If you have ever seen fireworks at a distance or ever seen lightning then you might have experienced how fast light actually is! You might have noticed that the lightning during a rainstorm and the fireworks being blasted into those beautiful sparkly colors are seen first and then their sound is heard. But have you ever wondered what is happening here, why is it that the sound comes after you have seen the whole thing happen?
If you have never seen this or might not have wondered then take a moment, DO NOT READ FURTHER, and think about this. If you are as curious as me then also try to come up with a reason for why is this the case.
Thought? Now confirm if you were right or wrong.
Why do we see lightning first and hear thunder later ?
It is because the speed of light is much much faster than that of sound, in fact its the fastest thing in the universe. But how do we know that?
The reason behind this is actually quite simple. It is because Einstein told us! He told us that its the fastest thing in the universe and you cannot go beyond the speed of light which is 3 x 100000000 m/s (3 x 10^8 m/s) approximately across the universe( that is Everywhere). If you are satisfied with one person saying that its the universal speed limit, may that person be EINSTEIN himself. With respect to the speed of sound that is 340 m/s (at Sea Level), that’s quite a difference. But how can we be sure that its the fastest and we cannot go beyond that speed?
After all there are no speed limit boards in the universe after every 5 km saying “SPEED LIMIT 299,792,458 m/s. Have a safe flight.”
CONFIRMING ‘SPEED OF LIGHT’ IS THE UNIVERSAL SPEED LIMIT BY THOUGHT EXPERIMENT.
Einstein said its the fastest thing but he was not mad, he was a physicist, then he must have had a reason to say this. Indeed he did, in his autobiographical papers he describe how at the age of 16 he did a THOUGHT EXPERIMENT and that’s what we will do to see why is the speed of light the fastest. We will do the same experiment that the Maestro of Physics did, he was lazy or not we do not know but this experiment can be done without any hassle.
Imagine you are going at the speed of light. On your side there is a light beam streaming down with the same speed which is the speed of light, 3 x 100000000 m/s. Then imagine how would that look like to you? If you can’t imagine that much try to picture that you are in a car going at about 100 km/s and there is another car right beside you going at the same speed i.e. 100 km/s. Then how would the car look to you? It would look stationary as both of you are travelling at the same speed, which makes the relative velocity 0 m/s. Then you would think that should also happen when you are travelling at the speed of light. You should see light to be standing beside you similar to what happened with the car. That’s what Einstein thought but then he (after a lot of thinking) found that would not be possible therefore he concluded that it must be impossible to go beyond the speed of light.
Now you must be thinking what happened that made Einstein contradict his own assumptions and deduce a result almost opposite to his own thinking.
When you have excluded the impossible whatever remains, however improbable, must be the truth.
Einstein might have thought this?
He might not have thought exactly this but we can say he followed the same principle.
So what made Einstein change his initial thinking ?
There is this principle in physics which paved the way for the idea that its impossible to go beyond the speed of light. It played a similar role as the falling apple did in Newtons theory of gravity (also it is interesting to note that in reality as newton described he had the idea first and then saw the apple in the way he was thinking, so actually the apple only confirmed the idea that he had). The principle is called the Heisenberg’s Uncertainty Principle. To understand how it affects light lets get to know more about it.
So what is the Heisenberg’s Uncertainty Principle ?
It basically states that there is uncertainty between the momentum and position of a particle. What this means is that you cannot determine the position and the momentum of a particle simultaneously. If you somehow calculate the exact position of a particle then you cannot determine its momentum. (if you can then you deserve a noble prize)
If you have noticed by now they i.e. the uncertainty in position and momentum have inverse relation, which means, for example, if the accuracy of measurement of the uncertainty in position would increases then the accuracy of measurement of the uncertainty in momentum would decrease.
To put it simply, if Momentum goes up then Position goes down.
Now if we apply this principle to our experiment, which we left incomplete. Last we were flying at the same speed of light and were seeing the beam of light to be stationary. But now if we see the experiment it would not make sense, something’s not right. If we are at the same speed as light then we know then we know the momentum of light to high accuracy then we must not know its position to high accuracy which is a direct result of Heisenberg’s Uncertainty Principle. This would mean we would not see light to be standing or stationary.
Now you would say but that’s not what happened in my head but we have to remember, that was according to our incomplete experiment. Now that we know we cannot see light to be standing this would mean it would be impossible to go at the speed of light. Now the result of our experiment matches with that of Einstein’s.
For our thought experiment we somewhat cheated and made the experiment easier by using the Heisenberg’s Uncertainty Principle but Einstein did not use this, in fact he did the experiment when he was 16 and that was in 1895 and the principle was given in 1929. He had already predicted the result of the experiment without this principle. He used the wave theory of light as the principle for this experiment. This same experiment became the basis of one of his most famous theories, The Theory Of Special Relativity, which he published 10 years later in his Miracle year of 1905. Einstein was indeed a Genius!
by Harjass Gambhir