Talk:Mass in special relativity

Physics: Relativity Start‑class Mid‑importance

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Vague Sentence

I think this sentence is a bit vague: "The invariant mass is calculated excluding the kinetic energy of the system as a whole, while the relativistic mass is calculated including it." Although this sentence is true, it might lead readers to believe that invariant mass completely excludes kinetic energy, when in fact, it is important whether the constitute parts of a system are actually moving with respect to each other. Any opinions on whether this sentence should be changed? --Armaetin (talk) 08:27, 10 March 2009 (UTC)[reply]

Well, it does say kinetic energy of the system as a whole. What this means is kinetic energy of the system-as-a-whole as calculated using the velocity of the center-of-mass. I'll tweak it. S B H arris 02:37, 25 October 2009 (UTC)[reply]

Okun references this article

In his article in the May issue of AJP, Okun includes a reference to this wikipedia article. /Pieter Kuiper (talk) 22:27, 14 May 2009 (UTC)[reply]

Experimental proofs missing

Article does not contain any experimental proofs or corresponding references. The "proofs" included are only theoretical derivations. Article should contain the proofs of mass increase and experimental proofs of quantitative validity of relativistic mass equation. I am not saying, that the relativistic mass physical effect does not exist. I am saying, that there are no experimental proofs of quantitative validity of relativistic mass equation. Without that relativistic mass equation is only a theory. Softvision (talk) 17:55, 12 August 2009 (UTC)[reply]

Even if "relativistic mass equation" were only a wild hunch, if it is sufficiently referenced, it is ok. Sufficient references are provided for "relativistic mass equation" as the consequence of a theory, so it is ok. Now, look at the pointers on your talk page and start aquainting yourself with the Wikipedia policies. In other words, please stop abusing article talk pages. Thank you. DVdm (talk) 18:37, 12 August 2009 (UTC)[reply]

DVdm, I do not consider you as serious person. Experimental proofs are pride of any theory. If such experimental proofs exist, it could be useful to present them in article. Softvision (talk) 18:54, 12 August 2009 (UTC)[reply]

Yes, If you have a reference for an experimental proof, then provide it, but don't complain about the article not having one.

Other han that, I don't really care how you consider me. Just start trying to consider the Wikipedia policies as serious policies. DVdm (talk) 19:01, 12 August 2009 (UTC)[reply]

Ok. Message to editors : Please add experimental proofs of quantitative validity of relativistic mass equation, it would enhance the quality of the article. The same relates to relativistic momentum. Softvision (talk) 19:06, 12 August 2009 (UTC)[reply]

DVdm, this is the proof ? I know that you don't care. Softvision (talk) 19:38, 12 August 2009 (UTC)[reply]

This article has become amazingly POV

This article used to be a good example of how a fair and neutral discussion of such a topic should be - which is also what Wikipedia stands for. But although this HAS BEEN so about one year ago (see the archives), now it is undeniably making propaganda for a Single Point Of View, complete with manipulative claims like "The term relativistic mass has also been used historically" (erroneous, as at least last year in discussions here it was fund to still be in use!) and "mistakenly used that way by some" which constitutes Original Research. In short, currently this article may serve as a good counter example of how a Wikipedia article should NOT be (see WP:NPOV of what needs to be done in order to respect Wikipedia requirements). Harald88 (talk) 07:58, 21 October 2009 (UTC)[reply]

Go ahead, help yourself and then remove the POV-tag. - DVdm (talk) 08:54, 21 October 2009 (UTC)[reply]

OK, I admit that my approach to ask others to do the corrections sounds a bit lazy - it's just that this month I'm very busy. Harald88 (talk) 15:52, 22 October 2009 (UTC)[reply]

DVdm were you being sarcastic when you gave the above link to authors who themselves are apparently not able to do a Google book search on relativistic mass? Anyway, as a reminder I copy here again what may well be the most recent (although not peer reviewed) statistics, provided by an anti-relativistic mass author: [1]. Amazingly, although slowly less physics books use "relativistic mass", he also found that in the period 2000-2005 there were twice as many books on relativity that used relativistic mass than books that did not! Harald88 (talk) 17:36, 31 October 2009 (UTC)[reply]

We could do one more re-write, if you want to treat both types of energy and mass equivalently. There are two types of mass: relativistic mass (M_r) and invariant/rest mass (M_i). The last is Lorentz invariant, the first is not, but both are conserved for single observers, through time, in isolated systems. To go with them, there are two types of corresponding energy: relativistic/total energy (E_r) and rest/invariant energy (E_i).

