|
Post by Progenitor A on Nov 20, 2010 8:45:36 GMT 1
Would someone like to attempt to explain to me the observable difference between two celestial bodis receding from one another at velocity v and the 'expansion of space' betwen them at velocity v? In other words, why should I accept 'expansion of space'? Is it necessary? Is it superfluous? Is it a porky? Certainly a magistrate would not accept my innocence plea for a speeding offence if I claimed I wasn't even moving M'lud, it was the space between me and the camera expanding. Leave c out of the explanation to start with, and we willl come back to that. As far as I can tell, you see, the 'expansion of space' is simply a cop-out. But never mind, we will look at the explanations first. STA can stay out of this because I am fed up with her gobbledygook , muddled thinking and ghastly English.
Anyone like to have a go?
|
|
|
Post by eamonnshute on Nov 20, 2010 9:23:25 GMT 1
The problem is that there is no absolute frame of reference, so everyone is stationary in their own reference frame - that is why the speed of light is the same for everyone.
so when you say that a galaxy has speed v you have to specify what the speed is relative to, and it can not be relative to space. A galaxy might be receding from us at half the speed of light, but relative to space, and its neighbours, it has no speed and it us us who are receding at half the speed of light. People in the other galaxy would measure light travelling at exactly the same speed in any direction, and deduce that they are not moving.
Since both we and the galaxy are stationary, and the distance between us is increasing, the space in between must be increasing.
|
|
|
Post by Progenitor A on Nov 20, 2010 9:34:36 GMT 1
The problem is that there is no absolute frame of reference, so everyone is stationary in their own reference frame - that is why the speed of light is the same for everyone. so when you say that a galaxy has speed v you have to specify what the speed is relative to, and it can not be relative to space. Well thanks for trying Eammon I would think that specifying the speed relative to something else is simple in that case? Surely its speed is relative to us. A galaxy might be receding from us at half the speed of light, but relative to space, and its neighbours, it has no speed Does this assume that we are not its neighbour? Are you saying that there are galaxies that have no relative recessional speed? Really I would like to keep c out of this discussion until later.
|
|
|
Post by eamonnshute on Nov 20, 2010 9:46:56 GMT 1
I would think that specifying the speed relative to something else is simple in that case? Surely its speed is relative to us. So we could say that we are stationary. The people in the other galaxy would say that they are stationary. Both viewpoints are equally correct. Galaxies have relatively small speed relative to their nearest neighbour, a tiny fraction of the speed of light. The distant galaxy is not a neighbour.
|
|
|
Post by Progenitor A on Nov 20, 2010 9:51:28 GMT 1
I would think that specifying the speed relative to something else is simple in that case? Surely its speed is relative to us. So we could say that we are stationary. The people in the other galaxy would say that they are stationary. Both viewpoints are equally correct. Galaxies have relatively small speed relative to their nearest neighbour, a tiny fraction of the speed of light. The distant galaxy is not a neighbour. Fine, but what is the observable difference between assuming that the increasing separation is due to expansion of space and assuming that the galaxies are simply speeding apart? (Again, if we can let c alone for the moment)
|
|
|
Post by abacus9900 on Nov 20, 2010 10:18:22 GMT 1
We are also told that some galaxies collide with one another, for example, some time in the distant future the Andromeda galaxy and the Milky way are due to collide. Now, if galaxies are continually expanding away from one another due to the expansion of space how come they still collide with one another?
But I think what eamonnshute meant was a galaxy doesn't move within its space - it is the space itself that is expanding between galaxies which increases the distance between them. So, from the point of view of an observer his galaxy is standing still and it appears all the other galaxies are receding from him and this is true wherever you are in the universe. Light from receding galaxies still maintains its normal speed but its wavelength is 'stretched' the faster the recession or, to use the technical term, 'red-shifted.'
|
|
|
Post by eamonnshute on Nov 20, 2010 10:19:43 GMT 1
The expansion of space is a prediction of Relativity, and Relativity has been tested experimentally with great success. For example, frame dragging, in which moving bodies (such as our distant galaxy) drag spacetime along with themselves.
|
|
|
Post by eamonnshute on Nov 20, 2010 10:23:26 GMT 1
We are also told that some galaxies collide with one another, for example, some time in the distant future the Andromeda galaxy and the Milky way are due to collide. Now, if galaxies are continually expanding away from one another due to the expansion of space how come they still collide with one another? Galaxies form clusters, their mutual gravity overcomes the expansion of space.
|
|
|
Post by abacus9900 on Nov 20, 2010 10:28:41 GMT 1
Really? I did not know that and am quite surprised.
Thanks.
|
|
|
Post by Progenitor A on Nov 20, 2010 10:29:10 GMT 1
The expansion of space is a prediction of Relativity, and Relativity has been tested experimentally with great success. For example, frame dragging, in which moving bodies (such as our distant galaxy) drag spacetime along with themselves. OK, but what observable difference is there between expansion of space and galaxies speeding away from one another? What error will I make if I assume that they are actually racing apart? On your second point, no-one has actually observed ST being dragged along have they?
|
|
|
Post by Progenitor A on Nov 20, 2010 10:32:43 GMT 1
Really? I did not know that and am quite surprised. Thanks. I know this is moving off-topic, but if the gravity of galaxy clusters overcomes the expansion of space, then why, at about t=0+ a little bit , when the gravitational forces were absolutely massive, did that not prevent the Universe from expanding?
|
|
|
Post by eamonnshute on Nov 20, 2010 11:25:59 GMT 1
What error will I make if I assume that they are actually racing apart? On your second point, no-one has actually observed ST being dragged along have they? You would be violating the principle that there is no special point in space. Why should one galaxy be regarded as moving while another is at rest? "The only equitable solution is to say that they are both at rest, since there is no reason to give us special status. There is experimental evidence for frame dragging, and astronomical evidence: en.wikipedia.org/wiki/Frame-dragging#Experimental_tests_of_frame-dragging
|
|
|
Post by eamonnshute on Nov 20, 2010 11:29:24 GMT 1
I know this is moving off-topic, but if the gravity of galaxy clusters overcomes the expansion of space, then why, at about t=0+ a little bit , when the gravitational forces were absolutely massive, did that not prevent the Universe from expanding? In the early universe matter was evenly distrubuted, so gravity pulled everything equally in all directions, giving no net force.
|
|
|
Post by Progenitor A on Nov 20, 2010 13:08:53 GMT 1
What error will I make if I assume that they are actually racing apart? On your second point, no-one has actually observed ST being dragged along have they? You would be violating the principle that there is no special point in space. Why should one galaxy be regarded as moving while another is at rest? "The only equitable solution is to say that they are both at rest, since there is no reason to give us special status. Fine again, but what error of observation or calculation would there be? Thanks I will look at that later
|
|
|
Post by Progenitor A on Nov 20, 2010 13:10:13 GMT 1
I know this is moving off-topic, but if the gravity of galaxy clusters overcomes the expansion of space, then why, at about t=0+ a little bit , when the gravitational forces were absolutely massive, did that not prevent the Universe from expanding? In the early universe matter was evenly distrubuted, so gravity pulled everything equally in all directions, giving no net force. If it was evenly distributed then what was its shape?
|
|