# My question is simple: Are there any massless

1. Jan 12, 2012

### Allojubrious

My question is simple: Are there any massless

My question is simple: Are there any massless particles?

Because I've now found out that the photon, which I always thought to be massless, actually has an extremely small amount of mass. It is something along the lines of a billionth of a billionth of an eV. Anyway, now I'm wondering whether there are any actual and true massless particles.

Thanks,
Al

2. Jan 12, 2012

### Pengwuino

Re: Massless-ness??

What you probably saw was an experimentally determined upper bound. You can never say any property of a particle or really, anything, has a given value to an infinite amount of precision. There exists no experiment where the velocity of light can be shown to be exactly zero. There are always experimental inaccuracies.

When a source says that the mass is $m_{photon} < 10^{-18} {{eV}\over{c^2}}$, that means an experiment was done that can measure a photon mass of no less than $10^{-18} {{eV}\over{c^2}}$ but was unable to detect a mass.

3. Jan 13, 2012

### Allojubrious

Re: Massless-ness??

Oh I see. That is interesting.
So then there really are not any massless particles that exist?

4. Jan 13, 2012

### Pengwuino

Re: Massless-ness??

No, that's not what I said. The photon could be massless, but there exists no experiment that could tell you with certainty that the photons mass is 0.0000000..ad infinitum. All you can say is that "if the photon has a mass, it has to be less than X because X is the lowest mass we can measure by experiment". Every experiment will have a threshold as to how small a mass it can measure. The photon has always been below those thresholds so we say it appears to be massless with the caveat that we can only measure with a certain precision.

Imagine trying to measure the speed of a car at rest. Let's say you point a radar detector at it. Now, that radar detector has a certain precision; let's say it can only measure a speed to within 0.1mph. If you point the thing at a car and it registers 0, it's not technically proper to say that the car is moving at 0mph, you can only really say that it's moving at < 0.1mph because that's all your radar detector can tell you. It may be truly at rest, but you can't build a radar detector to measure with perfect precision.

5. Jan 13, 2012

### Allojubrious

Re: Massless-ness??

Ohhhhhh now I get it, well thank you very much, you have answered my question.