From apples to gravitational waves: a brief history of gravity

A mere few billion years after life emerged from the primeval ooze, we the human race detected ripples in spacetime. Very impressive, considering that it has been only a few hundred years after a first understanding of gravity, when physicists junked the Aristotelian “the apple wants to go home" myth.

Einstein triumphs once again. Through him, we heard the last gasp of two black holesfatally attracted to each other 1.3 billion years ago.

We know of four fundamental forces in the universe, known as gravity, electromagnetism, the strong, and the weak. The physical world is a finely choreographed dance starring all four forces, or more correctly, interactions. Here is a handy summary of what they do:

• Gravity keeps you from flying up to bang your head on the ceiling.
• Electromagnetism prevents you from falling through the floor and dropping in on your neighbours if you live in an apartment. (Plus a lot of other good deeds. Electromagnetism holds atoms together, causes chemical reactions, allows us to surf the web, and last but not least, stops us from walking through walls.)
• The strong interaction causes the Sun to provide us light and energy free of charge.
• The weak interaction stops the Sun from blowing up in our face.

I don't quite remember, but I would suppose that, due to buoyancy (a force driven in fact by gravity as the fluid around you fought for a better deal by getting lower), we were not aware of gravity while in our mothers' wombs. But as soon as you entered the world, you knew about gravity, especially if the obstetrician grabbed you by the ankles and hanged you upside down. Then that quick slap on your bottom caused you to cry out and to open your eyes, thus discovering electromagnetism.

Henceforth, to your young self, the world appears to be full of mysterious forces and interactions.

Only four? Our ancestors shaking off the ooze might have thought that there must be thousands, if not millions, of forces in the world. Thus, our ability to state that there are only four fundamental forces is awesome to the max, a feat summarising centuries of painstaking investigations. In particular, behold Maxwell's realisation that electromagnetism was responsible for light!

A force acting over large distances: “so great an absurdity"

While the proverbial guy and gal in the street are plenty acquainted with gravity and electromagnetism, they have no personal experience with the strong and the weak interactions. The reason they don't is because the strong and the weak interactions between two particles, a proton and a neutron say, drop off precipitously to zero as soon as the two particles are separated by more than a distance characteristic of the atomic nucleus. In sharp contrast, gravity and electromagnetism are long ranged (as every schoolchild knows, between two bodies, both gravity and electromagnetism fall off slowly, as the inverse square of the distance separating the two bodies).

We are so used to Newton's shocking idea of gravity that we almost take it for granted. But imagine yourself Newton's contemporary. What!? You mean to tell me that a fiery ball of gas that we call the Sun, could reach across the vastness of space, and tug on this rock we call home, and keep it going around and around! How absurd, without even a rod or a super duper string attaching the Earth to the Sun!

When the Earth pulls the apple down, no hand comes out of the Earth grabbing the apple as in a horror movie. Gravity is invisible, thus all the more horrifying to the aging starlet.

Physics textbooks typically first tell students about the Newtonian concept of action at a distance, and then point out to the bewildered that action at a distance is weird, thus setting up poor Newton as a straw man to be criticised.

Very unfair! In fact, Newton wrote to a friend in 1693 that action at a distance is “to me so great an absurdity that I believe no man has in philosophical matters any competent faculty of thinking can ever fall into it."

Tell me, when you first learned about the inverse square law, did you not find it bizarre? Were you lacking in “faculty of thinking"?

Another strange feature of Newtonian gravity is that time does not enter. How can any change in the position of the Earth be instantaneously communicated to the Sun? Newton left this conundrum “to the consideration of the reader". The “reader" who took it up was Albert Einstein.

Faraday's field of force and our mother's milk

Every child familiar with interstellar warfare knows about the force field. You too, I presume. If so, may the force field be with you!

But this important and fruitful concept was unknown till Michael Faraday, who rose out of Dickensian poverty to become one of the greatest experimental physicists of all time, introduced it to theoretical physicists. Let Einstein sing his praise: “For us, who took in Faraday's ideas so to speak with our mother's milk, it is hard to appreciate their greatness and audacity." Today, quantum field theory remains our most profound and established theory in physics.

Grappling with the philosophical problem posed by action at a distance, Faraday tried to visualise what was going on around a wire carrying an electric current by sprinkling iron filings on a piece of paper next to the wire. When a current was turned on, the iron filings would obediently form a pattern (another pattern was formed when the filings were brought close to a magnet). Eventually, Faraday proposed that a magnet or an electric current produced what became known as a magnetic field, which exerted a force on the iron filings.

Similarly, an electric charge produces around it an electric field of force. When another charge is introduced into this electric field, the field acts on this charge, exerting on it a force. In effect, Faraday introduced an intermediary: two charges do not act “directly" on each other but they each produce an electric field that acts on the other charge.

A pragmatic physicist might be inclined to dismiss all this as just talk that did not advance our knowledge one whit. Faraday's notion does not explain the force between two charges; rather, it appears to be merely another way of describing what was already known.

But this view misses the point. Faraday thought of the electric field as an entity in itself: the electric field produced by an electric charge exists, regardless of whether another charge is introduced to feel the effect of the field.

The real content of Faraday’s picture, as it turns out, lies in the fact that the electromagnetic field not only can be thought of as a separate entity, it is a separate physical entity. Amazingly, the electromagnetic field could leave the charge and current behind, take off on its own and travel far, far away through spacetime, much like university-bound students leaving their parents.

I curved time, you now curve space also

Two giants of physics, one laying down the law of gravity, the other extending and generalising it. A cryptic exchange between two intellects: “I curved time!" “Yes, Isaac, and now I curve space as well!"

In the modern formulation of gravity, an elegant way of summarising Newton's work is to say that he curved time, but not space. Since Einstein had already unified time with space in his 1905 work on special relativity, he necessarily must curve space also. Time and space are just too intimately linked for physicists to be able to curve one without curving the other.

The last rigid entity to fall

Think of a long metal rod. Hit one end. The regular arrangement of atoms at that end is compressed, if only ever so slightly. By bouncing back to their appointed positions a moment later, the atoms crowd their neighbours down the line, who are in turn compressed. Thus information gets transmitted down the rod in a compressional wave. The speed with which the wave propagates is determined by elasticity, or equivalently by its inverse, rigidity. The more rigid the rod, the faster the wave moves.

Theoretical physicists love to contemplate taking extremes. Consider an infinitely rigid rod. Then by definition, when you hit one end, the whole thing moves as a whole, and the information that the rod is being hit at one end is transmitted to the other end instantaneously. In Einstein's special relativity, energy and information cannot move faster than c, the speed of light (by the way, c stands for celeritas. This notation was first introduced by Weber and Kohlrausch in 1856, long before Albert was born. Celeritas, being Latin, is not related to celery, which comes from the Greek word for parsley). It follows that infinitely rigid rods are not allowed in physics.

By definition, Newtonian spacetime is absolutely rigid. Gravity is transmitted instantaneously.

Once Einstein declared that spacetime is elastic, not absolutely rigid, gravitational waves became inevitable. This is why the overwhelming majority of theoretical physicists had long been convinced of the existence of gravity waves (when the rumours of the impending discovery of gravity wave started flying around cyberspace, I emailed my correspondents to name me a theoretical physicist who does not believe in gravity waves. Nobody could come up with a name. Still, it is crucial that physics is based on observational evidence).

That waves and rigidity clash is readily understood in everyday terms. Undulation - think belly dancing - is all about flexibility, and a stiff and stern man could hardly be expected to wave.

Think of spacetime as the last rigid entity in classical physics to fall: spacetime undulates.