General Relativity Explained simply & visually

Einstein's Special Theory of Relativity 00:00

• When Albert Einstein first published the Special Theory of relativity in 1905, it was either vehemently ridiculed or ignored.
• People thought it was too weird and radical to be real.
• Einstein was unhappy because the theory only applied to observers moving in a straight line at a constant speed.
• The theory did not apply if gravity was present or the observer was accelerating.

Einstein's Thought Experiment 00:51

• While observing a window washer on a ladder near his patent office, Einstein had one of his famous thought experiments that would go on to change the course of scientific history.
• Einstein imagined what would happen if the worker were to fall and put himself in the window washer's perspective.
• He realized that if falling, gravity would be the only force acting on him.
• He would be accelerating towards the ground, but since the ground was not pushing up on his body, he would feel no weight.
• With wind resistance, he would be in free fall, and this would be no different than being weightless in space.
• In a way, gravity and acceleration were different ways to describe the same thing.

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Einstein's Insight 02:25

• Einstein thought about what would happen if he took a flashlight or laser beam and pointed it from one side of the room to other, as the spaceship was accelerating upwards.
• If he had a sensitive measuring device, he could measure the height of light on the other side of the room.
• He realized that the height he would find on the other side of the room would be slightly lower than the source of light.
• Einstein thought it can't be because it violates the principle of equivalence.
• Acceleration of the room on a space should be no different than under the influence of gravity on earth.
• He realized that this meant light must bend in the presence of a gravitational field.
• But how could this be, because light always takes the shortest path between two points?
• Then he realized that maybe the light IS taking the shortest path between two points, and maybe the shortest path is not a straight line.
• He hypothesized that in space, perhaps a straight line is NOT the shortest path between two points, and that in the presence of mass and energy, space somehow becomes curved, so that the shortest path that light can take is a curved path.
• This was the key insight that Einstein had about gravity.

Einstein Contacts Marcel Grossman 06:19

• Einstein contacted an old buddy of his from college days, mathematician Marcel Grossman.
• Grossman had just finished his PhD dissertation on the topic of, wouldn’t you know it, geometry of curved spaces, called Reimannian Geometry.
• With his help, Einstein figured out the mathematics of curved space-time, and this geometry is really the basis of General relativity.
• Einstein’s theory was now that gravity is not a force between massive objects but something that emerges from the interaction of space and massive objects.

Orbits of Planets 07:36

• Orbits of planets could now be explained not by some mysterious force that acts at a distance but rather an interaction that takes place locally with mass or energy and the space around it.
• This can be visually represented by the kind of graphic you see here to show how massive objects like planets form orbits around other objects.

Importance of understanding the 3D interaction 08:03

• The trampoline analogy used for visualization purposes only depicts a 2D plane.
• The interaction between objects actually occurs in three dimensions.
• A graphic shows a more accurate representation of the interaction between objects.
• Although more difficult to visualize and animate, the graphic is more accurate.

Mercury's orbit mystery 08:29

• Mercury's orbit had been a mystery for decades because it was unusual.
• All planets orbited the sun in an ellipse, but Mercury's orbit had a precession.
• The point of Mercury's orbit farthest from the sun advances a little bit every time Mercury goes around the sun.
• No one could ever figure this out using Newton's equations.

Einstein's theory matches observation 09:03

• Einstein's new curved space theory predicted the precession that Mercury actually has.
• This was the first prediction that could be tested and not explained in any other way.
• Einstein's theory perfectly matched observation which had been a mystery for decades.
• Einstein was the only person in the world who realized that the universe actually works this way.

Confirmation of Einstein's theory 09:29

• Many scientists had doubts about Einstein's theory, but the most fool-proof confirmation came 4 years after he published it.
• A team led by English Astronomer, Arthur Eddington photographed stars near the sun during a total solar eclipse in 1919.
• The position of stars near the sun would appear different than predicted location based on where they should be as seen at night if Einstein's theory was right.
• This was because as light passed near the sun, it should be bent by the curvature of space due to gravity.
• He found exactly what he predicted, confirming that the theory was correct.
• This is the moment Einstein became a celebrity.

Space-time 10:15

• Einstein's first theory, special relativity comes in to explain how time enters into the picture.
• The essential presumption in special relativity is that light always moves at the same speed regardless of perspective or reference frame.
• This means that light will have the same speed in an accelerating reference frame as it will in a resting frame.
• Since the distance traveled by beam of light in a gravitational field is longer due to the curving of space, time itself must pass slower in the gravitational field relative to time in empty space.
• This implies that time is distorted by gravity along with space.
• Time increases proportionately with the curvature of space near a gravitational field, compared to empty space, to keep the speed of light constant in both reference frames.

Implications of space-time 11:33

• The observer experiencing no gravity at all sees a clock in a gravitational field running slower.
• The clocks on earth run slightly slower than the international space station.
• This effect has been confirmed by many experiments and is taken into account in order to keep the clocks of GPS satellites sync with those on earth.
• Otherwise, GPS apps like google maps would give inaccurate locations.

General Relativity limitations 12:06

• Although General relativity is an astounding achievement by one of the greatest scientists of all time, it does not answer everything.
• Questions remain, such as what exactly gravity is and why do massive objects distort space-time.
• It also predicts regions of space where time can get so distorted that nothing escapes, including light.
• This is called a black hole.
• General relativity fails to work at the singularity, where mass is concentrated to an infinitely small point with infinite density.
• To figure out what happens at these really small scales, we need to bring General relativity and quantum mechanics together to create a new theory called quantum gravity.