Physics Help Forum Time and gravitational field.

 Special and General Relativity Special and General Relativity Physics Help Forum

 Jun 27th 2017, 04:09 AM #1 Member   Join Date: Feb 2017 Posts: 82 Time and gravitational field. We all know that if there is a mountain, at the base of this mountain time will move slowly than at the peak of this mountain. So this gravity that causes time to move slowly is infinite in a black hole. So time stops at the black hole. Since a lot more gravity means slower time passes by. so if black holes have infinite gravity time stops. But my question is that there are 2 things. 1. Time stops near a black hole. 2. Time stops as we move at the speed of light. How are these two related?
 Jun 27th 2017, 04:20 AM #2 Member   Join Date: Feb 2017 Posts: 82 Gravitational acceleration. I thought about it so here i try to answer my question. The escape velocity from the surface (i.e., the event horizon) of a Black Hole is exactly c, the speed of light. So that means if you throw an object inside the black hole it will accelerate at the speed of light and as it does so if we have a clock attached to this object that is thrown in then this clock will stop. Time will stop in a black hole. This will happen because if we travel at the speed of light time stops. Is this correct? Last edited by avito009; Jun 27th 2017 at 04:22 AM.
 Jun 27th 2017, 05:16 AM #3 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 517 You can't accelerate an object with mass to the speed of light. Gravity, black holes is part of general relativity, not special relativity, which is a bit beyond me so I will let someone else answer the other parts of your question. topsquark likes this.
Jun 27th 2017, 06:14 AM   #4
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 Originally Posted by avito009 I thought about it so here i try to answer my question. The escape velocity from the surface (i.e., the event horizon) of a Black Hole is exactly c, the speed of light. So that means if you throw an object inside the black hole it will accelerate at the speed of light and as it does so if we have a clock attached to this object that is thrown in then this clock will stop. Time will stop in a black hole. This will happen because if we travel at the speed of light time stops. Is this correct?
First, "accelerate at the speed of light" makes no sense. The speed of light is a speed, not an acceleration. Second, saying that the escape velocity is c only means that an object would have to be going at least c (which is, of course, impossible) in order to escape the black hole. It does not mean that an object that falls into a black hole will suddenly be moving at c.

Jul 2nd 2017, 10:47 AM   #5
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 Originally Posted by avito009 We all know that if there is a mountain, at the base of this mountain time will move slowly than at the peak of this mountain. So this gravity that causes time to move slowly is infinite in a black hole. So time stops at the black hole. Since a lot more gravity means slower time passes by. so if black holes have infinite gravity time stops. But my question is that there are 2 things. 1. Time stops near a black hole. 2. Time stops as we move at the speed of light. How are these two related?
It's an operational impossibility to demonstrate time stopping at the evenr horizon of a black hole. But time most certainly does not stop near a black hole. Same applies to moving at the speed of light. No clock can move at that speed, nor can any physical process exist in a frame of reference moving that fast so it too is meaningless to speak of time stopping at the speed of light.

Jul 2nd 2017, 12:25 PM   #6

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 Originally Posted by avito009 So this gravity that causes time to move slowly is infinite in a black hole. So time stops at the black hole. Since a lot more gravity means slower time passes by. so if black holes have infinite gravity time stops.
What are you calling "infinite gravity?" If there is indeed a singularity inside the hole then we can't ever measure it. For all intents and purposes an object dropped into a black hole leaves our space-time, never to return. If we can't measure something we have to leave it to posterity to find a way to do it. (Though I'm betting that this one is going to be impossible to crack.)

-Dan
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Jul 5th 2017, 01:43 PM   #7
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 Originally Posted by topsquark What are you calling "infinite gravity?" If there is indeed a singularity inside the hole then we can't ever measure it. For all intents and purposes an object dropped into a black hole leaves our space-time, never to return. If we can't measure something we have to leave it to posterity to find a way to do it. (Though I'm betting that this one is going to be impossible to crack.) -Dan
You're quite right, Dan. Great catch! A friend of mine put it nicely, and he's the author of a well-known GR text and well-known in the field of relativity. As he said, we don't know exactly what happens to particles that fall into a black hole. From the perspective of an external observer, the in falling particles never cross the horizon in any finite amount of time--they slow down as they approach the horizon, and only approach it asymptotically. So the question about what happens to in falling particles in a black hole is meaningless for an external observer. Presumably an observer who falls with the particles will find out what happens, but he can't communicate this knowledge to the external observers. This raises fundamental questions about the operational meaning of any predictions made from theory about what happens inside a black hole. Are such statements in the realm of physics or of metaphysics?

 Jul 9th 2017, 07:28 AM #8 Member   Join Date: Feb 2017 Posts: 82 Einstein equivalence principle. Please help me out figure how do I use the equivalence principle in this context. Gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration, an idea called the “principle of equivalence." So does this mean that the infinite gravity inside a black hole is the same as acceleration of an object that goes inside a black hole?
 Jul 15th 2017, 08:26 PM #9 Junior Member   Join Date: May 2017 Posts: 12 I've got to chime in on this because i love Einstein's thought experiments. The clock on the train is perpendicular to the direction of travel, and the (theoretical) train is moving at the speed of light. The clock face is at a size so that the outside end of the second hand moves 1/8 of an inch each second. So the observer on the train sees the clock's second hand move normally; 1/8 of an inch per second. But, a stationary (also theoretical) observer sees the clock's hand move 186,000 miles for every second that passes on the train. Thus, we have 1/8 divided by 186,000 movement of the clock's hand for each second of the stationary observer. Admittedly a very small number, no where near as small as a planck, but not stopped by any means. Using myself as the casual observer and my limited sight it might appear that time had stopped on the train, but the math says differently. As far as defining what's inside a black hole, another universe, dimension, the future, the past, it's all speculation; But my opinion is that the two are not related because i don't know what time or the speed of light is inside a black hole.
Jul 15th 2017, 09:08 PM   #10
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 Originally Posted by avito009 Please help me out figure how do I use the equivalence principle in this context. Gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration, an idea called the “principle of equivalence."
That is a common misunderstanding. Einstein certainly never thought that. This has to do with what's known as inertial forces. Some people like to think that inertial forces "aren't real" forces. But as I said, Einstein wasn't one of them.

Here's how Einstein viewed gravity. It also explains why inertial force like gravity are "real" forces.

Inertial Force

I could explain it myself but I know its controversial and its certain to invoke responses like "Everyone knows gravity isn't a real force" and I hate those kinds of responses because its used to avoid real discussion and reasons. can provide all the reasoning required but then it will be said to be my personal opinion. So I took quotations from popular GR texts and placed them in that page.

 Originally Posted by avito009 So does this mean that the infinite gravity inside a black hole is the same as acceleration of an object that goes inside a black hole?
I don't know what you mean by "gravity" so please explain. Some people mean "tidal accelerations" which is synonymous with spacetime curvature while others, like Einstein and myself, mean a measure of the inertial acceleration of an object placed at a point.

But there's no infinite gravity inside a black hole except at the origin where there is a singularity.

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