I have a new question. Stephen Hawking said in one of his "Grand Design" series that clock would come to rest inside a black hole. Not only this, he also said that time does not exist inside a black hole. Can anyone explain why does that happen?

Black Holes are pretty massive (i.e. very very...). Every mass curves the space-time (general relativity). As the curvature increases the time moves more and more slowly (Time moves slower on the earth surface-due to this huge earth-than far away in the satellites- though only by a small fraction). A black hole causes infinite curvature to space-time. Now 1/∞=0. Therefore, time stops!!!
This is what I understood. Need not be completely true.

"The effect of time dilation in a black hole is caused by the movement from a high gravitational potential to a lower one (basically, getting closer to a massive object). It is predicted by relativity, but impossible to prove empirically since we cannot go in and out of a black hole at will. It is only the observer within the hole for which time stops, though.

Time is a relative issue. The rate at which time moves forward slows down when you're near a very heavy object compared to the rate at which time moves forward for someone who is not near that very heavy object.

If you would be inside a black hole, a day would still last a day for you. However the time outside that black hole would have passed at a much higher rate.
Would you be truly at the center of a black hole where gravity is infinite, then time would stop compared to the world outside the black hole. So in a fraction of a second inside the black hole, eternity has passed outside the black hole."
- taken from wikianswers.com

lokesh sharma.....the gravity at the center would never be infinite....as its opposite...it would be zero...you will feel ur self weightless at the center of the earth......assumption of cavity says so...

Who said time stops? This question is slightly more subtle than presented here. It boils down to what you mean by time, and who holds the clocks by which you count time. Consider Alice and Bob. Alice and Bob sit way outside a very large black hole and synchronize a clock that each of them carries. They do this by having their clocks be light bulbs that blink on and off at the same rate. Alice then waves goodbye to Bob and falls into the black hole. As Alice passes through the horizon of the black hole she still has a clock, it still blinks, she still sees it, etc. Nothing particularly strange happens to her notion of time. (Eventually she'll hit a singularity though, which will be unpleasant.)

What does Bob see? Bob sees the light signals from Alice's clock blink for a while. As she gets closer to the black hole, the light frequency that Bob sees decreases. The frequency decreases because the kinetic energy of light is proportional to its frequency, and by conservation of energy kinetic energy is lost as the light from Alice's clock travels from near the black hole to Bob due to the gravitational potential energy difference. So Bob sees Alice's clock blink at a lower and lower frequency, i.e. it takes longer and longer for each blink to be recorded by Bob. Her time has infinitely slowed down according to Bob and so according to him her time has 'stopped'.

HOWEVER, according to Alice her time has been perfectly fine. My point is: you can't ask these types of questions as 'does time stop' because that's not enough information in relativity. You have to ask questions about the relative time measured by two observers doing different things. Those questions are what are truly physically meaningful.

well, that was pretty much of it. But let me put my query in a better form:
I was actually watching a documentary by Hawking regarding God making the universe or not; he made a point that before there was anything, there was only a black hole. Then it disintegrated into many parts, one of which is the planet we walk over. Anyhow, he said, that before anything there was black hole and there was no time there, so in other words there was no time for God to create the universe. My question now can be put in the other way:

According to which "somebody" the time in black hole was not existing? Who was outside the black hole to measure the time?

Another point which i want you to clarify is : WHAT EXACTLY IS THIS TIME??
What's its fundamental definition?

hat does it mean to have "no time"?
This concept is very hard to imagine. It's literally beyond our perception.
If we considered another unilateral concept, like "distance" for example. To say an entity has no distance, being exempt from distance constraints, this implies that the smallest unit of distance is equal to the largest. A micron isn't different from "infinite incidents of microns" for such an entity.
Assuming the same applies to time, a traveller through a black-hole will observe events slowing down as he gets nearer to the center, once he's at the center time will not come to halt, it will collapse. Such traveller's perception -hence the essence of his observation- will take a leap and be exempt from time constraints. Events wouldn't have to happen consecutively as we expect. If time dimension is excluded from events as it should be for an observer at the center of a blackhole, everything would have happened already. Even the process of "observing" as we understand it will be wholestic and instantaneous.
It's very hard to imagine a human observer surviving that kind of change. I guess the poor guy will die from tidal forces as Rettaw mentioned, or if by some means he survived that, he'll lose consciousness as we know it in the process of timedilation to zero/infinite-time.

