PDA

View Full Version : massless particles?



Kebsis
2003-Jun-29, 09:57 AM
How is it that any particle is massless? Like light particles or anything. How can something exist without taking up space?

Kaptain K
2003-Jun-29, 12:19 PM
You are confusing mass with volume.

Kebsis
2003-Jun-29, 06:45 PM
So light particles do have mass? Then how can they move at the speed of light?

David Hall
2003-Jun-29, 07:03 PM
I think particle is kind of a misnomer. They're not exactly like little lumps of matter. The way I think of it (and I may be mistaken) is that photons and other massless particles are massless because they are pure energy. In the E=MC^2 equation they fall entirely on the "E" side, and therefore have no actual Mass to slow them down.

QuagmaPhage
2003-Jun-29, 07:17 PM
Sometimes it is best to think of light as waves that propagate in the electomagnetic field instead of particles.

The Supreme Canuck
2003-Jun-29, 10:52 PM
According to string theory (or more correctly, M theory) you can't think of any particle as a particle. All basic particles are actually loops of "super-string" that appear to be particles when observed from a distance. (Massive Over-simplification) Just adding a small amount of confusion to the argument. :wink:

If you're interested in this, I would suggest reading "The Elegant Universe" by Brian Greene. He explains it much better than I ever could. (He is a physicist after all!)

Senor Molinero
2003-Jun-29, 10:52 PM
ANOMALY ALERT!!!
If light (photons) are massless, then why does a gravitational lens work?
Why does light bend around massive objects?

IMHO light (and all EMR) follows the straight lines of the space-time continuum which itself is warped by massive (actually all) objects.

Simple questions do not always have simple answers. Much debate may be about to ensue.

Donnie B.
2003-Jun-30, 12:59 AM
Photons, like all particles (including macroscopic ones) exhibit both particle-like and wavelike behavior, depending on what you measure and how you measure it. It's not at all intuitive, since on our scale the wavelike behavior is unmeasurably tiny, so we have no direct experience of it. But it's real and pervasive in the subatomic realm.

It may seem weird, but we just have to get used to it. Besides, the computers we're communicating with rely on those weird properties to function, so I guess we should be glad they're there.

Donnie B.
2003-Jun-30, 01:01 AM
So light particles do have mass? Then how can they move at the speed of light?
No, they have no rest mass. However, they have no volume - they have no physical dimensions as, say, an electron does.

Tobin Dax
2003-Jun-30, 01:18 AM
However, they have no volume - they have no physical dimensions as, say, an electron does.

Do we have proof that electrons have a volume? Last I heard, there was only and upper limit on their size, and they were still being treated as point particles.

Tensor
2003-Jun-30, 01:58 AM
I think particle is kind of a misnomer. They're not exactly like little lumps of matter. The way I think of it (and I may be mistaken) is that photons and other massless particles are massless because they are pure energy. In the E=MC^2 equation they fall entirely on the "E" side, and therefore have no actual Mass to slow them down.

Not quite if I may pick a nit. Using realativiy theory, the full equation is E^2=(mc^2)^2+(pc)^2 where p is the momentum. Since the equation is normally used to describe a mass at rest, it reduces to E = mc^2. When dealing with photons (or other massless particles), the equation reduces to E=pc. The momentum of a photon can be expressed as p = hf/c (f is the photons frequency and h is Planck's constant.). From this, you can see that the momentum of a photon is based on its frequecy, as is its energy, once the momentum is multiplied by c.

Kebsis
2003-Jun-30, 03:03 AM
So light particles do have mass? Then how can they move at the speed of light?
No, they have no rest mass. However, they have no volume - they have no physical dimensions as, say, an electron does.

Well thats where my question is...if they have no physical dimensions, then how can they exist?

The Supreme Canuck
2003-Jun-30, 03:32 AM
A blackhole's singularity has no physical dimensions yet it still exists.

Kebsis
2003-Jun-30, 04:16 AM
I know they exist. I'm asking how is it that something with no dimensions exists? How can something, energy, 'be' without taking up space somehow?

The Supreme Canuck
2003-Jun-30, 04:18 AM
Oh, got what you're saying now, sorry. Unfortunately, I don't have an answer. Sorry again.

Grey
2003-Jun-30, 10:47 AM
Do we have proof that electrons have a volume? Last I heard, there was only and upper limit on their size, and they were still being treated as point particles.
You're right. Any interactions with an electron that we've observed have suggested that it's a point particle.


I know they exist. I'm asking how is it that something with no dimensions exists? How can something, energy, 'be' without taking up space somehow?
Actually, it's kind of the other way around. To our best estimates, none of the elementary particles actually take up space. The only things that do take up space are composite particles, and this just seems to be because of the separation of their components. So, protons and neutrons show a physical size, but it seems to be because each is actually made of three quarks. Quarks themselves, along with electrons, neutrinos, photons, and so forth, show no evidence of having any physical extent.

So it may be that there aren't any particles that "take up space"; space is just a meaure of the separation between particles. If string theory is valid, everything has a length (although even that is orders of magnitude below our ability to measure), but still no width or depth, so we still run into this issue in some sense; even when we properly extend this to the extra dimensions string theories require.

It's perhaps not very satisfying that it doesn't follow our common sense, but why should we expect the universe to obligingly be easy to imagine? :D We should instead be asking why some particles appear to have a volume, since it's "natural" for particles to have no physical extent. What makes objects seem to be solid and occupy volume isn't really the size of the things making them up, it's the forces that control how close the composite elements can get to each other. At every level we observe, everything is mostly empty space.

tracer
2003-Jun-30, 06:34 PM
Do we have proof that electrons have a volume? Last I heard, there was only and upper limit on their size, and they were still being treated as point particles.
The thing that makes this so tough is all of that quantum-mechanical position uncertainty stuff. You can't measure exactly where an electron is at any given moment (unless you measure in a way that makes its momentum completely uncertain) -- you can only measure where the center of the probability distribution is for the electron's position at a given moment. And, of course, the more accurate this measurement is, the less accurate your predictions of where it'll appear next are going to be.

eburacum45
2003-Jun-30, 06:42 PM
What is wierder is that photons do have effective mass- when they are moving away from each other;
the light in the universe has a slight gravity which helps to counteract the expansion of the universe.

I don't know why one photon has no mass and two photons do- it's something to do with the energy potential or something...

Kebsis
2003-Jul-01, 07:45 AM
Oh, got what you're saying now, sorry. Unfortunately, I don't have an answer. Sorry again.

No problem, I appretiate your trying to answer it. The confusion is more my fault, since I often have trouble wording questions about things I have little knowledge of.

It's hard being interested with how the universe works and not knowing any of the terminology :(

mike alexander
2003-Jul-03, 11:28 PM
I've thought about this one for a while. Since a photon has no rest mass when one is formed it can 'accellerate' instantaneously to lightspeed (that is, as long as a photon exists it travels at lightspeed.) Since at true lightspeed temporal duration goes to zero, does this mean that a photon exists for zero time in its reference frame? And is this the same as saying that in its own frame a photon doesn't exist at all?

I like the idea that a particle can only exist outside of itself.

DStahl
2003-Jul-04, 12:25 AM
The answer I've read to "how a photon sees time" is that it's a meaningless question, like asking for the square root of blue. There's a bit on John Baez's Physics FAQ site that talks about it in more concrete terms than I can muster right now.