PDA

View Full Version : Photon mass and relativity



hollowman
2009-Jun-20, 10:15 AM
This is my first post, so if the answer to this query is in the archives, pls direct me -- thx.)

If plain ol' photons -- which have some mass -- travel "at" the speed of light (SOL), why don't they become "heavy" (and too heavy to travel), as Einstein's theory predicts?
I'm gonna guess the answer may be related to SOL being an absolute math construct (i.e., SOL is a pure constant equations spat back; in reality nothing -- not even photons -- ever get to the 100% constant) AND the fact that photons weigh so very little to begin with.

Related queries:

How close to SOL does an object have to get (%wise) before its mass skyrockets?

What's the diff. (if any) between energy traveling at/near SOL as opposed to a massive object (nutrino, etc.) traveling at/near SOL?

loglo
2009-Jun-20, 01:13 PM
Hi Hollowman,

Welcome to BAUT.

Q1. Photons don't have any mass at all, that is how they travel at c. You may be thinking of neutrinos which may have a slight amount of mass but that is still uncertain.

Q2 Only massless objects can move at the speed of light, and they can never move at less than the speed of light.
Massive objects can never move at the speed of light. The lower the mass the more easily they can reach near light speed but still never actually reach it.

Just to be explicit, you appear to be talking about relativistic mass which is a concept from Relativity which is not really taught these days. Physicists prefer to think of mass as invariant and the energy to be frame dependent. What that means is that you can always consider yourself to have unchanging mass because you can always consider yourself to be at rest. But when you look at an object moving within your frame then you can measure an increased energy/mass for that object.

But then again someone in a different frame will measure a different mass/energy for the same object so that isn't much use. Better to look for the invariants, what everyone measures to be the same, and that is the rest mass, and that never changes for an object.

Hope that helps.

slang
2009-Jun-20, 02:47 PM
Just to be explicit, you appear to be talking about relativistic mass which is a concept from Relativity which is not really taught these days.

In a recent thread this paper (http://arxiv.org/PS_cache/physics/pdf/0504/0504110v2.pdf) (PDF) was cited as explanation for what Ioglo said. I happen to have it open (slowly plugging through :) ), don't remember which thread it was in.

hollowman
2009-Jun-20, 04:02 PM
Q1. Photons don't have any mass at all, that is how they travel at c.Hmm...then why are they affected by gravity? Black holes, galaxy clusters (lensing), etc., are, TTBOMK (& PCMIIW), dramatic examples of light deflection. Grav., as some suggest, transcends into other dimensions (as per string theory). Also, having "no mass" (i.e., absolutely no mass) to me concomitantly suggests "matterlesss".

tusenfem
2009-Jun-20, 06:03 PM
Hmm...then why are they affected by gravity? Black holes, galaxy clusters (lensing), etc., are, TTBOMK (& PCMIIW), dramatic examples of light deflection. Grav., as some suggest, transcends into other dimensions (as per string theory). Also, having "no mass" (i.e., absolutely no mass) to me concomitantly suggests "matterlesss".

That is because gravity is not a force as usual, but can be described as a warping of space time. Close to heavy objects, e.g. like the sun, spacetime is curved more (remember the rubber sheet analog). Light follows so called geodesics, which in non-curved space would be straight lines, however in curved space, geodesics are not straight but bent, which leads to light being influenced by gravity.

No mass would basically be matterless, it all depends on your own definition. But light is not matter, so much is sure. Or like Homer Simpson said:


Relax!
What's mind? No matter.
What's matter? Never mind.

Amber Robot
2009-Jun-20, 07:03 PM
That is because gravity is not a force as usual, but can be described as a warping of space time.

Then why did Newtonian gravity also predict a bending of light past the Sun?

cjameshuff
2009-Jun-20, 08:07 PM
Note that Newtonian gravitational acceleration of a body is independent of the mass of that body. This calculation does not assume a mass for light.

And yes, photons have precisely zero rest mass. This not only means they can travel at exactly c, which massive particles can not do, it means they can not travel slower than c. I'm not certain, but those Newtonian calculations may have only assumed an initial velocity of c, with the photons accelerating and decelerating as they moved into and out of the gravity well.

hollowman
2009-Jul-23, 09:43 PM
The most succinct, best-remembered answer to my orig. query is:
The photon is a FORCE particle (as opposed to a MATTER particle).

