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The Lecture on Relativity

“Get back to your desks, professor Einstein is almost here”, shouted Feynman, the class monitor. In a matter of seconds, what could have been declared as a chaotic system of loitering chatterboxes was organised into a class of physics students eagerly waiting for the arrival of their Professor.

Einstein walks in. Walks up to Plank and slaps him right on his face! The entire class stares in disbelief. After two minutes of trying to control the sob, Plank asks, “Professor…(inaudible sob)…I was ‘sitting still’ in my place, just as you had ordered earlier!”. Einstein retreated, “NO YOU WERE NOT…”, pin drop silence in class, “…at least not from my frame of reference. You were moving relative to me as I walked into the class!”

The first lesson of the day was taught. The concept of rest and motion was relative! Being humans, the fact that a person can be taller than someone and shorter than someone else at the same time is totally acceptable. We comfortably define the comparative degree of adjectives to be relative to the observer. “Similarly”, said Einstein later in the lecture, “your speed is not absolute and is relative to the frame of reference of the observer. You can never say, ‘I was sitting still in my place’ unless you mention ‘relative to the classroom floor’!” The class was startled. They knew they could never again ride the bike at 80 kmph, they could only ‘ride the bike at 80 kmph relative to the ground’.

“If you think about it for a second, you will realise that the speed of yours, is not special. In fact, you can be travelling at any speed depending on how fast the observer is moving! Hence, the laws of Physics should not be dependent upon your speed. That is why, when I published my Special Theory of Relativity, I proposed the first postulate to be: The laws of physics are the same in all inertial frames of reference.”

“Sir, what is an inertial frame of reference?” asked Tesla from the fourth bench. “It is a body, which does not experience any net force and hence does not accelerate relative to another such inertial system! Like a bus moving with constant velocity with respect to ground. Such a body or system is always at rest or moving with constant velocity with respect to another such body.” “But Sir, what if it does accelerate?” pointed out Edison from the last bench. Einstein smiled and answered, “That took me 10 years to determine, we will discuss about it when I teach General theory of relativity, presently the topic is Special Theory of relativity. As I was saying, if the laws of physics are same for every inertial frame then, when you are standing on the ground and your friend is travelling in a completely enclosed car at a constant velocity with respect to you, and both of you do an experiment like swinging a pendulum, then the two of you will have a hard time agreeing who exactly is in motion, pertaining to the fact that motion is relative. What this means is that there exists no experiment known to mankind which can differentiate in between ABSOLUTE REST and UNIFORM MOTION!” Plank suddenly felt the pain on his cheeks disappear, as a beautiful scientific explanation can often obliterate your discomforts!

“Our World would have been a merry place, had I not proposed the second postulate….” “Did you, Professor?” interrupted Curie. “You bet, I did! And it was the second postulate which was the reason why the world’s most brilliant scientists failed to grasp the concept for about half the century. Honestly, teaching Relativity would have been much easier if the students were toddlers, yet to form the concrete interpretation of the world around them, than, if they were high school goers with their mentality already shaped by the apparent observations and absolutist mindset. But then again, isn’t that the case with every idea-to-be-taught!” winked the professor.

“My second postulate says: The speed of light in vacuum has the same value c in all inertial frames of reference. This was the main reason I troubled myself to come up with the entire theory as it explains a strange observation made by Michelson and Morley back in 1887. Although their intention was to investigate the existence of ether, they in turn observed something rather strange. The Speed of light seemed to be constant! Should you measure it from a bus moving at 50 kmph or even when you are sipping coffee at rest on your desk! This had no explanation. We all know a bus moving at 50 kmph with respect to ground is moving at the speed of 25 kmph with respect to a car, which in turn is moving at 25 kmph with respect to ground. Even if you were in an aeroplane travelling at 200,000 kmps (!) with respect to ground you would still observe light to be travelling at 300,000 kmps, which was the same speed observed when you were at rest on the ground! Light, it seemed, was adamant and would only travel at 299792.458 kmps (which is rounded up to 3 * 10^5 kmps and denoted as ‘c’ by lazy people!). The speed of light, unlike speed of any other object is, wait for it, ABSOLUTE!”

“This spelled disaster for science. Hundreds of years old kinematics, developed by Galileo and nurtured by Newton was jeopardised. Scientists could point out paradox after paradox resulting due to the Michelson Morley Experiment, which could no longer be explained by classical physics. For example, if the speed of light is constant to any observer, then what if someone travels faster than light? Will light still travel at c for him? What if light is emitted from a moving (from now on when I say moving/at rest I will mean, ‘moving/at rest with respect to ground’) vehicle? Won’t the speed of vehicle add up to speed of light?” “Sir, if I emit a light pulse from the centre of a uniformly moving rocket, I will see the light wave strike both the front and back of the rocket at the same time. But my friend, standing still on ground will observe the light hit the back first, as the back has already moved closer and the front away! Clearly, our observations will not agree.” said Gamow. “Well spotted, indeed, you made the list of paradoxes longer, simultaneity now seems to be lost. In other words, no two events could any longer be said to have happened at the same time. As if laws of motion were not enough, the constancy of speed of light messed up the definition of time itself! Clearly, we could have discarded the experiment, but no one could question the authenticity of the carefully conducted Michelson Morley Experiment and hence the Physicists were faced with two choices: Either Stay Blind or Come Up With A New Theory. And that is what I did!” All the students clapped simultaneously ( or did they?)

