Talk:Special relativity/Archive 1

This page is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Article Bloat

Read through the article last night, so you could say I slept on it. Anyway.. my problem with this article is it's bloated, and also a poor intro for anybody who doesn't already understand SR. The "motivation" is way too long (1 page+ on an intro to what's essentially a simple and elegant theory), the "compatibility" is a huge list of bullet points (would the average reader really be interested in so many examples?) and my biggest problem is... no train/mirror example!? I did physics a while ago and more or less got my head around SR when my teacher explained it because she was a good teacher. I then read up on it again much later in a pop sci book, and only then came across the train/mirror example.

I was amazed at how intuitive it presented SR as, and feel this example (or one very much like it) should really be in this article, perhaps even as the first section - an introduction of WHAT SR IS. This article has lots of great info - it's very in depth - but it somehow gets lost in the detail I feel. While generally I recommend wikipedia as the source of most knowledge, if it came to recommending a good page to read about SR to friends who aren't physics buffs, I don't think I'd point them to this page.

Finally, "This page is 81 kilobytes long" - the discussion page needs some fat trimming off it too :)

Anyway, a fair bit of criticism there, hope I haven't stepped on anyone's toes and I apologise if I have - my only intention is to try and provide constructive crit. as to how this article could be improved and a theory that must surely be immensely thought provoking and interesting to the general public be accessible to them. That's what Wikipedia is all about, right?

Davedx

Prologue

Tom Van Flandern argues that the Lorentz Relativity theory, the primary competitor to special relativity, does a better job of explaining the Speed of gravity, and is suppored by all the existing experimental evidence that supports special relativity.

There are a hundred and three alternatives to special relativity, and next to noone takes them seriously. Discussion of the particular alternatives definitely does not belong on a main page like special relativity, any more than discussion of Velikovsky belongs on history or discussion of Nostradamus belongs on theology.

While Josh Grosse may feel this way, I'd like him to at least name five of these hundred and three alternatives that are known to fit all the existing experimental evidence. I have my doubts that they exist, based on what I have read about physics. Having read Tom Van Flandern's article, I think Ben was right in putting this note in the article on special relativity. (hmm. maybe I should let Ben fight his own battles though...)

Ok, so I was a little quick to judge, and I apologize. On a careful reading Flandern is indeed working with standard Lorentz ether theory, something which gives the same results as special relativity and so can't be dismissed experimentally, yet which physicists continually reject anyways (having talked to some). The reason is that it is essentially SR plus the concept of a preferred frame, and yet the symmetries in the equation make this frame indistinguishable from the others. Flandern argues that the gravitational field established a preferred frame, but this is no different than saying the observer establishes a preferred frame - it is not in fact a difficulty with frame equivalence. The gravitational field doesn't transform properly in SR, of course, but that is because SR is the gravityless case of general relativity, where there is no preferred frame. But this is beside the point. Flandern's work is not a mainstream position, nor is it a noteable competitor to the mainstream position except in so far as it is a particular argument for Lorentz ether theory, and there are plenty of others - for instance, the supposition that an ether is in fact falsifiable because it places strong constraints on spatial topology. I think it might be worth discussing on a page about ether theory or about Flandern, but it isn't really that relevant to SR in particular.

The following was posted to sci.physics.relativity:

Well, I can tell you that you're not going to get far with "real

experts" if the only person you reference (besides Einstein) is

Tom Van Flandern - a notorious crank, who has been effectively

refuted many times (see, e.g., the article in

http://www.salon.com/people/feature/2000/07/06/einstein/index.html)

--LMS

I've removed the following from the article:

(Asterisk)* To postulate means that one supposes that what one has postulated could happen in nature; Einstein supposedly postulated one-way, two-clock light speed isotropy and invariance; however, since such a speed calls not only for two clocks, but for two synchronized clocks, and since nature cannot synchronize clocks, there cannot be any such speed, so there cannot be any scientific postulate calling for its isotropy and invariance. Proof: The one-way, two-clock light speed measurement has never been made because it cannot be made. (The above note was added by Brian D. Jones bdj@access-4-free.com Aug-14-2002)

because it's essentially completely unsupported. Brian, feel free to put it back in if you can give a good reason why "nature cannot synchronize clocks" (other than plain assertion, and no, simply stating "X has not been done because X cannot be done" is not proof of anything - if you can show that it's a physical impossibility to make this measurement, please explain how). Anyway, I really don't understand what you're driving at here. SR only requires the existence of a time coordinate, not necessarily an actual physical clock. --Bth 14:17 Aug 14, 2002 (PDT)