the simplest answer is this - do not think of time as a separate entity, space and time are permanently linked together. now you are supposed to know that clocks run slower on the surface of earth than on the top floor of a high building. if you have two clocks and you take one on the roof of empire state building and keep one on the ground floor, you will find that the clock on ground floor runs slow. and that is because of stronger gravity. so gravity makes clocks run slow and boy you know the gravity is so so strong inside the black hole. the reason gravity makes time slow is because gravity is the result of curvature of space time as explained by general theory of relativity. and if you look at a black hole the space time is infinitely curved. time does not stop inside the black hole, time starts slowing down as you approach the black hole and time stops at the singularity because at singularity the curvature of space time (or the gravity) is infinite. there is no space and hence there is no time. hope that helps.

Perhaps look up the Schwarzschild metric for the mathematics if you're interested.

Mostly lifted (unashamedly) from Leonard Susskind's excellent book 'The Black Hole Wars', here's an (I hope) intuitive explanation as to why, according to an observer far away from the black hole, a clock falling into said black hole will tick slower and slower, asymptotically reaching a frequency of 0 Hertz at the event horizon.

Imagine this clock worked by emitting a pulse of light every microsecond (according to itself). Note that this is a perfectly legitimate way of making a clock. In flat spacetime, the travel times of two adjacent pulses to reach you will cancel, leaving a time difference between the two. (Note: If the clock is has a nonzero velocity, use the special relativistic Doppler effect to work out the discrepancy between the time difference of sending out pulses (1 microsecond) and you receiving the pulses. If it's stationary, you will measure the same interval as the clock does (namely, 1 microsecond)).

Very well, but what happens in a gravitational field? The equivalence principle (see here) can help. It basically means that gravity is the same in all respects as any other type of mechanical acceleration. (This is a big deal, as before it seemed almost coincidental that gravitational acceleration for a given force was inversely proportional to the same thing (mass) as mechanical acceleration (that is, that \( F=mg \) and \(F=ma \) ). So, similar to the classical example, as the clock's velocity away from you increases, the period of ticks increases (frequency decreases) (more on it here ).

You could look at it without GR (although with QM).

The gravitational potential energy at \( r \) away from the mass is \( U=\frac{-MmG}{r} \). Using the de Broglie hypothesis, \( mc^2=hf_0 \), and so \( U=\frac{-Mhf_0}{rc^2} \). The Schwartzchild radius (event horizon) of a black hole of mass \( M \) is \( R_S=\frac{2GM}{c^2} \).

Note that as the photon leaves the gravitational potential well, it loses all the gravitational potential energy it 'had' (it's a photon and has no mass, but this works! ("shut up and calculate"!)).
So,
\( hf= hf_0 (1-\frac{Mh}{rc^2}), \frac{f}{f_0}=(1-\frac{Mh}{rc^2}) \)
Where \( r \) is the radius the clock was upon emitting the light. Note that at the Swartzchild radius, \( f=0 \) (derivation: set escape velocity to the speed of light), meaning that \( T \rightarrow \infty \): time seems to have stopped on the clock according to the observer outside.

Note that the outside observer will keep receiving signals for infinite time, but the time intervals between them will become so far apart (and the short pulses redshifted to have a much longer wavelength until they're impossible to detect) that no instrument can observe them.

