Einstein discovered that gravity is not a force but a curvature

Albert Einstein called it “the happiest thought” of his life.

It was almost certainly the most revolutionary — and that, in the case of Einstein, is saying a lot.

At the time, the young scientist was struggling to broaden the framework of his special theory of relativity, which explains the behaviour of bodies in constant motion with respect to each other but does not account for acceleration, deceleration or the effects of gravity.

In a burst of intuition that would later yield an extraordinary new insight into the nature of the universe, it occurred to Einstein that the sensation of riding in an ascending elevator is similar to the sensation of gravity.

That was the intuition.

And this, a short while later, was the insight: acceleration and gravity are not just similar.

They are the same thing.

Einstein reached this conclusion in 1909, when he was 30 years old.

It would take him six more years of hard mental labour to prove the theory on paper, mathematically — an enterprise of sometimes excruciating complexity.

“You must help me or I’ll go crazy,” he implored a friend during one period of frustration, and there were to be many of those.

Max Planck, himself a leading German physicist, advised Einstein to abandon his quest for this grander theory of relativity, for he was bound to fail.

But by 1916, Einstein had succeeded.

The resulting depiction of gravity was stunningly different from the orthodox view that had prevailed since the time of Sir Isaac Newton, who first devised a coherent explanation of the phenomenon, one that accounted for the behaviour of familiar objects on Earth as well as the interaction of planets and stars.

Up to a point.

The classical theory of gravity had been a huge breakthrough in its time, and it still provides a good means of predicting the motion of objects. But it is wrong, in part because Newton misunderstood what gravity is.

He considered gravity to be a force that objects exert upon each other.

But, in a burst of brilliance, Einstein realized that no such force is required and in fact no such force exists.

He imagined occupying a windowless chest in outer space, a container that was being accelerated at a uniform rate by some propulsion device. At a certain sustained rate of acceleration, it would become impossible for the occupant to tell whether he was stationary on Earth or gaining speed in a distant void, for all physical operations inside the chest would be identical.

He could pour himself a cocktail, juggle several balls, stand up, sit down, dance the bossa nova and even weigh himself on a set of scales. The results, in both situations, would be the same.

Based on this thought experiment, Einstein concluded that gravity is not a force of attraction, for no such force is required. Instead, it is something quite different — a curvature in the fabric of space-time.

We “feel” the force of gravity only because we are perched upon a surface that gives us weight. Remove that surface and gravity would no longer feel like anything at all. We would be weightless.

By this way of reckoning, the Earth only seems to be turning in circles around the sun. In fact, it is going straight, but straight along a space-time surface that is itself curved — warped by the mass of the sun.

If this seems to make no sense, imagine driving “straight” from Toronto to Montreal. The truth is, you can’t do it, not even on the 401. The “straightest” route between the two cities — in fact, any route between the two cities — is curved by the surface of the Earth. In fact, if it goes on long enough, any “straight-line” journey along the surface of the Earth will eventually describe a circle.

The same goes for any “straight-line” journey in the vicinity of the sun.

“Newton would have said that an apple fell to Earth because there was a mutual force of gravitational attraction,” writes Simon Singh in his book Big Bang. “But Einstein now felt that he had a deeper understanding of what was driving this attraction: the apple fell to Earth because it was falling into the deep hollow in space-time caused by the mass of the Earth.”

Although it might not seem like it, the Earth is right now falling into the even deeper hollow in space-time caused by the mass of the sun. The only thing preventing a collision is the Earth’s velocity, which is about 107,000 km/h relative to the sun, or just enough to ensure that, in its never-ending downward spiral toward the fiery centre of the planetary system, our fine blue orb keeps missing its target — fortunately for us.

This constant state of free fall, coupled with an appropriate velocity, is what constitutes an orbit.

Astronauts aboard the orbiting International Space Station appear to be weightless — in fact, they are weightless — but not because they have escaped the reach of the Earth’s gravity. Instead, they and their space station are in a state of free fall toward the Earth. They avoid striking the planet for the same reason the Earth doesn’t crash into the sun — because they are going pretty fast. The space station travels at about 19,000 km/h relative to the Earth, or just enough to prevent a collision.

