A recent German television program said a spaceship could not approach the speed of light because its relativistic mass would be so great that it would collapse under the force of its own gravity. This is clearly nonsense, because if it were true, it would allow occupants to know they were in motion without reference to an external frame of reference, hence contradicting the principle of relativity. But how is it that observers at rest relative to said spaceship -- who should be able to observe its relativistic change of mass -- would not observe this effect either (presumably).

I did not of course mean "observers at rest relative to said spaceship" but "observers (at rest) of said spaceship"

Welcome to the BABB, escoville.

I think you are right.

But this should be in the Bad TV! Bad Movies! forum.

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Welcome to the board, escoville

The spaceship would have additional energy, and mass and energy both have "gravity"

hold the presses, I'm getting an inspiration. Feed...the...tunas... mayonnaise.

The spaceship would be a frame of reference and the gravitational pressure exerted on it.

I dont understand bigsplit's answer. The people in the spaceship would measure their proper mass, which is their rest mass. This would exercise the gravitational attraction it had when at rest. They would not perceive the thing collapsing under its own gravity. People outside the spaceship watching it rushing past would measure its relativistic mass, which would be vast. If it has (to them) such a vast mass, why don't the outsiders see it exercise a crushing gravitational attraction on itself? It can't both collapse and not collapse.

Because the spaceship is moving relative to them, so they would interpret the enormous relativistic mass as mainly kinetic energy, rather than gravitational mass? :-k

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I haven´t taken the time to check out the maths involved, but I´d say that time dilation would be so huge that an object would appear utterly frozen before reaching the speeds needed to make it massive enough to collapse under its own gravity. Observers in a different frame would have to wait like forever to see it collapsing.

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Half-right, half-wrong. The relativistic mass does increase, but this is a purely inertial mass measure and not a gravitational mass measure. The gravitational mass needs to be calculated from the stress-energy tensor when particles become relativistic. At that point, pressure actually adds a component to the gravity.

If you are traveling at the speed of light and are massive, the stress-energy tensor will have infinite terms included. This indeed will cause a catastrophic effect, but infinities cause blow ups in other places that are more reasonable to think about, so it's not clear that the producers of the show understood the implications of their arguments.

I dont understand bigsplit's answer. The people in the spaceship would measure their proper mass, which is their rest mass. This would exercise the gravitational attraction it had when at rest. They would not perceive the thing collapsing under its own gravity. People outside the spaceship watching it rushing past would measure its relativistic mass, which would be vast. If it has (to them) such a vast mass, why don't the outsiders see it exercise a crushing gravitational attraction on itself? It can't both collapse and not collapse.[/quote]

I haven't done the calculations, so I am guessing. However, here goes. The people in the earth frame see the object as not only having a gravitational attraction, it also has a huge Thirring field associated with it. The Thirring field is also called frame dragging, but I am thinking in a purely SR mode here. Anyway, they see that the spaceship is going at a large velocity through its own Thirring field. And the Thirring field is going to be pushing outward, cancelling out the gravitational attraction.

I am thinking in analogy to an electric fuse. If a huge current of electricity is sent through a wire, the wire implodes due to the huge magnetic field that is generated. It doesn't explode due to the heat, at least not right away. It seems to me that a fast spaceship would experience a similar force from the Thirring field, because the Thirring field is analogous in many ways to a magnetic field and the fast spaceship is analogous a large current of electricity. The big difference is that the force will push the parts of the ship outward, because in gravity like "charges" attract. A gravitational charge is the mass.

This is just plain wrong! As others have stated, the occupants of the ship detect nothing out of the ordinary, as motion is indeed relative.

What people in a different frame would see is a bit beyond my layman's knowledge, although I think a better understanding of what exactly "relativistic mass" is meant to represent would help here. Also, the ship itself would be extremely flattened in the direction of its relative motion though as seen from the other frame (lorenze contraction). Maybe that plays into the whole thing as well. 8-[