Can u please explain qualitatively without equations..this post was useful..but to understand fully in all aspects can you explain in a logical manner but not intuitive..if possible

The second part uses a gravitational field model (i.e. Newtonian model) rather than curvature of spacetime, but the conclusions are similar to relativity's. Basically, a photon loses energy as it gains gravitational potential energy leaving the black hole's gravitational potential well, and the only way it can lose this energy is through reducing its frequency (as \( E=hf \) ). If it lost so much energy that the frequency is 0, the light would be impossible to see, as it has an infinite period. Because this doesn't use relativity I suppose it skirts the question a bit, but it is similar to relativistic arguments.

As in the special theory of relativity the clock in the accelerating field ticks slowly with respect to the one at rest that means moving clock ticks slowly and gravity is the a kind of acceleration field.
In the case os black holes the gravity is very very very high ( say infinite) so it provides great acceleration and that's why the time stops

According to Albert Einstein.. dark is an absence of light.. we cant study dark but light can. so it means we don't know what will be the time inside the black hole. we can measure the light but dark can't. but other said that even the light cannot pass into the black hole, and Einstein said the stops when you approach the speed of light. i guess time stops inside the block hole or unless will move you to the past.

hey guys....tell me if i am right...
i think we are oscillating ..our universe is connected with two systems....one is black hole and one is white hole..at one side.....black holes tucks in the every material and white hole (we named big bang) is ommiting those things.....

flow of time depends on two things speed and gravitational force. but in a black hole it dostort space time fabric infinitesimally and flow of time slows down to zero value.

when a body comes in tha gravitional field than due to high gravitional force body moves with the speed nearly light so acording to theory of relativity when speed become more faster than scale of measuring time become smaller so at speed of light time stop.

Black Holes are pretty massive (i.e. very very...). Every mass curves the space-time (general relativity). As the curvature increases the time moves more and more slowly (Time moves slower on the earth surface-due to this huge earth-than far away in the satellites- though only by a small fraction). A black hole causes infinite curvature to space-time. Now 1/∞=0. Therefore, time stops!!! This is what I understood. Need not be completely tru

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TopNewestBlack Holes are pretty massive (i.e. very very...). Every mass curves the space-time (general relativity). As the curvature increases the time moves more and more slowly (Time moves slower on the earth surface-due to this huge earth-than far away in the satellites- though only by a small fraction). A black hole causes infinite curvature to space-time. Now 1/∞=0. Therefore, time stops!!! This is what I understood. Need not be completely true.

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"The effect of time dilation in a black hole is caused by the movement from a high gravitational potential to a lower one (basically, getting closer to a massive object). It is predicted by relativity, but impossible to prove empirically since we cannot go in and out of a black hole at will. It is only the observer within the hole for which time stops, though.

Time is a relative issue. The rate at which time moves forward slows down when you're near a very heavy object compared to the rate at which time moves forward for someone who is not near that very heavy object.

If you would be inside a black hole, a day would still last a day for you. However the time outside that black hole would have passed at a much higher rate. Would you be truly at the center of a black hole where gravity is infinite, then time would stop compared to the world outside the black hole. So in a fraction of a second inside the black hole, eternity has passed outside the black hole." - taken from wikianswers.com

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lokesh sharma.....the gravity at the center would never be infinite....as its opposite...it would be zero...you will feel ur self weightless at the center of the earth......assumption of cavity says so...

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That's what make black holes special.

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Who said time stops? This question is slightly more subtle than presented here. It boils down to what you mean by time, and who holds the clocks by which you count time. Consider Alice and Bob. Alice and Bob sit way outside a very large black hole and synchronize a clock that each of them carries. They do this by having their clocks be light bulbs that blink on and off at the same rate. Alice then waves goodbye to Bob and falls into the black hole. As Alice passes through the horizon of the black hole she still has a clock, it still blinks, she still sees it, etc. Nothing particularly strange happens to her notion of time. (Eventually she'll hit a singularity though, which will be unpleasant.)

