Chapter 8: Traveling Alongside A Beam of Light
Every now and then, human knowledge gets a boost from a genius’s mind. Their work results in new foundations that science can stand on and look in directions previously unknown. Just like Galileo’s telescope and Newton’s laws of motion, Einstein’s theory of relativity and quantum mechanics became two pillars of modern physics. In Einstein’s theater, light plays the protagonist who puts an end to the idea of absolute time & space.
Einstein from an early age was a quick learner and gravitated towards physics and mathematics. He rebelled against the prevalent educational method of rote learning. By age 12, he took it upon himself to understand calculus and mastered it by age 14. After graduating in 1900, Einstein spent a couple of frustrating years looking for a teaching job or get a doctoral degree. When Einstein finally joined the Swiss Patent office, through friends’ help, he suddenly had access to groundbreaking work with real world applications and importantly, time to think about the theory behind those patents.
Just learning quickly doesn’t make you a genius though. It certainly makes you smart. Mix that with passion and you can achieve great things. It’s about having a unique thought, finding a novel solution to an existing problem and going beyond conventions. Most of the scientific community thrives on these principals. Some are able to take it to the next level. Important to caveat, Einstein had close collaborators who made his theories better. Like his first wife, Mileva Maric, who was also his classmate in college.
During Einstein’s formative years, Physics was struggling to get classical mechanics (Newton) and electromagnetism (Maxwell) to agree with each other. Danish astronomer Ole Roemer had discovered in 1676 that light travelled at great speed by looking at Jupiters’ moons as they passed behind it. This seemed to happen at different rates, depending upon how far Earth & Jupiter were — Roemer noticed that eclipses of Jupiters moon happened later the further we were from Jupiter. So light travelled at a great, but calculable, speed. A lot of effort was put into figuring out if the speed of light was different relative to a made up “ether”, or to the speed of the source or of the observer. But to everyone’s surprise, Michelson & Morley’s experiment showed that there was no difference in the speed of light regardless of the situation.
Einstein’s thought experiment in this realm was to imagine what it would be like to travel alongside a beam of light. Something extraordinary emerged from this. Einstein reasoned, by traveling at exactly the speed of light, he would see light, the electromagnetic field, hanging right next to him, seemingly stationary in space. But that was impossible as neither Maxwell’s equations or any experiment on Earth had shown that to be the case. There’s another reason it’s impossible to catch up to light, that he’ll discovered later.
Everyone also believed in absolute time — i.e. different observers would agree on the interval of time between events. Time was thought to be completely independent of space. Einstein’s next thought experiment was to think about lightening striking two ends of a train station. Let’s say there are two observers to this event. One stationary on the platform and the other on a train moving between the lightening strikes. Now as the two lightening bolts strike, the stationary observer sees them happen simultaneously. But to the moving observer on the train, the strike happening in front would seem to happen quicker than the one behind. How do you reconcile the difference in two observations? Einstein proposed that we get rid of absolute time. Both observers measured the time between the two strikes correctly according to their motion. Neither is preferred or correct vs the other.
The idea of absolute space can be removed by a similar experiment, but with ping pong balls. Let’s say the person on the train is bouncing a ping pong ball on their bat as the train moves forward. For this observer, the ball is bouncing on the same spot. But for the observer on the stationary platform, each bounce is happening at different spot as the train moves forward. Again, both observers see the bounce happen according to their motion. Neither is preferred or correct vs the other. Everyone has their own measure of time & space.
In 1905, building upon his thought experiments, Einstein put forward an astonishing idea that set the scientific community on fire — the laws of physics and the speed of light must be the same for all uniformly moving observers, regardless of their relative motion. For this to be true, space and time can no longer be independent. Rather, they are “converted” into each other in such a way as to keep the speed of light constant for all observers. This paper was titled — On the electrodynamics of moving bodies. We know it as “Special Relativity”.
It was Minkowski who concluded that space and time could be combined into a single “four-dimensional” spacetime fabric. Human beings use it all the time. Like to attend any meeting, we always ask “when” & “where” to meet. Without this information we’ll be either in a wrong place or arrive at wrong time.
Part of these miracle year papers was Einstein’s effort to explain photoelectric effect. His idea was motivated by Max Planck’s earlier derivation of the law of black-body radiation — energy is exchanged only in discrete amounts (quanta). Einstein postulated that light itself consists of discrete packets or quanta amounts. This theory led to adoption of photons as light particles, creation of quantum mechanics as new branch of physics and eventually to things like laser, that are embedded in our daily lives. But for a class 3 patent examiner to do this is truly remarkable and probably adds to the mystique of his character. Einstein was awarded Nobel prize in 1921 and this work was specifically cited in it.
Einstein kept thinking about his relativity paper and put forward another radical idea the same year about the mass-energy equivalence. The result is the most famous equation ever written, E = mc2 (where E is energy, m is mass & c is the speed of light). This is also the reason why one cannot reach the speed of light. This is because the energy an object has due to its motion will also add to its mass. This is only significant when an object approaches the speed of light, say 90% of the speed, its mass rises ever more quickly and it takes even more energy to speed up. Only massless objects can travel at speed of light. Everything else is restricted by this speed limit.
Another consequence of that equation was that tiny amount of mass can release tremendous amount of energy. This fact was obviously used for the development of Atomic bombs. But the equation goes beyond nuclear forces, as Brian Greene explains it —
The part about not including gravity in his theory of relativity will keep bothering Einstein for a while. Until he has the happiest thought of his life a few years later. More on that soon!