Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John369 on 20/08/2020 14:22:09

Title: Does spacetime bend only locally around objects?
Post by: John369 on 20/08/2020 14:22:09
Please I need help. Does a body with mass curve space and dilate time only locally, eg 5 km? Or is the whole universe affected by it even though in some very small insignificant way since gravity is universal? Does changing the mass of a body or moving an object hence increasing its kinetic energy has a very small negligible effect on the entire universe or only the space surrounding that object? How can a sensitive theoretical device measure such a small gravitational disturbance from light years away from that object?

Also, from extremely theoretical point of view of 11D supergravity, would a small gravitational disturbance in our 4D universe(eg an athlete kicks football) affect all of the dimensions in some small way? Is our 4D universe at the surface of a 5D hypersphere?
Title: Re: Does spacetime bend only locally around objects?
Post by: Colin2B on 20/08/2020 15:03:32
The influence of a gravitational field falls off as an inverse square law, so although it has a theoretical influence to infinity in practice it becomes very small (relative to its local intensity) very quickly. Having said that, a large black hole will influence the path of large stars at a considerable distance.
If you think about our sun at 150 million km it still influences the orbit of the earth and also our tides, and at 1.3 billion km it influences saturn.

By comparison, kicking a football is a negligible effect.
Whether our 4D is considered to be the surface of a hypersphere really depends on which particular theory you are looking at. Remember these are models and there are a lot of them  ;)
Title: Re: Does spacetime bend only locally around objects?
Post by: chiralSPO on 20/08/2020 15:30:17
Great answer from Colin2B!

I will also add: we can detect gravitational waves from stellar (black hole and neutron star) collisions from billions of light years away!! It takes the most sensitive instruments on the planet to detect the motions of the most massive bodies in the universe—but it clearly demonstrates that massive bodies can distort space (space-time) across cosmic distances.
Title: Re: Does spacetime bend only locally around objects?
Post by: Halc on 20/08/2020 15:32:55
Does a body with mass curve space and dilate time only locally, eg 5 km?
'Locally' means 'here', and spacetime is locally Minkowskian (flat) anywhere where there isn't a physical singularity.  So quite the opposite: A body with mass does not exist locally (Earth is not all right 'here'), and does not curve spacetime locally, but it does exist and curve spacetime on larger scales. As Colin points out, there is no limited range to this.

To illustrate: I can draw a chalk triangle in a sidewalk square and the angles will add up to 180° since spacetime is flat locally.  But the larger the triangle I draw, the more the angles add up to something else.

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Does changing the mass of a body or moving an object hence increasing its kinetic energy has a very small negligible effect on the entire universe or only the space surrounding that object?
Mass is conserved, so you can't just change the mass of an object. You can only merge it with some other mass that is already there, and spacetime is already curved because of that other mass.  So the merging doesn't really change much.  Rearrangement of mass (such as Earth orbiting the sun) does affect the gravitational field, and those changes (not the gravity, just the changes) propagate through the universe at light speed via gravitational waves.

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How can a sensitive theoretical device measure such a small gravitational disturbance from light years away from that object?
Look up the wiki site on LIGO which does just that.
Title: Re: Does spacetime bend only locally around objects?
Post by: John369 on 21/08/2020 07:48:20
Suppose an athlete hits a football, did the athlete change the whole universe in some very small way just by doing that act on earth moving across space? Would this act change the gravitational disturbance in some insignificant way light years away? Did increase in kinetic energy of the ball by athlete slightly change curvature of spacetime around it universally? Could this change be detected if we had sensitive enough instrument? Did the athlete change the effect universally as both observers and the observed are in inertial/non inertial reference frame? Can we 3D beings confined to movements in between specific coordinates each interval affect regions of space universally? Did the athlete affect(in some small way) a distant planet? A distant black hole in some negligible way? Objects in motion do change mass slightly, would this cause slight gravitational disturbance universally? Since gravitational waves can only travel at the speed of light, would a change in gravity caused by the athlete be universal but only detectable after years for the detectors being light years away from athlete? But the change made by athlete did change the universe in some small way?
But since the universe is accelerating in expansion faster than speed of light, the gravitational disturbance would never be able to completely fill the universe and be never detected?

