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For People in a Hurry: What was Einstein's Special Relativity?


1905 is considered to be the miracle year in Albert’s Einstein’s life. His phenomenal contributions to physics came at a time when he was working at the patent office. In April 1905, he submitted his doctoral thesis, ‘A New Determination of Molecular Dimensions’ to the university in Zurich. The remarkable outpouring of Einstein’s brilliant ideas changed science in such a profound way that nobody before and ever since have managed to do the same. His achievements included his first and second paper on special relativity – ‘On the electrodynamics of moving bodies’ and ‘Does the inertia of a body depend upon its energy-content’- where he used his imagination rather than fancy math to come up with his most famous equations. The theory of special relativity is one of the fundamental theories of physics. It has successfully graduated from all the experiment of extreme conditions and unbelievable accuracy. Einstein’s Special Relativity has a reputation of being extremely difficult. However, it is not difficult, at least mathematically.

The idea of relativity in physics is fairly a straightforward one. What it says is that the laws of physics is applicable and definitely remain the same for all moving observers. What do we mean by something being in state of motion?

From Galileo’s Perceptive

Back in 1632, Galileo explored the idea that all motion is relative and it only makes sense when something moves in relative to something else. He urged that two observe moving at constant speed and direction with respect to one another will obtain same result for any mechanical experiment. He asked those who objected him that if there were any way in which a passenger inside of inside a windowless cabin of a ship moving at a constant can determine that the ship is moving or at rest. Fairly enough, the answer is no. There was no experiment that could be performed in the ship that would give different result to the same experiment carried out in the shore.

We can call a tennis game inside the ship as Galilean tennis ;-)

We can call a tennis game inside the ship as Galilean tennis ;-)

Frames of Reference and defying Newton’s notion

Centuries after Mr. Galileo, Albert Einstein proclaimed that motion has no meaning without a frame of reference. It is the heart of his theories of relativity. Isaac Newton’s laws of classical physics allowed the existence of absolute rest, absolute space and absolute motion. He wrote –

Absolute motion is the translation of a body from one absolute space into another. And relative motion is the translation from one relative space into another

Angry Einstein completely did away with the idea of Newton. On one occasion, he asked a bemused ticket inspector, ‘Does Oxford stop at this train?’ (We don’t know what happened to the inceptor. I hope the poor guy found a way out of his misery)

Introducing Special Relativity

Einstein was very familiar with electromagnetism. His introduction of the notion of relativity to explain magnetic effects in current carrying wire was ground-breaking. Since the time of Faraday, it was believed that moving magnet producing a current and the moving coil producing a current were two discreet explanations. The idea led scientist to believe that there was a state of absolute rest with respect to Ether. Einstein was completely devastated by the idea. He said, it was their relative motion to each other produced a current. That led him to conclude that the idea of absolute rest and absolute were flawed. That’s how he dismissed the idea of ether and set out his principle of relativity. He argued that absolute time and space should be replaced with absolute spacetime. The mathematical reality showed that time and space are entangled and both alter as one approach near to the speed of light.

He said that the laws of physics are the same in all inertial reference. No matter how fast or slow you are moving. That is what Galileo was saying back in 1632. It was Einstein's first postulate. In the meantime, Morley and Michelson failed to find any evidence of ether. Einstein won.

Michelson and Morley Experiment

Michelson and Morley Experiment

What is spacetime?

Einstein balked at Newton's ideas of absolute time and absolute space. What he came up with is his own idea of spacetime. Three-dimensional space and one-dimensional time make a four-dimensional spacetime. To help visualise a path through spacetime, physicists employ a concept called the block universe, which maps all events past, present and future. In 1907, Herman Minkowki developed another way to represent spacetime. It is called Minkowki spacetime diagrams. It genuinely give us a graphical way to illustrate some of the puzzling effects of relativity, eg Time Dilation, Length contraction

Four-dimensional spacetime

Four-dimensional spacetime

The constancy of light

Einstein continued to challenge 200-year-old Newtonian Physics. He pondered the behaviour of light and concluded that the speed of light is a constant. Some things may be relative, but the speed of light is absolute. The speed of light never depends on the motion of observe or source of emission. This made no sense in Newtonian physics – where speeds are added up togather. Einstein’s light postulate was proven by Michelson and Morley – they found that the speed of light is same no matter how they measured. Einstein won. Again.

