# Electron change?

1. Apr 17, 2013

### michb

I was wondering if the electrons change to Tau electrons when an object reaches close to the speed of light. From my limited understanding, the mass as well as the energy increases to almost an infinite mass/energy. Tau electron have a heavier mass, right?

2. Apr 17, 2013

### Staff: Mentor

No.

When particle physicists talk about the mass of an elementary particle, it's always the "invariant mass" a.k.a. "rest mass" which is a fundamental property of the particle.

3. Apr 17, 2013

### michb

Thank you. :) Trying to learn and figured I would ask......

4. Apr 17, 2013

### michb

If the mass of an elementary particle is at the "rest" state, how does the mass increase so much when it travels near the speed of light?

5. Apr 17, 2013

### Parlyne

It doesn't.

Energy increases without bound as speed approaches c.

In the early days of relativity, some physicists found it convenient to define a quantity called "relativistic mass", which is really just energy divided by c2, because its use made the relativistic formula for momentum look like the classical one. But, there's no sense in which relativistic mass is actually the mass of the object.

6. Apr 18, 2013

### michb

Thank you so much. I really appreciate all the feedback :)

7. Apr 18, 2013

### Jano L.

That is not true. The thing is rather that there are some people who do not have anything better to do than to ridicule the notion of relativistic mass. But the notion is useful in many circumstances. This article explains it very well:

http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/mass.html

8. Apr 18, 2013

### Staff: Mentor

That's a bit too strong of a statement - relativistic mass is still useful for some problems. It is, for example, the easiest way of calculating the trajectory of a relativistic particle subjected to a transverse force where F=mrela works as expected. This might have been more interesting a century ago, when experimental verification of the simplest predictions of SR was still an important problem.

However this is a bit of a digression, as there's a real misunderstanding at play here:
None of the interesting relativistic effects - time dilation, length contraction, energy increase, mass increase if you're old-fashioned - are observed by the moving object itself. As far it is concerned, it is at rest while the rest of the universe is moving rapidly in the other direction... so nothing changes for it, and OP's original question about whether relativistic effects will cause the particle to change its basic attributes is misplaced.

9. Apr 20, 2013

### Parlyne

So basically, the article is saying that relativistic mass is useful for hiding the places where relativistic dynamics are actually different from what would be expected classically. Again, though, most of those cases come right back to insisting that momentum is mv. And, the invoking of GR is even worse; since, once you're in the context of GR, the quantity that would show up in the places that relativistic mass shows up in SR is not generically the same as γm.

The low point of the article, though, is where it claims that, in rejecting relativistic mass, one asserts that a heated object does not gain mass. This claim is, put lightly, a load of bovine excrement. In insisting that rest mass is the only thing that can validly be called mass, what one is really saying more technically is that mass is (up to appropriate factors of c, which I will ignore because we're cavalier like that in particle physics) the magnitude of an objects 4-momentum. This works just as well for composite objects as for fundamental ones; and, it will generally be the case that, in the frame where total momentum is 0, the mass of a composite object will be the sum of the energies of its constituent parts. (Well, again that's in SR; but, the basic point here holds generically.) When talking about a composite system like this, we generally call this the system's "invariant mass."

10. Apr 20, 2013

### Parlyne

Or, we could just admit that Newton's original formulation was maybe not the best way to express the dynamical idea in the first place. Even in classical physics we can find places where F = ma doesn't work. But, $\vec{F} = \frac{d\vec{p}}{dt}$ will; and, it will continue to work even in SR.

I think, to some degree, that the desire to keep relativistic mass around arises from the mistaken idea that energy and momentum are derived quantities (leaving the kinematic quantities, mass, and force as fundamental), when a better understanding of physics tells us that energy and momentum are the things that really have a deeper significance in the structure of physical reality.

11. Apr 20, 2013