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## Main Question or Discussion Point

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

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

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mathman

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

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From the sticky threadIs it true that no experimental evidence exists for length contraction?

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Length_Contraction

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Thanks

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PAllen

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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.

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I recently learned about time dilation through the muon experiment, so thy was helpful.

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

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

https://www.physicsforums.com/showthread.php?t=642188

Zz.

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HallsofIvy

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jtbell

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

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

(with the usual caveats about the reliability of Wikipedia)

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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.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.

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 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."

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

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

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jtbell

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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.why even bother teaching relativistic mass then?

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

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Nugatory

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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.

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

The equations of SR do allow us to define the relativistic mass m

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:

E

The two things to rememebr here are:

1) The m in E=mc

2) For an object at rest, m

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Dale

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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.He then adds in a footnote: "Since E and m_{rel}differ only by a constant factor c^{2}, there's nothing to be gained by keeping both terms in circulation, and m_{rel}has gone the way of the two-dollar bill."

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

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jtbell

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BruceW

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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.

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