As we have described a black hole, we never can observe one of them because they would not reflect or emit any radiation or particle. But there are certain effects that can be detected. One of these effects is the gravitational effect on a neighboring star.
Suppose a binary star system (two stars so close to turning the one around the other) in which one of the stars is visible and than we can calculate its distance from Earth, and its mass. The visible star will make a few oscillatory movements in space due to the gravitational attraction of the invisible star. From these movements we can calculate the mass of the invisible star .
If this invisible star exceeds a mass of about 2.5 times the mass of our Sun, we must assume that it’s a black hole.
According to the general relativity theory of Einstein, near a large mass, time happens slowly due to the gravitational action.
Einstein deduced (as we read in his book “The Meaning of Relativity”) the following formula
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It is possible to find the relationship between the mass and the radius of a spherical black hole keeping in mind that the maximum speed that could reach an object, according to the theory of relativity, it is the speed of light.
The scape velocity in the surface of a spherical star will be the maximum velocity that could reach an object in order to stay in orbit around the star. This will happen when the kinetic energy of one object is equal to the potential energy due to the star gravitational attraction. Continue reading
It is said that the gravitational waves are a consequence of the theory of relativity, of Albert Einstein, but it is difficult to find texts in which they argue why this relationship, except that they are arising from the study of the Einstein field equations. Let’s try to apply a little logic and some knowledge of relativity in this issue.
The key question is the following question: is possible to convey the effect of gravity faster than light?
Let’s assume for a moment that that is possible. For simplicity suppose that gravity affects instantaneously, or at infinite speed. That has a very important consequence, that is that you could create an experiment to transmit information instantaneously through instantaneous changes of gravitational effects. …more about gravitational waves