Also, is each and every person constantly contributing towards increase in entropy and hence eventually heat death of the universe?
Title: Re: Does spacetime bend only locally around objects?
Post by: Halc on 21/08/2020 12:49:05
Suppose an athlete hits a football, did the athlete change the whole universe in some very small way just by doing that act on earth moving across space? Would this act change the gravitational disturbance in some insignificant way light years away?
The change in mass distribution would technically cause some gravitational waves to be emitted, but the entire mass of the Earth moving at 30 km/sec around the sun generates only about 200 watts of gravitational waves, so that's a lot of zeros for the football.

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Did increase in kinetic energy of the ball by athlete slightly change curvature of spacetime around it universally?
No.  Spacetime is curved due to the presence of mass, not kinetic energy.  The center of mass of Earth system was not effected at all by the kick, so the entire effect was a tiny change to the shape of the mass distribution of Earth.

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Could this change be detected if we had sensitive enough instrument?
If they hypothetically built something that could detect a single graviton, sure, but not sure how you'd pick up that specific one against the background noise of all the gravitons continuously coming from all directions. It's not like photons that stop when they hit something. Anyway, practical answer is no: No instrument can be that sensitive.

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Did the athlete change the effect universally as both observers and the observed are in inertial/non inertial reference frame?
The athlete definitely does cause distant effects by kicking the ball. Most of this is through means like light, and not so much the trivial difference in the bending of spacetime.
I hate to use 'change' to describe that since the word means a difference in state from an earlier time compared to a later time, and you're using it to compare some state with a different state at the same time that might otherwise have been.

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Can we 3D beings confined to movements in between specific coordinates each interval affect regions of space universally?
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But since the universe is accelerating in expansion faster than speed of light, the gravitational disturbance would never be able to completely fill the universe and be never detected?
Light/information indeed can only get as far as the event horizon, about 16 billion light years away.  We cannot have any effect ever on space 'currently' beyond that distance. For locations beyond that, accelerating expansion will simply increase the distance between our influence and that location at a rate that the expanding bubble of effects from the ball being kicked today cannot ever overtake.

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Did the athlete affect(in some small way) a distant planet? A distant black hole in some negligible way?
If they're close enough to see the ball, then yes. It just takes just one photon. Each individual photon we see from super-distant places is from some small event less significant that the ball being kicked, and we have instruments sensitive enough to pick out just one photon.
But we're getting into light now, and not the changes to spacetime geometry that you're talking about. That has pretty much nil effect at great distances since no change in mass was made, only a trivial rearrangement of it.

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Objects in motion do change mass slightly, would this cause slight gravitational disturbance universally?
They do not change mass.  If they gain mass from external input of energy, that energy/mass had to be taken from something else. Mass/energy is conserved, so there can be no change to the amount of it, only change to its form and distribution.

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Since gravitational waves can only travel at the speed of light, would a change in gravity caused by the athlete be universal but only detectable after years for the detectors being light years away from athlete?
Yes, the waves travel at light speed. There's no change in the total gravity since there's no mass change, and gravitational waves convey the changes, but do not convey the gravity itself. No gravitons are needed to pull you to Earth when you step off a chair. Spacetime is already bent and your freefalling worldline just traces a straight line (a geodesic) through it.