What does Bob see? Bob sees the light signals from Alice's clock blink for a while. As she gets closer to the black hole, the light frequency that Bob sees decreases. The frequency decreases because the kinetic energy of light is proportional to its frequency, and by conservation of energy kinetic energy is lost as the light from Alice's clock travels from near the black hole to Bob due to the gravitational potential energy difference. So Bob sees Alice's clock blink at a lower and lower frequency, i.e. it takes longer and longer for each blink to be recorded by Bob. Her time has infinitely slowed down according to Bob and so according to him her time has 'stopped'.

HOWEVER, according to Alice her time has been perfectly fine. My point is: you can't ask these types of questions as 'does time stop' because that's not enough information in relativity. You have to ask questions about the relative time measured by two observers doing different things. Those questions are what are truly physically meaningful.

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Why gravity distorts space and time?

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it proves the existence of mass there...every type of quantity has some type of response..; )

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well, that was pretty much of it. But let me put my query in a better form: I was actually watching a documentary by Hawking regarding God making the universe or not; he made a point that before there was anything, there was only a black hole. Then it disintegrated into many parts, one of which is the planet we walk over. Anyhow, he said, that before anything there was black hole and there was no time there, so in other words there was no time for God to create the universe. My question now can be put in the other way:

According to which "somebody" the time in black hole was not existing? Who was outside the black hole to measure the time?

Another point which i want you to clarify is : WHAT EXACTLY IS THIS TIME?? What's its fundamental definition?

Log in to reply

hat does it mean to have "no time"? This concept is very hard to imagine. It's literally beyond our perception. If we considered another unilateral concept, like "distance" for example. To say an entity has no distance, being exempt from distance constraints, this implies that the smallest unit of distance is equal to the largest. A micron isn't different from "infinite incidents of microns" for such an entity. Assuming the same applies to time, a traveller through a black-hole will observe events slowing down as he gets nearer to the center, once he's at the center time will not come to halt, it will collapse. Such traveller's perception -hence the essence of his observation- will take a leap and be exempt from time constraints. Events wouldn't have to happen consecutively as we expect. If time dimension is excluded from events as it should be for an observer at the center of a blackhole, everything would have happened already. Even the process of "observing" as we understand it will be wholestic and instantaneous. It's very hard to imagine a human observer surviving that kind of change. I guess the poor guy will die from tidal forces as Rettaw mentioned, or if by some means he survived that, he'll lose consciousness as we know it in the process of timedilation to zero/infinite-time.

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the simplest answer is this - do not think of time as a separate entity, space and time are permanently linked together. now you are supposed to know that clocks run slower on the surface of earth than on the top floor of a high building. if you have two clocks and you take one on the roof of empire state building and keep one on the ground floor, you will find that the clock on ground floor runs slow. and that is because of stronger gravity. so gravity makes clocks run slow and boy you know the gravity is so so strong inside the black hole. the reason gravity makes time slow is because gravity is the result of curvature of space time as explained by general theory of relativity. and if you look at a black hole the space time is infinitely curved. time does not stop inside the black hole, time starts slowing down as you approach the black hole and time stops at the singularity because at singularity the curvature of space time (or the gravity) is infinite. there is no space and hence there is no time. hope that helps.

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Perhaps look up the Schwarzschild metric for the mathematics if you're interested.

Mostly lifted (unashamedly) from Leonard Susskind's excellent book 'The Black Hole Wars', here's an (I hope) intuitive explanation as to why, according to an observer far away from the black hole, a clock falling into said black hole will tick slower and slower, asymptotically reaching a frequency of 0 Hertz at the event horizon.

Imagine this clock worked by emitting a pulse of light every microsecond (according to itself). Note that this is a perfectly legitimate way of making a clock. In flat spacetime, the travel times of two adjacent pulses to reach you will cancel, leaving a time difference between the two. (Note: If the clock is has a nonzero velocity, use the special relativistic Doppler effect to work out the discrepancy between the time difference of sending out pulses (1 microsecond) and you receiving the pulses. If it's stationary, you will measure the same interval as the clock does (namely, 1 microsecond)).

