# Time and Gravity

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 27461.  Wed Oct 19, 2005 4:43 pm Apologies if this has been mentioned elsewhere. Steven said, "The further away from gravity you get, the slower time runs, and with no gravity time stops. This was proved by taking an atomic clock up in an airplane and when it landed it was running slightly slower." WRONG! There is no gravity between the stars, if time stopped, how would the light, or anything else, reach us? Although it is true about the airplane it is actually accelerations which have a time dilation effect, the faster you travel, the more slowly time passes for you. If you hitch a lift on a photon (of light) then time for you freezes.

 32406.  Fri Nov 18, 2005 1:27 pm No confirmation or contradiction? Gasp!! :)

 32433.  Fri Nov 18, 2005 2:36 pm I was wondering why I had never heard of this link before.

 32443.  Fri Nov 18, 2005 2:55 pm Harry, Time has stopped hereabouts. A scientist will be back to you shortly.

 32469.  Fri Nov 18, 2005 4:57 pm Failing that, I'll have a bash... Some of what Stephen said was accurate, and some not. The same is true of your statements, in fact - I'll try to explain what's what, what. I don't have a transcription of exactly what Stephen said, so I'll take your word for it (I'm sure Flash can help us out...) This is all Einsteinian General Relativity, as I'm sure you're aware. Stephen: "The further away from gravity you get, the slower time runs" - this is quite correct. Down on the surface of the Earth, time runs more quickly than if you were 1,000km up. Stephen: "and with no gravity time stops" - this is nonsense, of course, for the reasons you give and many more. Stephen caught a-busking there, I feel. Stephen: "This was proved by taking an atomic clock up in an airplane and when it landed it was running slightly slower." - the 'atomic clock in an aeroplane' experiment did return to Earth with a clock that was running slightly slower than its Earth-bound counterpart, but not only because it went high up - it was also because it was travelling relatively quickly. So half right. HarryAlffa: "There is no gravity between the stars" - this isn't quite true - there is gravity everywhere: you can't ever get completely away from it. Between the stars it's just very weak. HarryAlffa: "Although it is true about the airplane it is actually accelerations which have a time dilation effect, the faster you travel, the more slowly time passes for you." - again, partially correct. High relative velocity gives a time-dilation effect (because the constancy of the velocity of light 'squashes' any frames of reference that approach it). 'Faster' means 'with more velocity', whereas 'acceleration' means 'with a changing velocity' (and is equivalent to a gravitational force). HarryAlffa: "If you hitch a lift on a photon (of light) then time for you freezes." - quite right. In fact, if you ever got to the speed of light, the universe (from your point of view) would immediately come to an end. You're right to point Stephen's inaccuracies, though. Relativity is a highly unintuitive area...

32474.  Fri Nov 18, 2005 5:14 pm

 Gray wrote: Stephen: "The further away from gravity you get, the slower time runs" - this is quite correct. Down on the surface of the Earth, time runs more quickly than if you were 1,000km up.

I don't think that this quite correct. What does 'the further away from gravity' mean?

Time runs more slowly for bodies 1,000 km up than for a body on the surface because the higher body is moving faster. This is because it has a greater distance to travel as the Earth makes one complete revolution.

32479.  Fri Nov 18, 2005 5:36 pm

 Gray wrote: Some of what Stephen said was accurate, and some not. The same is true of your statements, in fact - I'll try to explain what's what, what. I don't have a transcription of exactly what Stephen said, so I'll take your word for it (I'm sure Flash can help us out...) Stephen: "and with no gravity time stops" - this is nonsense, of course, for the reasons you give and many more. Stephen caught a-busking there, I feel.

Actually, to say that 'without gravity time stops' would be correct, though maybe not for the reason you think.

Gravity is caused by matter causing a curvature in the space-time continuum. So if there were no matter there would be no gravity. But, if there were no matter there would be no time. Without changes in matter there would be no way of distinguishing one moment from another and so time would not exist.

32559.  Sat Nov 19, 2005 5:16 am

Okay, a few more misunderstandings here.

 Quote: I don't think that this quite correct. What does 'the further away from gravity' mean? Time runs more slowly for bodies 1,000 km up than for a body on the surface because the higher body is moving faster. This is because it has a greater distance to travel as the Earth makes one complete revolution.

'Further away from gravity' means (here) higher up the gravitational well - i.e. further away from the centre of gravity of an object.

A body 1,000km up and relatively still compared to the Earth will STILL run more slowly because the spacetime its sitting in is less curved than if it were on the Earth's surface. The speed that it's going (if you assume it's in orbit) will have an additional effect, but it's the 'spacetime gradient' that we're talking about here.

 Quote: Gravity is caused by matter causing a curvature in the space-time continuum.
Quite correct.