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Also, is each and every person constantly contributing towards increase in entropy and hence eventually heat death of the universe?
I guess. It's not like we're really hastening it. The universe doesn't end at some specific moment that comes all the sooner because I drew breath today.
Title: Re: Does spacetime bend only locally around objects?
Post by: John369 on 22/08/2020 10:58:16
Thanks for the great answer. I have some final conclusive questions:

1. Do we at all times emit black body radiation? Do some of that undetectable light escape the earth and travel indefinitely to outer space as photons?
2. In the athlete kicking the ball example, is there no change in gravitational field of earth even a tiny bit? If the change did happen, it has the potential to be detected after 1 year by a detector 1 light year away?
3. Is there some way we affect the whole universe by doing our daily activities? Like increasing entropy macroscopically? The goal is to show we have a sort of universal range and we change the universe slightly. Could a change be made instantaneously across large distances? Like we are made up of a large number of particles, is there any chance some of them regularly exchange information via entanglement with particles billions of light years away?
4. In a universe accelerating in expansion and space between galaxies getting larger, are gravitational waves and photons affected by expanding space? Could they travel slightly faster than light in between galaxies where space is expanding?
Title: Re: Does spacetime bend only locally around objects?
Post by: Halc on 22/08/2020 12:09:51
1. Do we at all times emit black body radiation? Do some of that undetectable light escape the earth and travel indefinitely to outer space as photons?
We emit radiation at all times, yes. We also reflect much of ambient light, so it isn't really blackbody radiation.

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2. In the athlete kicking the ball example, is there no change in gravitational field of earth even a tiny bit? If the change did happen, it has the potential to be detected after 1 year by a detector 1 light year away?
Again, yes, it changes the field, and that change must propagate at c, so in theory, it is detectable.

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3. Is there some way we affect the whole universe by doing our daily activities?
Only part within our future light cone, which hardly covers the whole universe.
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Like increasing entropy macroscopically?
Do we do that?  It's not like the entropy wouldn't have happened without our presence, so all we do is rearrange the timing of it a little.

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The goal is to show we have a sort of universal range and we change the universe slightly.
If Earth was viewed from afar, it would stand out like a beacon in certain light frequencies that don't occur so much naturally. But Earth would effect the universe anyway without needing to be different in some way, so the answer is yes: It is pretty much impossible not to have effects on parts of the universe in our future light cone.

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Could a change be made instantaneously across large distances?
Relativity says there is no 'instantly'.  There is not a specific distant event that is simultaneous with a given event here. Quantum mechanics has local interpretations that say no event can cause an effect at a different event separated from the first in a space-like manner. There are also non-local interpretations (like Bohmian mechanics) that require faster-than-light effects. So such interpretations need to deal with relativity of simultaneity. In the end, it cannot, which means that non-local interpretations also need to allow effect to precede cause: A choice made today has effects arbitrarily far in the past.

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Like we are made up of a large number of particles, is there any chance some of them regularly exchange information via entanglement with particles billions of light years away?
No. Information cannot be exchanged via entanglement. Cause and effect maybe, but not information. Nobody has ever sent a FTL message.

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4. In a universe accelerating in expansion and space between galaxies getting larger, are gravitational waves and photons affected by expanding space? Could they travel slightly faster than light in between galaxies where space is expanding?
The proper distance between us and some distant thing (photon, rock, whatever) is increasing at a rate proportional to its distance from us. For light, that means any light receding from us is increasing its distance from us at a pace greater than c, and beyond a sufficiently large distance, the light coming straight at us is not getting any closer over time.  It might still eventually get here.
No light we see today has ever been as far away from here as 6 billion LY.  Any light more distant than that simply hasn't had time to get here yet.  If it was emitted recently, then it hasn't had time to cross that distance. If it was emitted much further back (say 13.4 billion years ago, the oldest non-CMB light we've found so far), then it was actually emitted quite close by (about 2.8 GLY) but it has had to fight to catch up to Earth moving at 3x light speed relative to the object that emitted it.  That means that in a way, the visible universe is under 6GLY in radius. That's not how they usually express the size of the visible universe.
Title: Re: Does spacetime bend only locally around objects?
Post by: John369 on 23/08/2020 10:52:51
Hold on, does the infrared radiation emitted by us ever leave the earth atmosphere(with all its greenhouse gases and clouds) to outer space? Does a person's emitted EM radiation ever leave earth or just get absorbed by its gases? If they do leave, how often does that happen? Is it very rare? What percentage of our emitted radiation successfully goes to outer space? And will they keep traveling forever as individual photons? If a person is 20 years old, did the light emitted by that person traveled 20 light year distance? Since universe is very empty, photons can travel successfully to billions of light years before getting absorbed or reflected by anything? What things in space could halt the progress of these photons?