“We grow up thinking that space and time is absolute. We think if the college is 1 km from home, it will be at the same distance tomorrow, and even the day after or an year later. Similarly, an hour long length of time is equally long be it in 2017 BC or 2017 AD. But guess what, it is not. Dimensions of Space and time, like the speed of ordinary objects, and unlike speed of light is RELATIVE!” This time the entire class was slapped, verbally. Everything they knew about reality to be absolute seemed to be relative! “Logic is simple, but the consequences are a bit…..well…..weird!” He walked up to the board and started writing and speaking simultaneously. “Speed is distance over time. As speed increases, distance increases and time decreases. It is not just the magnitude of the distance of time that alters but also the perception of it. A ‘Metre’ seems to be longer and a ‘second’ shorter. But a metre and second are quantities defined by us not by nature. Hence, to ensure that a metre seem longer and second seem shorter, at high speeds, our space contracts (in the direction of motion) and time dilates!”

Students were heard groaning and struggling to interpret what they had just heard. Professor turned for a second and rephrased what he had just said, “It means, the faster you move, the shorter your car gets and slower your clock gets!”

“The only reason you are finding it hard to believe is because, you have never experienced it, since the speed of light is extremely, extremely, extremely fast, we never reach such a speed in daily life and never suffer from any appreciable length contraction or time dilation.” “But sir, do you have any real life example?” enquired Fermi. “You know that every subatomic particle has a definite half life. But when they are accelerated in LHC to approximately the speed of light, they exist longer, why? Because time was slowed down for them. Now imagine you are in a rocket that can accelerate to great speeds. You are planning to break the light barrier and travel faster than light and see what are the effects, purely for academic purposes! When you start the rocket, the velocity increases with the burning of fuel. The more you burn, the faster you go. You’ll observe that initially burning a given amount of fuel sped you up immensely, but as time progresses, you will somehow find that the rocket is no more accelerating as much, as if the rocket has grown heavier, in other words, your inertia has increased! With the same exact force as earlier, the acceleration turns out be minimal now. There will come a point when you no longer feel like you are accelerating, the more you accelerate by a little bit, the more difficult it gets to accelerate further. If you take my advice, do not look outside the window, for what lays beyond your accelerating rocket can pretty much end you up in a stellar rehab for mentally ill people! You try harder and harder to accelerate further but to no avail. It will seem to you that there exists a speed limit in our universe, limit here being a mathematical limit. You can get closer forever but you can never overtake it. That speed my friends, is ‘c’, and if you observe your speedometer you will be astonished to find out that you indeed are failing to cross that speed limit despite using a lot of energy. Your friend however, is on Earth and observing you with a telescope. For him (or her?) the scenario will be entirely different. He will see you accelerate initially, and as you gain speed, he will observe that the rocket you are travelling in growing shorter, until it appears to be only like a disk. What your friend is observing is actually, Length Contraction. If you two were communicating by some method, then initially both of you would be pinging each other alternately at equal intervals, then slowly your friend will start to observe that your pings are coming at greater intervals. You will observe that your friend has gone crazy and is pinging you continuously with progressively diminishing intervals! It’s not that you are becoming lazy, or your friend hyperactive, it is just that time is slowing down for you unlike your friend. This is time dilation. What we learn from here is that, to make sense of the Michelson Morley experiment, you will need to accept the laws that I had formulated, and a consequence of those laws is that a speed barrier is created. No one can claim firmly that theory of relativity is damn correct but the reason it is so widely believed is because it explains the observations better than other competent theories (not that there are many!). It is difficult for anyone to convince you whether Relativity is correct because it deals with scenarios which are not observed in daily lives but nevertheless, effects of relativity though minute are omnipresent in our daily lives. Which means if you travel more in a car you live longer than if you walk, by maybe a nanosecond!”

“Sir, you said our mass increases! How is that even possible?” asked Hawking. “Mass increases by the same factor as length decreases and time slows, the increase of mass is what prevents you from crossing the light barrier. As mass is inversely proportional to acceleration when a constant force is acting, the more energy you put into the rocket, it all adds up to the mass of it rather than increasing the speed. That way Energy seems to be proportional to mass. Oh yeah, and the constant of proportionality is the square of speed of light. E=mc^2 !” Professor dropped the mic, though he did not have any. Some students felt the atom bomb drop. Finally, they have learned how that little, elegant, devilishly popular formula fit into the entirety of Special Theory of Relativity. Einstein was done with the lesson, he was starting to move out of the classroom, or as he might say the classroom was starting to move behind with respect to him, right when Dirac asked, “Professor, what about General Relativity?” Einstein stopped, and without turning back, said, “Ever been in an accelerating rocket in space? You will feel like gravity is acting backwards though there isn’t any gravity! Ever been in a freely falling lift on the Earth? You will feel like there is no gravity even though there is indeed! Acceleration and Gravity are indistinguishable in a closed system!”

– “That’s it?”

– “Nope, that’s the beginning, and the end says…….dramatic pause…..SPACE AND TIME are curved and could be warped!” He left.

The class did not want to believe what they had just heard, but they were in a dilemma, for Professor Einstein had his ways to make them believe! They sat still in their places with respect to the ground!

After ‘The Lecture on Relativity’, the class of Physics Students had somehow forgotten to revert back into the chaotic system of loitering chatterboxes!

PS: The names of the students have been taken from the surnames of popular scientists, kindly do not mistakenly identify them as the real scientists, as they are just students in the class Professor Einstein teaches. This lecture is considered to have been given in the recent times by Professor Einstein, who has been assumed to be a teacher of Physics for a group of Physics Students! Also there are no images and approximately no equations in the post as I deliberately wanted the words to do the talking. Kindly share this with Everyone. Professor Einstein will be happy. BTW, this post was written in July 2017!

Arkadeep Mukhopadhyay