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Relativity In The Universe

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nimble
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I'm not a science buff, but from what I understand about general relativity all objects in the universe have no objective position, their position is merely relative to other objects around them? Is that it?

If so how do we know the universe is expanding (I know this is probably a dumb question)? Since it would seem like maybe the objects are just moving away from us, and not actually all moving away from some local center? For instance, let's say an ant can measure how far the sun is. He does it once, and gets a measurement, then does it again throughout his life and concludes that the sun is moving away from us, and he also notices that so is mars and venus, so he concludes the solar system is expanding. Then the ant dies. If he would have lived longer he would have noticed that really those objects move away from us, but they come back eventually too. And thus the solar system doesn't really expand, just objects move about within it. If we tie that analogy to us and the universe, I don't see how you could absolutely conclude the universe is expanding, unless more evidence falls onto xray and microwave detections which I would know nothing about.

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I'm not a science buff, but from what I understand about general relativity all objects in the universe have no objective position, their position is merely relative to other objects around them? Is that it?

Not quite.

All bodies in the universe are constantly moving. None are fixed. Therefore their locations are all relative to each other and change through time. This means there is no fixed location. However, since the location of anything can be measured and expressed in unambiguous terms, all such positions are objective.

As to the expanding Universe, look it up in any astronomy book. The explanation isn't complex, but it takes some time to tell.

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I'm not a science buff, but from what I understand about general relativity all objects in the universe have no objective position, their position is merely relative to other objects around them? Is that it?

This is correct, but it isnt really general relativity, the denial of priviledged frames of reference was first put forwards by Galileo, and is built into Newtonian mechanis. Special relativity took things slightly further, but the idea of there being (eg) no such thing as absolute rest has been accepted for centuries.

Saying 'theres no such thing as objective position' is a very misleading way of putting things though - position (and speed) is still objective within the context of any given reference frame. To take an example - when I look out my window, and see a car going past at 30mph, its objectively correct that the car is moving at 30mph, measured from where I am standing. Now, this doesnt mean that the car is 'really' moving at 30mph, because its speed will change depening on where you look at it from. Someone in a car moving at 20mph in the opposite direction will measure the original car as moving at 50mph, and someone looking at earth from the moon will measure the car as moving at several thousand mph. All these different measurements of the speed are equally correct, within the (different) frames of reference they are being taken in. And theres no 'real' speed/position of the car above and beyond all these different measurements.

If so how do we know the universe is expanding (I know this is probably a dumb question)? Since it would seem like maybe the objects are just moving away from us, and not actually all moving away from some local center?
Well, the idea that everything in the universe is moving away from Earth strikes me being implausible. This would imply that we were somehow at the centre of the universe, which seems a bit egocentric and a regression to the Christian worldview. But aside from this, I'm under the impression that data suggests the distance between all things in the universe is increasing, not just the distance between them and earth (I'm not sure about this however, having never studied cosmology in any detail).

I don't see how you could absolutely conclude the universe is expanding, unless more evidence falls onto xray and microwave detections which I would know nothing about.
The discovery of Cosmic background radiation is generally taken to be the piece of evidence which swung most people's opinions conclusively in the favour of the big bang theory, and which killed off most mainstream research into steady-state explanations. Edited by Hal
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The Earth is not expanding (at least not as part of a universal expansion). The Solar System is not expanding. The Milky-way Galaxy is not expanding. Our local cluster of galaxies is not expanding.

The Expansion of the Universe is an increase in the distances BETWEEN CLUSTERS OF GALAXIES. The galactic clusters have been moving away from each other since they formed following the Big Bang. And they are moving so fast compared to their masses that they have exceeded escape velocity -- they will never stop moving away and fall back, they will just keep going.

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The Earth is not expanding (at least not as part of a universal expansion). The Solar System is not expanding. The Milky-way Galaxy is not expanding. Our local cluster of galaxies is not expanding.

The Expansion of the Universe is an increase in the distances BETWEEN CLUSTERS OF GALAXIES. The galactic clusters have been moving away from each other since they formed following the Big Bang. And they are moving so fast compared to their masses that they have exceeded escape velocity -- they will never stop moving away and fall back, they will just keep going.

I think this is wrong, althuogh I'm not positive. I was under the impression that its only with clusters of galaxies that the increase in distance becomes noticable. Otherwise we'd have some bizarre theory where its only after objects reach a certain size that they begin to move away from all other objects (a sort of cosmological dualism, with no gradual transition between the large and the small). Why would there be anything special about 'clusters of galaxies' that makes them obey different physical laws from the rest of the universe?

I'm not sure since Ive never studied cosmology, but I thought the standard theory said (effectively) that the distance between all points in the universe was continually increasing, but we didnt notice this when performing measurements on earth because the expansion also affected our measuring instruments. Again though, I could just be making this up.

Edited by Hal
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I think this is wrong, althuogh I'm not positive. I was under the impression that its only with clusters of galaxies that the increase in distance becomes noticable. Otherwise we'd have some bizarre theory where its only after objects reach a certain size that they begin to move away from all other objects (a sort of cosmological dualism, with no gradual transition between the large and the small). Why would there be anything special about 'clusters of galaxies' that makes them obey different physical laws from the rest of the universe?