Also, does the expansion of space affect those traveling photons making them travel faster than light since space is expanding everywhere? So suppose space expanded between the source of radiation(human) and his emitted radiation(20 light years away), then the radiation would seemingly be traveling faster than light? Can gravitational waves also travel faster than light by this way of expanding space?
Title: Re: Does spacetime bend only locally around objects?
Post by: Halc on 23/08/2020 12:37:24
Hold on, does the infrared radiation emitted by us ever leave the earth atmosphere(with all its greenhouse gases and clouds) to outer space? Does a person's emitted EM radiation ever leave earth or just get absorbed by its gases?
Both actually. Most of my personal EM radiation is probably absorbed by my clothes for instance, but gases do eventually absorb most of it, and from there it radiates into space.  Just because I don't emit much EM directly into space doesn't mean it never gets there.
Look at the center of Earth: There is much light generated there, but light gets absolutely nowhere inside the Earth, so it just warms it up in there. That thermal energy then moves to the surface mostly by conduction and convection and very little by radiation. But to the surface it eventually gets or else the planet would not be in thermal equilibrium. It eventually radiates into space. Heat from people (and from nuclear combustion in the sun) works the same way.

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If they do leave, how often does that happen? Is it very rare?
Earth continuously radiates into space at least as much energy as comes in by the sun, else it would not be in thermal equilibrium, and would heat up incredibly fast.

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What percentage of our emitted radiation successfully goes to outer space?
All of it, but probably less than 1% directly. I cannot think of another place it could go. Most individual photons hit something (my clothes) and that energy moves by other photons or by convection, eventually getting radiated away into space.

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And will they keep traveling forever as individual photons? If a person is 20 years old, did the light emitted by that person traveled 20 light year distance?
Only the photons that went directly into space. Most just heated up the environment, at which point it is just heat, and not specifically 'your heat', so there's no specific photon that makes it way into space that is specifically yours.
Is a photon that passes through glass the same photon that entered it? Quantum mechanics doesn't provide a clear answer. Any local theory says the photon doesn't exist at all until it hits something and causes an effect somewhere. But yes, in general, photons in space can be thought of as travelling forever until they hit something. There are just different interpretations of 'hit something', which is why I brought up the glass.

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Since universe is very empty, photons can travel successfully to billions of light years before getting absorbed or reflected by anything? What things in space could halt the progress of these photons?
We can still see the oldest photons in the universe (the CMB), so that's pretty much forever, yes.  The longer the wavelength, the less likely it is to be absorbed by something like dust. Most light from the center of the galaxy don't get here. Nothing in the visible light (nanometer) range. To see in there, they have to look at light with with wavelengths in the hundreds of meters which is capable of getting through the dense material between here and there.

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Also, does the expansion of space affect those traveling photons making them travel faster than light since space is expanding everywhere?
Light from our heat might start at micro-meters, but over time (billions of years) it keeps getting longer relative to the local comoving objects, so the further it gets, the less likely it is to be absorbed by anything. Thus much of it will go forever and never be absorbed.
Relative to us, you can word it like you do there. Light always moves locally at c, but far from here is not local, so the proper distance between us and receding light always grows at a rate greater than c. I have a resistance to call that 'travelling faster than c', but it's one way of expressing it. I'd say light always travels at c, but the space between us and distant things is also growing, increasing our proper separation in addition to its actual relative motion.

All that said, there is an event horizon, and light emitted by you can never reach a comoving object at our event horizon. That continuously accelerating (relative to us) object is receding faster than what light can ever reach. To illustrate, if I were to continuously accelerate in a rocket at 1g to the right, then light emitted from a light-year to the left will never reach me. That's the nature of continuous acceleration.

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Can gravitational waves also travel faster than light by this way of expanding space?
Measured that way, yes, same as light, and those waves don't ever stop. They just distort things a bit as they pass by.