B D Jones is happy to reply, as follows:

Here is my simple explanation re the fact that nature cannot possibly synchronize clocks (although, as you will see after this explanation, it is almost too obvious to need any explanation):

Clock synchronization requires three physical operations, none of which can be accomplished by Mother Nature (as will be self-evident).

Given, two spatially-separated, unstarted clocks, these three operations are as follow:

[1] deciding upon their temporal relationship

(i.e., devising a definition of synchronization),

and

[2] adjusting their hands/dials in accordance with the definition,

and

[3] somehow starting the clocks.

Re [1]: Since Nature has no brain, She obviously cannot possible concoct a clock synchronization procedure.

Re [2]: Since Nature has no hands (or any other means), She obviously cannot possibly adjust the hands of clocks.

Re [3]: Since Nature has no brain or hands, She obviously cannot possibly start clocks at the proper moment and in the proper manner (i.e., in accordance with some clock synchronization procedure).

Clearly, only man can synchronize clocks.

However, it is just as clear that all two-clock measurements are dependent upon how the clocks are synchronized. Indeed, no two-clock measurement can be made unless the clocks have been synchronized somehow.

Therefore, it is impossible to measure light's one-way, two-clock speed without clock synchronization, and the result is dependent upon the given synchronization.

Clearly, only if Nature Herself has synchronized the clocks can the result of any two-clock measurement be a natural result.

Then instead have two identical particle transformations occur that happen to start at the same time and not in the same place. Sabejias

[large deletion by author Brian D Jones - something about 
"pearls before swine," and all that rot!]

The Special Theory of Relativity was constructed to explain earlier experiment results, and the most significant of them was the Michelson-Morley experiment. However M-M experiment was performed in air, giving the famous null result. But SR uses speed of light in vacuum in its postulate, so it can't explain at all M-M experiment (because the theory can not be derived using the speed of light in air).

So let's take a closer look to the M-M experiment and consider it performed using following mediums where light propagates:

(i) vacuum (speed of light is 299 792 458 m/s) (ii) air (speed of light is 299 702 547 m/s) (iii) water (speed of light is 225 407 863 m/s)

(iv) consider also the experiment performed using electrons instead of photons, travelling much lower speed (say 100 m/s)

It is quite well known that (i) produces null result. In 1887 Michelson concluded null result from (ii) with then known methods, but what is result got now? If M-M experiment supports SR, (ii) shall produce non null result. Also (iii) shall produce unambigious non null result, because of bigger speed different. But it is quite clear, that (iv) produces null result. So, which speeds of information carriers in medium produce null result and which not?

In fact, if (ii) and (iii) produce null result, this contradicts SR.

- TS

As I said on Talk:Michelson-Morley experiment, take it tosci.physics.relativity. Or any of the other physics newsgroups. Just not here, because talk pages are not intended for this kind of discussion. -- Tim Starling 12:06 Mar 12, 2003 (UTC)

What's that bit about cellular automata in aid of? It's not relevant at all. -- CYD

Cellular automata: I put that section in, so I take responsibility for it. It's there for pedagogical purposes. It is completely relevant. In contrast to pretentious intellectual babble, it actually transmits information, which is exactly what language is for, last I checked. Cellular automata is a paradigm that is perfectly appropriate for special relativity. The point of using it is to get the point across(i.e. educate). So long as it helps build the appropriate neural pathways, it is completely relevant, for if the criteria for relevance stood independant of such a goal, the 'relevant' material would be doomed to meaninglessness. -- Kevin Baas

It doesn't get the point across to me. Perhaps you could elaborate? In fact that whole section, starting from "Geometrically", seems suspect to me. "All 'points' along the null dual-cone represent the same point in space-time" - say what? -- DrBob 16:50 Apr 30, 2003 (UTC)

The idea that all points on the cone are the same simply because the interval between them is zero is incorrect. Pseudometrics don't work the same as metrics, and you can see this by noting that there are points with non-zero separation that both have zero separation from some appropriate third point.

