# Thread: General theory of relativity - in simple words?

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## General theory of relativity - in simple words?

I have read about the general theory of relativity in Wikipedia and I understood the example with a rocket and constant gravity pull, however I still don't seem to see what is the theory about or, more exactly, why is it so important. Can you please explain it in simple words if it is possible? Maybe it's not the simple words that matter, but just a bit different style of telling which will let me understand better.

2. I'm not sure if I can state things correctly, but I think I can give you an idea of what is special about it. You start with the idea that the speed of light is the same for any observer, and other things fall in place.

Normally, if you are traveling 10 kph and I am going 20 kph, and a cheetah runs by us at 80 kph, you will perceive it to be going 70 kph and I will perceive it to be going 60 kph. That is Newtonian physics. But that is not the way it works with the speed of light. We will both see it going the same speed. So if I am going 0.25 C and you are going 0.5 C, and a photon comes flying past us, we will both see it going the same speed, C. And I think the other things fall into place from that.

3. Originally Posted by Jens
I'm not sure if I can state things correctly, but I think I can give you an idea of what is special about it. You start with the idea that the speed of light is the same for any observer, and other things fall in place.
That's the special theory of relativity.

The OP asks about the general theory. Of course, the general theory includes the special theory, and the idea of equivalence of gravity and acceleration mentioned in the OP.

Why is it so important? It is an improvement of Newton's laws of gravitation, that allow us to account for not only the measurements and experiments that Newton's laws account for, but also others that Newton's laws seem to fail for. It takes us one step closer (maybe a big step ) to an understanding of the physical world.

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Newton's law of gravity describes gravity as a force between masses.

Einstein's general theory of relativity describes gravity as an effect of
the shape of space, where the shape of space is determined by the
presence of energy.

Einstein's theory describes gravity more accurately than Newton's,
especially where the gravity fields are strong, and it describes other
phenomenae besides gravity, since it is based on the special theory
of relativity, which is in turn based on classical electrodynamics.

-- Jeff, in Minneapolis

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Originally Posted by Louigi Verona
I have read about the general theory of relativity in Wikipedia and I understood the example with a rocket and constant gravity pull, however I still don't seem to see what is the theory about or, more exactly, why is it so important. Can you please explain it in simple words if it is possible? Maybe it's not the simple words that matter, but just a bit different style of telling which will let me understand better.
Matter (or really energy) causes spacetime to curve. That curvature is gravity which supercedes Newtonian gravity. Matter moves through spacetime depending on how spactime is curved by the matter (energy) in it. To quote John Wheeler, and make it extremely simple, 'Matter tells space how to curve. Space tells matter how to move.'

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Thanks, guys. I think I am beginning to get the idea. A few more questions then:
1. What exactly is meant when they say that space curves. Can we feel that our space is being curved when we are inside it?
2. HOw on Earth can you come up with that idea? I mean, are there any observations/mesurements which suggest this?

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Originally Posted by Louigi Verona
1. What exactly is meant when they say that space curves. (1.2) Can we feel that our space is being curved when we are inside it?
2. HOw on Earth can you come up with that idea? I mean, are there any observations/mesurements which suggest this?
1. That space becomes distorted. What it exactly means, is not something that's easily described by words.

1.2: Yes, it makes a feeling of acceleration and pulls masses together. In one word, it's gravity.

2. Propably plenty. Most of them are forgotten by genereal public, since people only remember what came out of them. Yet, most importantly, theories like that come from observing the ways universe works, and triyng to explain it.

8. Originally Posted by Louigi Verona
2. How on Earth can you come up with that idea? I mean, are there any observations/mesurements which suggest this?
Well it began with Galileo, who realised that in a closed room with no windows there is no way to tell if the room is at rest or if it is moving at a certain speed in a fixed direction.

Then came Newtons Laws of Motion describing how bodies move in response to forces. Newtons first law, the Principle of Inertia, says that a body experiencing no force moves at a steady speed in a straight line. There was a re-evaluation of Galilean Relativity, into the form: The Laws of Physics are the same in a uniformly moving room as they are in a room at rest. Physicists thereafter defined a room with calibrated measurements on the walls and a clock in it - an inertial frame of reference.

Then came Maxwell, who gave us the equations describing how electric and magnetic fields interact and give rise to each other, and these equations predicted that waves made up of electric and magnetic fields moved at 186,300 miles a second. It was immediately realised that light must be one of these waving electric and magnetic fields.

Then came Einstein, who stated that if the Laws of Physics are the same in a uniformly moving room as they are in a room at rest then the Laws of Physics are the same in all Inertial Frames. This means that any measurement of the speed of light in any inertial frame will always give 186,300 miles per second and led to startling conclusions - that if two observers moving at different relative speeds both measure light as 186,300 miles per second, their definitions of time and distance (their clocks and rulers) are being changed due to their relative speeds in order to keep light at that constant speed. This was Special Relativity.

Then came Einstein's General Relativity, where he realised that gravity and acceleration were equivalent and so he tied everything above together! It unifies special relativity, Newton's law of universal gravitation, and the insight that gravitational acceleration can be described by the curvature of space and time, this latter being produced by the mass-energy and momentum content of the matter in spacetime. 'Matter tells space how to curve. Space tells matter how to move.'

Initially, Einstein proposed three tests of general relativity.

1. the perihelion precession of Mercury's orbit
2. the deflection of light by the Sun
3. the gravitational redshift of light

The precession of Mercury's orbit had always been a problem with Newtonian physics. As a planet orbits a star, that orbit describes an ellipse that has a closest and furthest point from the star. In a very simple model with just a star and a planet, the orbit describes the same path each revolution, and so the closest point remains in the same place, relative to the star. In a complex solar system though, other gravitational forces come into play and that closest point actually moves around the sun a little each orbit and this is called precession.

Unfortunately the precession of Mercury was different to what Newtons Laws predicted and had been a problem in astrophysics for a while, but General Relativity describes the precession pretty much perfectly (within the limits of observational error).

I won't go into so much detail about the others (phew!) but the deflection of light by the Sun was finally shown to be in full agreement with General Relativity in the late 1960's after years of wrangling over earlier results, and gravitational redshift was tested successfully in 1959.

Since then we have tested other predictions of GR like gravitational lensing, light travel time delay testing, the equivalence principle and the gravitational time dilation which requires the clocks in all but one* of the GPS satellites in orbit around the Earth to be set to run a little slower than clocks here on the ground.

*The first GPS satellite was sent up with a standard clock and it gained 38 microseconds per day, as predicted by GR.
Last edited by speedfreek; 2008-Jan-22 at 12:44 AM.

9. General Relativity also limits the transfer of energy, or information, to the speed of light. It's the fastest anything can pass through space.

10. I knew I had missed something important!

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Thanks everybody!

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