Can an object be both a black hole and not a black hole?

Click For Summary

Discussion Overview

The discussion centers around the concept of whether an object can simultaneously be classified as a black hole and not a black hole, particularly in the context of Special Relativity and relativistic mass. Participants explore the implications of an object's speed on its mass and the conditions under which black holes form.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that an object moving at relativistic speeds could gain enough mass density to become a black hole, while remaining non-black hole from the perspective of an observer in its frame of reference.
  • Others argue against this notion, stating that a black hole's formation depends on rest mass rather than relativistic mass, suggesting that the concept of relativistic mass can lead to confusion.
  • A participant clarifies that modern terminology typically refers to "mass" as rest mass, and that gravity in relativity does not depend on relativistic mass, but rather on the stress-energy tensor.
  • One participant references a FAQ from John Baez to further explore the implications of speed on black hole classification.

Areas of Agreement / Disagreement

Participants express disagreement regarding the relationship between relativistic mass and black hole formation, with no consensus reached on whether an object can be both a black hole and not a black hole.

Contextual Notes

There are limitations in the discussion regarding the definitions of mass, the role of relativistic effects, and the assumptions underlying the formation of black holes, which remain unresolved.

friend
Messages
1,448
Reaction score
9
According to Special Relativity, objects moving fast have more mass than objects at rest. So what if there is an object having a mass density near that required to create a black hole. Now if it were moving fast enough as well, it would gain enough mass density to become black hole. But to the observer moving in its frame of reference it would not. How can an object both be a black hole and not be a black hole? To an observer moving fast enough, every object would seem to be a black how, right?
 
Physics news on Phys.org
friend said:
To an observer moving fast enough, every object would seem to be a black hole, right?
Not right.
A black hole forms or not according to how large the rest mass is, not the relativistic mass.
 
friend said:
According to Special Relativity, objects moving fast have more mass than objects at rest.
This statement is common, but wrong. Moving objects have more relativistic mass, but their rest mass is unchanged. Modern usage (i.e. pretty much any paper in the last half century) is that "mass" means rest mass, and relativistic mass is pretty much never used. It's just total energy divided by ##c^2## anyway, and calling it mass leads to confusion of the type you have here.

Pop sci is more concerned wigh sounding cool and less concerned with clear communication, so has yet to catch up with that.
friend said:
How can an object both be a black hole and not be a black hole?
It can't. Gravity in relativity does not depend on relativistic mass, and people assuming it is basically Newtonian gravity with relativistic mass plugged in is one more reason to abandon the concept. The source term for gravity in relativity is the stress-energy tensor, which includes mass and other terms related to momentum and pressure. But they don't simply add to the mass in any naive way.
 

Similar threads

  • · Replies 31 ·
2
Replies
31
Views
2K
  • · Replies 40 ·
2
Replies
40
Views
4K
  • · Replies 22 ·
Replies
22
Views
2K
  • · Replies 67 ·
3
Replies
67
Views
6K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 51 ·
2
Replies
51
Views
2K
  • · Replies 23 ·
Replies
23
Views
4K
  • · Replies 4 ·
Replies
4
Views
2K