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Where is most "gravity", inside or out?
- Thread starter nebel
- Start date
By definition the 'bottom of the well' is the minimum gravitational potential. That makes sense because the PE of the mass is the lowest at this point. You are probably thinking that the bottom of the well has the highest potential because that is the place where the greatest amount of energy is required to exit the well. But that is simply not the convention.
Isn't the answer sort of obvious? Time dilation is not a result of the measured gravitational acceleration.
You should also probably substitute the word time, where you say clock. When you say things like "drag on the clock" that leaves the impression that time dilation is effecting clocks instead of effecting time.
exchemist
Valued Senior Member
NOBODY said potential was the opposite of field strength. You've made that up and it's crap.
Potential was explained in post 319, in a post of mine that Origin commended for its brevity and clarity
. I suggest you read it again.Gravitational Potential is potential energy, the the ability to do work by allowing a mass to "fall" under the influence of gravity. How a hydroelectric power power station gets energy from the fall in water from a reservoir.
Conversely, the further "down" you are, the more work it takes to climb "up", right? As when you climb a mountain.
The minimum in the potential is at the centre of the Earth, because that is the point that requires the most work to climb "up" from. The work you do is done against the force of gravity.
Work done = Force x Distance gone against that force. Elementary school mechanics, yes? That is why I said in post 319 potential is the integral of force with respect to distance, r. In algebra:- V = ⎰F dr , where F =mMG/r² outside the Earth and some other function I can't be bothered to figure out, but linear in r, inside it.
Gravitational time dilation is a function of that work that needs to be done, i.e. of the potential. Not the force. The potential. The reason why is all General Relativity stuff. I am not going to try to start talking to you about the metric tensor, as I do not understand how it works myself, so the chance of you doing so is zippo. But in the GE picture we do not have a gravitational force any more. We have instead mass causing spacetime (as expressed in the metric tensor) to change.
It is a totally different way of conceiving of gravitation from the Newtonian picture with its forces etc. Just accept it, would be my advice, and focus instead on getting your head round the difference between a force and a potential.
yes, and I stumble over that term, "work to be done", because the work to be done is at the maximum at the center., to get out of the pit. re:relativity, force: Newton works quite well to get satellites into orbit, Kepler to find TNOs, perhaps there is a simple way to explain the center-earth disconnect to a simpleton like me? because
judging from the three clock lines, it is only in the interior that has a total contradiction between "force" and "potential" results, a result being time dilation slowing clocks sub question, if both potential and strength are at zero at the center, why is time dilation not giving the clock full reign?
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Can you explain that to me? I am affected by gravity, (often in a bad way). but a clock is not? does gravity not affect both time and clocks? and the clock measures time. gravity affects through relativity the dimensions in the clock so it slows allegedly in the pit, showing that the time has slowed, when
in zero gravity measured it should have sped up?
exchemist
Valued Senior Member
A minimum -ve value is not zero. Read post 319 again.
exchemist
Valued Senior Member
What you call "zero gravity" mean zero force, not zero potential. And it is potential that affects the rate of clocks, not force.
Forget this crap about dimensions of the clock. It is not some mechanical effect, induced under the influence of a force, it is spacetime itself that is shaped by the presence of mass. Spacetime gets a dent put in it by a mass - not a dent with a spike in it at the centre of the mass - and that has a number of effects, amongst which is that time runs more slowly. It has nothing to do with the Newtonian idea of the "force" of gravity.
from post# 319.
so, potential has fallen to zero at the center,
That field strength has fallen to zero at the center too, according to origin's graph, so, with the least. zero gravity at the center, every wich way you look at it, --gravity slowing clocks, and absence of gravity letting them run normal, why the exception, the opposite, the slowing of the clock at the center ?
I noticed the curve in pos# 319 goes at the surface point from a convex top on the outside to concave inside the mass to the center.
exchemist
Valued Senior Member
You are a timewasting idiot and I'm putting you back on Ignore.
sorry, you are not the only one frustrated. well, hopefully someone will step up to the plate and explain this quote in post# 351.
there is no gravity, and therefore there should be no slowing down of the clocks through time dilation (or lengthening of pendulums). nor in the newtonian model of origin's graph. --but
the green line shows a slowing of the clock. so :
given zero gravity at the center, what justifies, causes the exception, results contrary to the normal outside effect proven in satellites?
One suggestion was that overlapping gravitational fields, , pulling in different directions do cancel the effect, but continue to co-exist, ( in the realm of relativity so to speak) giving gravity a dual nature
Obviously the scientist that who wrote it, confirmed that indeed theren is no "no accelerated reference frame in the center", talking relativity. so:
there is no gravity, and therefore there should be no slowing down of the clocks through time dilation (or lengthening of pendulums). nor in the newtonian model of origin's graph. --but
the green line shows a slowing of the clock. so :
given zero gravity at the center, what justifies, causes the exception, results contrary to the normal outside effect proven in satellites?
One suggestion was that overlapping gravitational fields, , pulling in different directions do cancel the effect, but continue to co-exist, ( in the realm of relativity so to speak) giving gravity a dual nature
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DaveC426913
Valued Senior Member
OK, after asking around a bit from some folks with eggier heads than me, I've written an explanation I can get my head around.
nebel:
Time dilation is not A Thing That Exists at the centre of the Earth. Time dilation is - like other relativistic phenomena - a relative effect dependent on two reference points separated by a distance.
