# How exactly does time slow down near a black hole?

To oversimplify the explanation, you have to understand the
curvature of space time around a black hole. The basic principle is that
because of the curvature of spacetime around a black hole, the amount of
"distance" a beam of light has to cover is greater near a black hole.
However, to an observer in that gravitational field, light must appear to
always be 300,000 km/sec, time has to slow down for that individual as compared
to someone outside that gravitational field as related by the time/distance
relationship of speed.

If acceleration is equivalent to gravitation, it follows that
the predictions of Special Relativity must also be valid for very strong
gravitational fields. The curvature of spacetime by matter therefore not only
stretches or shrinks distances, depending on their direction with respect to
the gravitational field, but also appears to slow down the flow of time. This
effect is called gravitational time dilation. In most circumstances, such
gravitational time dilation is minuscule and hardly observable, but it can
become very significant when spacetime is curved by a massive object, such as a
black hole.

A black
hole is the most compact matter imaginable. It is an extremely massive and
dense object in space that is thought to be formed by a star collapsing under
its own gravity. Black holes are black, because nothing, not even light, can
escape from its extreme gravity. The existence of black holes is not yet firmly
established. Major advances in computation are only now enabling scientists to
simulate how black holes form, evolve, and interact. They are betting on
powerful instruments now under construction to confirm that these exotic
objects actually exist.

Consider the question: Time slows
down? Do we know what is time? That is the first problem. Without going into
details consider this definition: "Time is presence of motion and
forces." This definition is derived as follows: Consider two masses
orbiting around each other held together by a force. Now imagine that time slow
down in this system. You expect slowing of motion. What about forces between
the two masses they should weaken at the same rate to keep in the same orbit.
If the time and motion stopped completely the force should disappear.

Now the next question is where does
time come from or how is it produced? We live in an expanding universe and time
is slower near intense gravity where expansion of space is slower. So time is
related to expansion of space or caused by expansion of space. So around
neutron stars and black holes expansion of space is slow causing slowing of
time. You may ask why does time slow with motion? Well if total motion allowed
by expansion of space is constant then if you increase the external motion of a
mass then internal motion slows down. This we seee as slowing of time.

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