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DaCaptain
2018-Sep-24, 07:15 PM
Silly question???

swampyankee
2018-Sep-24, 07:57 PM
No. Mass is everywhere.

Noclevername
2018-Sep-24, 10:12 PM
Silly question???

Nope. Gravity fields are IIRC not limited in range, only in detectability. There's always going to be some gravity present, even at great distances from mass.

Jens
2018-Sep-24, 10:50 PM
To add what others have said, gravity is an inverse square law, so if you double the distance the gravity is four times as weak. What that means of course is that it can never reach zero, only come close to it.


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ngc3314
2018-Sep-25, 01:53 AM
Indulging in a license for pedantry - there are arrangements of masses which can give arbitrarily low gravitation acceleration over finite areas. Robert Forward described such an arrangement in a technical paper (https://journals.aps.org/prd/abstract/10.1103/PhysRevD.26.735) as well as his novel Dragon's Egg. Doing this over big regions near an astronomically significant mass starts to take a lot of well-controlled mass.

Copernicus
2018-Sep-25, 02:02 PM
I suspect that gravity is like everything else. It is the difference that counts. We don't understand the quantum of gravity. We don't understand, if there are two quantum of gravity, next to each other, would they have gravity between them, or gravity as a unit.

Swift
2018-Sep-25, 02:29 PM
I suspect that gravity is like everything else. It is the difference that counts. We don't understand the quantum of gravity. We don't understand, if there are two quantum of gravity, next to each other, would they have gravity between them, or gravity as a unit.
Copernicus

Q&A is not the place for speculation or non-mainstream ideas. Don't do it again.

Copernicus
2018-Sep-25, 03:07 PM
The gravity at the center of the earth is zero, but that does not include everything else pulling on that center, like the sun, other planets etc. https://www.quora.com/Why-is-gravity-zero-at-the-centre-of-earth

DaCaptain
2018-Sep-25, 05:40 PM
The gravity at the center of the earth is zero, but that does not include everything else pulling on that center, like the sun, other planets etc. https://www.quora.com/Why-is-gravity-zero-at-the-centre-of-earth

Interesting. So there's really no where were there's no gravity. You can equalize the gravitational effects of nearby masses by equal and opposite masses. But there will still be gravitational pull in all directions. Gravity can't be blocked or cancelled. It's simply omnipresent.

DaveC426913
2018-Sep-25, 11:33 PM
The gravity at the center of the earth is zero, but that does not include everything else pulling on that center, like the sun, other planets etc. https://www.quora.com/Why-is-gravity-zero-at-the-centre-of-earth
The gravity at the centre of the Earth is high. You would be able to detect time dilation.
But its force cancels out, since it's pulling from all directions equally.

DaCaptain
2018-Sep-26, 12:48 AM
The gravity at the centre of the Earth is high. You would be able to detect time dilation.
But its force cancels out, since it's pulling from all directions equally.

Does gravity come from the center of earth??? Or is it more the combined force of all earths mass? So it wouldn't be it's center but somewhere offset from center but directly below you.

Noclevername
2018-Sep-26, 12:51 AM
So it wouldn't be it's center but somewhere offset from center but directly below you.

I don't understand this statement. The center of mass is where all the gravity balances out.

DaCaptain
2018-Sep-26, 01:12 AM
I don't understand this statement. The center of mass is where all the gravity balances out.

Oops, my mistake. Not sure what I was thinking. Guess I was thinking that if I was standing on earth that there would be more mass between me and the center than on the other side of earths center. Not true.

But then it brings up the thought if gravity is zero at the center, why are so many heavy elements gathered there?

Noclevername
2018-Sep-26, 01:17 AM
Oops, my mistake. Not sure what I was thinking. Guess I was thinking that if I was standing on earth that there would be more mass between me and the center than on the other side of earths center. Not true.

But then it brings up the thought if gravity is zero at the center, why are so many heavy elements gathered there?

Because the gravity is not really zero. It's like a Trojan point, matter tends to collect there.

