Question about GR - movement in space-time

1. Mar 22, 2005

X-43D

Is it true that nothing physical can move in space-time?

As a layman in GR i really wondered whether anything physical can move in space-time because movement in space-time is self-referential and will contradict the basic definition of space and time.

2. Mar 22, 2005

HallsofIvy

Staff Emeritus
Where did you get the idea that moving in space_time is "self-referential". In fact, since time goes on, no matter what we do, we can't help moving in space-time!

3. Mar 22, 2005

dextercioby

Yes,space-time digrams are the most intuitive way to approach Special Relativity.What Halls said can be explained through a simple space-time driagram.

So yes,everything "moves in spacetime".Points in space-time are called events.

Daniel.

4. Mar 22, 2005

Garth

Hi X-43D, it is a matter of perspective.

SR encourages us to have a truly 4D perspective of space-time. As time itself is 'within' a space-time diagram, and our experience of movement requires a 'passing of time', then as that link of yours quotes 'nothing moves in space-time'. It is a 'block universe'.

However that is only an artefact of the diagram and its 4D perspective. As we are conscious beings we observe the universe from our own 3D+1 perspective and slice space-time into our own perception of space and time.

Different observers slice up space-time (foliate it) in different ways, so, for example, the plane of simultaneity is frame dependent. From our perspective, or that of any other observer, time does 'pass' and there is movement.

It is important not to confuse the two perspectives in describing the times and positions of different events in the universe. You just have to do it consistently.

Garth

Last edited: Mar 22, 2005
5. Mar 22, 2005

jcsd

It dpend swhat ypou mean, you can't move through spacetime in the same way as you move through spacetime. You can look at objects in spacetime in such a way where to tlak about movemnt makes little sense (thisd is IMHO the best way as you look as your just using your intution from 3-D geometry in 3+1 dimensonal spaces) or you cna look at it in a way where there is soemthing analogus to movemnt (though all this 'movement' really represnts is the moevmnt of objects through space and time in diffeernt refenrce frames rather than through spacetime).

Last edited: Mar 22, 2005
6. Mar 22, 2005

X-43D

Ok but it's correct to say that space-time is an abstract mathematical entity. Also since the t-coordinate is invariant (time itself does not change), time travel through wormholes is physically impossible.

Hi Garth,

So according to my understanding it depends on the definition. I couldn't find any mathematical difference between the thing that we call 'curved space-time' and the other thing that we call the gravitational field. Therefore it's not really space-time which is curved, it's the gravitational field under the influence of masses.

Therefore i guess it's correct to say that time itself does not change because motion/change is observed but time is never observed, it's abstract. Therefore time dilation (sometimes a misnomer) and process slowdown are basically one and the same.

Last edited: Mar 22, 2005
7. Mar 22, 2005

dextercioby

Nope.Curved space-time is just a mathematical abstraction,just like any other Lorentzian manifold.We humans use it in physics just as any other math.tool.While a gravity field is something whose existence is physically provable.You can do experiments to prove the existance of the gravity field.Space-time is curved.Period.

Daniel.