When we say the word “Relativity”, the first name that comes to mind is the name: “Albert Einstein”. However, many do not know that relativity is an old concept that dates back to the time before Galileo. Relativity in physics simply means that measurements may have different outcomes depending on the relative motion of the observers. To illustrate this, imagine a person sitting inside a moving train who throws a ball straight up. Relative to that person (and everyone inside the train), this ball goes straight up, stops, and falls straight back down. But what would that ball look like from the perspective of a person at rest who is outside the moving train?
Relative to that outside observer, the ball does not only go up and then down, but also moves horizontally along the same line as the train. Which observation is correct then? Relativity tells us that both observations are valid and that no observation is more accurate than the other, as long as both observers are either at rest or moving at a constant velocity (not accelerating).
What does Einstein have to do with relativity then? Einstein’s Special Theory of Relativity, or Special Relativity for short, is a special case of the old concept of relativity, in the sense that it deals with observations at speeds close to the speed of light. Einstein came up with this idea by doing a simple thought experiment. He asked the question:
“What would happen if I rode a beam of light?”
For some context, by that time James Clerk Maxwell had already published his theory on Electromagnetism. In that theory, he explained how light is composed of both electric and magnetic waves creating each other while propagating through space. He has also found that the speed that light should propagate should always be equal to 299,792,458 meters per second, which is known as the speed of light. According to relativity, if you would ride a beam of light, you would see both the electric and magnetic waves at rest because they are moving with you. This is not possible, however as shown by Maxwell’s theory of electromagnetism.
Einstein’s Special Relativity is quite simple with only two postulates:
- The laws of physics are the same in each reference frame, independent of the motion of this reference frame.
- The speed of light, c, is the same in every reference frame.
The first postulate simply means that the laws of physics work similarly in all reference frames. Let us look back at our previous example of a ball being thrown upward, inside a moving train. Relative to a person inside the train, the ball goes upward, stops and then falls back down because of gravity. Relative to a person at rest outside the train, we see the ball not only go up but also move horizontally with the train. Its trajectory forms a parabola. This is also due to gravity and is called projectile motion. We see here that gravity works similarly from the two reference frames.
What makes special relativity groundbreaking, however, is the second postulate and its consequences. At first, it may not sound very simple, but think about the consequences of that. There is a concept in relativity called the addition of velocities. An example of this is a person inside a car moving slowly at 1 m/s. He throws a ball forward at the same speed 1 m/s. Relative to him, the ball will move forward at 1 m/s, but relative to a person observing at rest, the ball will have been moved at 2 m/s. It also works in the opposite direction, if the ball is thrown backwards instead, at the same speed, relative to the person resting and observing, the ball will have a velocity of 0, meaning it will seem to be stationary.
Mythbusters have actually done this experiment by shooting a soccer ball from the back of a moving truck. The soccer ball is released when the car accelerates to a speed which is equal to the velocity with which the soccer ball is released. Relative to the camera on the ground, the cannonball appears to be stationary and drops straight down.
For light, however, this cannot happen as the second postulate of Special Relativity tells us that the speed of light will be the same at all reference frames. For example, a spaceship is moving at half the speed of light when it fires its headlights. Relative somebody at rest outside the spaceship, they would still see light travelling at the speed of light, and not 1.5 times the speed of light (speed of space ship plus speed of light). How can this be possible?
Einstein solved this problem by saying that even though all observers agree that light is always travelling at the speed of light, they may not agree on both the time and distance in which it happened. In other words, to make different observers agree on the same speed of light (remember that speed is just distance divided by time) both the distance and the time should be different for both observers. The changing distance and time are called length contraction and time dilation, respectively.
Length contraction is the phenomenon where, relative to an outside observer, a moving object will have its length in the direction of its motion contracted. For example, a 1-meter stick moving at 87% the speed of light will be observed by an outside observer to have a length of around half of its proper length, or 0.5 meters. At a speed of 99.5% the speed of light, the meter stick would appear to have a length of 10 centimeters, or 1/10 of its proper length. The effect of length contraction grows exponentially towards infinity as the speed of the moving object gets closer and closer to the speed of light. This means that an object moving at the speed of light would have zero length in its direction of motion.
Time dilation is the more interesting of the two consequences of the second postulate of Special Relativity. It is the slowing down of time for a moving object relative to an outside observer. Its effect would have the same magnitude as the effect of length contraction, so 1 year for an object moving at 87% the speed of light would be 2 years for an outside observer. As a matter of fact, GPS satellites, which orbit the earth at great speeds, have to account for the slowing down of their own clocks relative to our clocks on earth, to be able to give an accurate position.
I mentioned that time dilation is more interesting than length contraction because of one thing – time travel. Time dilation gives us a proven and working mechanism for time travel, at least in terms of the future. If you ride a space ship for 1 year in space at 87% the speed of light, when you return, 2 years will have passed on Earth. Increase the speed to 99.5% the speed of light, and 10 years will have passed on Earth once you return. You would be travelling to the future! This is quite different from our concept of time travel to the future from movies. You would not be able to see your future self, because you wouldn’t be on Earth during your trip to the future. It would be the same as how Captain America travelled to the present day from the past, by being frozen in ice.
Since the time of Isaac Newton, physicists believed that space and time are universal, which simply means that 1 meter and 1 second on earth will be equal to 1 meter and 1 second anywhere in the universe. But as Einstein showed in his Special Relativity, both space and time are relative. They both depend on the relative motion of the observer. This is what made Special Relativity, and thus, also Einstein, so great and groundbreaking. It changed the way we perceive space and time.
Einstein has also made another great discovery in his Special Relativity theory that turned into the most famous equation in all of physics, E = mc2, although only a few might understand its true meaning. The E in this equation means energy, while the m stands for mass. This equation means that energy can be converted to mass and vice versa. Physicists before Einstein have always viewed energy and mass to be very distinct from each other, but this equation has shown that they are like two sides of the same coin. This equation has also explained where a star gets its energy from, which is from converting some of its mass to energy through nuclear fusion at its core. The sun fuses Hydrogen gas to Helium, and in the process a little bit of mass is lost and converted to energy. This is the same principle behind nuclear reactors and nuclear bombs, although they work through nuclear fission rather than fusion. In this process, instead of combining 2 elements in nuclear fusion, an element is split into two. This also converts some of the element’s mass into energy.
Special Relativity did not just astound the world through its groundbreaking predictions and discoveries. It also showed that anyone, even just an unknown patent clerk at that time named Albert Einstein, can contribute amazing things to the world. In this way, Albert Einstein, the greatest physicist that has ever lived, can be an inspiration to all of us.