Very well, but what happens in a gravitational field? The equivalence principle (see here) can help. It basically means that gravity is the same in all respects as any other type of mechanical acceleration. (This is a big deal, as before it seemed almost coincidental that gravitational acceleration for a given force was inversely proportional to the same thing (mass) as mechanical acceleration (that is, that \( F=mg \)

and\(F=ma \) ). So, similar to the classical example, as the clock's velocity away from you increases, the period of ticks increases (frequency decreases) (more on it here ).You could look at it without GR (although with QM).

The gravitational potential energy at \( r \) away from the mass is \( U=\frac{-MmG}{r} \). Using the de Broglie hypothesis, \( mc^2=hf_0 \), and so \( U=\frac{-Mhf_0}{rc^2} \). The Schwartzchild radius (event horizon) of a black hole of mass \( M \) is \( R_S=\frac{2GM}{c^2} \).

Note that as the photon leaves the gravitational potential well, it loses all the gravitational potential energy it 'had' (it's a photon and has no mass, but this works! ("shut up and calculate"!)). So, \( hf= hf_0 (1-\frac{Mh}{rc^2}), \frac{f}{f_0}=(1-\frac{Mh}{rc^2}) \) Where \( r \) is the radius the clock was upon emitting the light. Note that at the Swartzchild radius, \( f=0 \) (derivation: set escape velocity to the speed of light), meaning that \( T \rightarrow \infty \): time seems to have stopped on the clock according to the observer outside.

Note that the outside observer will keep receiving signals for infinite time, but the time intervals between them will become so far apart (and the short pulses redshifted to have a

muchlonger wavelength until they're impossible to detect) that no instrument can observe them.Source 1 Source 2

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Can u please explain qualitatively without equations..this post was useful..but to understand fully in all aspects can you explain in a logical manner but not intuitive..if possible

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The second part uses a gravitational field model (i.e. Newtonian model) rather than curvature of spacetime, but the conclusions are similar to relativity's. Basically, a photon loses energy as it gains gravitational potential energy leaving the black hole's gravitational potential well, and the only way it can lose this energy is through reducing its frequency (as \( E=hf \) ). If it lost so much energy that the frequency is 0, the light would be impossible to see, as it has an infinite period. Because this doesn't use relativity I suppose it skirts the question a bit, but it

issimilar to relativistic arguments.Log in to reply

As in the special theory of relativity the clock in the accelerating field ticks slowly with respect to the one at rest that means moving clock ticks slowly and gravity is the a kind of acceleration field. In the case os black holes the gravity is very very very high ( say infinite) so it provides great acceleration and that's why the time stops

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According to Albert Einstein.. dark is an absence of light.. we cant study dark but light can. so it means we don't know what will be the time inside the black hole. we can measure the light but dark can't. but other said that even the light cannot pass into the black hole, and Einstein said the stops when you approach the speed of light. i guess time stops inside the block hole or unless will move you to the past.

Log in to reply

hey guys....tell me if i am right... i think we are oscillating ..our universe is connected with two systems....one is black hole and one is white hole..at one side.....black holes tucks in the every material and white hole (we named big bang) is ommiting those things.....

Log in to reply

flow of time depends on two things speed and gravitational force. but in a black hole it dostort space time fabric infinitesimally and flow of time slows down to zero value.

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AS AT BH LIGHT CANT COME OUT N AT LIGHT SPEED TIME IS STEADY SO IN BH TIME DOES NOT EXIST.....AM I RIGHT

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when a body comes in tha gravitional field than due to high gravitional force body moves with the speed nearly light so acording to theory of relativity when speed become more faster than scale of measuring time become smaller so at speed of light time stop.

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what is a black hole

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Black Holes are pretty massive (i.e. very very...). Every mass curves the space-time (general relativity). As the curvature increases the time moves more and more slowly (Time moves slower on the earth surface-due to this huge earth-than far away in the satellites- though only by a small fraction). A black hole causes infinite curvature to space-time. Now 1/∞=0. Therefore, time stops!!! This is what I understood. Need not be completely tru

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do u copy paste d solns....

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