# Is relativistic mass an illusion

### #1

Posted 8 February 2009 - 08:40 PM

http://www.einstein-...logy/index.html

### #2

Posted 8 February 2009 - 08:46 PM

same way they won't agree on the speed the object is travelling but they will all be right.

"Special" Relativity, stupid ideas seem smarter when they come at you really fast.

### #3

Posted 9 February 2009 - 12:38 AM

*Minutus cantorum, minutus balorum, minutus carborata descendum pantorum* **To shake my vodka martini, click the up arrow ^**

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### #4

Posted 9 February 2009 - 02:39 AM

http://www.einstein-...logy/index.html

### #5

Posted 9 February 2009 - 03:24 AM

### #6

Posted 9 February 2009 - 04:05 AM

i thought this was a good description of the matter

http://math.ucr.edu/...ty/SR/mass.html

It makes it sound like gamma is some kind of resistance in space despite the fact that a continuous 1G acceleration will cover more distance at a faster and faster rate according to your clocks on the ship. Earth time is the one that seems to be stuck at the speed of light. That could be a relevant factor on a trip.

http://www.einstein-...logy/index.html

### #7

Posted 9 February 2009 - 04:39 AM

### #8

Posted 9 February 2009 - 04:51 AM

Exactly, the kinetic energy and relativistic mass are equally varible depending on what you are comparing your speed to....Of course kinetic energy depends on how fast you consider the object to be moving.

http://www.einstein-...logy/index.html

### #9

Posted 9 February 2009 - 12:25 PM

### #10

Posted 9 February 2009 - 03:07 PM

Just because some quantity depends on the observer's reference frame does not mean that that quantity is illusory. It just means it is frame-dependent.

And as it is frame dependent you must not really attach any great meaning to it. Only frame independent things have some "deeper" meaning.

Mathematical Ramblings.

### #11

Posted 9 February 2009 - 03:37 PM

The problem with relativistic mass is not that it is illusory. Velocity and energy are equally frame-dependent but are quite useful and very non-illusory concepts. The chief problem with relativistic mass is that it is synonymous with energy (literally synonymous; m=E/c

^{2}). The concept of relativistic mass doesn't add much of value (just use energy) but it certainly does add confusion.

### #12

Posted 9 February 2009 - 03:45 PM

http://www.einstein-...logy/index.html

### #13

Posted 9 February 2009 - 04:14 PM

A physical system will come with some automorphisms. The only "meaningful" observables are those that are invariant under these automorphisms. If lets say observable A can be transformed into observable B via an automorphism then how can we attach a real physical meaning to A (or indeed B).

That is not to say that A is not useful in some situations, but it cannot have some deeper meaning. The example of relativistic mass springs to mind here. It may be useful for some things, but as we have seen it is not an "absolute" and the frames need to be specified. (This is reminiscent of gauge fixing).

For the case of special relativity this is the Poincare group.

For gauge theories this is the relevant gauge group (and if relativistic the Poincare group also)

Mathematical Ramblings.

### #14

Posted 14 March 2009 - 11:26 PM

### #15

Posted 15 March 2009 - 02:33 AM

### #16

Posted 16 March 2009 - 07:49 AM

### #17

Posted 16 March 2009 - 01:14 PM

Try this thought experiment. We start with three stationary references separated a distance d in a triangle. We label one and give it velocity V. From the point of view of either stationary reference, we see one stationary and one reference with velocity. From the point of view of the moving reference, we see two stationary references appearing to move. One can see the energy balance is different, so relative can cause problems in terms of energy conservation. With two references, it is harder to do an energy balance, so we have 50/50 odds of picking the correct reference that is moving because of energy (has the relativistic mass).

This goes back to your question, is relativistic mass real or just a mathematical concept? If it was real, we should be able to measure it and differentiate relative reference. If it is just a concept, we are stuck at 50/50 odds of getting it right with respect to energy balance reality. We could pick what is convenient for us, and then have 50/50 odds. We can still learn things but we might assume to much or to little energy. This will have an impact on subsequent assumptions.

It may be important to figure out how to measure relativistic mass, from a distance, to confirm if we have the energy balance correct. If we go back to our three reference scenario, if we could measure relativistic mass, the moving reference would see V but no m. The stationary would both see V and m. Based on that both know who is stationary and moving, even if there are special effects in t, d.

### #18

Posted 16 March 2009 - 02:24 PM

I have a struggle to understand how relativistic mass is real if the observer that is traveling close to the speed of light sees the rest of the universe as massive and time dilated but the at rest observer sees only the speeding object as time dilated and massive. How can both be correct?

"How can both be correct" isn't a meaningful question unless you give some apparent reason why they can't.

also when the fast observer is slowed down only his time dilation proves to be real, the rest of the universe has aged at the same rate it always has. Only the time dilation of the fast observer is real when he slows down.

This is because the "fast" observer has done all the accelerating.

### #19

Posted 16 March 2009 - 02:39 PM

SR has three relationships, one for time, distance and mass. The relativistic mass is important because it allows us to do an energy balance and therefore differentiate between relative references. The M is connected to E=MC2 with E connected to kinetic energy.

Or you could have an invariant mass, and then the kinetic energy is the frame-dependent term. Relativistic mass is by no means required, and screws up the representation of the momentum 4-vector.

*Minutus cantorum, minutus balorum, minutus carborata descendum pantorum* **To shake my vodka martini, click the up arrow ^**

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My SFN blog: Swans on Tea

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