What happens to density as speed increases?

[Pragz]

Assuming a ship is approaching the speed of light, what happens to its density? Is there any notable change at all?

 

[Creator]

Assuming a ship is approaching the speed of light, what happens to its density? Is there any notable change at all?

As measured from which inertial frame?

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[Rap]

As measured from which inertial frame?
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Right. In the rocket's own frame, it is motionless, so its density remains the same. As it approaches the speed of light with respect to some observer, the density will increase.

 

[Pragz]

Sorry, I was in a rush when I wrote this. I meant if you were an external observer measuring the density of the ship.

Does this also work backwards in the sense that if you are on this ship and measure the density of an object going much slower, is its density lighter?

Ultimately what I'm trying to resolve is something a friend said to me while discussing space travel:

"A major problem with traveling at near lightspeed is that if you're going a 99% the speed of light and your ship hits a rock floating in space, it would tear through the ship just as if it was hit by a rock traveling at 99% the speed of light."

But I was thinking that the ship would effectively be superdense relative to the rock, so it wouldn't necessarily just rip straight through it.

 

[HallsofIvy]

If an observer were able to measure the density of an object moving very fast with respect to himself, he find the density to have increased noticably. The volume would have decreased and the mass would have increased.

"Does this also work backwards in the sense that if you are on this ship and measure the density of an object going much slower, is its density lighter?"
No. I think you have the wrong idea about relative speeds. If A is moving very fast relative to B (so that B is moving much slower that A from B's point of view), then B is also moving very fast relative to A. From the point of view of A, it is not moving, all other things are moving relative to it. All other thing are faster than A.

 

[Rap]

"Does this also work backwards in the sense that if you are on this ship and measure the density of an object going much slower, is its density lighter?"
No. I think you have the wrong idea about relative speeds. If A is moving very fast relative to B (so that B is moving much slower that A from B's point of view), then B is also moving very fast relative to A. From the point of view of A, it is not moving, all other things are moving relative to it. All other thing are faster than A.

In other words, no, its density will be larger as well, according to you on the rocket ship, by the same factor that your density seems larger to it.