# Special Relativity and Black Holes

• 2.71828
In summary, the conversation discusses the relationship between Einstein's special relativity and the existence of black holes. It is determined that black holes are relativistic in nature due to the fact that mass alone does not generate gravity, rather it is the stress-energy tensor that determines the presence of a black hole. Additionally, it is noted that the energy density at a point increases by a factor of gamma^2 due to a "boost", which is a relativistic velocity transformation. The conversation concludes with the forum member expressing their enjoyment of the website and their interest in learning more about general relativity.
2.71828
Hello Forum,

This is my first post :)

Einstein's special relativity finds that length is contracted if the observer is moving at high speeds. Now, assume there is a mass at rest in space that is below critical density of a black hole. If an observer were traveling at high speeds past this mass, the object will contract; therefore, placing the object above critical density. Are the existence of black holes relativistic?

Yes. In GR, mass, or density, alone does not generate gravity. Gravity is generated by the stress-energy tensor, which is a fully relativistic object. The density is a component of the stress-energy tensor, but its only one component. So while the density term does increase, other terms appear and the net result is that you don't wind up with a black hole.

It may (or may not) be interesting to note that the energy density at a point (T_00 of the stress-energy tensor) increases by a factor of gamma^2 due to a "boost" (a relativistic velocity transformation), because the stress-energy tensor measure energy, and not only do you have a smaller volume after the boost, but the smaller volume contains more energy due to the motion of the particles contained within. Here gamma is 1/sqrt(1-v^2/c^2), the usual relativistic term.

pervect said:
Yes. In GR, mass, or density, alone does not generate gravity. Gravity is generated by the stress-energy tensor, which is a fully relativistic object.

Thanks Pervect. Looks like I got some reading to do on GR. I'm really enjoying this forum website :)

## 1. What is special relativity?

Special relativity is a theory developed by Albert Einstein that describes the relationship between space and time in the presence of high speeds. It states that the laws of physics are the same for all observers moving at a constant velocity, and that the speed of light is the same for all observers regardless of their relative motion.

## 2. What is a black hole?

A black hole is a region of space with a gravitational pull so strong that nothing, not even light, can escape from it. It is formed when a large amount of mass is compressed into a small volume, causing a strong curvature in space-time.

## 3. How does special relativity affect black holes?

Special relativity plays a key role in understanding the behavior of black holes. It predicts the existence of event horizons, the boundary beyond which not even light can escape the gravitational pull of a black hole. It also explains the time dilation and length contraction observed near black holes.

## 4. Can anything escape from a black hole?

According to our current understanding, nothing can escape from a black hole once it has crossed the event horizon. However, there is still ongoing research and debate about the possibility of theoretical phenomena such as Hawking radiation, which could potentially allow particles to escape from a black hole.

## 5. What are the implications of special relativity and black holes?

The study of special relativity and black holes has led to a better understanding of the nature of space and time, as well as the behavior of matter under extreme conditions. It has also opened up new areas of research in astrophysics and cosmology, and has practical applications in technologies such as GPS satellites and particle accelerators.

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