Now the equation E = mc^2 (or E = m if you use units c=1) is generally true so long as you remember to equate the right types of energy with the right types of mass, so E_r = M_r, and E_i = m_i. All you need to remember is that when mixing the terms, you need to add momentum terms to get from E_r to m_i, and thus, all four energy and mass types are equal only when total system momentum is zero:

E_r = M_r = E_i = m_i (when p = 0). Which handily leads to E_r = m_i in the COM frame only, something that leads to much confusion for those who think E_r = m_i must be generally true.

Anybody interested in a re-write that treats these all on an equal footing, from the beginning?

S B H arris 19:04, 20 December 2009 (UTC)[reply]

Math Font for Greek symbols

Momentum is generally represented by the Greek rho (ρ) rather than the lowercase Latin p, but there is no lowercase rho available in the math font without it throwing a parsing error. This seems like a critical weakness given the abundance of lowercase Greek letters within mathematics. —Preceding unsigned comment added by Fulvius (talk • contribs) 11:02, 4 January 2010 (UTC)[reply]

As you can verify in the article and in any physics book, momentum is generally represented by the Latin p. DVdm (talk) 11:23, 4 January 2010 (UTC)[reply]

style of article

This article is in my opinion not intelligible to the intelligent layperson for whom it is presumably intended, as experts will have their own sources. It may be a difficult topic, but the excellent article on the (not unrelated) Twin Paradox shows that it can be done. Escoville (talk) 16:09, 19 June 2010 (UTC)[reply]

What do you think of mass energy equivalence, which can be read in parallel, or as an introduction? This article is basically about mass-energy equivalence but with the additional complication of needing to explain the two kinds of mass-definition (rest/invariant mass vs. relativistic mass). That inevitably adds complexity here. S B H arris 18:20, 19 June 2010 (UTC)[reply]

Fundamental problem with this article and related ones

Conservation of mass is a concept which is a bit of a cheat with respect to today's understanding of energy, mass, and the mathematical descriptions of the particles and interactions which constitute a physical system. To get an idea of what I am talking about, please see this short article by Frank Wilczek.

To quote Wilczek, "Mass is a property of isolated particles, whose masses are intrinsic properties -- that is, all protons have one mass, all electrons have another, and so on. [...] There is no separate principle of mass conservation." This is evident in the treatment of mass in quantum theory, where a mass is a multiplicative factor in an interaction term of the Lagrangian. Mass has a completely different character than energy, which is an eigenvalue of the Hamiltonian. Conserved quantities in quantum theory are observables (Hermitian operators) which commute with the Hamiltonian, and their eigenvalues are the possible measured values of these conserved quantities. Mass measurements are not to my knowledge quantities which can be represented as eigenvalues of such operators, and therefore mass is not a fundamentally conserved quantity. Tim Shuba (talk) 02:49, 1 October 2010 (UTC)[reply]

Invariant mass is obviously conserved, since it's invariant. It can't change in any frame, so long as nothing is added or removed from the system. Relativistic mass is frame-dependant, but it's just (relativistic energy)/c^2, so it's as conserved as the energy is. No, there's no separate principle of mass conservation, as it's the same thing as energy conservation (no matter what sort of energy you're talking about). See the Mandelstam variables. The invariant mass is just the square root of Mandelstam's s. S B H arris 03:35, 1 October 2010 (UTC)[reply]

I agree. The article by Wilczek is more of an interest in order to understand the classical limit, than special relativity. About measurement, I would say that the four-momentum p of a system is obviously a bona fide observable that commutes with the Hamiltonian of an isolated system (because the lagrangian should be invariant under translations) and we have that m^2 = p^2, so mass is obviously an observable. But it could be that I need to rethink my argument about the classical limit posted here. Count Iblis (talk) 03:53, 1 October 2010 (UTC)[reply]

The momentum p as an operator is an observable, but p = -i hbar d/dx. Much different than the classical four-momentum. Tim Shuba (talk) 05:27, 1 October 2010 (UTC)[reply]

Invariant mass or relativistic mass?

In the section 'Modern view' it says:

The invariant mass is the ratio of four-momentum to four-velocity: : $p^{\mu }=m_{0}v^{\mu }\,$

This seems an error (unless I interpret this equation incorrectly). Should be relativistic mass rather than invariant mass, right? JocK (talk) 03:29, 17 December 2010 (UTC)[reply]

No it is indeed invariant mass that is the ratio of four-momentum to four-velocity. See the article on four-momentum-- especially in the end sections where it talks about its invariance and relationship to four-velocity. S B H arris 10:08, 19 December 2010 (UTC)[reply]

I have restored the section and added a source. DVdm (talk) 11:14, 19 December 2010 (UTC)[reply]

Thanks, that makes it much clearer. (I interpreted the four-velocity as (c, vx, vy, vx), i.e. the four-vector that contains the bare velocity components in the spatial directions, and hence without the time dilation factor gamma.) JocK (talk) 15:39, 20 December 2010 (UTC)[reply]