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Thanks so far. Let's revise the scenario. Two apparently identical spaceships A and B are in deepest space. They cannot see each other, they are not accelerating, and have no record of ever having accelerated, and are too far away from any celestial body to judge whether they are moving. Then they enter each other's field of view and it is clear that they are moving relative to each other, and thus that at least one is moving relative to celestial bodies. The relative motion is very fast, so the usual relativistic effects are apparent -- Lorentz contraction and time dilation. These are of course reciprocal. They can easily be understood as effects of perspective in four dimensions. However Special Relativity theory also requires that each should see the other as more massive. How they could do this is uncertain. One way might be as follows: an occupant of A sees an occupant of B toss a tennis-ball at a paper screen, tearing it and passing through. Because of time-dilation, though, the velocity of the ball in its own frame of reference is seen as very slow. But the momentum is still enough to wreck the screen, so the mass component must have increased correspondingly. This effect is also reciprocal: an occupant of B would see the same thing happen in A. Each might by extrapolation deduce that the mass of the total system of the other had increased to the point where it ought, by the gravity field it creates, to put a strain on the construction. But clearly this cannot be, for otherwise each would notice the mass of his own ship increase, which contradicts the principle of relativity. Is there not a paradox here? Note also that the occupants of either ship cannot know that they have any kinetic energy whatever. Indeed, as either might be stationary, it is not clear that they have any.

But in this discussion Later in that thread it is explained some more, and it relates directly to the subject of this thread.

--John

Two months ago, chiaroscuro25 did do that very calculation. See the thread I mentioned in my previous note.

GR is not SR, any more than SR is Newtonian. Once you are moving so fast that your relativistic mass produces significant gravity, you can't use SR anymore but have to use GR. The intuative result of SR is no more correct than the intuative result of Newtonian.

Short answer: saying that the reletivistic mass doesn't contribute to gravity (therefore it doesn't make a black hole) is wrong. Rather, only the gravity coming from the rest mass counts toward making a black hole.

Thanks to all for for referring me to the previous thread (whose title didn't lead me to think it was relevant) and especially the very enlightening exchange between John Dlugosz and chiaroscuro25 on precisely this point. From my point of view the subject is closed.

Don't know how I missed it myself.
What is your conclusion, as it relates to the OP?

My conclusion is, that as far as space-ships are concerned, they must have been accelerated to this speed and it is probably therefore meaningless to look at this problem in terms of special relativity. However I am surprised that a physicist as eminent as Fred Hoyle could overlook this (his so-called sci-fi "novel" Into Deepest Space is a thinly disguised layman's guide to this sort of thing, and an awful novel), and so did Poul Andersen in Tau Zero (also said to be based on real science, and a vastly better book). On the other point, how two bodies travelling at uniform near-light speeds could pass each other and measure each other's mass as near infinite but their own as no more than their rest mass, with no apparent gravitational effects, I am still in the dark, but so it seems to me is everyone else. I get the impression that kinetic energy is a mysterious memory of having at some time been accelerated (in the same way that homoepathic remedies preserve a mysterious memory of having once contained a measurable concentration of the active principle). But as Sir Arthur Eddington said in "The Nature of the Physical World", physics is a matter of making the books balance, and should not be seen as a literal description of reality, whatever that is.

Then why would you consider the subject closed?
Kinetic energy is like homeopathic remedies?

To point one above: because I don't think there's anything that can be added, in particular to chiaroscuro25's points made in the other thread. Of course others may think it is worth pursuing.

To point two: in a sense. Kinetic energy is not an intrinsic feature of a body, but the concept is needed to balance the books. It can be regarded metaphorically as the body's "memory" of having been accelerated. Homoeopathic remedies are diluted (literally) beyond recognition. If they work, they have retained a "memory" of what they once contained. This was not meant to be a close analogy, but a salutary reminder that physics is an instrument to help us handle reality, not reality itself.

Just as long as you realise that it's highly dependant on which frame of reference you're measuring it in. And the same goes for potential energy; they both lack an absolute value. Gravitational mass is absolute, inertial mass isn't. Get it?

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we seem to be doing ok
You mean, instead of its spaciotemporal relationship to other bodies? I think that's a better analogy.
If? o boy

Agreed. Kinetic energy doesn't depend on which body "accelerated" in relation to a second "at rest" body. To each, the other can equally said to have "kinetic" energy due to its relative motion. To refer to it as being some kind of "memory" of a body's acceleration to me sounds incorrect.

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. . . My moustache is touching my brain!!!!

Please don't take my metaphors too literally. But if you haven't done so already, I would recommend that you read Eddington (not the bits about the current state of science, which are out of date, but about the nature of physics as a science). His credentials are I think beyond dispute.
Perhaps I shouldn't have dragged in alternative medicine. It gets people worked up.

well, there's alternative medicine, and then there's homeopathy

It's a sign of intelligence or creativity to connect what others would see as unrelated. But it's still surprising where a conversation can lead sometimes! Definitly off-topic for this board (except for being against the mainstream), but here are a couple links, and no additional comments on the subject.

http://www.randi.org/jr/02-02-2001.html

http://www.skepdic.com/homeo.html

http://www.quackwatch.org/01Quackery...ics/homeo.html