Ken G
2009-Jul-23, 10:54 PM
Not sure that will cut it, because there are other force particles that do have mass! The particles that mediate the strong force are gluons, and they, like photons, should be massless, but the force particles that mediate the weak force are W and Z bosons, and they are quite massive indeed. Because they are only virtual particles when they are mediating forces, particles of such high mass can only exist for very short times, so the weak force is short range.

hollowman
2009-Jul-26, 10:08 AM
Not sure that will cut it, because there are other force particles that do have mass! The particles that mediate the strong force are gluons, and they, like photons, should be massless, but the force particles that mediate the weak force are W and Z bosons, and they are quite massive indeed. Because they are only virtual particles when they are mediating forces, particles of such high mass can only exist for very short times, so the weak force is short range.Okay, but we still need a succinct, one-sentence, understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".

Ken G
2009-Jul-26, 08:17 PM
Okay, but we still need a succinct, one-sentence, understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".I'm not sure that's possible. We could say it's because electromagnetic forces are long range, but one could as easily draw the conclusions the other way around, it's just begging the question. It is because we observe it to be, maybe someone with better knowledge of photons, gluons, and W and Z bosons can give a "reason" that will not sound like complete gobbledy-gook (i.e., will not sound like "because the associated gauge symmetry is partially but not explicitly broken", or some such thing).

a1call
2009-Jul-26, 09:08 PM
Okay, but we still need a succinct, ... understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".

I really have no business budding in this thread after the board gurus have replied, but :whistle:

Haven't you provided the answer in your OP?




If plain ol' photons -- which have had some rest mass -- travel "at" the speed of light (SOL), why don't wouldn't they become "heavy" (and too heavy to travel), as Einstein's theory predicts?

AndrewJ
2009-Jul-26, 09:35 PM
Okay, but we still need a succinct, one-sentence, understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".

I've wondered that for a long time. I figured that if "energy is discrete" (quanta can't be infinitely small) maybe mass only kicks in at a certain scale that a single photon wouldn't have. I don't know if that's relevant but it made the concept of a massless particle seem less alien.

blueshift
2009-Jul-26, 10:05 PM
A photon is massless because the only way anything can move at the same speed with respect to all reference frames is for that anything not to have any mass. This is also why the graviton must be massless because gravitational fields are not relative either. If they were then Kepler's Laws become falsified and so do Newton's.

Jeff Root
2009-Jul-26, 10:05 PM
You can wonder why photons are massless, or you can wonder why other
particles have mass.

Some particles have electric charge; others don't. Some particles have
color charge; others don't. Some particles have mass; others don't.

-- Jeff, in Minneapolis

AndrewJ
2009-Jul-27, 06:07 AM
You can wonder why photons are massless, or you can wonder why other
particles have mass.

Some particles have electric charge; others don't. Some particles have
color charge; others don't. Some particles have mass; others don't.

-- Jeff, in Minneapolis

I think that leaving concepts such as electric charge and mass unexplained is less illuminating than referring to first principles that might then be incomprehensible to the layman. I once asked my teacher how a magnet knew another was nearby so that they could interact - I would have been appeased by QM jargon.

trinitree88
2009-Jul-27, 02:44 PM
Not sure that will cut it, because there are other force particles that do have mass! The particles that mediate the strong force are gluons, and they, like photons, should be massless, but the force particles that mediate the weak force are W and Z bosons, and they are quite massive indeed. Because they are only virtual particles when they are mediating forces, particles of such high mass can only exist for very short times, so the weak force is short range.

