H
hecd2
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…and I said nothing about it in my reply - see above.hecd2:![]()
First off - I said nothing about the earth rotating.Your argument was about a Newtonian system. In a strictly Newtonian system, and given the relative strengths of the sun’s and other stars’ gravitational fields at the earth, an unmoving earth is not viable - a) the rest of the universe cannot “balance” the sun’s gravitational field (ie the earth cannot be at the L1 Lagrange point of the sun and the rest of the universe given the proximity of the earth to the sun) - am happy to provide numbers if you like - b) no extended object (ie earth) can remain permanently at the centre of mass of the universe - the existence of a fixed centre of mass in a Newtonian system of freely moving particles subject to gravitation and elastic interactions alone should not be confused with the claim that a point on an extended object (eg the earth) is at that fixed centre of mass, which it can be instantaneously but not for a finitely long time - and this ignores non gravitational forces such as electromagnetic interactions and non-elastic interactions which in any real system mean that even the centre of mass is not fixed.
Yes, but what *is *scientifically possible is to demonstrate that the earth cannot permanently occupy that centre for the two reasons I laid out above, viz that in a Newtonian universe it cannot be at rest because it is far away from a gravitationally null field (the sun’s gravitational field at the earth is demonstrably and hugely greater than the rest of the universe’s) and because an object of finite extent and mass in a Newtonian many body system cannot occupy any single location in co-ordinate space indefinitely.And unless you happen to know all of the masses (and their distances) in the universe - which you don’t, there is no way to calculate where the center of mass of the universe actually is. Or where it will be in an hour. Or where it was an hour ago. So to say that “It definitely wasn’t / isn’t / won’t be here” is not scientifically possible.
You don’t need to calculate where the centre of mass of the universe is, which is I agree is impossible to do also because we can’t see the fiull extent of the universe, to know that the earth cannot be at that point indefinitely.And even if you had all the masses and their distance and directions, it’s an impossible calculation anyway.
The concept of a finite speed of gravity is a general relativistic one, so is moot in a discussion about a Newtonian system. But in any case you don’t have to know where the centre of mass of the universe is to determine for the two reasons I laid out that points in the earth can only coincide with it instantaneously. So you can say that it is possible in a Newtonian system that the earth can coincide with the centre of mass of the universe a finite number of times, but you cannot say that the earth is permanently at rest at that centre.But suppose you had a super computer that could do it. You’d have to know at what speed gravity (waves?) travel. Do you know?
Sure there are, but you started this discussion with a post talking about the centre of mass of the earth-sun system - your logic was built on the assumption of centre of mass of the universe. You can imagine other definitions, such as the geometric centre of a spherically symmetric finite universe, but so long as whatever you define as the centre is at rest in a co-ordinate system, points in the earth cannot coincide with it except instantaneously.And even then, you are assuming that the center of the universe is the same as the center of mass of the universe - which I suppose is one way to look at it, but there are others I’m sure.
Alec
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