# Photons and Mass

1. Feb 6, 2012

### Andrew Aidan

What I have been taught in high school physics says that all particles can be interpreted as waves, and all waves as particles. By definition, particles have volume, and therefore must have mass (in a physical sense). Photons are always regarded as waves, but therefore could always be interpreted as particles. Because they're particles, wouldn't that mean that they have volume, and therefore mass? or do photons behave differently than other particles/waves?

2. Feb 6, 2012

### MrAnchovy

Why do you assume that anything with a volume has a mass? What is the mass of 1m3 of vacuum?

3. Feb 6, 2012

### zhermes

true

Less-true, but sure.

No. Nothing about the definition of a particle requires it to have volume in any concrete (or exclusive) sense. Generally electrons (for example) are modeled as points, and the realization that they seem to have finite size is a fairly recent one.
Furthermore, even if something has a volume, why must it have mass?

Photons are by definition the particle quanta of electromagnetic radiation; i.e. the counterpoint to the electromagnetic wave.

Photons and gluons are massless particles (the only ones we know of); and if there is a graviton, it is expected to be massless as-well.

4. Feb 6, 2012

### Staff: Mentor

The best way of thinking about it that I've seen is as follows. Light is an electromagnetic wave that happens to interact and transfer energy in discrete quanta called Photons.

5. Feb 7, 2012

### Andrew Aidan

Thanks, zhermes. I just always assumed that a physical body with volume had to contain mass, otherwise it wouldn't be considered a physical body but a lack thereof. I see my error now. But I have one more question, since we always saw electrons as points until recently, wouldn't the same realization be possible with photons? I just don't understand how a physical body can have volume but no mass.

6. Feb 7, 2012

### ZapperZ

Staff Emeritus
Whoa! Back off a bit. Where is this "until recently" part? What did we saw?

In fact, the latest experimental evidence in trying to find the electric dipole distribution of the electron saw NOTHING to change that view of a point particle with no internal structure!

In this forum, you cannot throw off statements like that without giving ample citations to back up such claims!

Zz.

7. Feb 7, 2012

### Andrew Aidan

But zhermes said:

I didn't make the original claim, I was basing my question off of a claim made by another user.

8. Feb 7, 2012

### zhermes

I was indeed mistaken about the most up-to-date experiments; but at the same time it seems there are always going to be strong lower limits on the electron radius---based on the de broglie wavelength and the schwarzschild radius (significantly larger than the planck-length, so it should still apply). Also, if the electron were truly a point-particle wouldn't the Abraham–Lorentz force be divergent?

But back to the overall point: there is no strict (or at least known) connection between mass and volume. There are possible theoretical lower limits to the density of a particle, but nothing really known. Also, something like a photon does have in connection with it the concept of occupying a certain region of spacetime, and thus some vague concept of a volume---while I don't think any specific definition could be made---and yet no mass.

9. Feb 7, 2012

### James_Harford

Let $R_S$ be the electron's Schwartzchild radius, $G$ the universal gravitational constant, $l_P$ the Plank length, $m_P$ the Plank mass, ${m}_e$ the electron's mass, and $\lambda_C = \frac{{h}}{{m}_e c} = \frac{l_P m_p}{m_e}$ its Compton wavelength.

Then,
$\begin{eqnarray} R_S &=& 2 {G} \frac{{m}_e}{c^2} \nonumber \\ &=& 2 ({\frac{l_P}{{m}_P}}c^2) \frac{\frac{l_P m_p}{λ_C}}{c^2} \nonumber \\ &=& 2(l_P)^2/\lambda_C \end{eqnarray}$

An electron's compton wavelength is quite a bit larger than the Plank length. Therefore, this pretty relationship shows that the electron's Schwartzchild radius is not larger then the Plank length, but vastly smaller.

Last edited: Feb 7, 2012
10. Feb 7, 2012

### ZapperZ

Staff Emeritus
You should be very concerned that you base your entire knowledge simply on what you read on here, AND, without any kind of support to validate such a thing.

Then submit a rebuttal to a peer-reviewed journal and get it published FIRST. Till you do that, and till we have other experimental evidence to the contrary, you have less of a leg to stand on to make such claims contrary to QED.

Zz.

11. Feb 7, 2012

### zhermes

Wow. Someone's cranky.
Clearly Andrew's entire world view is shaped entirely and exclusively by what he reads from forum posts.
@Andrew, my apologies for misleading you.

Yeah, I was clearly suggesting that my knowledge is publishable, irrefutable and a necessary improvement to QED. That's why I said something 'seemed' a certain way, and asked a question about another thing.

For the record, in Eric Poisson's reviews in Living Reviews of Relativity, and CQGra
(see: http://adsabs.harvard.edu/abs/2011LRR....14....7P), he points out the divergence of the Abraham-Lorentz force acting on a particle in curved space-time, specifically stating
The discussion is entirely out of my pay-grade (and even more so, my understanding), so I have no idea how it applies; but I thought I recalled the concept from a class at some point, and it seems the idea is published.

I think Robert Wald makes a similar point in his 'General Relativity'.

Until there is a quantum theory of gravity such points are purely academic; but at least the string theories I've heard of, again don't allow point particles.

ZzZzZz

12. Feb 7, 2012

### Andrew Aidan

That's ok, I tend to believe things when other people know what they're talking about and I have less of an understanding. My world view isn't shaped entirely by that, but when I don't understand it I tend to be more likely to believe others. I'm sorry if I upset anyone, that wasn't my intention.