As I understand it, Kevin, what you are trying to communicate with the reference to CAs is that the structure of SR is local, i.e. repeated at every point. But they really don't have much in common - space-time is neither cellular nor has any of the properties of automata associated with it. I think it suggests far more that is not true than it does that is true. Surely if the point is so important there is some other way to state it?

DrBob-I refer you to tensors, and null geodesics. You should also study special relativity, ofcourse. The classical text on tensor calculus, which includes a decent( thou small and partial) section on special relativity., is Synge's "Tensor Calculus". What I mean by that specific statement is that the measure between any two points on the null geodesic is zero. Minkowski space, and, in general, Non-Euclidean(Riemannian geometry), does not operate the same way as Euclidean geometry. If you're still confused, or are too stubborn to reference these materials, you could also read Einstein's own book on special relativity, which is written for the layman.

Regarding intervals: 1. Refer to Topology regarding the mathematical meaningfullness of a distinction between 'two' points which cannot be distinguished mathematically by any means whatsoever. 2. By a pseudometric I assume you mean a metric on an unorientable manifolds? I would imagine that if you measure the distance incorrectly the two distances cancel out. However, if you measure them correctly, you'll see that, according to standard procedure, there is no such thing as a negative distance(measure). Negative only implies that one is measuring in the opposite direction, or with a complementary orientation. Furthermore, the proper way to measure distance in Riemannian geometry, is via variational calculus, which produces a unique and positive-definite result.

Regarding space having local structure: yes, that is what I am trying to say. But let me quote someone who I think said it more clearly:

"Another radical consequence is the rejection of the notion of an absolute, unique, frame of reference. Previously it had been believed that the universe traveled through a substance known as "aether" (absolute space), against which speeds could be measured. However, the results of various experiments, culminating in the famous Michelson-Morley experiment, suggested that either the Earth was always stationary (which is absurd), or the notion of an absolute frame of reference was mistaken and must be discarded."

Try to figure out where I took that quote from.

The speed of light is the same in every inertial reference frame. An inertial reference frame is a 'cell'. I think the analogy is pretty obvious and simple. The implication is simultaneity.

--Kevin Baas 2004.05.01

Kevin, no.

A pseudometric is like a metric, but it is not positive-definite, and as a result various properties cease to hold. The Minkowski metric falls into this category. Thus, for instance, there are two completely different sorts of non-zero intervals, ones where ds² is positive and ones where it is negative. These correspond to the interior and the exterior of the cone, and are completely distinct, not just different in orientation. Also, the points on the cone can't be identified just because they have zero interval between them.

A simple thought experiment should confirm this. At event A, I send out a pulse of light. At event B, it is reflected by a mirror. At event C, I receive it again. Obviously if events A and C are distinct, then at least one must distinct from event B - but they are both connected to event B by null geodesics. Really, though, a decent treatment of Minkowski space should explain such things. Your assumption that the people who disagree with you simply haven't had enough exposure to Riemannian geometry is, I can assure you, false.

As for the cells, the analogy is badly stretched. A frame of reference in relativity is a local coordinate system, describing some neighborhood of a point. A cell in a cellular automaton is a particular structure associated with a point, i.e. an automaton whose inputs are the states in the neighboring cells. I think the comparison is more misleading than enlightening.

Assuming one considers 'automaton' a valid philosophical construction. Insofar as the validity of an 'automaton' is a stretch, I concede that the analogy is a stretch. However, putting aside the false sentiment of a 'thing-in-itself' implied by an 'automaton', i mantain that insofar as SR implies that information propagates at the speed of light, the -t half-plane of minkowski space can be considered the 'input' or the past which is observable from thereference frame, and the 'output' of the reference frame is that which is in the -t half-plane of all other referencee frames, which is by inverse relation in the +t half plane of the reference frame. Again, I am not stating that an object or event at any point 'exists' in a reference frame, or even that such a proposition would be meaningfull.