KenG. Not quite. While it is true that there are massive weakons, the W+. the W-, and the massive Z0, with Mev/c2 ~ 80-90 Mev, that's not all. The Z can be any particle/antiparticle pair, and is correctly viewed as fluctuating between different particle/antiparticle pairs as it travels....kind of like a blinking firefly, blinking into different shizophrenic existences, with different probabilities of finding it as a particular pair (branching ratios). Below 1.022 Mev, however, the Z can only be massless as either a photon/antiphoton, or a neutrino/antineutrino. So the low energy Z, Gamow's candidate for a graviton, travels explicitly at c like gravitational waves. Using the Heisenberg Uncertainty Principle, this makes the range of the weak interaction at energies less than 1.022 Mev, infinite, just like gravitational effects.

see:http://en.wikipedia.org/wiki/W_and_Z_bosons

stutefish
2009-Jul-27, 04:14 PM
Okay, but we still need a succinct, one-sentence, understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".
First, why do we need this? ETA: Are the "unwashed" doing a lot of sophisticated work with photons in their daily lives, that would be improved by such an answer?

Second, have you considered the possibility that the topic is too complicated to allow for simple, easy-to-understand answers?

I mean, it's not like there's a simple version of differential calculus: The only way to understand it is the long, hard complicated way.

Ken G
2009-Jul-27, 06:12 PM
I'm not sure I agree with that. I think any concept should have an "unwashed" answer that should never be "you can't understand that." The only valid answer like that is "I don't yet understand it well enough myself to be able to give you an answer that would give you a useful way to picture this." However, there certainly is also the answer "I don't think that question has an answer, but if it does, I certainly don't know what it is." The reason we need "unwashed" answers is that we want to share at least the flavor of massless photons, or the flavor of differential calculus, with everyone. They don't need to understand it like we do, or be able to do calculations, but they should be given a picture they can understand of what it is-- or else we don't understand it suitably ourselves.

Ken G
2009-Jul-27, 06:15 PM
Below 1.022 Mev, however, the Z can only be massless as either a photon/antiphoton, or a neutrino/antineutrino. So the low energy Z, Gamow's candidate for a graviton, travels explicitly at c like gravitational waves. Using the Heisenberg Uncertainty Principle, this makes the range of the weak interaction at energies less than 1.022 Mev, infinite, just like gravitational effects.Thank you for illuminating me, I did not know that about Z bosons. I confess to knowing little particle physics, having never taught it!

trinitree88
2009-Jul-27, 07:53 PM
Thank you for illuminating me, I did not know that about Z bosons. I confess to knowing little particle physics, having never taught it!

KenG. You're welcome. Learned it from Nigel Calder in his text "Key to the Universe".circa 1978. While the popular treatise often mention the resonances seen at the high mass/energy bumps, they are to be viewed as separate energy levels in the "spectrum" of Z's, much like Lyman, Balmer, Brackett, Paschen, Pfund series of spectral lines for the hydrogen atom ....most of them leave this little tidbit out. It's why I harp on Gamow's comment.
The recent thread I started on Koides formulas for masses of the hadrons by Carl Brannen and Marni Sheppeard gives such explicitly close experimental matches to the formulas, that one would have to expect discrete Fourier transforms are going to play a role in determining the mass of the putative Higgs. I've been quite surprised nobody has been wowed by that paper....fell like a feather. :shifty: pete

Jeff Root
2009-Jul-28, 01:32 AM
Learned it from Nigel Calder in his text "Key to the Universe".circa 1978.
That was the second book I took off the shelf to try to find anything
more about the Z particles. I looked in the index but there is nothing
for Z. The W is in there, though.

-- Jeff, in Minneapolis

NorthernBoy
2009-Jul-28, 11:45 AM
W and Z were both discovered at my lab. Wikipedia has a reasonable coverage of the story;

http://en.wikipedia.org/wiki/W_and_Z_bosons#Discovery

It is not surprising that a book written in the '70s is a bit sketchy on particles discovered in the '80s...

trinitree88
2009-Jul-28, 04:18 PM
W and Z were both discovered at my lab. Wikipedia has a reasonable coverage of the story;

http://en.wikipedia.org/wiki/W_and_Z_bosons#Discovery

It is not surprising that a book written in the '70s is a bit sketchy on particles discovered in the '80s...

