Question about relativistic mass

OmegaGuy · Post by **OmegaGuy** » 2006-09-20 12:58pm

Okay, I know that if an object with mass is moving, the closer it gets to lightspeed, the more massive it gets.

Would it be theoretically possible for a projectile to travel so fast that it becomes so massive it collapses in on itself and becomes a black hole?

Admiral Valdemar · Post by **Admiral Valdemar** » 2006-09-20 03:26pm

No, since the projectile does not have that mass in its own frame of reference. The projectile simply achieves "imaginary" mass, as it were to any outside observer, though get beyond 86% c and you get a projectile that has more energy than if its mass was pure energy like a M/AM bomb.

Kuroneko · Post by **Kuroneko** » 2006-09-20 03:58pm

No. A black hole is defined globally, i.e., by the fact that there is an absolute horizon. Relativistic mass dilation, on the other hand, is an observer-dependent effect only--relative to one observer, the relativistic mass of the object may be very high; relative to another, it may be rather low. There will be no horizon at all merely because the object is traveling fast. However, if enough force is applied to the object, the associated acceleration horizon may form within the object itself. Of course, for relistic materials, it will break apart long before that happens.

Admiral Valdemar · Post by **Admiral Valdemar** » 2006-09-20 04:06pm

This also shows the folly of singularity projectiles, since the extra energy required to acclerate them would be better suited to a normal projectile being fired instead. It's not like a black hole "bullet" would be worth the effort.

OmegaGuy · Post by **OmegaGuy** » 2006-09-20 04:16pm

Okay, thanks.

Molyneux · Post by **Molyneux** » 2006-09-20 07:36pm

Admiral Valdemar wrote:This also shows the folly of singularity projectiles, since the extra energy required to acclerate them would be better suited to a normal projectile being fired instead. It's not like a black hole "bullet" would be worth the effort.

How about a singularity *bomb*? After all, once past a certain size a black hole can 'feed' itself and grow in strength, can't it?

drachefly · Post by **drachefly** » 2006-09-21 11:31am

There's nothing imaginary about that extra mass. The catch is that mass is not the only part of the stress tensor, and it's the complete stress tensor that determines gravity. The rapid movement of the object contributes other parts, which in turn prevent black hole formation.

OmegaGuy · Post by **OmegaGuy** » 2006-09-21 12:42pm

You see that's one of the possibilities I was thinking of, but which explanation is correct?

Post by **Ghost Rider** » 2006-09-21 12:48pm

OmegaGuy wrote:You see that's one of the possibilities I was thinking of, but which explanation is correct?

You did read the part where drachefly is sayng it wouldn't happen as a black hole event, right?

It's not gonna happen the way you are thinking it would.

Admiral Valdemar · Post by **Admiral Valdemar** » 2006-09-21 04:25pm

drachefly wrote:There's nothing imaginary about that extra mass. The catch is that mass is not the only part of the stress tensor, and it's the complete stress tensor that determines gravity. The rapid movement of the object contributes other parts, which in turn prevent black hole formation.

Yeah, that's what I should've said. Imaginary mass is only with regards to certain particles (unsure whether physicists still use it with regards to photons anymore), though the idea that in one frame of reference the projectile gains arbitrary mass becoming a black hole being wrong was conveyed.

drachefly · Post by **drachefly** » 2006-09-21 04:45pm

Admiral Valdemar wrote:(unsure whether physicists still use it with regards to photons anymore)

Not that I've heard.

Thinkmarble · Post by **Thinkmarble** » 2006-09-21 06:29pm

Admiral Valdemar wrote: Yeah, that's what I should've said. Imaginary mass is only with regards to certain particles (unsure whether physicists still use it with regards to photons anymore), though the idea that in one frame of reference the projectile gains arbitrary mass becoming a black hole being wrong was conveyed.

Mhm, if I hear imaginary mass I do not think relativistic mass or dynamic mass I think mass which is imaginary. Dynamic mass falls by the wayside in anything above an introductionary course into SRT, as it is misleading.
So you just get mass which is the energy an system has in its rest system.
As no rest system for photons exists they do not in fact have mass.

One nitpick, pure energy is a nonsensical term.