In your thought experiment, event A, B, and C are clearly distinct because the origins of the null geodesics that you used in your model are clearly distinct.(for instance, on frame A, the origin of frame C is at t=k(and does not lie on a null geodesic), where k is nonzero) Ofcourse, the fact that they are local Minkowski spaces is shared by all three.

I concede that A and B are connected via a null geodesic, and that B and C are -- in fact, I do not need to concede that because it is my point.

Ofcourse, SR is not cellular because it is not discrete, and insofar as a cell is a 'structure' I concede that the analogy is stretched, ect.. But that's just what an analogy is: an exposition of the likeness of two entities which are not exactly similiar. What I mean to imply by the analogy is purely and simply the intuitive spatio-temporal geometry, which I think would be the primary concept that would be carried over by the association. If you can find a more fitting analogy that enables higher-level thinking and is appropriate to an audience that is not already familiar with the subtleties of SR, then I'm all for it. But I think you're overstating the dangers of sacrificing a few subtleties for a clear, accessible, and concept-building presentation.

btw, forgive me for my patronizing. it was inappropriate. i had a bad day. :(.

--Kevin Baas 2004.05.01

Ok, I don't see what it is you are trying to say about null geodesics. The article says that all points along the "null dual-cone" are actually the same point. That should identify B with A, and C with B, since they are connected by light paths. But here you say you aren't doing that. So what are you doing? I must be misunderstanding the comment from the article, then.

As for the analogy, I think that if I missed the point, CYD missed the point, and DrBob missed the point, then the point can't be that obvious. You have done a good job explaining what is meant here on the Talk page, I'll see if I can transport some of that to the article.

Cool. I am not a miser with my words. The main point of me writting that section was for there to be such a section: a clear and generally accessible section which develops a basic geometric understanding of SR.

Perhaps the misunderstanding came from this "point". I did not mean that the null geodesic represented the same point in a global reference frame, or so called 'space'; aether (for, as the theory states, one cannot be validly constructed), but rather that it was the same point within the local reference frame, according to it's intrinsic geometry, in much the same way that (1,0) and (1,2*pi) are the same point in a polar coordinate system.

I agree that if three people missed the point to the extent that they made a point of that fact, then it is evident that the(my) presentation is not effectively communicating the point. I would like to see it more clearly presented.

--Kevin Baas

Add theory number 104 - Wisp Unification Theory - to the alternatives to SR. It produces a general Doppler equation set that is fundamentally different to Einstein's, and yet when a limit process is added it produces all of SR's equation and matches or betters its experimental predictions. I have opened up a "one-way light speed test" discussion on the metaresearch site at www.metaresearch.org/msgboard/ >genral matters>meta science>SR & one-way light speed tests. I am a firm believer that a simple Earth based one-way test using two clocks will prove SR wrong. But I do not support the concept of a local gravitational field to explain matters. My theory is based purely on commonsense principles and is available free from http://www.kevin.harkess.btinternet.co.uk

Kevin Harkess (2 Aug 2003)

Special relativity includes the sentence:

For an example, see the relativisic rocket problem.

What is that? LarryW 19:13, 2 Aug 2003 (UTC)

No idea, maybe the twin paradox... Exactly one hit on google. Ксйп Cyp 21:08, 2 Aug 2003 (UTC)

That sentence in Special relativity may need to be removed if someone doesn't clarify it after a week or so. LarryW 00:36, 3 Aug 2003 (UTC)

It should be $E_{0}=mc^{2}$ and not $E=mc^{2}$ , i.e. the energy in the rest frame (energy depends on the frame of reference) equals mass (this is independent on the frame of reference) times velocity of light squared. But it would make necessary to change a lot more than that.

Ok, this is a really long page already and I'll confess not having read the abode. If BDJ wants to give us all permission to pare it down that would be good. Anyway, I have a point to make.

The Michelson-Morley_experiment seems to prove that accelerating in one direction has no more effect on one's time flow rate than accelerating in another direction would. In other words, to accelerate away from earth towards Alpha Centauri would not affect one's timeflow experience any more than accelerating at the same rate in the opposite direction.