In 1983, the speed of light was defined to be

c = 299 792 458 m / s

Scientists use c as a symbol. Which comes from latin word 'celeritas' which stands for swiftness!

Einstein’s way of understanding Time

Imagine yourself in a space station and firing pulse of light. Your friend resides in a spaceship which is heading away from you at the speed of c/2 (c= 3 × 108 ms-1 ) What is the speed of light your friend measures? Our mere common-sense may conclude the answer to be c/2. But it is WRONG! According to Einstein’s light postulate the light will arrive at the spaceship at the speed of c. We know that, v= d/t. In order to keep the speed of light constant for all observes, the distance and time should be change.


Δt = the observer time

Δt0 = the proper time

According to special relativity, the faster one move through the space, the slower travel through time. Time between the intervals of events seems to slow down as one approaches the speed of light. This phenomenon is called time dilation. That means the measurement of time is observer dependent. In experiment, such as those carried out in LHC at CERN, where subatomic particles are collided at nearly the speed of light, or in space-travel, or even in GPS – the effects of time dilation is considered. These relativistic effect on time is greater the closer we are to the speed of light. Experiments carried out in atomic clocks demonstrated that clocks flown on a plane tick a few hundred billionths of a second slower than similar clocks that remained on the ground

Einstein’s explanation to the Lorentz-Fitzgerald contraction:

Einstein’s quest to bend time didn’t satisfied him. He desired to sharnk the space too. He observed that a motion moving closer to the speed of light appears to shrink along the direction of motion. It is called the Lorentz-FitzGerald contraction after two physicists who proposed it as a solution to the failure of the Michelson-Morley experiment. Einstein showed that it was real and it is a consequence of the properties of space and time. In order to keep the speed of light constant, the space must shrink in size. Any objects moving closer to the speed of light is only shortened in the direction of its motion! It can be said, mathematically -


What is Mass-energy?

In 1840, Physicist William Thomson aka Lord Kelvin realized that the power that drove many different processes could be explained in terms of the transfer of energy from one system to another. For instance, the chemical energy stored in your energy allows you to move - in different word the chemical energy is converted to kinetic energy. If an object is said to have potential or described to be energetic, it means it can do work. The amount of energy in the universe is limited - can never be created nor destroyed.

Newt's view of conversation of energy says that the energy is conversed in a system. The energy is stored in the field of conservative force, rather in the objects! 200-years later, Einstein came up with his idea of mass-energy conservation, which implies that it is mass-energy which is conversed in a system. Mass and energy are two sides of the same coin. If an object gains mass, it loses energy. On the other hand, if an object loses mass it gains energy. That is how Einstein brought up his most famous equation E=mc^2 !

What If we could travel at the speed of light?

Our current technologies do not allow us to travel at c. In fact, nothing can travel at the speed of light - well apart from light, definitely. If could, distance would shrink to zero, and the atomic clocks could tick no more. There would be no time. What if universe were contracted into zero length. What subatomic particles didn't exist? That is way beyond common man's comprehension. But Einstein said something that may put a smile in your face

The most beautiful experience we can have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. Whoever does not know it can no longer wonder, no longer marvel, is as good as dead, and his eyes are dimmed

— Albert Einstein

Special Relativity is famous for its elegance both theoretically and practically. It is needless to say that the applications of Special Relativity have reshaped our world very much. Our understanding of space, time, mass and energy is much greater now than it was at the turn of the century. Special Relativity is logically self contradictory despite its valid reasoning from theoretical and mathematical premises. That's what makes it weird and odd. That's the beauty of Physics.

© 2021 Yusrat Sadia Nailat

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