# Is there such thing as rest mass.

1. Apr 23, 2010

### Simon43254

Is there actually a superfluous reason for rest mass. I only ask because clearly as far as I know, all particles have spin, and im led, (possibly falsly) to assume that so do things such as photons (although we know photons dont have rest mass anyway). Thus if "everything" has spin, (lets just leave this as nucleons for now) then it has an acceleration, and therefore isnt at rest at all even without it accelerating by defintion. So is it therefore conceivable to suggest this notion of what we describe as rest mass is either nonsensicle or perhaps false convention if it existed as terminology before spin was introduced to our knowledge??

2. Apr 23, 2010

### HallsofIvy

I think you are misinterpreting "spin". That is a quantum physics concept that does NOT mean the particle is actually "spinning" like a top. You also seem to be misinterpreting "rest". There is always some coordinate system in which a body is at rest.

3. Apr 23, 2010

### Simon43254

yes but what i mean is that spin refers to its angular momentum. Like how a gyroscope works. so clearly if it has any momentum or acceleration then it's total mass or atleast part of its mass can be said to be due to it's spin. So how can say a proton have a quoted rest mass if it's not strictly speaking at rest. Or is it my defintion of at rest that is the problem?

4. Apr 23, 2010

### Pengwuino

Why do you say this?

5. Apr 23, 2010

### Simon43254

The best way I can explain why I think this is by using an example. An electron has a finite, set, rest mass. Using Einsteins theory, as that electron approaches the speed of light, its mass increases. Thus a certain amount of its mass is due to the velocity it is traveling at. Now an object that has angular momentum, you can say has a tangential velocity, and therefore a centripetal acceleration, indicating that it's spin has contributed to if you like it's mass.

By the way, this was just a question, I don't know if I have any of this right or if I have gained a wrong definition of something along the way. If I'm going far out in the wrong direction please let me know where I am going wrong.