WHAT’S UP WITH GRAVITY?

Some problems are so important that scientists have been trying to solve them for centuries. No matter if they are ever found, the search for the answers provides us with new knowledge.

Gravity is predictable in its effects, but we still don’t know exactly how or why it works. It’s arguably science’s greatest mystery.

It keeps our feet planted solidly on the ground and planets orbiting the Sun at distances of millions of km. Nevertheless, gravity is very much weaker than the other forces of nature, and it remains a mystery to scientists. The search for the particle which carries its force has been in vain so far. Perhaps the answer to the mystery of gravity is not a particle, rather it is hidden in seven so far unseen dimensions.

Afew days after astronaut Jack Lousma had returned to Earth after spending two months in the US Skylab space station, he put down his aftershave bottle in the air beside him. A loud bang and lots of broken glass suddenly reminded him that it was a bad idea. In the space station, he had gotten used to everything flying about in a state of weightlessness. But in a bathroom on Earth, other rules apply. We are subjected to gravity, and although it is not particularly “loud”, it rules everything – including aftershave.

When you lift your coffee cup from the table, you sense the invisible force. When your smartphone hits the asphalt, it is due to gravity. And when you step down from your bathroom scales, it decides the number of kg indicated. Indeed, Earth and the universe would not exist, if it were not for gravity.

Almost 14 billion years ago, after the Big Bang, gravity made sure to contract matter, so stars and planets were formed. The fact that Earth is circular, is also due to gravity. Gravity tries to attract everything that planets are made of to their centres, but as the material cannot be totally compressed, they are shaped like balls.

In short, gravity is the ruler of the universe. But although it might seems evident, it is one of the major scientific mysteries. Every time scientists have managed to lift a bit of the veil, they have run straight into new problems. The major question that still remains unanswered is how gravity is carried. Some scientists assume that a particle carries the force, but although they have already named the particle – a graviton – they have never managed to capture it in spite of persistent efforts.

Thanks to geniuses such as Isaac Newton and Albert Einstein, we now know how gravity works between Earth and a rocket, etc., and how it makes planets orbit their stars. But how it works at the atomic level remains a mystery, which scientists are still struggling to solve. If they are successful, we might get the very “manual” of the universe – from the tiniest of elementary particles to the largest of galaxies.

Rocks and water long to be back on Earth

Around 1600 AD, Galileo Galilei of Italy climbed to the top of a tower to throw down two metal balls. That was the beginning of scientific research concerning gravity. Galileo was highly sceptical of the existing view of the world, which dated back to around 350 BC.

At that time, Greek philosopher Aristotle realised that objects falling towards the ground had to do so for a reason – and according to Aristotle, the explanation was obvious. Objects fall towards the ground, because they try to get back to the place they originally came from. A rock comes from Earth, and so, a falling rock will try to get back to Earth. The same is true for water, which is also native to our planet. Fire and air, on the other hand, are not earthly, so they will rise, according to Aristotle. Moreover, he was convinced that the heavier an object is, the faster it will return to its starting point to be reunited with its element. In short, heavy objects will fall faster than light ones, according to the Greek philosopher.

The theory seemed so evident that about 2,000 years passed, before anyone questioned it. That was when Galileo entered the scene. Among the professors of the University of Pisa, Galileo Galileo had a reputation for being a very bright student, but he was also very stubborn indeed. He questioned everything. In 1582, at the age of 17, he began to study medicine, but after all, he loved mathematics and mechanics more, and something was bothering him. Every time his teachers talked about Aristotle’s theories, Galileo objected. He refused to accept that the weight of an object has anything at all to do with the speed of its fall. In a vacuum, in which there is no air resistance, any body will fall at the exact same speed – and a rock will not fall any faster than a feather, according to Galileo.

Around 1600, he decided to put theory into practice. He carried a heavy and a light metal ball up the stairs of a tower – the Leaning Tower of Pisa, according to the myth – to make an experiment. Hundreds of curious people came to the base of the tower to watch the rebellious Galileo make a fool of himself.

