A question about time

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I have a hard time understanding what physics thinks time is and what its effects are hence the following question.

If time is slowed by gravity like a black hole, why don't objects approaching a black hole's event horizon appear to slow down to an outside observer?
 

Drakkith

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Are you sure they don't appear to slow down?
 
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I have a hard time understanding what physics thinks time is and what its effects are hence the following question.

If time is slowed by gravity like a black hole, why don't objects approaching a black hole's event horizon appear to slow down to an outside observer?
Giving out a definition of time depends as to how you see it. Speaking in general relativity,time is considered in reference frames, object A relative to object B,etc.
As far as the objects light is considered,yes they do slow down in fact it's one of the relativistic property also known as Time dilation.Which in essence says that the closer an objects velocity reaches to 'c' the obvious time dilation occurs.Once the objects goes through the EH, to the relative stationary observer the object being sucked in would appear frozen while in the reference frame of the falling object it'd obviously be annihilated metaphorically speaking.
Relativity is made up of four dimensions of which three are space and the last one is not time but time x square root of -1 ( excuse my lack of latex usability)
Time was said to be constant during the mechanistic era until it was found out that speed of light isn't infinite,constant,hence the notion of constant time was rolling down.

Note :Gravity doesn't slow time but wherever dense matter is present significant space-time curves.

Regards,
ibysaiyan
 

Drakkith

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Note :Gravity doesn't slow time but wherever dense matter is present significant space-time curves.
What do you mean? Isn't that all one in the same? IE isn't gravity the curvature of spacetime?
 
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What do you mean? Isn't that all one in the same? IE isn't gravity the curvature of spacetime?
Sorry,that was a mistake on my part.Yes gravity is a property of space-time.I was actually referring to objects with greater mass = greater curvature,more area for light to travel.
Regards,
ibysaiyan
 

Drakkith

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Sorry,that was a mistake on my part.Yes gravity is a property of space-time.I was actually referring to objects with greater mass = greater curvature,more area for light to travel.
Regards,
ibysaiyan
Ah, ok.
 
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So as the object approached the black hole, it would slow down. Would an observer actually see the object 'go into' the black hole? It seems to me the object would appear to be stationary at the event horizon to an external observer; whereas the object would actually be 'in' the black hole. Is that correct?
 

DaveC426913

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So as the object approached the black hole, it would slow down. Would an observer actually see the object 'go into' the black hole? It seems to me the object would appear to be stationary at the event horizon to an external observer; whereas the object would actually be 'in' the black hole. Is that correct?
Yes. That is correct.

Though it has less to do with time dilation, and more to do with light trying to climb out of the well.
 

Drakkith

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Yes. That is correct.

Though it has less to do with time dilation, and more to do with light trying to climb out of the well.
What does the light escaping the gravity have to do with time dilation? I thought that only affected the frequency of the light.
 

DaveC426913

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What does the light escaping the gravity have to do with time dilation?
It's called gravitational time dilation.

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[43] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.
http://en.wikipedia.org/wiki/Black_hole#Event_horizon
 

Drakkith

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If this is correct than to an outside observer nothing would ever appear to enter a black hole? Everything would always look stuck outside the event horizon?

The gravity source for the central black hole of a galaxy must be tremendous, wouldn't the stars close to it appear to be stopped? Wouldn't there be a frozen halo of suns around it? I thought we could observe suns near the galaxy core and I never heard anyone say anything about observing this stuff. Not slow orbits even. Or are these effects too slight to see?
 

DaveC426913

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If this is correct than to an outside observer nothing would ever appear to enter a black hole? Everything would always look stuck outside the event horizon?
In theory, yes. In practrice it dims and redshifts to invisibility first.

Remember, the image you see of it is stretched over time. i.e. the last second of photons leaving the object is what is seen over the rest of eternity, so they are necessarily more and more attentuated. Pretty soon you're getting, like, one photon a second. i.e. too dim to see. Additionally redshifting means they're lower and lower energy - radio waves, not light.


The gravity source for the central black hole of a galaxy must be tremendous, wouldn't the stars close to it appear to be stopped? Wouldn't there be a frozen halo of suns around it? I thought we could observe suns near the galaxy core and I never heard anyone say anything about observing this stuff. Not slow orbits even. Or are these effects too slight to see?
1] We can't actually see the centre of galaxies.
2] BHs are surrounded by a halo of infalling matter, which radiates very strongly as it gets squeezed. What we see is a lot of activity in the X-ray band.
 
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In theory, yes. In practrice it dims and redshifts to invisibility first.

Remember, the image you see of it is stretched over time. i.e. the last second of photons leaving the object is what is seen over the rest of eternity, so they are necessarily more and more attentuated. Pretty soon you're getting, like, one photon a second. i.e. too dim to see. Additionally redshifting means they're lower and lower energy - radio waves, not light.



1] We can't actually see the centre of galaxies.
2] BHs are surrounded by a halo of infalling matter, which radiates very strongly as it gets squeezed. What we see is a lot of activity in the X-ray band.
That's what I mean, why is that matter infalling? How can a black hole even grow if matter is stuck in time outside it? Something doesn't make sense, there seems to be a contradiction here or our idea of time is wrong.

