RELATIVITY OF SIMULTANEITY

Time-we are well aware of this physical entity which according to many waits for none. Several theories were developed on the concept of 'time'.

It was during the mid-sixteenth century that Sir Isaac Newton published his masterpiece, The Principia Mathematica, in which he introduced the concept of 'absolute time'. 

Figure 1: Unparalleled geniuses, Isaac Newton and Albert Einstein.

Though this Newtonian concept was famous and widely accepted in the beginning, it faced a lot of opposition during the early twentieth century from a patent clerk who was working on a new concept of space and time. This patent clerk would publish his famous papers on relativity, in a scientific masterpiece, 'Relativity- Special and the General Theory'.

The patent clerk was none other than, Albert Einstein who believed in the concept of 'relativistic time'. Poor Einstein had to face a lot of criticism when he published his findings solely because, it questioned the findings of Sir Isaac Newton, who by then had attained a cult-status and was widely regarded as the 'father of science'.

Figure 2: The patent clerk who challenged Newton's ideas of time.

The notions of nature, if true, can never be altered by any force. The mathematics and theoretical knowledge that Einstein applied in his theory was so unique and exhilarating that he could successfully silence all those who criticised him of his theory. The power of Einstein's equations is very well explained by Michio Kaku in his following quote,

''When we physicists look at Einstein's equations, we cry. We cry because they are so gorgeous''

-Michio Kaku, theoretical physicist

In his book, 'Relativity- Special and General Theory, Einstein puts forward a concept called, 'Relativity of Simultaneity'. It is using this simple concept that Einstein overthrew the age-old Newtonian concept of absolute time.

Theory of Simultaneity

''Look up at the stars. Not down at your feet''

- Professor Stephen Hawking

Let us understand this concept with Einstein's own example.

Figure 3: A page from Einstein's Relativity- The Special and the General Theory, explaining the Theory of Simultaneity.

Now this diagram seems very complicated. So we have to develop a much simpler diagram for better understanding.

Figure 4: A simple demonstrative example for explaining the 'Theory of Simultaneity'

Here we must consider two cases:

CASE 1

Situation analysed: A person and his friend are outside in the open watching a train moving at a distance.

Here you and your friend will see the train moving with a certain velocity, say v. 

Now think what would happen if two lightning strokes strikes the train, a 100m apart.

Since both you and your friend are at rest outside the moving train, the two lightning strokes appear to be simultaneous, that is, both you and your friend will see the lightning strokes exactly at the same time. Now this is in accordance with Newton's Absolute time theory.

But things are about to change!

CASE 2

Situation analysed: A person is standing outside in the open, while his friend is inside the train.

Here you are outside in the open while your friend is inside the moving train with a velocity, say, v'.

Both your friend and the train will obviously have the same velocity, i.e.

v=v'

where,

v is the velocity of the train and 

v' is the velocity of the person in the moving train (your friend)

Now consider the exact same scenario, i.e. two lightning bolts striking at a distance 100m apart on the moving train. You will see the two bolts at the same time, i.e. you will see them simultaneously. But your friend would see one bolt first followed by the other, because he is moving towards one and away from the other.

So if L₁and L₂ represent the two lightning bolts, then you will see both L₁ and L₂ at the same time for you are outside the train. But your friend whose motion is relative to the motion of the moving train will see either L₁ or L₂ first, depending on which direction the train is moving. 

If L₁ strikes at the left side and L₂ strikes at the right side. Then if the train is moving towards the right your friend will see L₂ first followed by L₁ as he is moving in the direction of L₂  and vice-versa.

This result of  CASE 2, strictly questions Absolute time theory given by Newton.

Einstein asked a simple question,

''How can the two of us (you and your friend) see the same scenario at different times? Its evident that time has to be relative rather than absolute''

I highly recommend you to watch the following video designed by students at the Indian Institute of Technology, Bombay (IITB) for further detailed understanding of this theory. 

Video 1: A small presentation from students at IIT Bombay to explain the concept of Relativity.

Newton's interpretations

'' If I have seen further than others it is by standing on the shoulders of giants''

- Isaac Newton

An obvious question that you may ask now is, ''where did Newton go wrong?''

According to Newton, the rate at which time flows is same everywhere in the universe regardless of the point of reference.

Sounds complicated right. No worries. This was perhaps one reason other than the scientific idea that this theory got deposed!

But Einstein couldn't digest all these words at a go. He theoretically explained that, the rate at which time flows depends on the frame of reference in which you are.

In simple words, Newton believed that time ran exactly the same way for all persons regardless of where they are however from the practical explanations mentioned in the previous section we have shown that time is not the same for all persons, because time depends on velocity.

In CASE 2, we considered this aspect were we found that 'faster a person or a body moves, slower will be the time change and vice-versa as momentum bends space-time.

All these notions are really complicated and is dealt in advanced physics. We usually don't learn them at school because school students may find it very difficult to digest.

It is worth mentioning here that when Einstein published his papers on relativity, even the greatest of scientists couldn't understand it. It is widely believed that only three people could understand this monumental work, probably Einstein would be one among them!

Since it is very complicated topic it is best not to proceed further as it might entangle your nerves. Einstein even admitted that he suffered from mental breakdowns when he used his brain continuously, analysing these strange, unique and stupendous phenomena!

References:

1. Relativity: Special and General Theory, by Albert Einstein.

2. Mathematical Principles of Natural Philosophy, by Isaac Newton

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