# Einstein and Relativity

1. Feb 6, 2013

### quicksilver123

Was Einstein aware of the implications of relativity (such as length contraction) before he concocted it? Or were these implied concepts just happy accidents?

2. Feb 6, 2013

### mathman

The Lorentz contraction was known (that's why it has that name). Special relativity supplied an explanation.

3. Feb 6, 2013

### quicksilver123

Is it true that no experimental evidence exists for length contraction?

4. Feb 6, 2013

5. Feb 6, 2013

Thanks

6. Feb 6, 2013

### PAllen

This, from wikipedia, is actually a pretty good write up, with references for validation:

http://en.wikipedia.org/wiki/Length_contraction#Experimental_verifications

There are many indirect validations. One of the simplest is the same muon data used for time dilation. From a frame in which the muon is at rest, the sole reason the muon reaches the ground is length contraction.

7. Feb 6, 2013

### quicksilver123

Thanks; that was an interesting read.
I recently learned about time dilation through the muon experiment, so thy was helpful.

8. Feb 7, 2013

### quicksilver123

relating back to my first post, what about relativistic mass?

9. Feb 7, 2013

### ZapperZ

Staff Emeritus
10. Feb 7, 2013

### HallsofIvy

What about it? Your first post said nothing about relativisitic mass. If you are asking if Einstein was aware that his theory implied that mass was dependent upon frame of reference, yes, he derived that equation along with the Lorentz contract. Now, however, we would talk about "mass-energy" and use the term "mass", alone, to refer to rest mass only.

11. Feb 7, 2013

### Staff: Mentor

I assume you're referring to the historical aspect of relativistic mass, that is, did the concept of relativistic mass, or something similar to it, exist before Einstein published his theory? Apparently the answer is "yes."

http://en.wikipedia.org/wiki/Mass_in_special_relativity#The_relativistic_mass_concept

(with the usual caveats about the reliability of Wikipedia)

12. Feb 7, 2013

### quicksilver123

Yes, that's what I was asking. I'll have to learn more about Lorentz since Einstein seems to have looked to him... and he's not taught to us in this course.

We're being taught in this course that relativistic mass is the way things will be taught in high level physics. Apparently this isn't true.

I've read over the link zapper posted (just the first page) and it seems that relativistic mass isn't used at all anymore?
Instead, "proper mass" or stationary/rest mass is used instead. This seems to make things simpler but why even bother teaching relativistic mass then?
I'm told that the reason a particle cannot be accelerated to "c" is because of relativistic mass. As the velocity approaches "c," the mass will approach infinity; and one cannot accelerate an infinite mass.
Does a mass at a high velocity (relative to the speed of light) have a gravitational effect due to the increased "relative mass?"
In short - what's the point?

/relativisticmassnihilism

13. Feb 7, 2013

### quicksilver123

A better question would be:

are relativistic mass calculations out of favour, Lorentz transformations being used instead?

14. Feb 7, 2013

### Staff: Mentor

Inertia. Not the physics kind of inertia, but the human / psychological / cultural kind. Many or most commonly-used introductory physics textbooks date back to the 1950s or 1960s, with revisions since then, and probably were not written or updated by people who actually use relativity in their daily physics work.

When I was in graduate school in the late 1970s to early 1980s, working in experimental high-energy particle physics, where most particles are relativistic, nobody I worked with used or mentioned "relativistic mass." To us, "mass" always meant "rest mass" a.k.a. "invariant mass." But how many particle physicists write introductory general-physics textbooks? Real textbooks, that is, not pop-sci books?

The introductory modern physics book by Beiser (6th ed., 2003) uses only rest mass, but mentions relativistic mass in a sidebar paragraph, as "the view often taken in the past, at one time even by Einstein." An earlier edition of this book (late 1980s) did use relativistic mass.

I also looked in both of the upper-level undergraduate electromagnetism textbooks that I have at hand. Pollack & Stump (2002) use only the rest mass, and in a footnote comment that "In some discussions of special relativity, a velocity-dependent mass is introduced. In our treatment of the theory, m always denotes the rest mass [...]" Griffiths (3rd ed., 1999) comments that "Einstein called $m_{rel} / \sqrt{1 - u^2/c^2}$ the relativistic mass [...] but modern usage has abandoned this terminology in favor of relativistic energy: $E = mc^2 / \sqrt{1 - u^2/c^2}$." He then adds in a footnote: "Since E and mrel differ only by a constant factor c2, there's nothing to be gained by keeping both terms in circulation, and mrel has gone the way of the two-dollar bill."

Last edited: Feb 7, 2013
15. Feb 7, 2013

### Staff: Mentor

Lorentz transformations have been in favor all along and they aren't going anywhere. They're how you relate the notions of time and distance between observers; the time dilation and length contraction formulas are derived from them; and they're essential to any understanding of SR.

The equations of SR do allow us to define the relativistic mass mrel=γm0 where m0 is the rest mass of an object. In the early days of SR it seemed natural to use this quantity because it fit in naturally with the earlier work referenced in that wikipedia article.

However, the relativistic mass fits very awkwardly into the newer mathematical formulation of SR based on Minkowski spacetime. In that framework it's much easier to work with a constant rest mass and a momentum that varies with velocity:

E2 = (m0c2)2+(pc)2

The two things to rememebr here are:
1) The m in E=mc2 is the mrel defined above
2) For an object at rest, m0 is equal to mrel so E=mc2 is consistent with more modern formulation. They're different ways of representing the same physics.

Last edited: Feb 7, 2013
16. Feb 7, 2013

### Staff: Mentor

I like that quote! We should make a "relativistic mass" fee. In order to use the term on this site you need to send a two dollar bill to Greg Bernhardt.

I just can't decide if it should be a per use fee or an annual fee.

17. Feb 7, 2013

### Staff: Mentor

I've now looked at a couple of introductory physics textbooks on my shelf. Halliday / Resnick / Walker's "Fundamentals of Physics" (6th ed., 2003) does not mention "relativistic mass" at all, only the invariant mass which it simply calls "mass". Likewise with Knight / Jones / Field, "College Physics" (2nd ed., 2013).

18. Feb 7, 2013

### BruceW

yeah, I think that once someone is used to the ideas of relativity, you prefer the term 'Energy' to 'relativistic mass'. But I guess the term 'relativistic mass' is useful for explaining relativistic phenomena to someone who is not so familiar with relativity.

For example, when a particle is accelerated, its relativistic mass increases, and it becomes increasingly difficult to increase its speed further. This is intuitive, because we used the word mass. People who don't know relativity can relate the idea of relativistic mass to the idea of how difficult it is to change the speed of something.

If we used the word 'Energy' instead, we are saying the same thing, but someone who doesn't know relativity probably won't make the connection as easily to the idea of how difficult it is to further increase the speed.