1. Einstein’s theories of Relativity
Albert Einstein (Al) is best known for his two theories of relativity
Special theory of relativity (dealing with high velocities)
General theory of relativity (dealing with gravity)

2. Special theory of Relativity
Proposed in 1905 to support the observations of A.A. Michelson and E. W. Morley that they could not see the effects of the ether.
Einstein reckoned if you can’t measure it, then it does not exist.
No matter what our speed, we always measure the speed of light as 299, km/s.

3. The special theory of relativity allows us to extend our laws of Physics from low speeds to speeds approaching the speed of light.
Some predications
Moving objects appear to contract in direction of motion
Moving clocks run slow
Mass appears to increase
All predictions have been repeatedly verified to very high accuracy.

4. General theory of relativity
General relativity explains what gravity is and how it fits in the special relativity framework.
Proposed in 1915.
It equates gravity to accelerating reference frames. There is no experiment you could perform to tell the difference except looking outside.
As with the ether, Einstein reckoned if you can’t measure it or tell the difference, there is no difference.

5. To incorporate accelerating frames into special relativity, Einstein had to treat time just like he treated space and thus our new coordinate frame is called space-time.
It is a four dimensional space.
Thus matter curves space-time and objects follow that curvature and we call the effect Gravity.

6. Consequences of Relativity Theories
The study of black holes
Let us suppose we get funding to send an unmanned space probe towards a black hole. The probe will radio back information like temperature, time, etc.

7. We discover that as the spacecraft approaches the black hole, the frequency we have been communicating with slowly changes to a lower frequency.
This is because the light losses energy as it tries to escape from the region around the black hole. As time goes on the frequency shifts so low we can no longer communicate with it. It has slipped past the event horizon.

8. We also notice that has the probe approaches the black hole, the time reported back continues to be slow. It is as if we are aging faster than the probe is. This is called time dilation.
As the probe enters the event horizon it appears that its clock slows to a stop and we never actually see the probe enter the horizon.

9. Other GTR predictions
The extra amount of precession of planet Mercury’s orbit is explained by GTR.
The bending of light by gravity is predicted

10. Gravity from other galaxies bend the light from other galaxies to form Einstein rings and crosses.