Relativity & Quasars: Spatial Contraction & Density

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Discussion Overview

The discussion revolves around the effects of relativistic speeds on the density of objects, specifically addressing whether an object's density increases due to spatial contraction as perceived by an observer in motion. The conversation also explores the implications of these effects in the context of quasars and the conditions under which they might become black holes.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions whether an object's density increases when observed at relativistic speeds, specifically in the context of spatial contraction.
  • Another participant asserts that the density does increase, citing that energy density increases by a factor of roughly gamma^2, influenced by length contraction and the kinetic energy of relativistically moving objects.
  • A further inquiry is made regarding quasars, asking if a quasar traveling at sufficient speed could become a black hole from the perspective of an observer moving at relativistic speeds.
  • A link is provided to an external resource discussing the relationship between relativity and black holes, though no specific claims from that resource are discussed in the thread.

Areas of Agreement / Disagreement

There is a partial agreement on the increase of density due to relativistic effects, but the discussion regarding quasars and their potential to become black holes remains unresolved, with no consensus on the conditions required for such a transformation.

Contextual Notes

The discussion does not clarify the assumptions regarding the specific conditions under which density changes occur or the precise definitions of terms like "black hole" in this relativistic context.

b2386
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Hi all,

Just a quick question.

Suppose I am traveling at relativistic speeds. An object moving with respect to my frame has undergone spatial contraction. In my frame, does the object's density increase?
 
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Yes, the density does increase.

Energy density, the sort used in the stress-energy tensor, increases by a factor of rougly gamma^2, gamma = 1/sqrt(1-(v/c)^2). This happens by a combination of length contraction and the fact that relativisticly moving objects have kinetic energies on the order of their rest mass.
 
OK, let's change the situation slightly.

Suppose I am traveling at relativistic speeds. A quasar moving with respect to my frame has undergone spatial contraction. In my frame, does the quasar become a black hole if it is traveling at sufficient speed?
 
(link) http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/BlackHoles/black_fast.html
 

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