**E**instein’s theory of relativity is another great example of how seemingly complicated and far-fetched concepts are very much part of our daily lives. Whether applied to natural processes or high-tech devices, relativity has enabled us to make the picture we have of the world much more accurate. I don’t deny that the theory can be baffling but that is only because it deals with phenomena which we are not necessarily familiar with. But once we have an inkling as to what lies behind the theory then it should all seem clearer and simpler.

The crux of the theory has to do with light. Interestingly enough, it is the speed of light that determines the effects we observe from the theory of relativity. The faster an object moves, the more it gains in mass! The faster it moves, the more it shrinks in size! The faster it moves, the slower time will pass for it! Exactly! Strange, isn’t it? Now why should that be the case? Why should you gain mass the faster you move? Let’s dig into this a bit more…

We knew that the speed of light is a universal constant even before Einstein proposed his theory of relativity. That concept came out of Maxwell’s theory on electromagnetism. James Clerk Maxwell is another one of the giants of science to have changed the way we perceive the world. His theory beautifully combines electricity and magnetism into one quantity called electromagnetism, all supported by a rigorous mathematical edifice. And out of the mathematics, these little gems, these little Maxwell’s Equations came to be; giving us the foundation to the nature of electromagnetism. And light, which is just one manifestation of the electromagnetic radiation (X-rays and Infrared are other manifestations of EM radiation – please see Spectrum), also abides to the rule laid down by the mathematics of electromagnetism. Perhaps the most important outcome of these equations is the fact that we can determine exactly what the speed of light is in a vacuum. And that speed is defined exactly by other constants making the speed of light, itself, another universal constant. An unchanging quantity, a value set-in-stone, if you will. That value turns out to be exactly this: 299,792,458 metre/second. Fine. Great, we all now know the speed of light in vacuum. We all know that this value cannot be changed – unless we alter the other constants, of course. (But that would mean tweaking yet other constants and definitions so much that we might as well be in a different universe altogether where the parameters are completely different!)

The genius of Einstein was to ponder on that very peculiar speed. He was fond of what we call ‘thought experiments’. That is, he would think up scenarios of made up situations and figure out what would happen. It was as though he was conducting a real experiment, such as measuring the acceleration due to gravity, in his mind. In cases where he could not physically carry out those experiments, say if the conditions imposed where too extreme or hard to set up, then he would imagine, as realistically as possible, the outcomes of such thought experiments. So one of those experiments had to do with reflecting upon moving at the speed of light. We can all entertain the idea of moving alongside a cyclist or a truck; these speeds are very familiar to us. But what if we were to move alongside a beam of light? What would that be like?

When you start to investigate these questions then you pretty much run into the world of strangeness and weirdness. Einstein imagined himself moving alongside a propagating wave of light. But then, relative to him, because he’s moving at exactly the same speed as the wave, he would observe the wave to be motionless. This is down to the simple notion of relativity of motion. When you and your companion are walking side-by-side, both at the same pace, then relative to you, your companion will appear to be stationary; i.e. they won’t appear to be going past you any quicker or slower than you. However, a third person observing the two of you will notice that you are both in motion and both traveling at a given speed. Relative to that third person, neither you nor your companion would appear to be stationary. Speed, thus, is all relative. Relative to the observer. Relative to what we call the *frame of reference*. Two trains approaching each other from opposite direction will appear to be moving faster relative to each other. Speed, or I should say *velocity*, is additive. But here’s the caveat: it’s not as straightforward as this when it comes to speed which are close to that of light.

Imagine driving along a busy road where a car with its lights on is approaching you from the opposite direction. Relative to your own frame of reference, you’re stationary; it’s just the things around you which are whizzing past you. Fine. The car approaching you would do so at a speed equal to the sum of your speeds. Again, so far, so good. The beam of light from the car, however, isn’t approaching you at a speed equal to the sum of your speed and that of light! Here lies the difference. Whenever the speed of light is involved, one simply cannot add speeds. In fact, the beam of light would approach you at the speed of light itself; not faster, not slower. If you were moving alongside a beam of light, it will still be moving at the speed of light, regardless of your own speed. The addition of speed works differently in this context.

So if the speed of light doesn’t change, if the speed of light* cannot* be altered then what does that mean? Well, as Einstein has found out, something else has to give. Some other quantity has to change so as to preserve the immutability of the speed of light. It turns out that quantities such as time, length and mass are the victims of such a strict condition. If the speed of light were to remain unchanged then time appears to slow down, length appears to contract, mass appears to increase *relative* to the observer.

And if we take it to the extreme then, assuming one could travel at exactly the speed of light, time would be at a standstill relative to the observer. But for the one traveling at the speed of light, nothing would appear unusual; instead, relative to them, everything else would appear to be in motion. Now, the consequence of this is not purely for our intellectual enjoyment of thought experiments and mathematical acrobatics. There are real life implications of such an unusual behaviour.

We have observed and measured and confirmed the increase in mass of particles moving at speeds close to that of light. In particle colliders, such as the one in CERN where the Higgs boson was discovered, we have been able to verify Einstein’s theory of relativity. The masses of such fast-moving particles corresponding precisely to the value predicted by Einstein’s theory. So, what’s exceptional about this theory is that, prior to Einstein, everyone thought that quantities such as time and mass were absolute. We used to think that time was being measured, or at least experienced, in the same way for every observer. Well, as it turned out, all of these quantities are relative. And that realisation completely changed the way we looked at Physics and the world. It revolutionised our foundations of Physics. And this is one of the reasons why Albert Einstein is often quoted as a genius.

There is more to relativity than this curious phenomenon of mass increase. But the point I wanted to make here is that it is the speed of light, that universal constant, which gives rise to such weird behaviours. Interesting, isn’t it?

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