitwistors: "The rest of your post was too incoherent for me to respond to." (Who rattled his cage?). I'm not trying to battle technicalities. I seriously thought that regarding everything more or less as energy might help you overcome your difficulty with "massless" photons. The idea is that if you regard mass as just another manifestation of energy that you get pulled away from the idea you seem to be stuck with that only "material" things can exert influences.

Note that no one has argued against the contention that it is energy that warps space-time, rather than mass.


"AND E EQUAL MC SQUARED."

Which equation, of course, includes ALL mass, no matter how "generated".

"I think that we are far beyond accomplishing anything here."

Please yourself. It's "your" thread.

"It's very simple really."

No, it's not.

"Does a photon have mass? Yes or No."

No it doesn't. Entirely logical if you just regard mass as one form of energy and "photons" as another. If so, your question becomes: "Does one form of energy comprise another form of energy". A question that ridicules itself. As in: does sound have mass: no it doesn't, but it does have energy (OK ... don't stretch this too far ;-).


"Use the right math and you'll get the right answer."

Erm ... just who's to decide, or how do you decide just what the "right" math is? There's an awful lot of it out there ... and it's all "right" within limitations. In fact, it's hard to for me to see how math can be wrong. It might be incorrectly applied, but it can't be wrong in itself - it's an abstraction ( er, may be it's not ;-).

"No names, no theorums, just good ol' fashion math and common sense will get you the answer."

Well, er, theorems are part of "good ol' fashion math"; and common sense really gets you nowhere, unless "common sense" indicates that time slows down as you go faster (whatever that means, time actually speeds up if you hop on a west-bound airplane), you get "heavier", and you get smaller.

I don't think any of those fall within most people's definition of common sense. What does get you somewhere are observations, and then trying to create mathematical frameworks to fit those observations, as well, of course, as Einstein said, imagination.

"May I recommend that a Administrator close this thread."

Yes, you may.

;-)

Ambitwistor:

Yes, thanks for your explanations of "invariant"/"conserved". That's more or less how I thought it was. I'm a bit rusty on some things.

That said the split between "invariant mass" and "relativistic mass" seems artificial to me. Isn't one alien space-being's invariant mass another alien space-being's relativistic mass? How would they tell the difference? Are there any equations that indicate a difference; I mean for all conceivable observers (from whichever galaxy)?

Gluons are massless?:-

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html

(may be that's all old stuff)

Gluons are still not totally hypothetical? Not just another way of explaining energy exchange? Or perhaps a way to insist, with really no definite evidence, that a nucleus somehow actually looks a bit like a bunch of the things it is known it can decompose to: neutrons, protons - as if an "electron" in an orbital actually "looks like" or behaves much like a free "electron". It doesn't.

Yes, but you're talking about relativistic mass (which most people today just call "energy" or "mass-energy").

But ... is there any way of telling the difference between "invariant mass" and "relativistic mass" ... I don't mean by sending the "mass" off in a spaceship, and work it out from the equations. I mean by measurements. So, I've got a lump of metal, I'm moving in some way, because I'm on the Earth (a spaceship!), what experiment can I do to tell me how much of the "mass" of the metal is of the "invariant" type, and how much of it is of the "relativistic" type? Presumably also, this ratio changes as the Earth's rotation slows down. But ... is there anyway of detecting it?

The rest of your post was too incoherent for me to respond to.

Really? I thought it was quite coherent. That doesn't mean anything in it might in any way approximate to reality, but I think that also goes for what "most people today" think. Perhaps you just didn't understand my meaning.

 

[Ambitwistor]

Originally posted by GijXiXj
That said the split between "invariant mass" and "relativistic mass" seems artificial to me. Isn't one alien space-being's invariant mass another alien space-being's relativistic mass?

No. Like I said, everybody agrees on what the invariant mass is, but they don't agree on what the relativistic mass is. They're not the same thing. Some experiments measure relativistic mass, some measure invariant mass. If you want to measure relativistic mass, there are various ways of doing that, depending on whether you're talking about inertial, passive gravitational, or active gravitational mass. (And note that inertial mass is really a tensor, of which the traditional "relativistic mass" is just a component.) If you want to measure its invariant mass, there are lots of ways particle physicists do that too.

Gluons are massless?

Yes, although it's kind of hard to define the "mass" of a confined particle:

http://www.lns.cornell.edu/spr/2000-05/msg0024932.html

Gluons are still not totally hypothetical?

That depends on what you mean by "totally hypothetical". Nobody's ever seen an isolated gluon, just like nobody's ever seen an isolated quark: they don't exist in isolation. But physical predictions of quantum chromodynamics are borne out by experiment.

 

[GijXiXj]

Fair point.

If you want to measure its invariant mass, there are lots of ways particle physicists do that too

Fair point. I guess they can just count the particles; or in a different situation the two alien space beings can just count the atoms in their lumps of "mass" of the same composition. And I'm sure they'd agree that those numbers had significance. Silly me.

Your other point, well, I guess "what really happens", ie: the result of experiment is the only test we have.

 

[trex1950]

The key to the why the photon can travel and be bent by gravity has nothing to do with it's mass. Because a photon has no mass. The answer is related to curvature to space-time due to light moving around an massive object. It is just following the curvature of space-time produced by the mass of a planet or star. It does not have any force, pressure, or gravity.

 

[Nano-Passion]

The key to the why the photon can travel and be bent by gravity has nothing to do with it's mass. Because a photon has no mass. The answer is related to curvature to space-time due to light moving around an massive object. It is just following the curvature of space-time produced by the mass of a planet or star. It does not have any force, pressure, or gravity.

You just bumped a 5 year old thread.

Doesn't photon get bent through space due to the electrical forces acted upon it?

 

[kirtg]

doesn't the kinetic energy of a photon give it a "mass" if energy=mass and mass=energy?

 

[kirtg]

if a photon is massless like the neutrino and has not charge like the neutrino, why doesn't it penetrate matter like the neutrino?

 

[Spourk]

if a photon is massless like the neutrino and has not charge like the neutrino, why doesn't it penetrate matter like the neutrino?

Neutrinos have mass.

 

[kirtg]

So if a nearly massless neutrino with no charge can penetrate matter with little likelihood of a collision, why doesn't a photon do the same thing"

 

[Matterwave]

The photon interacts with charged particles (like the electrons in atoms) via QED (EM forces basically). Neutrinos only interact with particles via the weak force.

Photons do not have rest mass.

In general, it's better to read the faq if you have some basic misunderstandings. If you can't find the answer in the faq or a quick search, then post a new thread, don't bump a ~decade old thread (holy crap PF is ~decade old!?).