Copernicus
2018-Sep-26, 01:27 AM
Because the gravity is not really zero. It's like a Trojan point, matter tends to collect there.

No the gravity is zero at the center of earth. There is no time dilation.

Noclevername
2018-Sep-26, 01:42 AM
No the gravity is zero at the center of earth. There is no time dilation.

DaveC426913 directly contradicted that. Post 10.

So it comes down to, who's got citations?

Jens
2018-Sep-26, 02:41 AM
DaveC426913 directly contradicted that. Post 10.

So it comes down to, who's got citations?

I'm sure that somebody who actually understands this can clarify. But I believe that DaveC426913 is correct, because it is the gravitational potential rather than the actual felt gravity that is important. But as I said, I'll wait until someone with an actual grasp of physics chimes in!

JCoyote
2018-Sep-26, 04:01 AM
It would require that portion of the universe to be so distant from all matter and energy that the point would exist at the center of an absolutely empty sphere around 14 billion light years in radius.
Except quantum mechanics would laugh at this and make at least one particle appear in there by now.
On the one hand the universe could be infinite, but on the other there is no reason to assume such a large void is possible out of any origin theory we have.

And Copernicus, gravity doesn't effect itself, I asked this question in astronomy in college long ago because otherwise black holes would roll inward as gravity couldn't escape itself.

DaCaptain
2018-Sep-26, 10:59 AM
And Copernicus, gravity doesn't effect itself, I asked this question in astronomy in college long ago because otherwise black holes would roll inward as gravity couldn't escape itself.

Please explain, gravity doesn't effect itself.

Copernicus
2018-Sep-26, 04:00 PM
It would require that portion of the universe to be so distant from all matter and energy that the point would exist at the center of an absolutely empty sphere around 14 billion light years in radius.
Except quantum mechanics would laugh at this and make at least one particle appear in there by now.
On the one hand the universe could be infinite, but on the other there is no reason to assume such a large void is possible out of any origin theory we have.

And Copernicus, gravity doesn't effect itself, I asked this question in astronomy in college long ago because otherwise black holes would roll inward as gravity couldn't escape itself.

I don't know where, "gravity doesn't effect itself came from. As I stated before. The gravity at the center of the earth is zero, with respect to the earth. It obviously is not the exact center because there are variations in density and shape of the earth. The time dilation comes from outside the center of the earth. The strongest gravity is at the surface of the earth. The ideal graph is shown in the following link.

https://www.google.com/search?q=gravity+field+of+a+sphere&rlz=1C1CHBF_enUS774US774&source=lnms&tbm=isch&sa=X&ved=0ahUKEwis4qPrg9ndAhURXK0KHcEcAqwQ_AUIDigB&biw=1350&bih=616#imgrc=mjN5Zbxxp0T5YM:

This does not mean there is not gravity at the center of the earth, because there is gravity from all over the universe.

Hornblower
2018-Sep-26, 05:01 PM
I am seeing enough errors and misunderstandings in this thread that it is hard to tell where to start. Let me address the readership at large.

As far as we can tell, there is matter everywhere, so there is gravity everywhere. Even if we come to a place beyond which there is no matter, the gravitational effects of the matter that is elsewhere will extend without limit, according to our prevailing theory.

At the center of a uniformly spherical planet the gravitational action of the surrounding matter acts equally in all directions, thus cancelling out so there is no net acceleration of an object at that point.

At the surface of that planet the resultant gravity is the same as if it were all at the center. Newton found the means to calculate that as an exercise in calculus. I would not say that the gravity is coming from the center. It comes from all over the planet and merely gives the same net result as if it came from the center.

Suppose the planet is transparent so we can see a clock at the center from afar. That clock is not being accelerated but it is still at the bottom of a gravitational well and would appear time-dilated relative to a clock on the surface. Likewise the surface clock will be time dilated compared to one above the surface. The latter has been observed with clocks aboard airplanes and satellites.

I could go on and on but I think this is enough for now. These are my understandings and as always don't take them as gospel. If any of you think I am mistaken or missing something, please speak up.