But the important point for relativity theory is that space-time is curved by mass.

“A star or a planet or any hunk of mass warps space and time,” says Robert Mann, a physicist at the University of Waterloo.

This may sound bizarre, but it is really not that difficult to envision.

Think of a child on a swing, alternately plunging into and accelerating out of the depression in space-time created by the mass of the Earth.

Or think of daredevil Nik Wallenda, balancing on a tightrope strung across the Niagara Gorge. From a bird’s-eye perspective, Wallenda walks “straight” to his destination on the other side of the river, just as you may think you can drive “straight” from Toronto to Montreal.

In both cases, this is an illusion. In fact, if you chart Wallenda’s progress from a vantage point on the same horizontal plane, you can see that the acrobat’s tightrope sags significantly as Wallenda is drawn into the hollow in space-time caused by the mass of the Earth. His journey is curved.

We are all drawn into that same hollow — although we’re mostly sane enough not to do it on a tightrope — and this explains why we don’t drift away into space. In effect, we are all in the act of falling toward the centre of the Earth, or we would be if only the planet’s surface didn’t get in the way.

In the decades since Einstein outlined his radical new vision of the universe, general relativity has repeatedly proved itself to be more accurate than Newton’s classical gravitational laws, accounting precisely for certain minute perturbations in the orbit of Mercury, for example, and also showing that beams of light are themselves warped in proximity to massive objects, such as stars.

Here’s another way to think of gravity. Let’s say you jump out of an airplane at 15,000 feet, without a parachute. Now, roll onto your back and look up. You could easily imagine yourself riding through space, at rest. Granted, there’s a fairly stiff wind beating up against you, but it’s the wind that seems to be moving, not you.

Now, roll onto your chest and look down. Uh-oh. Turns out there’s a good-sized planet rushing straight up at you.

Still, it’s the planet that seems to be moving, so it will be the planet’s fault when you collide — if that’s any consolation.

Assuming you survive, you can try a similar experiment while standing on the ground. Normally, if you think about these things at all, you probably think that you, standing on the ground, are “pushing down” against the Earth with your weight and that it’s your weight that keeps you from flying away.

But you could just as accurately turn this perception around and imagine it is the Earth that is “pushing up” against you, giving you the illusion of weight, just as the floor of an upward accelerating elevator pushes up against its occupants.

This realization — that gravity and acceleration are really the same thing — provides the central insight of general relativity. That work was Einstein’s crowning scientific achievement, and it won him international celebrity, not just among scientists but among the wider population, too.

Still, towering intellect though he was, Einstein did not re-imagine the cosmos and their contents alone.

During the past 100 years or so, a succession of physicists and astronomers has brought humankind closer and closer to a complete understanding of the universe.

Granted, huge conundrums remain, and we may never solve them entirely. It may never be possible to comprehend the universe at the moment of its origin, for example, much less at a moment before.

But we keep venturing closer.

In 1911, a New Zealand-born scientist named Ernest Rutherford discovered that the atom — until then considered the smallest object in existence — could be broken down further, into a nucleus circled by an array of orbiting electrons.

The central positive charge, or nucleus, is breathtakingly small, representing only a millionth or so of the volume of the atom, which is composed overwhelmingly of empty space.

The jarring sense of solidity we experience when we walk head first into a lamp post, for example, is really a kind of fallacy produced by our immense size compared to sub-atomic particles, which are mainly emptiness and energy rather than what we think of as impermeable matter.

Even food is composed almost entirely of nothing, which definitely makes you question the merits of dieting.

Later, it would turn out that even the nucleus could be broken down further, into protons and neutrons, particles of ridiculously minute mass — 10 to the power of minus-23 grams. That’s a decimal point followed by 23 zeroes and then, finally, a measly “1.”

