# Laymen’s view of what general relativity is?

1. Apr 1, 2004

### JonF

Will someone please give me a laymen’s view of what general relativity is?

2. Apr 1, 2004

### chroot

Staff Emeritus
3. Apr 1, 2004

### chroot

Staff Emeritus
It's rather difficult to provide a layman's description of an entire theory as expansive as GR. Let's see if I can (maybe someone can do better!).

In one sentence: "The General Theory of Relativity is a theory of gravity based on the assumption that mass curves the space (and time) around it; other masses interact with that curvature, following it in the straightest way possible."

- Warren

4. Apr 2, 2004

### JonF

Wow, how would mass curve time around it? Isn’t mass three dimensional, and time linear?

5. Apr 2, 2004

### chroot

Staff Emeritus
Well, you've just opened the pandora's box.

Most people -- and, in fact, everyone alive prior to the days of Lorentz and Einstein -- consider space to be three-dimensional, and time to be a totally separate entity. Time, however, is as much a dimensionful quantity as position. Think about this way: if I wanted to meet you for dinner, I should say "Meet me at the pizza parlor at 7 pm." If I omitted the time, and just said "Meet me at the pizza parlor," I wouldn't have provided enough information. Such a meeting, properly organized, is an example of an event -- the agreement provides three spatial coordinates, those of the pizza parlor, and one time coordinate, namely 7 pm, for a total of four coordinates. Four coordinates, one for each of four dimensions.

In relativity theory, neither space nor time are absolute. There is no such thing as a master ruler against which all distances can be authoritatively measured, and there is no such thing as a master clock against which all intervals of time can be authoritatively measured. Time and space are local quantities, experienced independently by each meterstick and each clock, and by the people using them. If you go flying off in a spaceship at a very significant fraction of the speed of light, as seen by an earth-bound buddy, your clock and your earth-bound buddy's clock will not tick at the same rate. Furthermore, your notion of distances will also change -- if you go fast enough in your spaceship (extremely close to the speed of light), you'll measure the stars as being only feet, or inches, apart, as compared to the trusty meterstick you keep in the cockpit. That's lesson number one in relativity: time and space are not absolute quantities. They are... relative!

Now, there's another way to change one's notion of time and space that does not require messy spaceflight and large velocities. Gravitational fields affect space, as well. (Everything I've said up to this point deals only with the special theory of relativity, which is only applicable in the absence of gravitational fields. When you introduce gravity, you have to use the full splendor of the larger and more complicated general theory of relativity, which includes the special theory as a special case.)

Deep in a gravitational well, clocks tick slowly, as compared to distant clocks. Deep in a gravitational well, distances are equally affected. These changes in time and space measurements go hand in hand: one is called time dilation, and the other is called length contraction. The two effects always occur together, and in similar amounts. The two effects are really just two sides of the same coin.

Relativity considers time and space to be "intertwined," in a way. Instead of treating time and space as being entirely different concepts, relativity theory uses them together as part of a larger concept: spacetime. When I said that mass curves "space and time," I was being somewhat loose with terminology: mass curves spacetime. Spacetime is a four-dimensional sort of thing: three of space and one of time.

It's difficult to "detect" the curvature. In a small neighborhood, all the dimensions of space appear to be exactly perpindicular to each other, for example. You have to do experiments with clocks, or light beams, or other bits of apparatus (and many, many have been done) to detect the curvature of something so mild as the Sun, or the Earth.

In a more extreme example, you could quite easily detect the strong curvature near a black hole. A black hole's gravity is so strong, and thus the nearby spacetime so curved, that light can actually orbit the black hole. If you were standing in just the right place, you could shine a flashlight forward and see the back of your own head!

Light, you see, and all other objects moving freely, will try to follow the straightest possible lines in curved spacetime. There are no perfectly straight lines in curved spacetime, however. When light orbits a black hole, it is in fact moving in the straightest possible way -- but that way, from a distance, appears decidedly non-straight.

Another more familiar example of the curvature of spacetime is the trajectory of a baseball thrown into the air. The familiar parabolic curve is, you guessed it, the straightest possible line the baseball can take through the mildly curved space around the Earth.

- Warren

6. Apr 2, 2004

### JonF

thanks, that makes alot more sense now

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