# Could you say that GR is a theory of four or five dimensions?

1. Jan 1, 2012

### zheng89120

I don't really know much about GR, but I was just wondering about this theoretical question. I am asking this, because I think Special Relativity is a theory of four dimensions, and wasn't sure how many dimensions GR involved.

2. Jan 1, 2012

### Pengwuino

Four. Why would it be 5?

3. Jan 1, 2012

### zheng89120

Because, since space is "curved" in GR, I thought may be an addition dimension could be introduced to describe the curvature of space, but perhaps that is not counted as another dimension.

4. Jan 1, 2012

### elfmotat

Four: x, y, z, and t. What's the 5th dimension?

5. Jan 1, 2012

### maverick_starstrider

Essentially it's all integrals with 4 elements, what makes it GRey is that the volume element changes with position in space-time

6. Jan 1, 2012

E???

7. Jan 1, 2012

### Pengwuino

The 4 dimensions do describe the curvature of the space-time. For example, let's say you have a can of soda or a ball or something in the 3-dimensional world we're accustomed to. You can fully describe the curvature of those objects by using those 3 dimensions. A 4th dimension need not be introduced in this example.

Same idea with GR, except you now are dealing with a 3+1 dimensional spacetime instead of the 3 dimensional Euclidean space of the soda can/ball example.

If E = energy, why would E be a dimension? It doesn't measure space-time displacements.

8. Jan 1, 2012

### TimeContinuum

Sure it does. E alters our perception of distance.

9. Jan 1, 2012

### Pengwuino

The energy-momentum tensor in GR tells gravity, as many put it, how to curve, but that certainly does not mean it is qualified to be a dimension. A dimension has very specific meaning.

10. Jan 1, 2012

### Staff: Mentor

OK, given an inertially moving rocket in flat spacetime where, from the rocket's perspective, would the 1 J hyperplane be located?