The stars, clouds of dust, planets, asteroids, comets, black holes, pullsars and assorted other objects within a galaxy, are all bound gravitationally into a rather stable unit. Likewise, clusters of galaxies are bound to each other. Therefore when space expands, these units and clusters move together.

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Ok, this is mightly misleading. As space itself, (spacetime) expands the gravitational attractive forces acting on each body become less because the space (spacetime) between each mass is greater. so if we use newton's universal law of gravitation, then since F=G(M1+M2)/(d^2), the force pulling the bodies together is actually

equal to dF/dt. Evidence: We are losing our moon approximately 2mi/year, or so... (i think that's approximately correct.) Also, we measure the distance between two objects by the length of time (using dilation to account for gravitational fields and such) it takes for a light beam to reach us from us, to there, and back, divided by wo (me thinky). The other celecstial bodies do not move from us, but space is expanding and so, essentially, all celestial bodies are moving away from all points of space at all times. I think about it as a buch of bowling balls sitting on a latex mat with equal sized blocks on it; if you pull the mat tighter, it gets thinner and drifts and the blocks et bigger. this is not a perfect analogy, but it conveys my very basic meaning.

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Evidence: We are losing our moon approximately 2mi/year, or so... (i think that's approximately correct.)

Except that that's not a relativistic effect. It's due to the tidal forces of the Moon. As the Earth rotates, it pulls the tidal bulges created by the Moon's gravity out of the line connecting the centers of the Earth and Moon; the bulges in turn pull the Moon forward in its orbit (because they aren't symmetrical about the Earth-Moon line), which increases its energy and thus the size of its orbit by about 3 cm/yr. (It also slows the Earth's rotation by about 1 second every 60,000 years because of the friction caused by the Earth rotating under the tidal bulges.)

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Ok, this is mightly misleading. As space itself, (spacetime) expands the gravitational attractive forces acting on each body become less because the space (spacetime) between each mass is greater.

Not quite.

Let's try an analogy. Suppose the Universe is a balloon. You place some objects on its outer surface and inflate it. As the balloon inflates, the outer surface expands, yes? And the objects you placed on it grow farther apart. But suppose two of the objects were joined by a string. The two would gorw apart from everything else, but their distance to each other would remain unchanged.

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Not quite.

Let's try an analogy. Suppose the Universe is a balloon. You place some objects on its outer surface and inflate it. As the balloon inflates, the outer surface expands, yes? And the objects you placed on it grow farther apart. But suppose two of the objects were joined by a string. The two would gorw apart from everything else, but their distance to each other would remain unchanged.

Another analogy(can you have too many?) that simplifies part of it for me is to imagine 3 objects-say cars moving in the same direction where the furthest car is traveling at 50 kph, the second car at 30kph and the 3rd at 10kph. They all distance themselves from one another. So if you have 12 cars travelling 3 north 3 south 3 east and 3 west they would still be moving apart. Extrapolating that into 3 dimensions obviously yields the same result.

Regarding whether or not it is only galaxy clusters moving apart, I understood that all matter is essentially moving apart, it's just not percievable at smaller levels. The universe is ultimately entropoic...loosing energy...which I take to mean that all existent forces will eventually weaken and finally cease to exist(albeit far to far in the future to concern most of us). It seems right, otherwise the universe would have to be a big perpetual motion machine.

What i am wondering is if this means that all matter would cease to exist as well since it is held together by nucleur forces? If any physicist(professional or otherwise) inhabit this board, I'd be interested to know if this is the case and if not why not.

Best regards

Gordon

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I was under the impression that its only with clusters of galaxies that the increase in distance becomes noticeable. Otherwise we'd have some bizarre theory where its only after objects reach a certain size that they begin to move away from all other objects (a sort of cosmological dualism, with no gradual transition between the large and the small). Why would there be anything special about 'clusters of galaxies' that makes them obey different physical laws from the rest of the universe?

Objects in the universe are just moving under the normal influences of their inertia (momentum) and various forces, especially gravity. The expansion is not some magical thing imposed upon the normal order. It is a CONSEQUENCE of the normal order of things.

Immediately after the Big Bang, everything was flying apart with the motions roughly proportional to the distances. We see a remnant of this in the Hubble equation for the expansion. But you should understand that this is just an approximate pattern, not a separate and firm natural law.

Local concentrations of material occurred as a result of random fluctuations. These were amplified by their mutual gravitational attraction of their parts. Some of these concentrations became gravitationally bound, i.e. they started revolving around each other instead of continuing to fly apart. These eventually became the clusters of galaxies we see today.

I'm not sure since Ive never studied cosmology, but I thought the standard theory said (effectively) that the distance between all points in the universe was continually increasing, but we didn't notice this when performing measurements on earth because the expansion also affected our measuring instruments.

This would make the expansion meaningless. If everything, measurer and measured alike, was expanding, then the ratios of the distances would all be unaffected by the expansion. This does not make sense. You should not assume that such an important theory does not make sense. Rather you should presume that it DOES make sense and that you have just got it wrong. Ask questions about it instead of making baseless pronouncements.

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