Copernicus
2018-Sep-26, 08:01 PM
As regards to time. Time will move fastest at the center of the earth. Gravity slows down time. Where the field strength is highest, time moves slowest.

Copernicus
2018-Sep-26, 08:11 PM
As regards to time. Time will move fastest at the center of the earth. Gravity slows down time. Where the field strength is highest, time moves slowest.

It looks like I am wrong about this.

DaCaptain
2018-Sep-26, 11:25 PM
..... Suppose the planet is transparent so we can see a clock at the center from afar. That clock is not being accelerated but it is still at the bottom of a gravitational well and would appear time-dilated relative to a clock on the surface. Likewise the surface clock will be time dilated compared to one above the surface. The latter has been observed with clocks aboard airplanes and satellites.....

Wow, so what would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?

DaveC426913
2018-Sep-26, 11:25 PM
Yeah. Time is dilated (slower) at the centre of a massive object, as viewed from higher in the g-well.

DaveC426913
2018-Sep-26, 11:30 PM
Wow, so would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?
Neither.

We cannot observe such a clock, as no light could escape the BH to reach us.

What would happen is that, when we dropped the clock into the BH, from a distance, it would fall toward the event horizon. As it did so, it would appear to slow down, and it would also red shift.
No matter how long we observed it, that's all we would see. It would get closer and closer the EH, getting slower and slower, as well as redder and redder and dimmer and dimmer, until we could no detect enough photons coming from it to even call it an image. We would never see it cross the EH, even if we waited forever.

Noclevername
2018-Sep-26, 11:35 PM
Wow, so what would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?

You can't see the inside of a BH from the outside. No light escapes. That's why it's black.

mkline55
2018-Sep-27, 12:08 PM
Forget about the "light cannot. . . " diversion. What about a "clock" that was generating gravitational waves? Would it appear to be faster, slower, or non-existent compared to a similar "clock" outside the BH?

<edited to change the word choice>

Jens
2018-Sep-27, 12:44 PM
Forget about the "light cannot. . . " diversion. What about a "clock" that was generating gravitational waves? Would it appear to be faster, slower, or non-existent compared to a similar "clock" outside the BH?


A gravitational wave travels at the speed of light, so it won’t escape any better than a photon. The gravitational waves we see are of things collapsing into black holes, not black holes themselves.


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Jens
2018-Sep-27, 12:45 PM
The only things that escape from black holes are our hopes and expectations.


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Noclevername
2018-Sep-27, 01:06 PM
Forget about the "light cannot. . . " diversion. What about a "clock" that was generating gravitational waves? Would it appear to be faster, slower, or non-existent compared to a similar "clock" outside the BH?

<edited to change the word choice>

We cannot get any form of information from inside a BH. None. Your question just can't be answered.

mkline55
2018-Sep-27, 06:56 PM
The question from DaCaptain was, "Wow, so what would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?"

All I see are attempts NOT to answer that question. Let's take a different approach. Lots of you like to play games with semantics instead of directly answering questions. "Place" doesn't mean "toss". To place a clock in the center of a black bole, you would have to be within that black hole. You would have to physically put it there and leave it there. There is no assumption of the size of that black hole. So for this question, I choose a black hole exactly 20 billion light years in radius. The matter is not very dense, and therefore not crushing you as you place the clock at the center. The forces of gravity are pretty nearly balanced out in all directions. Mathematically (not practically) would I, no longer at the precise center of this black hole be able to calculate the rate of time on the clock to be any different than that on on my watch?

Hornblower
2018-Sep-27, 09:37 PM
The question from DaCaptain was, "Wow, so what would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?"

All I see are attempts NOT to answer that question. Let's take a different approach. Lots of you like to play games with semantics instead of directly answering questions. "Place" doesn't mean "toss". To place a clock in the center of a black bole, you would have to be within that black hole. You would have to physically put it there and leave it there. There is no assumption of the size of that black hole. So for this question, I choose a black hole exactly 20 billion light years in radius. The matter is not very dense, and therefore not crushing you as you place the clock at the center. The forces of gravity are pretty nearly balanced out in all directions. Mathematically (not practically) would I, no longer at the precise center of this black hole be able to calculate the rate of time on the clock to be any different than that on on my watch?

