This is perhaps the most commonly-known equation. In 1905, Albert Einstein published a paper which introduced what is now known as the Special Theory of Relativity. Ten years later, he published another paper that introduced the General Theory of Relativity. The Special Theory is essentially a special case of the General Theory (hence the name ‘special’, get it?) and does not consider the effects of gravity.

Dr Bennett’s book, “What is Relativity?” explains the intricacies of the concepts of relativity with numerous, easy to understand examples. He elaborated on a few of them during the talk which took place on Friday, September 25 in the Gleason Auditorium as part of the monthly Public Science Lecture Series.

Since the elaborate explanation of these theories would make this article extremely long, I would like to focus on some of the things that struck a chord with me during the talk.

**Misconceptions, Common Sense and Intuition**

Misconceptions are overcome when we realize that something isn’t making sense. In other words, it disagrees with our logic, intuition or common sense. Why then, does relativity appear to defy our common sense? Well, we do not have an intuitive grasp of relativity because we do not normally encounter the speeds at which the effects of relativity are pronounced enough for humans to be able to observe them. Consider this analogy-New Horizons flew past Pluto at fifty thousand kilometers per hour, which is about one-twenty thousandth the speed of light. We have never encountered such speeds and hence, do not have any common sense about it.

Dr Bennett explained this with a few analogies: Imagine that we do not yet know that the Earth is round. Two explorers are sent out in opposite directions and told to travel as far as they could in a perfectly straight line and to return only when they discover something extraordinary. Well, the travelers, oblivious to the spherical shape of the Earth, are very surprised to meet each other when they began their journey in exact opposite directions. It only ceased baffling them when they considered that the Earth might be round. So, even thought they left in opposite directions and traveled in a straight line path, they met each other halfway across the sphere.

**The absolute and the relative**

‘Relativity’ is rather a misnomer as the special theory of relativity hinges on two absolutes:

- The laws of nature are the same in all inertial frames of reference.
- The speed of light is constant regardless of which frame of reference it is measured in.

Some important things to be careful about when dealing with extremely high speeds are:

- Simultaneity: Things that are perceived to occur simultaneously from one frame of reference may not be simultaneous when observed from another frame of reference.
- Time and space are no longer constant: An observer travelling at a speed close to c perceives the time of a stationary person to run slower (time dilation). Distances appear to have decreased in a direction parallel to the direction of motion (length contraction). The distances in a direction perpendicular to the direction of motion are not affected.

Understanding the theories of relativity requires us to extend our current and intuitive knowledge of time and space. Once we have done that, these theories are simple and elegant. They only require geometry and algebraic calculations and are just beautiful in all their simplicity.

**The General and the Special Theories of Relativity**

First of all, Dr Bennett clarified that the word *theory* describes an idea that has been experimentally tested and verified to be true. A theory should not be confused with a hypothesis which represents an idea that has not been verified experimentally.

Einstein began thinking what would it be like if one could ride on a beam of light. Would one ever be able to outrun a beam of light? So, what provoked people to question whether or not there was a limit to the speed of light? Maxwell’s equations have the speed of light ‘*c*‘ but a reference frame for its measurement is not specified. Turns out, due to the absoluteness of the speed of light, one does not require a reference frame for its measurement! Physics makes sense, and it’s an incredible experience when things make sense.

The General Theory of Relativity improves on Newton’s “action at a distance” explanation of gravity by introducing space-time. Space and time can no longer be considered separately. Gravity curves space-time and objects under its influence and even light follows the shortest straight-line path, even thought that path may appear curved, just like the story of the two explorers meeting halfway across the earth. The sun, huge galaxies thus, bend light from stars and galaxies lying right behind them when viewed from the Earth and this phenomena is called* *g*ravitational lensing. *

**Black Holes**

Using the rubber sheet analogy, an object with mass bends space-time just like a marble bends a rubber sheet. Now a canon ball would bend it much more than a marble would. Black holes concentrate an immense amount of mass in a tiny region of space, thus creating a huge bend-almost a hole-in the rubber sheet we compare space-time to. Their density is equivalent to that of the mass of the Sun occupying a region about six kilometers in diameter. (The Sun’s diameter is about 1400000 kilometers). One of the most common misconceptions about black holes is that they ‘suck’ objects around them. They don’t. You have to aim very carefully in order to enter the black hole. Additionally, space is so vast and black holes are so tiny in comparison that Dr Bennett says that they are the ‘hardest things to fall into by accident’. He compares falling into a black hole to falling from the edge of a waterfall. You’re simply ‘falling’ not being ‘sucked in’. The immense mass of a black hole gravitationally attracts other objects which begin to orbit it-forming its accretion disk. These particles experience friction with other particles in their orbits and lose angular momentum causing their orbits to decay and end up falling into the black hole.

**The value of relativity**

Dr Bennett ends his book with four justifications on why everyone should know about relativity; and he spoke about them at the end of his talk as well:

- Space, time and gravity are really everything there is. If you want to know everything there is, you need to know relativity.
- For a hundred years now, we have known that space and time are not the way one would ordinarily think they are and it’s necessary to know that in order to have an understanding of reality.
- Einstein lived through terrible times and yet, remained an incredible optimist about the potential of the human race and what all we could achieve if we used our brain power wisely.
- Everything you do, every action, becomes a permanent event in the four dimensions of space-time. Every moment of our existence is leaving an indelible mark in the Universe.