Physics Questions

Hi, I’ve got some questions for anyone well-versed in physics:

Do photons have mass? Do they have no mass at rest? Why would they have mass moving but not at rest, if this is the case?

Are all forces (IIRC, there are only 4 forces: Electro-Magnetic, Gravity, Strong and Weak Nuclear… is that right?) conveyed by particles? If photons convey the EM, what particles convey the other three?

What exactly are Quarks, Leptons, Mesons, and Bosons?

If possible, could someone direct me to a source understandable by someone wayyyy out of their field?
:confused:
-Steve


“Heyyyy sexy mama! Wanna kill all humans?” -Bender, Futurama

“Your game shows reward knowledge. Ours punish ignorance!” -The Simpsons

I’ll take a shot at some of them:

Yes, but only becaase of the energy they carry while they are moving.

As far as we can tell, no. Also as far as we can tell, there is no way for a photon to actually be at rest. They always travel at “c”, the speed of light in vacuum. Over macroscopic distances, they apppear to move slower through other media (air, water, …) but actually do not; they interact with the atoms of the media and the apparent slowing is due to time lags between absorption and emission of photons.

Ah, why is a really tough question, and there are several widely-held but differing opinions. I believe Feynman’s explanation, which is roughly:

We don’t kow why, and will never know why. The quantum world is so weird that we have no hope of understanding it in terms of our everyday experiences. All we can do is accept that it is and we can predict it mathematically.

We think so. Photons carry the electromagnetic force, gravitons (which have never been oserved and the theories about them are very far from complete) carry gravity, W and Z particles carry the weak nuclear force, and gluons carry the stron nuclear force.

That’s worthy of a long dissertation. Try Elementary Particles, The Particle Adventure, The Sub-Atomic Zoo

I hope some of the links I posted, and some of the sites to which they link, help. If you are interested in a book, I feel compelled to reccomend QED: The Strange Theory of Light and Matter, even though it is peripheral to your direct questions.


jrf

Many, many thanks for your helpful response! The websites were useful, and I’ll look into the Feynman book. You’ve helped clear up a lot of stuff for me.
:slight_smile:
-Steve

The concept of a “moving mass” and a “rest mass” is a bit misleading. Photons have momentum, but not mass.

Momentum happens to look a whole lot like mass, but it does not carry the same gravitational consequences.

I read and loved ‘The Particle Garden,’ very easy reading and will explain alot about subatomic particles.

Quarks: fundamental fermion particles which undergo strong interactions. Six flavors: up, down, charm, strange, b (bottom or beauty, and top.

Leptons: fundamental fermion particles which do NOT undergo strong interactions. Six flavors: electron, muon, tau, electron neutrino, muon neutrino, tau neutrino.

Meson: a particle composed of one quark and one anti-quark.

Baryon: a particle composed of three quarks. Anti-baryons are composed of three anti-quarks.

(Quarks are never seen alone.)

Bosons: particles which have integer spin, and thus all may have the exact same wave function, and you may get a Bose condensate.

Fermions: particles which have half-interger spin, and thus can never have the exact same wave function, which is why you can only have 2 electrons (1 spin up and 1 spin down) in an atom’s “shell”.


-k-
Karen Lingel, Physicist

Photons have energy. Energy is mass. Mass has gravity.

Photons have been observed to change direction in a gravitational field. Indeed, this phenomenon was the first experimental verification of General Relativity, when, during an eclipse, the position of stars were observed to change as their photons passed close to the sun.

Sorry, accidentally deleted this from the previous post.

Photons do not have rest mass. This is why they can go exactly the speed of light (indeed, photons are the light the speed of which we’re measuring). Photons, however, never exist “at rest.” In any and all reference frames, an observer will always measure their speed at the same value. It’s General Relativity which describes the mass/energy and speed of light.

186,000 MPS: It’s not just a good idea, it’s the law!

Einstein proposed that light has more mass than you might think. If light has mass than it can be effected by gravity. This is why light does not escape black holes.

We haven’t discovered the exact particle which mediates gravity, but if we do it we do it will be called the gravitron. The strong force is carried by gluons (cause they’re like the most perfect imaginable glue). And the weak nuclear force is carried by weak bosons.