As I think I understand mainstream explanations of black holes, the clock at the center would be crushed beyond recognition in a finite amount of time as reckoned locally, and you would be done in likewise in a finite amount of time as reckoned by your watch. During the descent from the event horizon to "crush depth", if the clock is between you and the center you would observe it to be time dilated. An observer outside the event horizon would see nothing.

In a thought exercise we could have a low-density spherical body which has just contracted into its event horizon, and is still low density throughout. This would only be temporary, as the material would fall headlong into the center. An observer outside the event horizon would see or feel nothing during this final infall. An observer falling in with the aforementioned material would feel nothing initially but would start becoming spaghettified within a finite time as reckoned from his watch. We could not have a steady state here, if I understand the explanations of the theory correctly.

DaCaptain
2018-Sep-27, 10:07 PM
The question from DaCaptain was, "Wow, so what would happen if we could place that same clock at the center of a black hole? Would we would be seeing into the future or into the past?"

All I see are attempts NOT to answer that question. Let's take a different approach. Lots of you like to play games with semantics instead of directly answering questions. "Place" doesn't mean "toss". To place a clock in the center of a black bole, you would have to be within that black hole. You would have to physically put it there and leave it there. There is no assumption of the size of that black hole. So for this question, I choose a black hole exactly 20 billion light years in radius. The matter is not very dense, and therefore not crushing you as you place the clock at the center. The forces of gravity are pretty nearly balanced out in all directions. Mathematically (not practically) would I, no longer at the precise center of this black hole be able to calculate the rate of time on the clock to be any different than that on on my watch?

Thank you mkline55. All I asked for was an answer using a little imagination. Where would we be without it?

Noclevername
2018-Sep-27, 10:41 PM
Thank you mkline55. All I asked for was an answer using a little imagination. Where would we be without it?

But any imagined answer would be a fantasy based on an unreal scenario. Basically you'd have to change the BH until it wasn't a BH, for the question to reflect any possible scenario where it was answerable.

DaveC426913
2018-Sep-27, 11:27 PM
Thank you mkline55. All I asked for was an answer using a little imagination. Where would we be without it?
Nobody's being unimaginative. And nobody's attempting to not answer the question.

The only meaningful answer is that - as the clock nears the EH - it will be increasingly time dilated.


If what you're asking is: what would the laws of physics do if we changed the laws of physics?
Then the answer is whatever you want them to do! Unicorns appear.
(I'm not being facetious. That question often comes up on science forums, and this is a standard answer.)

DaCaptain
2018-Sep-27, 11:49 PM
Nobody's being unimaginative. And nobody's attempting to not answer the question.

The only meaningful answer is that - as the clock nears the EH - it will be increasingly time dilated.


If what you're asking is: what would the laws of physics do if we changed the laws of physics?
Then the answer is whatever you want them to do! Unicorns appear.
(I'm not being facetious. That question often comes up on science forums, and this is a standard answer.)

Very true. My apologies. It was one of those questions that I asked on the spur of the moment. My imagination got the best of me.

DaveC426913
2018-Sep-29, 03:55 AM
Very true. My apologies. It was one of those questions that I asked on the spur of the moment. My imagination got the best of me.
I should add a qualifier:

There's absolutely nothing wrong with creating Thought Experiments that explore an idea that we can't actually test. One can take liberties with certain impracticalities that would get in the way of a good exploration. But it's a skill to determine what details can be bent for the sake of the thought experiment and what details cannot be violated.

eg: What if the Sun instantly disappeared? Would the Earth feel it right away?
We can bend the physics of how the sun could be instantly "snuffed out", but we can't ignore the fact that the mass cannot simply teleport away. Getting snuffed out doesn't explicitly violate a law of physics that's relevant to the experiment, but pretending the mass can vanish without physically dispersing (and the consequent ripples in space time) would definitely be a break from the laws of physics that would render any subsequent conclusions invalid.

Keep up the creative thinking.