What happens to density as speed increases?

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Discussion Overview

The discussion revolves around the effects of relativistic speeds on the density of objects, particularly focusing on a hypothetical ship approaching the speed of light. Participants explore how density is perceived from different inertial frames and the implications of these perceptions in the context of high-speed space travel.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that as a ship approaches the speed of light, its density will increase when measured from an external observer's frame.
  • Others clarify that in the ship's own frame, it remains motionless, and thus its density does not change.
  • A participant questions whether the density of a slower-moving object would appear lighter when measured from the ship, suggesting a need for clarification on relative speeds.
  • Some participants assert that if an observer measures the density of a fast-moving object, they would find its density increased due to decreased volume and increased mass.
  • There is a contention regarding the interpretation of relative speeds, with some arguing that the density of an object moving slower relative to the ship would not appear lighter to an observer on the ship.

Areas of Agreement / Disagreement

Participants express differing views on how density is affected by relative motion, with no consensus reached on the implications of these effects in practical scenarios, such as collisions at relativistic speeds.

Contextual Notes

Discussions include assumptions about inertial frames and the effects of relativistic speeds on mass and volume, but these assumptions remain unresolved and depend on the definitions used by participants.

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

..
 
Creator said:
As measured from which inertial frame?
..

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.
 
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.
 
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.
 
HallsofIvy said:
"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.
 

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