And yet this seemingly irrelevant amount of next-to-nothingness is nearly all that an atom has got going for it in the mass department: a miniscule nucleus girded at a vast distance by orbiting electrons.

“If we had an atom and wished to see the nucleus, we would have to magnify it until the whole atom was the size of a large room, and then the nucleus would be a bare speck which you could just about make out with the eye,” wrote the late Richard Feynman, a celebrated physicist, in his book about particle physics entitled Six Easy Pieces. “But very nearly all of the weight of the atom is in that infinitesimal nucleus.”

Scientific advances in recent decades have not been restricted to the realm of the extremely small. They have also broached the equally daunting domain of the unimaginably vast.

Around the time of Einstein’s greatest advances, the universe seemed to be a stable and dependable place, consisting of just one configuration of stars — the celestial neighbourhood we know as the Milky Way — an expanse that was static and probably eternal. It had no beginning and apparently no end and had always been the same size. That was what we thought.

Almost all of these assumptions have turned out to be wrong, and we have a U.S. astronomer named Edwin Hubble to thank for reordering our understanding of most of them.

During the early 1930s, working mostly at the Mount Wilson Observatory in California, Hubble determined that the universe is vastly larger than previously suspected, composed not of just one galaxy but of hundreds — and even that would turn out to be too modest.

“We now know there are billions,” writes Barry Parker in his book Einstein’s Brainchild.

About 400 billion galaxies, according to the current consensus.

Not stars. Galaxies.

Not only that, but the galaxies are moving away from each other. By 1936, Hubble was convinced that the universe is not static at all. It was, and still is, expanding.

That, of course, means that the universe must once have been smaller than it is now, which suggests that it had a beginning, which caused some scientists to speculate that it was once infinitely small, a view that spawned the theory of creation as a colossal explosion of space and time.

In other words, the Big Bang.

In the 1960s, a pair of U.S. researchers in New Jersey — Arno Penzias and Robert Wilson — inadvertently detected a phenomenon now known as the cosmic microwave background radiation, a series of high-frequency waves that swept through space when the universe was 300,000 years old, an age when it had finally cooled enough so that electromagnetic radiation could pass through it.

Although extremely weak, those same waves still pervade the heavens — an electro-magnetic souvenir of the Big Bang and further proof that the universe really did start out from a very small point, in fact an infinitely small point that’s now referred to as a singularity. It has been expanding ever since.

And not just expanding. We now know that its expansion is accelerating.

“Not only are distant galaxies getting farther and farther away; they are getting farther away faster,” says Mann.

We also know that the universe is about 14 billion years old, that its average temperature is 2.73 degrees centigrade above absolute zero, and that its radius extends for about 78 billion light-years and is getting larger all the time.

The huge disparity between the universe’s age and its size certainly seems contradictory — and it is. After all, a universe that’s 14 billion years old “ought” to have a radius of about 14 billion light-years.

If you peered through a telescope capable of resolving images that far away — technically, it probably isn’t possible — you would expect to observe the beginning of time, because the light you would be seeing departed its source about 14 billion years ago, just when the universe was being born.

By conventional reckoning, the radius of the universe shouldn’t be any larger than the distance that light could travel since the beginning of time — and yet it is bigger. Much bigger.

Scientists surmise that the distances travelled by light beams in the past have been “stretched” by the universe’s expansion, and not just by a little but by a lot.

If that seems bizarre, just wait.

“Nature, as we understand it today, behaves in such a way that it is fundamentally impossible to make a precise prediction of exactly what will happen in a given experiment,” Feynman writes in his book on particle physics.

A Nobel laureate who died in 1988, Feynman was referring to a field of scientific reasoning called quantum mechanics, a deeply unsettling realm where an explanation for the behaviour of sub-atomic particles can be achieved only by denying that the behaviour of sub-atomic particles can be explained.

Or, according to an apocryphal remark variously attributed, in one form or another, to any number of different physicists: “If you think you understand quantum mechanics, then you don’t understand quantum mechanics.”


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