Isaac Newton, we all learned, “discovered" gravity. An apple fell off a tree, the story goes, and it suddenly struck him. (The idea, not the apple, though perhaps the apple struck him too.) This was gravity, a force that attracted objects towards each other.

The more massive an object, the greater its gravity. Newton described the force mathematically, showing that its strength decreased as you moved further away; as mathematical models go, this was a first-class one. It accounted for why we stay rooted to the surface of our planet. It accounted as well for how objects in space revolve around each other (the Earth and its moon, for example). It was able to tell us how powerful a rocket we’d have to build if we wanted to escape the Earth’s gravity and explore space. Truly, this great scientist’s theory allowed humankind to understand our universe and our place in it.

And yet, it did not fully explain some phenomena we can observe. Like, what happens when objects move? Newton’s description of gravity is adequate when they move fairly slowly, but when they move at speeds approaching the speed of light—as many objects actually do—it breaks down. Why?

Besides, take my dilemma, studying physics in college. Here were Newton’s ideas about gravity, his laws about force and motion, the formula that let me calculate the attractive force between objects. I learned how to plug numbers into it to solve exam problems. Not too well—I was never very good at physics—but the knowledge at least settled into my brain. Yet, I could never shake a niggling, naïve, question: why does this force exist in the first place? That is, why should it be an intrinsic property of an object that it exerts a pull of gravity on other objects? Why are they attracted to each other? After all, nothing (and nobody) was particularly attracted to me.

I mean that last question only half-jokingly. Sure, I could live with the idea that you can actually detect gravity only with really massive objects, like the Earth. Newton’s formula made that clear to me. But was there no way to even comprehend the force as it emanated from me, especially as I grew ever more massive? (Only half-jokingly, I said.)

It took another giant, four centuries later, to fill in the gaps in Newton’s theory and offer me the understanding I yearned for. In 1915, Albert Einstein proposed his general theory of relativity, called that essentially because he considered the motion of objects in relation to each other. Of course, the theory is profound and complex. Yet, at its core is a remarkably simple, intuitive and still profound explanation for how gravity works.

Einstein suggested that space and time, considered together, constitute the fabric of our universe. That is, celestial objects don’t simply exist in some mysterious vacuum. They actually rest in the fabric, the continuum, of space-time.

Perhaps this sounds opaque? A good way to think about it is to imagine a thin rubber sheet—balloon material, say—stretched and held down at its edges. Once you have pictured that, imagine dropping a heavy sphere—say, a watermelon—on the sheet. What happens? The watermelon causes a depression in the sheet and settles there.

Now imagine that we take a smaller sphere—say, a marble—and roll it onto the sheet from one edge. If it gets sufficiently close to the watermelon, clearly it will not simply keep rolling in a straight line. Instead, it will follow the slope of the depression and roll towards the fruit. You might say it is attracted to the fruit.

Einstein’s remarkable insight was that this is how gravity works: that in their paths through the universe, objects simply follow the curves and depressions of space-time—curves that are caused by other objects. (Like me, too.) Remember that he was not suggesting that there actually is a vast sheet of rubber-like material stretched across the cosmos, on which sit you and me, the earth, the sun and galaxies. Space-time is an idea, a way of thinking—but very real, and with deep consequences.

And one such vital consequence is that there will be what Einstein called “gravitational waves"—meaning ripples in space-time caused by one event or another. Think, again, of dropping a pebble on one corner of that sheet and watching ripples spread across it. If Einstein is right, we should be able to detect such waves in space-time.

A worldwide experiment called LIGO (Laser Interferometer Gravitational-Wave Observatory) has been searching for such waves, without success, for nearly a quarter century. But as I write these words—in about two hours, in fact—LIGO will make a major announcement.

Many people believe they have finally detected gravitational waves. (You will know for sure by the time you read this.) This is the biggest news in physics in a century, confirming Einstein’s theories and insights.

In cynical times of terror and refugees and sordid politics, this is remarkable, soaring, inspiring news. Let’s hear it for science, all over again.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers explores the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at dilip@livemint.com. To read Dilip D’Souza’s previous columns, go to
www.livemint.com/dilipdsouza

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