I'm just an engineer but if I had to guess I would say that an outside observer would see nothing slowing down going into a black hole. The affect on time is just a local change in the time references for what's going in and apparent time would be slowed for it but to us it would go in just as fast as normal. That would seem to be the only explanation that would allow a black hole to grow and actually suck in matter.

Do we actually have any observational evidence one way or the other or are we relying on theory?
 

DaveC426913

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That's what I mean, why is that matter infalling? How can a black hole even grow if matter is stuck in time outside it? Something doesn't make sense, there seems to be a contradiction here or our idea of time is wrong.
No. The time dilation is only an observed phenomeon from a vantage point away from the BH. The infalling matter experiences no such problem, and will rapidly fall right to the centre of the BH.
 

WannabeNewton

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That's what I mean, why is that matter infalling? How can a black hole even grow if matter is stuck in time outside it? Something doesn't make sense, there seems to be a contradiction here or our idea of time is wrong.

I'm just an engineer but if I had to guess I would say that an outside observer would see nothing slowing down going into a black hole. The affect on time is just a local change in the time references for what's going in and apparent time would be slowed for it but to us it would go in just as fast as normal. That would seem to be the only explanation that would allow a black hole to grow and actually suck in matter.

Do we actually have any observational evidence one way or the other or are we relying on theory?
Remember that the property of nearby observers seeing matter falling into a black hole slow infinitely in time is only a property of schwarzchild space - time. This is because the schwarzchild metric is a static solution to the EFEs and as such the geometry of space - time should not have any apparent time evolution (the metric doesn't depend on time) and any reversals of time should preserve the structure of space - time. In falling matter into a black hole would cause it to grow as seen by an observer and therefore morph the space - time and since the schwarzchild solution doesn't have time evolution there is no way for the metric to describe changes in the geometry due to the growth of a black hole. Mind you there are metrics (much more complicated than schwarzchild's) that can describe what happens to space - time around a black hole when matter does fall in as seen by an observer.
 
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Remember that the property of nearby observers seeing matter falling into a black hole slow infinitely in time is only a property of schwarzchild space - time. This is because the schwarzchild metric is a static solution to the EFEs and as such the geometry of space - time should not have any apparent time evolution (the metric doesn't depend on time) and any reversals of time should preserve the structure of space - time. In falling matter into a black hole would cause it to grow as seen by an observer and therefore morph the space - time and since the schwarzchild solution doesn't have time evolution there is no way for the metric to describe changes in the geometry due to the growth of a black hole. Mind you there are metrics (much more complicated than schwarzchild's) that can describe what happens to space - time around a black hole when matter does fall in as seen by an observer.
How would you relate hawking radiation to this? I read somewhere that it faces some criticism.
Regards,
ibysaiyan
 
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Indeed understanding time in physics can be a bit confusing. First stop thinking that time is something that nothing can change and the speed is fixed. Time is very flexible and runs at different speeds at different places. Think of time as the forth dimension. And about the example you gave, think about this: If a spacecraft was travelling near the speed of light, a person outside the spacecraft would see time running much faster in the spacecraft. However the people in the spacecraft would feel time running much slower in the spacecraft while time outside would appear to run very fast. It's called relativity. It's relativity that stops us from actually seeing objects slow down before being sucked into a blackhole.
 

DaveC426913

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Indeed understanding time in physics can be a bit confusing. First stop thinking that time is something that nothing can change and the speed is fixed. Time is very flexible and runs at different speeds at different places. Think of time as the forth dimension. And about the example you gave, think about this: If a spacecraft was travelling near the speed of light, a person outside the spacecraft would see time running much faster in the spacecraft. However the people in the spacecraft would feel time running much slower in the spacecraft while time outside would appear to run very fast. It's called relativity. It's relativity that stops us from actually seeing objects slow down before being sucked into a blackhole.
You've got this all wrong.

From outside the spacecraft, observers will see time inside the craft moving slowly.
Occupants in the spacecraft will not experience time inside the craft as having changed at all.
From inside the spacecraft, occupants will see time outside craft moving slowly.
 
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Indeed understanding time in physics can be a bit confusing. First stop thinking that time is something that nothing can change and the speed is fixed. Time is very flexible and runs at different speeds at different places. Think of time as the forth dimension. And about the example you gave, think about this: If a spacecraft was travelling near the speed of light, a person outside the spacecraft would see time running much faster in the spacecraft. However the people in the spacecraft would feel time running much slower in the spacecraft while time outside would appear to run very fast. It's called relativity. It's relativity that stops us from actually seeing objects slow down before being sucked into a blackhole.
Adding to what the kind poster (DaveC) above me mentioned, for the occupants/observers inside the spacecraft,they will only feel space contraction i.e lorentz contraction.Time doesn't run,it is a parameter which is dependent over our perception, we call it frame of references or reference frames in special or general relativity.What actually runs are the objects within the reference frame due to which time appear to pass for e.g an object moves from coordinate A-> B and so on,in doing so its position is changed on the time coordinate.


Regards,
ibysaiyan
 
Last edited:

DaveC426913

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... for the occupants/observers inside the spacecraft,they will only feel space contraction i.e lorentz contraction.
No they will not.

Again, no test they can do inside their capsule (be it clock, ruler, pendulum or weigh scale) will indicate anything other than that they are perfectly stationary.
 

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