                        Later...

If at first you don’t succeed you’re about average.

Also, there was once a force called the electroweak force very early on (in the universe). It only exists at extreeeemly high energy levels.


If at first you don’t succeed you’re about average.

You can also read the sci.physics FAQ, which is available in several places, including here.


“No,” he replied, and smiled seraphically, as was his wont.

It is wrong to say that photons always travel at c (the speed of light). In fact they only travel at c in vacuo (break out your physics text and check). In experiments published in Science magazine recently (February 2000 I think), researchers succeeded in slowing light to about 5 mph.

As for the mass question, that has always troubled me, too. And it doesn’t help to say that it’s simply momentum, since momentum is MASS X kinetic energy). Worse, it should be impossible to bring ANY mass to the speed of light, because this would theoretically require infinite energy. So why does light bend in a gravitational field?

The best answer I can come up with is that it is difficult for mere non-physicists like us to grasp the idea that energy=mass. Photons do have measurable energy (ever get a sun tan?), so from this point of view, they must have mass (whatever that means).

If you find a better answer, please let me know!!!

In experiments published in Science magazine recently (February 2000 I think), researchers succeeded in slowing light to about 5 mph.

As for the mass question, that has always troubled me, too. And it doesn’t help to say that it’s simply momentum, since momentum is MASS X kinetic energy). Worse, it should be impossible to bring ANY mass to the speed of light, because this would theoretically require infinite energy. So why does light bend in a gravitational field?

The best answer I can come up with is that it is difficult for mere non-physicists like us to grasp the idea that energy=mass. Photons do have measurable energy (ever get a sun tan?), so from this point of view, they must have mass (whatever that means).

If you find a better answer, please let me know!!!

Duh!

If at first you don’t succeed you’re about average.

I stand by my original statement. Photons travel at c and appear to move slower in physical media because of interactions.


jrf

Could you provide an example or reference?

I am not aware of any situatuion in which you cannot regard E = mc^2 as an identity rather than a mere equality. Energy is mass is energy; there’s no way to tell the difference. We seem to see a difference, but that’s because our senses are inadequate.


jrf

If something has momentum, doesn’t it have to have mass, by definition?

Sorry kids, photons do not have mass. No mass. Sorry.

You need to learn a lot of quantum mechanics to get it all to gel in your own minds. Here are some hints.

Photons do have momentum. In quantum mechanics, momentum is a consequence of the wavelength of the quantum wave function. Since photons are also light waves, there is a momentum associated with the wavelength. This does not mean that photons have mass, any more than saying an object which has potential energy also has speed.

Photons appear to be affected by gravity. This is not because they have mass, but because gravity “warps” space, so it looks like photons are bending. Photons, by definition, travel in straight lines.

Saying E= mc^2 is saying that mass and energy can be interchanged, not that one is the other. For instance you wouldn’t say that money = VCRs, although you can trade money for VCRs and trade VCRs for money. Similarly, potential energy can be exchanged for kinetic energy which can be exchanged for heat energy, etc. The (kinetic) energy of very high energy photons can be exchanged for mass energy, for instance, when an X-ray passes through a metal and creates an electron-positron pair.

Finally, that photons have no mass is a requirement of relativity and particle physics.

No mass.

As for experimental evidence, as of 1998 the limit on the photon’s mass was < 2.E-16 eV. For comparison, the mass of an itty bitty electron is 511 eV.


-k-
Karen Lingel, Physicist

Karen Too I understand that photons have no invariant mass, but I thought that they have relativistic mass and so affect and are affected by gravity. I’ve got a question for you. If matter and antimatter contained in an insulating box (say an airplane’s black box) combine producing photons, what happens to the gravitational field surrounding the box? Are you saying the gravitational field would disappear?

The following is from “Introducing the Black Hole,” Physics Today 24 (1971) by John A. Wheeler and Remo Ruffini:

The above (at least to me) implies that photons do have mass-energy and affect gravity.

I always understood E=m*c^2 to mean that energy and mass are really manifestations of mass-energy. More like pennies and dollars are both money than like dollars and VCR’s.


I am not a physicist. I am just trying to understand.


Virtually yours,

DrMatrix