NorthernBoy. Actually, at the time they were still calling it the W0, not the Z0...so technically they say nothing about the "Z", but there is, as I recall, on the bottom of a page, a sequence of fluctuating particles, first the charged W's then the neutral W. The sequence with the neutral W is used to describe the neutral currents which had been found in Gargamelle, the liquid cryogenic hydrogen bubble chamber at CERN in UA1, I believe....I haven't read the text since ~ 1981. Somewhere in the vicinity of that series of pages is a picture of a knock-on electron....pushed away from a neutral hydrogen by an unseen incoming neutrino acting via the neutral current, and then the neutrino exits also without leaving a trail....very ghost-like. Today, few people use W, most use Z. pete

StupendousMan
2009-Jul-28, 05:08 PM
An isolated system consists of a neutron star orbited by a planet like the Earth at a distance around 1 AU. The two objects are so far from other stellar systems that gravitational disturbances from other bodies are negligible. One can measure the orbital parameters of the system very precisely.

Now, very far away, a GRB-like explosion sends a very strong beam of electromagnetic energy towards this isolated system. The beam is collimated well enough that its effective diameter when it reaches the system is much smaller than the orbital radius. The beam passes "vertically" through the system, perpendicular to the plane of the orbit. It does not strike the neutron star. It does not strike the planet. The beam continues to flow through the system for several days at high intensity.

While the beam is shining through the system, do the parameters of the planet's orbit change? Again, there is no direct interaction of the photons in the beam with the planet, nor with the neutron star, because the beam passes through the empty space between the planet and the neutron star (but within the radius of the planet's orbit).

NorthernBoy
2009-Jul-28, 05:33 PM
The sequence with the neutral W is used to describe the neutral currents which had been found in Gargamelle, the liquid cryogenic hydrogen bubble chamber at CERN in UA1, I believe.

Yep, that's the one. It now sits on a stick (the chamber, not the particle) outside my old office.

I was there just a month or so ago actually, and took a photograph of it.

GuyHill
2009-Jul-31, 07:23 PM
Okay, but we still need a succinct, one-sentence, understandable-by-the-unwashed "answer" of why (to the best of current scientific know-how) a photon is "massless".

The answer to that question is "We do not know", as physics does not deal with "why" questions. It isn't even clear whether the question even makes sense.

A more interesting question would be "how do we know that photons are massless?"

I am not entirely sure of the answer to this question. I guess that it is impossible to show that photons are massless, as opposed to them having a wee bit of mass.

What we do know, however, is that photons move with velocity c. Or very nearly c, as there's always a measurement error. We also know that all photons travel at the same velocity. Up to a few parts in a billion or so (again, measurement error). We also know that photons of arbitrarily low energy exist (well, not exactly. We know that photons with very little energy exist), which implies that photons have zero mass. So, based on the experimental evidence, it is far from unreasonable to conclude that photons have zero mass.

On the theory side of things, the answer to the question is much more simple: in modern particle theory, the photon is massless. Needless to say, this theory is in very good agreement with experiment.

GuyHill
2009-Jul-31, 07:30 PM
An isolated system consists of a neutron star orbited by a planet like the Earth at a distance around 1 AU. The two objects are so far from other stellar systems that gravitational disturbances from other bodies are negligible. One can measure the orbital parameters of the system very precisely.

Now, very far away, a GRB-like explosion sends a very strong beam of electromagnetic energy towards this isolated system. The beam is collimated well enough that its effective diameter when it reaches the system is much smaller than the orbital radius. The beam passes "vertically" through the system, perpendicular to the plane of the orbit. It does not strike the neutron star. It does not strike the planet. The beam continues to flow through the system for several days at high intensity.

While the beam is shining through the system, do the parameters of the planet's orbit change? Again, there is no direct interaction of the photons in the beam with the planet, nor with the neutron star, because the beam passes through the empty space between the planet and the neutron star (but within the radius of the planet's orbit).

If I understand your question correctly, you are asking whether a beam of light has a gravitational effect. The answer to that question is yes, according to the general theory of relativity. The reason for this is that energy (or more technically, the energy-momentum tensor) is the source of curvature of spacetime. Mass is a form of energy, and therefore mass curves spacetime. Since light is also a form of energy, it curves spacetime as well.

I am not aware of any experiment to measure this effect, however. One way of doing this would be to have two laser beams at straight angles that almost, but not quite intersect, and then to measure the deflection of one by the other. I suspect that the effect would be far too small to be detectable, however.