massless particles

[The Wrath]

Can someone explain to me (in layman's terms please) how a particle can have no mass? Supposedly, some theoretical particles like the graviton have no mass. Well, if they have no mass, then how can they be said to even exist? The only explanation I can think of is that they are made of energy...but can you really have a particle made out of energy? Even if you can, isn't matter basically just a condensed form of energy?

[LaszloWalrus]

The short answer: it can't. This is a sad consequence of the irrational epistemology of modern physics. Likewise, energy applies only to mass.

[bobsponge]

The short answer: it can't. This is a sad consequence of the irrational epistemology of modern physics. Likewise, energy applies only to mass.

Isn't the theory of the massless graviton particle a result of the math coming out to not work unless the massless particles exist? Either the math is flawed, or the graviton exists. And so far the math has been pretty good. I'd say that is a pretty good case for the graviton, as absurd as it may be.

[DavidOdden]

Can someone explain to me (in layman's terms please) how a particle can have no mass?

I'm not sure that that's a truly sensible request (nothing personal intended there), that is, I think that when you ask a question like that, it gets you square into non-layman's concepts. Such as, "what is 'mass'?" -- it's not weight. This page addresses the problem of the photon being massless, and it points out that mass has two definitions. Under the old definition, a photon has mass, and under the new definition, the familiar equation "E=mc2" is wrong.

I take it that you see a problem in the concepts of "mass" and "particle", so that for some reason, being a particle in some way entails having mass. I don't have that belief, but maybe that's because I haven't thought much about the concepts of "mass" and "particle". I am fairly convinced that there is good scientific reason to believe in photons and to believe that they are massless, which tells me that "massless particle" isn't an obvious contradiction.

[The Wrath]

To me "mass" is anything made of matter that takes up space. If something is massless, it isn't made of matter, and takes up no space. That's what doesn't make sense to me.

[DavidOdden]

To me "mass" is anything made of matter that takes up space. If something is massless, it isn't made of matter, and takes up no space. That's what doesn't make sense to me.

Ah. Well, gauge bosons (photons being an example) aren't made up of fermions, so under one definition of "matter", they aren't made up of matter. I'm not sure about this idea of "taking up space", but an electromagnetic field would seem to me to intuitively "take up space" without being made of matter or having mass.

[Adrian Hester]

To me "mass" is anything made of matter that takes up space. If something is massless, it isn't made of matter, and takes up no space. That's what doesn't make sense to me.

Which brings up the next question, In your view, what's matter?

[DavidV]

From a layman's perspective, we make various inductive conclusions, such as that all material objects have mass. However, without expertise in theoretical physics, we cannot make any such conclusions about realms that we have no direct perception of.

Philosophy can only exclude certain possibilities - such as that a particle both has and does not have mass. It cannot tell us a priori whether all particles have mass. Making assumptions about the microscopic level based on macroscopic observations is a reasonable place to start, but to treat such conclusions as axiomatic is a violation of context. "Mass", like "color", is a concept based on macro-level observation. On the quantum level it may denote a complex combination of concepts, just as color refers to the distribution of electromagnetic radiation given of by certain particles when they interact with photons.

For example, it is reasonable to begin with Newtonian physics when studying objects moving at relativistic speeds - but dogmatically applying Newtonian laws removed from the context they were formulated in blinds us to the bigger picture provided by relativity and quantum theory.

Edited September 19, 2006 by GreedyCapitalist

[punk]

The short answer:

There is no such thing as a massless particle and you have misunderstood what physicists are saying.

Never in nature will you ever encounter a massless particle.

You will (however) encounter particles with zero REST MASS.

Rest mass is different from mass.

All zero rest mass requires is that the theory state that you are never able to encounter the particle at rest. That is to say the zero rest mass particle will always be in motion relative to any observer.

A zero rest mass particle will always be observed to have mass, since it will always be observed in motion.

So take a photon (zero rest mass) which has frequency f, so that the energy of the photon is given by:

E = hf

Then its observed mass is:

mc^2 = hf

Or:

m = hf/c^2

This is all satisfied by requiring that a particle with zero rest mass move at the speed of light, which is the same as saying it will move at the speed of light relative to any observer (since observers can never go the speed of light).

[Robert J. Kolker]

Can someone explain to me (in layman's terms please) how a particle can have no mass? Supposedly, some theoretical particles like the graviton have no mass. Well, if they have no mass, then how can they be said to even exist? The only explanation I can think of is that they are made of energy...but can you really have a particle made out of energy? Even if you can, isn't matter basically just a condensed form of energy?

Zero rest mass. What particles like photons lack in rest mass (they are never at rest) they make up in energy. Einstein's well known equation specifies the equivalence between mass and energy. That is why light gravitates (see prediction of Light Bending by General Theory of Relativity).

Bob Kolker