However, I see an exception made here when it is said that a person accelerating towards Alpha Centauri would experience a reduction in time flow rate (age more slowly, think more slowly, etc.), but upon decelleration he would "normalize" back to experiencing time the same as any other "stationary" person would. In other words, time goes at the same rate for a star-traveler once he returns to earth and stops as it did when he left.

Now is that reasonable? Does it add up? The guy was slowed by accelerating and speeded back up for decellerating. Why? Is there a difference between acceleration and decelleration? I thought it was just exactly the same thing in different directions.

And another thing. If acceleration is different from gravity, as the twins paradox article states, that would mean that persons living in a centrifuge would live at a different rate from someone standing on the surface of a planet. A centrifuge (or rotating space colony) produces counterbalancing acceleration and decelleration constantly. And yet, you can hardly say the person isn't moving. Is he experiencing time at a slower rate at one side of the station than the other? ---Luke Parrish P.S. Sorry for not taking this to usenet. I like the idea of being able to delete/edit stuff after I say it, or let people do it for me. Makes for a more civil conversation. Also I am on a firewall. Why do they have to make usenet so inconvenient, I don't know. Guess I could move to Google Groups if I had to.

If you take a rabbit and a few carrots, attach synchronized clocks to the rabbit and the carrots, put the rabbit on a catapult capable of launching the rabbit in a random direction at 97.3% of the speed of light, launch the catapult with rabbit into space, at the same time launch the carrots into space in random directions, using a remote control set the catapult to launch the rabbit, wait for the rabbit to smell, locate and eat the carrots, the clocks on each carrot should appear to have been running faster than the rabbit's clock, at the time the rabbit eats each carrot. Is that relevant to the long discussion earlier on the page? Κσυπ Cyp 20:34, 21 Sep 2003 (UTC)

SORRY - edited page but could not upload image. Will try later. 82.35.65.1

In the motivation part for theory it is said: "For example, one may observe no magnetic field, yet another observes a magnetic field in the same physical area". I can't see any connection between this statement and the need for SR. On the other hand, there is no other motivations mentioned. So why was SR needed in 1905?

- TS

By "motivation", I don't think it is meant "purpose of". One does not say, "we need or should have x for the practical reasons of y", and then develop x.

Rather, by motivation, esp. in regard to scientfic theoretical motivation, it is something which points out an insufficiency in the current theory. The theory at the time demonstrate that what was mentioned should not happen; that, rather, the same magnetic field should be observed regardless of the observer. SR then remedied this problem. It motivated SR in so far as it suggested a problem that needed to be solved, and SR was, at least in part, created out of an attempt to solve this problem, which is succeeded in. In any case, SR is a "solution", and solutions cannot exist without "problems". -Kevin Baas 2004.01.08

What does the term 'mass' mean?

Sometime in the last couple of months or so, someone added:

As the velocity of an object increases, so does its mass, and the increase in mass is equal to 1/c2 times the increase in energy. I.e. E = mc2

This is wrong, or at least isn't consistent with how SR is currently taught (there's an older formulation of SR, which uses a different definition of mass, in which this is true - see rest mass). The trouble is, this is such a "well known fact" that people keep adding it to the article

Probably the article needs a description of the both formulations to prevent this happening. Anyone feel up to it? -- DrBob 01:37, 12 Mar 2004 (UTC)

I've written an article at relativistic mass that discusses this matter in detail. I will try to clean up the usage of mass in this article as well. -- Fropuff 21:42, 2004 Mar 19 (UTC)

Hmm. I haven't studied physics at University for some time now, but I don't believe that the term 'rest mass' has been deprecated. Physicists are often sloppy with terms and say 'mass' when they mean 'rest mass' etc, but that is because they expect you to know what they are talking about. Similarly mass & energy are separate concepts - they are not the same thing, but in some situations are sloppily treated as if they were. I think in this case (many of our readers are not expert physicists), it make sense to be careful with terms.