 Quote: So if there were no matter there would be no gravity.
Also good.

 Quote: But, if there were no matter there would be no time.

This, however, doesn't follow - or it at least depends on an inconsistent definition of what you mean by 'time'. In that paragraph, there's been a subtle switch between 'time as a physically measurable quantity' and 'time as a subjective concept to distinguish events in one's perception'.

Agreed, without matter there would be no time measuring equipment, but this is just a solipsistic point (like the old "if there's no-one to hear them, do falling trees make a noise?" argument. It relies on a subjective condition, and is therefore not something directly connected to scientific modelling).

32565.  Sat Nov 19, 2005 5:58 am

Gray wrote:

 Quote: Gravity is caused by matter causing a curvature in the space-time continuum.
Quite correct.

 Quote: So if there were no matter there would be no gravity.
Also good.

 Quote: But, if there were no matter there would be no time.

This, however, doesn't follow - or it at least depends on an inconsistent definition of what you mean by 'time'. In that paragraph, there's been a subtle switch between 'time as a physically measurable quantity' and 'time as a subjective concept to distinguish events in one's perception'.

Agreed, without matter there would be no time measuring equipment, but this is just a solipsistic point (like the old "if there's no-one to hear them, do falling trees make a noise?" argument. It relies on a subjective condition, and is therefore not something directly connected to scientific modelling).

I don't agree. If there were no matter there would be no time. The only way that we can say that time passes is through changes in matter. A clock is a particular arrangement of matter which measures time, but even if there were no clocks we would still need matter to indicate the passage of time.

Suppose there was universe with 3 particles of matter, A, B, and C. A and C were a fixed distance apart and B oscillated between the two. The fact that B was at some moments close to A and at other moments close to C would indicate the passage of time. We could call one moment AB and another BC.

Now suppose there were a universe deviod of matter (if such a concept makes sense which it probably doesn't) then there would be no way of distinguishing one moment from another. Each 'moment' would be indistinguishable from another. How could you say that an hour, or a second, or any time at all had passed?

 32937.  Sun Nov 20, 2005 11:38 am As I said, that is a solipsistic argument. If there were no matter, we wouldn't be around either, so the concept of time (and it really is purely a concept) wouldn't exist either - there would be no brains for it to lurk in. We were talking about time as it is represented in General Relativity, which is a different matter altogether. Nowhere does GR predict that absence of matter in an area of spacetime would preclude the 'passage' of time - if you could measure it. Why should it? If that were the case, then going to a point of very little gravitational attraction - say where Voyager I is now - woud mean that time would somehow stop, which is clearly not the case, as we're still receiving signals from it. And signals can only exist in time. These are two different contextualisations of what we mean by 'time'.

 32950.  Sun Nov 20, 2005 12:28 pm But Voyager is made of matter. The argument being made is (I think) that whilst time could pass, it might as well not.

33211.  Mon Nov 21, 2005 11:35 am

This is a very interesting discussion, but I feel the need to point out a very slight error that has been made here.

Time actually slows down in the presence of gravity, not the other way around.

Check out http://www.astro.uiuc.edu/~uias/s-m/sm-feb-95.html which says:

 Quote: Black holes have a few very strange properties. One of these is that when you are close to them, time slows down in comparison to an outside observer(time slows down when you are close to any gravitational body. We are even experiencing time slightly slower than satellites in orbit around the earth, but close to a black hole, this time dilation is greatly increased).

Of course, this is somewhat confused by the fact that relativity is, as the name suggests, all about relative speeds or gravitational fields. If you hitch a ride on a photon and travel at the speed of light, a stationary observer will see your time stop. However, from your point of view, the "stationary" observer will be moving at the speed of light relative to you and therefore his time will appear to stop.

 33213.  Mon Nov 21, 2005 11:38 am So if you moved at almost the speed of light in opposite directions, what then?

 33220.  Mon Nov 21, 2005 12:05 pm Nothing can travel faster than the speed of light. This is one of the principles of relativity. So if you have person A moving forwards at the speed of light, person B moving backwards at the speed of light, and person C standing still, the following would occur: Person A would see person B moving away from them at the speed of light. Presumably they should also see person C moving away from them at the speed of light too, which means that they would not see person B moving away from person C. This would make sense, since they would see time for person B and person C stop, since they're travelling at the speed of light. Thus, time would be frozen at the point where person B and person C are still standing together. Person B would see the same, except with person A and person C. Person C would, of course, see person A moving off in one direction at the speed of light, and person B moving in the other direction at the speed of light. If your head is starting to hurt at this point, welcome to the whacky world of relativity! :)

 33247.  Mon Nov 21, 2005 1:55 pm No, I meant towards each other at the speed of light.

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