Einstein’s Theory Of Relativity

The Speed of Light

A Danish astronomer by the name of Ole Römer, became the first person to ever measure the speed of light, albeit, completely by accident. In 1676, he was measuring the timing of eclipses of Jupiter’s moon, Io, from two points in earth’s orbit. After several years of collecting measurements, he realized the that time differences of the eclipse could be explained by the time that light had to travel to reach the earth in its particular point in orbit. Before this discovery, scientists believed that the speed of light was either infinite, or too fast to possibly be measured.

Some 200 years later, in 1905, Albert Einstein released his Theory of Special Relativity, which described the relationship between the speed of light and massive objects in the vacuum of space. The term ‘Special Relativity’ refers to how objects move and light behaves over the course of vast distances without the complications of the gravitational component.

Theory of Special Relativity

Einstein’s favorite physicist, Sir Isaac Newton, was the first to contemplate the laws of motion and gravity, but admitted that his theories did not answer all the surrounding questions. It wasn’t until Einstein published his Theory of Special Relativity that the landscape of physics was changed forever.

Einstein often pondered about trying to catch a beam of light. He imagined, the closer one approached the speed of light, the more energy would be required to accelerate mass towards the cosmic speed limit. Since light, an electromagnetic wave, has no mass, accelerating any object with mass to the speed of light would require an infinite amount of energy and so it remains an impossible feat. Not to mention, if one could travel faster than light, then light itself would never be able to reach their eyes, thus they would see nothing—complete darkness.

Einstein’s Theory of Special Relativity also proposed the existence of ‘gravitational lensing’; a phenomenon where light from a source is curved due to a massive body which can distort the apparent position of said light source. His prediction of gravitational lensing was later confirmed in 1979.

Gravitational lensing—the sun’s mass curves the light of the star located behind it as viewed from earth.

Time Dilation

One of the aspects of Einstein’s Theory of Special Relativity is time dilation; simply put, a moving object experiences time more slowly than an object at rest. First of all, there is no such thing as absolute rest, nor absolute motion. Everything is moving, in relation to everything else, hence the term ‘relativity’. For example, a car traveling at 60 mph is moving at 60 mph relative to an object at rest. At the same time, a car traveling 60 mph is moving at 20 mph relative to a second car traveling in the same direction at 40 mph.

Einstein concluded that light itself was relative; no matter how fast one could travel, light would travel away from you at the speed of light—it would appear to you that you were not moving at all. Because the speed of light is constant regardless of the frame of reference, what must change is—time. In fact, it is Einstein’s theory that allows Global Positioning System (GPS), to work today, as the timing must be corrected each day for the time difference between clocks on earth and the clocks onboard orbiting satellites. The differences in gravity affects the calculation of time and must be adjusted in order to provide precise calculations.

Time dilation is negligible at the speeds we have been able to travel as well as with the gravity on earth, but if you amplified the numbers, it becomes a much easier concept to grasp. A great visual example of this phenomena occurred in the movie Interstellar. The protagonists park their spacecraft outside nearby a planet, relatively close to a blackhole. Due to the immense gravity in the area, each hour spent on the planet equated to seven years of time passing on earth.

Theory of General Relativity

In 1915, a mere 10 years later, Albert Einstein revised his previous assertations and published his findings, known as the Theory of General Relativity. He asserted that space and time could be warped by objects of massive gravity and thus interwoven into a single entity—space-time. The standard visual used for many years was that of a bowling ball placed onto a trampoline which portrayed a warp in the trampoline. While this is a good example, it is often confusing as it only depicts space-time in a two-dimensional sense.

The sun and earth warping the fabric of space-time—two dimensional (2-D) depiction.

Unfortunately, the above example does not fully visualize the conception properly. The graphic below more accurately represents how mass affects the fabric of space-time.

Mass warping the fabric of space-time in a four-dimensional (4-D) visual. The clocks at each intersection display how mass distorts time.

Why don’t we notice relative motion?

As previously mentioned, everything is in motion relative to something else. When we are flying on an airplane traveling at 500 mph, without acceleration or turbulence, it feels as though you are sitting still. Standing, walking, or any other motions feel normal because the airplane is an inertial reference frame—meaning, everything in the airplane is moving together.

This is the same reason a vehicle can travel down the highway at 70 mph and a bee can fly around inside the vehicle with ease. Everything inside the vehicle is moving together and thus it feels as if it is not moving as there are no opposing forces. The reason we don’t feel the movement is because the air molecules in the vehicle are stationary relative to everything else inside the vehicle.

That being said, everything remains in motion. The earth rotates once a day on its axis at approximately 1,000 mph—it revolves around the sun at approximately 67,000 mph, while the sun and our solar system orbit the galaxy at approximately 490,000 mph—and throughout all of this, the Milky Way is barreling through the universe at a staggering 1.3 million mph. Essentially, the earth is our personal spaceship, our vehicle, moving through space-time at astronomical speeds; we just can’t feel it because the earth, the atmosphere, and everything we interact with are all inertial reference frames, traveling through space-time together.

“If you can't explain it simply, you don't understand it well enough”

— Albert Einstein