I think that the historical treatment of SR really has outlived it's usefulness. Why are we still talking about a 25 year wrong turn? Concepts like Aether can only confuse (and attract cranks). It would be nice just to get rid of the reference to Aether, Lorentz etc and move it to 'History of Relativity' or such-like. Do others agree? Oz 04:48, 19 Mar 2004 (UTC)

Certainly it's confusing, which I why I think we should be careful. Especially when different Wikipedia article use "mass" in mutually inconsistant ways. Generally though, "mass" meaning "rest mass" is the standard usage, nowadays. The problem is that popularizations and so forth tend to use the older style.

I agree that we could downplay the history a bit, perhaps just re-order the sections so that what SR is appears in the article before the history of its development. -- DrBob 16:11, 19 Mar 2004 (UTC)

Physicists are most certainly not slopply with the usage of the word mass in relativity. They know precisely what they mean. In fact, the terms rest mass and relativistic mass are virtually unused in modern literature (i.e. the past 60 or so years). For more discussion, see the relativistic mass article.

Regarding the topic of history: The history of special relativity is an extremely important aspect of the subject and deserves to be discussed somewhere. My preference would be to leave it on this page until it grows too large to be managable. Besides, the "crack pots" will find it no matter where you stick it. -- Fropuff 21:42, 2004 Mar 19 (UTC)

I don't why know the history is important. The theory stands alone without it. It's also a lot harder to check historical facts for authenticity. Newtonian physics, for example, isn't taught by first teaching incorrect theories as history. Anyway...

There are 2 concepts of mass: 'rest mass' & 'relavitistic mass'. I trained as a mathematican / phyisicts and I don't think either should be dissignated as THE 'mass'. Physicts use the term in context (THAT is what I meant by sloppy). In QM, mass will probably mean rest mass; In the formula E=mc^2, it means the relativistic mass. In an introductory text on SR, I think it is important to keep the distinction. Once it is appreciated, and the context established, the generic term 'mass' can be used without confusion.

It is true that special relativity stands on its own without regard to its history, but one can gain a much deeper understanding of it by learning how it evolved. The same is true of any physical theory — including Newtonian mechanics. And contrary to your statement, even Newtonian mechanics is taught by dispelling certain beliefs held by the ancient Greeks. At least, that's how I learned it and its how I continue to teach it.

Actually, most physicists take the m in E=mc^2 to be the rest mass. This means, of course, that the formula works only in the rest frame. A more general formula, E = γmc^2 works in all frames. I agree that if this equation were the only issue at hand it would be much nicer to use the relativistic mass. But doing so leads to all sorts of confusions in other places, which is why people don't. Unfortunately, the equation is so well known to non-physicists and so often misinterpreted that it is hard to explain to people how it is really used. -- Fropuff 01:03, 2004 Mar 20 (UTC)

It seems to me that E=mc^2 works just fine for the usual uses: explaining what'd happen if you dropped a jar of antimatter, and what binding energy is. –– wwoods 01:28, 20 Mar 2004 (UTC)

Of course it does. The equation isn't wrong: It's just a matter of what you choose to call the mass. Is it E/c^2 or is it E/(γc^2). It turns out the second is more convenient. -- Fropuff 01:36, 2004 Mar 20 (UTC)

I just meant that usually you're just talking about the equivalence of mass and energy; you aren't talking about masses moving at relativistic speeds, so \gamma is ~1. ––wwoods 01:43, 20 Mar 2004 (UTC)

Precisely. This is what I meant, when I said the equation E=mc^2 is only applied in the rest frame (or approximately when velocities are small).

Let me give an example just to clarify a potential confusion between rest mass and relativistic mass. Everyone "knows" the photon is massless. If one naively applies the formula E=mc^2 one is led to the incorrect conclusion that a photon has zero energy. It doesn't. Where's the error? It's in using the relativistic mass M = γ m. Here the rest mass m equals zero and γ is infinite (since photons travel at the speed of light). But infinity times zero is an indeterminate quantity. Hence the relativistic mass is undefined. -- Fropuff 01:56, 2004 Mar 20 (UTC)

In that case you have to go all the way back to E^2 = m^2c^4 + p^2c^2, of course. By coincidence, earlier today I took the relativistic mass out of kinetic energy. ––wwoods 02:11, 20 Mar 2004 (UTC)

Actually E=mc^2 works for all velocities short of c, where m is the relativistic mass. The photon is certainly capable of taking away energy (mass) so I guess this really supports the idea that the mass of the photon should not be equated with it's 'rest mass'. ;-).

