How is spacetime one?

1. Nov 20, 2014

rajeshmarndi

Why do we say spacetime is moving and not space is moving in time dimension. And how is spacetime one.

Also, there is only space, where changes(event) occur, and the rate of changes is describe with term time. That is, time is just a term and not actually a dimension like space(3 dimension).

2. Nov 20, 2014

ChrisVer

I'll start from your last comment. In that you take as a given fact there's only space where events occur and you use time as a parameter which can describe your system / describe your spatial dynamical variable and not as a dynamical variable itself. That's actually a classical/Newtonian way of seeing the world with an absolute time.

Now to the first which I think I don't understand. Are you asking why Special Relativity exists?
Because Lorentz Transformations mix time with space, making time a dynamical variable as well...Frame of reference dependent...

3. Nov 20, 2014

A.T.

You can interpret any independent parameter as a "dimension" in some abstract space. See for example:
http://en.wikipedia.org/wiki/Color_space

4. Nov 20, 2014

rajeshmarndi

I am asking how time is not separate from space? And therefore spacetime and not just space.

5. Nov 20, 2014

A.T.

Per definition. You define a manifold that contains both, et voilĂ .

Last edited: Nov 20, 2014
6. Nov 20, 2014

ChrisVer

Because eventually the Lorentz Transformations which connect the reference frames among themselves keeping the proper distance fixed, in contrast to Gallilean Transformations, mix the time with space. Since they are mixed, time and space should be one thing, that's why instead of 3 vectors you are working with 4 vectors, and thus events are described not by 3 but by 4 variables... Now asking why it's true, it's like asking why SR is true... Because it has been very well experimentally verified and established. As an example, it's the Lorentz transformations which keep the Maxwell equations invariant...

7. Nov 20, 2014

rajeshmarndi

What does it mean, space also has time in it.

8. Nov 20, 2014

A.T.

No, spacetime has time in it.

9. Nov 20, 2014

rajeshmarndi

And what does that actually mean. That is what I am not clear.

10. Nov 20, 2014

rajeshmarndi

Can you elaborate it, that will be helpful.

11. Nov 20, 2014

A.T.

There isn't much to elaborate on, beyond what has already been said.

12. Nov 20, 2014

ComplexVar89

This might help you. Minkowski space is the "abstract space" that SR is built upon. I'm not saying you need to understand the explicit mathematics right now, but the opening description might help you a bit, and you can follow links to terms you don't know right down to the point that you do understand, at least in a general "handwaving" sense, what they mean.

http://en.wikipedia.org/wiki/Minkowski_space

13. Nov 20, 2014

rajeshmarndi

Do we call individual sliced frame of events as spacetime. Or all past and future event are inside the spacetime. That is, spacetime is like a single loaf of bread which can be sliced differently into frame which differ by simultaneity.

14. Nov 20, 2014

DaveC426913

A dimension can be considered a degree of freedom. I can move in direction X independently of my movement in Y or Z.

Consider: you cannot define two points(such as, say, you and your date) to be co-incident with each other without defining four values.

You can tell your date to meet you at Broadview (X) and Main (Y) in the 6th floor (Z) coffee shop, but unless you tell her when (t), you will miss her.

Spacetime requires 4 coordinates to define a point.

15. Nov 20, 2014

Staff: Mentor

I don't think that anyone says spacetime is moving. In fact, the first postulate essentially says that such a concept is physically meaningless.

16. Nov 20, 2014

rajeshmarndi

I understand spacetime is the totality of all events inside it. So it become meaningless to say spacetime is moving, instead it is the successive sliced frame of the spacetime that are moving in the forward direction of time.

17. Nov 20, 2014

ComplexVar89

Spacetime is static, not "moving." Every past, present, and future event is already laid out. The loaf of bread in that analogy doesn't move, does it? That loaf is the stand-in for spacetime. The loaf itself doesn't move.

18. Nov 20, 2014

Khashishi

Rajesh, time and space are connected because different observers see different slices in spacetime. For observers moving at different velocities, the slices (which correspond to their space dimension) are at angles to each other. That is, one person's space dimension is another person's space dimension plus a little bit of time. What person A measures as a distance between two events, using a ruler, another person will measure as a distance and time using a ruler and a clock.

19. Nov 20, 2014

pervect

Staff Emeritus
Generally, people don't say "space moves in the time dimension". At least not that I've seen. Are you sure that this is the exact wording?

I frequently see people say that objects and/or people "move" through time, but this isn't literally any sort of motion. I am going to assume this is probably what you meant by your quesiton. Personally I prefer "progress" through time, but I don't think that phrase is very commonly used.

As far as why space and time are unified, it is for the same reason that we unify two different directions on a plane (say left-right and up-down. If an object is exactly on your left, and you turn a bit, it might be in front of you. Rotations can turn "left" into "front", they "mix" the two different directions together. So the different directions such as "left" and "front" are part of a bigger entity, and entity we call space.

Similarly, Lorentz boosts are a form of transformation in special relativity that are similar to rotations, but these these rotations mix together time and space in much the same way that rotations mix together left and front. Lorentz boosts are the mathematical description of a change in velocity in special relativity. For a more detailed example, read "The Parable of the Surveyor" , you can find an online version at http://spiff.rit.edu/classes/phys200/lectures/intro/parable.html or download chapter 1 of spacetime physics (the original textbook source) at http://www.eftaylor.com/download.html. The later is credited as the original source of the idea, the author generously makes the first few chapters of the 1965 first edition of his textbook available online.

Note that in general you can't really trust everything you read online about special relativity :(. Taylor and Wheeler's textbook is a good reference, though, and the online material presents the same message as the textbook does.

20. Nov 21, 2014

ComplexVar89

This is from the Relativity FAQ here on PhysicsForums, and while it invokes GR, it might help you realize what we're trying to tell you a little better.

Geodesic:

A geodesic is the world-line (a path in four-dimensional spacetime, or 4-curve) "followed" by a free-falling body, or by light.

For example, a rocket in orbit follows a geodesic, but a rocket using its engines does not, nor does a rocket on the ground.

There are also "faster-than-light" geodesics.

[Emphasis Mine] Technically, a particle does not move along a world-line: the world-line is a curve in spacetime, representing the whole history of the particle.