One does not experience time dilation - one only ever observes it occurring in a different reference frame.
Time dilation is an effect of the difference in states between two points.
And, for GR, what stands between them is a curvature of space. GR time dilation occurs when an observer on one side (say, Earth orbit) of a curvature of space (Earth's well) observes something on the other side (the centre of Earth). And the direction - whether they're looking down into it or up out of it - is what determines the "bias" of time dilation.
(And GR time dilation is biased - it's not subjective, like SR dilation is. The two observers are not equivalent - and they know it. Simply by observing each other's time dilation, they will each always know which one of them is "above" the other.)
So that's how an observer at the centre of the Earth e̶x̶p̶e̶r̶i̶e̶n̶c̶e̶s̶ observes time dilation - only by comparison to events outside the well - and to look outside the well for him means looking up that curvature. Likewise time dilation at the centre of Earth is only observed because we are outside looking in.
And that's why time dilation cannot be zero at the centre. No matter what he does during his normal day, an observer at the centre of the Earth ultimately must look up-well to his reference point - through the Earth's spacetime curvature - out to "flatter" space. Likewise, the only way to observe events at the centre of the Earth is to look down-well - down that curvature of space.
TL;DR: it's not "that" you're at the centre of the Earth that is the source of time dilation - it's where you are looking - what (curvature) you are looking through.
nebel:
Time dilation is not A Thing That Exists at the centre of the Earth. Time dilation is - like other relativistic phenomena - a relative effect dependent on two reference points separated by a distance.
One does not experience time dilation - one only ever observes it occurring in a different reference frame.
Time dilation is an effect of the difference in states between two points.
And, for GR, what stands between them is a curvature of space. GR time dilation occurs when an observer on one side (say, Earth orbit) of a curvature of space (Earth's well) observes something on the other side (the centre of Earth). And the direction - whether they're looking down into it or up out of it - is what determines the "bias" of time dilation.
(And GR time dilation is biased - it's not subjective, like SR dilation is. The two observers are not equivalent - and they know it. Simply by observing each other's time dilation, they will each always know which one of them is "above" the other.)
So that's how an observer at the centre of the Earth e̶x̶p̶e̶r̶i̶e̶n̶c̶e̶s̶ observes time dilation - only by comparison to events outside the well - and to look outside the well for him means looking up that curvature. Likewise time dilation at the centre of Earth is only observed because we are outside looking in.
And that's why time dilation cannot be zero at the centre. No matter what he does during his normal day, an observer at the centre of the Earth ultimately must look up-well to his reference point - through the Earth's spacetime curvature - out to "flatter" space. Likewise, the only way to observe events at the centre of the Earth is to look down-well - down that curvature of space.
TL;DR: it's not "that" you're at the centre of the Earth that is the source of time dilation - it's where you are looking - what (curvature) you are looking through.
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Your own ignorance of the basic fact of gravitational time dilation being a relational phenomenon was indeed revealed over at PhysicsForums:
https://www.physicsforums.com/threads/cause-of-time-dilation-when-net-gravity-is-zero.963892/
And yet it has all been covered many times in many previous threads both here at SF and over there at PF. Even in this thread, p6 settled it. In one ear, out the other.
And so the blind lead the blind here at SF. What's new.
DaveC426913
Valued Senior Member
My intent was to find a way to express it to nebel in a way he could understand it. No matter how many times it's been explained it to him, he hasn't gotten it.
And as I remarked back here: http://www.sciforums.com/threads/where-is-most-gravity-inside-or-out.155498/page-15#post-3558155
, he never will really 'get it' [Edit: Or if 'gotten' briefly, retained for any substantial time span!]. Merry-go-round.
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Dave,& Q-reeus, please remember, that I started this thread. a question, , about "gravity" not it's relativistic effects, about which I am not in an armchair, but a highchair. so, since I am not posting, not even aware of the good physics link.; how does it reveal my my ignorance?
Yes, I am vaguely aware of the being in, looking at different reference frame implications.-- but are clocks that have been transported into different speed or gravity situations not been found to have slowed for a while?, really turned back? , does the twin not actually have aged less? but we are way off topic here i.m.h.op. It appears the discussion has changed to:"
where is most time dilation inside or out?, not my obsession, so why not you open a topic on that?
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DaveC426913
Valued Senior Member
OK. You'll forgive me if your comments about nebel don't direct my actions.
DaveC426913
Valued Senior Member
Well, no. My post addresses the subject of gravity and its effects at the centre of a massive body - hopefully, in a way that puts your doubts to rest.
The green curve, below, is what you have been questioning. It is the amount of gravitational red-shifting - it must needs increase the further into a gravity well you go. It cannot decrease - as you suppose in your diagrams.
so, if you lowered that clock, gravity meter into to center of the Earth, made recordings, what would the record show? surely the time record would not be erased? if the instruments send data, we on the surface in highest gravity (base on the graph) should have records of what happened, appeared on our receivers? time delation observed only?
DaveC426913
Valued Senior Member
The retrieved clock would be a few nanoseconds behind the surface reference clock.
Just like a clock on the surface compared to a clock up in orbit (SR notwithstanding).
thank you, yes, in so far as the green curve, ,( gravity TIME DILATION) is related to the actual strength of the gravity field as such, but as the "Exemption" clause shows, that connection is only tentative. define the exemption (where the opposite happens)for us again.