Consider a stationary particle A of mass 4, which decays into a photon and a second particle B of rest mass 1 moving near the speed of light. It's relativistic mass is 2. The formula E=mc^2 works just fine if m is regarded as the relativistic mass, but fails if m is limited to mean the rest mass. The photon would take away 2 units of mass, particle B would take away 2 units of mass. I do see how there would be a problem if you interpreted m just to be the rest mass. ;-) Oz 03:37, 20 Mar 2004 (UTC)

As I said, if m is the rest mass the formula E=mc^2 applies only in the rest frame. -- Fropuff 04:15, 2004 Mar 20 (UTC)

There is some agreement: The term 'mass' can mean 2 things, and the usage depends on what is most convenient for YOU. I would argue that the 'relativistic mass' has a better claim to the term. Why?

Consider the following (artifical) problem: A 100m long train accelerates up to 0.5 of the speed of light. It approaches a tunnel 90m long. A short, dealy blast of Gamma rays (from an exploding star?) will arrive on earth when the end of the last carriage enters the tunnel. Will the whole train fit in the tunnel at the same time, and be protected from the blast?

The first step is to calculate it's length at 0.5c. If it is less than 90m, it will fit in the tunnel. The calculated (relativistic) length is the 'real' length since it determines whether some of those on the train live or die.

You could however argue that term length should only apply to the 'rest length', but those on the train would probably disagree. A similar argument applies to mass.

If your universe revolves around 4-vectors and solving differential equations, then you would never use the 'relativistic mass', or 'relativistic length'. You may regard the problem above as a 'popularisation', but it is good physics. If I was designing a rail gun, I would probably start with simple calculation like those above, and I would not believe complex calculations until I had checked them against the simple calculations.

'Popularisation' are valuable. The anchor theory to intuition. Einstein's famous thought experiments were used for just this purpose.

In many ways, this is a silly argument and only exists because of a faulty claim of uniqueness for the term 'mass'. Oz 02:49, 20 Mar 2004 (UTC)

Will the whole train fit in the tunnel at the same time, and be protected from the blast? my calculations say yes. On the assumption that, in the reference frame of the tunnel the gamma blast arrives at the same time at the end and at the start of the tunnel we have three different {\em events} in spacetime: $x_{0}$ is the event where/when the gamma blast hits the start of the tunnel and the end of the train. $x_{1}$ the end of the tunnel when the blast arrives, that is 100m and no time afar from $x_{0}$ in the reference frame of the tunnel, and $x_{1}^{'}$ is the place of the head of the train at the time when it leaves the tunnel, that is no time and 90m afar from $x_{0}$ in the reference frame of the train. $x_{1}^{'}$ transformed to the reference frame of the tunnel lies some time in the future of $x_{1}$ so the head of the train will be still inside the tunnel when the gamma blast arrives at the end of the tunnel. 217.230.29.21 21:43, 24 Oct 2004 (UTC)

Will the whole train fit in the tunnel at the same time, and be protected from the blast? From whose point of view, the guy on the ground or the guy on the train? Rememeber, at the same time is an observer dependent concept in SR. This is a variation of the classic Barn and pole paradox. Of course, who lives and who dies does not depend on what reference frame you're in, or what length you talk about.

I agree that popularizations are valuable, in general. I would argue, however, that this particular popularization leads to more confusion than enlightenment. One is free to disagree. In the end its not a matter of what I think or what you think is more convenient, its a matter of how the terms are used in the real world. Wikipedia should reflect that. We can and should mention both uses of the word mass, but we should also stick to the most common usage. -- Fropuff 03:19, 2004 Mar 20 (UTC)

Until all reputable physicts use the term mass to mean rest mass, in all situations, it would be misleading to claim that mass just means rest mass.

I'm going to make some changes to the article. We can fight about the wording if it pleases you. -- Fropuff 04:15, 2004 Mar 20 (UTC)