Heavy and light balls fall at the same speed

The crowd stared at the daring scientist, as he let go of the two metal balls, making them fall freely from the top of the tower. People cheered, as contrary to expectation, the heavy and the light ball hit the ground at the exact same time, proving Galileo right.

With this and a long series of similar experiments, Galileo pulled the rug from under the existing theories, demonstrating over and over again that gravity is characterized by the fact that that all bodies, disregarding their masses, fall at the same speed under its influence.

If he had lived for about 400 years, he would no doubt have celebrated an experiment which American Apollo 15 astronaut David Scott made during a lunar landing in August 1971. A few hours before the return, Scott took a falcon feather from his pocket and made the 30 g feather and a 1.3 kg hammer fall from the same altitude in the vacuum as a tribute to Galileo. And just as the late Italian had predicted, the feather and the hammer hit the moon dust at the very same time.

“Nothing like a little science on the Moon,” David Scott enthusiastically said from his outpost approximately 400,000 kilometres from Earth.

At the time when Galileo was engaged in his experiments with bodies in a free fall, German astronomer Johannes Kepler made a surprising discovery. Following many years of observations of planetary positions in the sky, he had to acknowledge that the planets travel in elliptical orbits, not in perfect circles, such as scholars used to believe.

That one body should act upon another... without the mediation of anything else is so great an absurdity that no man suited to do science... can ever fall into it... ISAAC NEWTON in a letter to a friend in the 1690s

Johannes Kepler introduced a series of laws concerning the way in which planets travel around the Sun, but he was unable to explain the reason why they travel in the fashion which they do.

Newton’s apple tree travels into space

Hardly anybody else in the world would have paid attention to how mature fruit falls to the ground, but 23-year-old Isaac Newton was an unusually gifted young man. Due to the plague, which caused havoc in Europe, he had fled Cambridge, where he was studying, for the countryside. One day in the late summer of 1666, he was sitting in the garden of his childhood home, drinking tea in the shadow of an apple tree, as his mind travelled. Suddenly, an apple fell and landed at his feet.

This very ordinary phenomenon made Newton wonder, why apples always fall vertically? Why do they not rise or move sideways, he thought. He imagined that some sort of attraction was at work. Earth attracted the apple and all other bodies near it, and perhaps the attraction had an even longer reach – as far as to the Moon and further into the universe. The realization was to have far-reaching consequences and take up all Newton’s time for many years to come.

Ever since he was a child, Newton had impressed people around him with his brilliant ideas. When he was a boy, he had invented a grain grinder powered by mice, he had designed clever clocks that measured time by means of water, and by watching his own shadow, he could immediately tell what time of day it was.

Moreover, if he had had the ability to look into the future, Isaac Newton would have known that the very apple tree which let go of one of its fruits in the late summer of 1666 in the garden of his childhood home, Woolsthorpe Manor, would at some point in the future be known as the Gravity Tree. He would also have known that a handful of seeds from the same, extremely die-hard apple tree would one day in December 2015 be launched with a rocket to escape the very force that had once made the apple fall down at Newton’s feet.

The seeds formed part of an experiment at the International Space Station, ISS, where Newton’s fellow countryman, astronaut Tim Peake, studied how space travel affected their growth.

Anything with a mass has attraction

Inspired by the fallen fruit, Newton thought about linking Kepler’s laws of planetary motion with Galileo’s laws concerning falling objects. The forces applying on Earth must also govern the universe, Newton realized. The force that makes the apple fall from the tree must be the exact same one which keeps the Moon in its orbit around Earth and the planets in their orbits around the Sun. And the reason why the planets do not crash into the Sun is that they travel so fast that they enter into an orbit around it.

In 1687, Isaac Newton published his ground-breaking theory of gravity in the Principia masterpiece, which would be known as one of the most important scientific

A feather is much lighter than an apple, so it will fall more slowly, as it is slowed more down by air resistance. In a vacuum such as on the Moon, apple and feather will fall at the same speed.

works ever written. In it, Newton in an intellectual tour de force introduced not only a mathematical theory of gravity, but also three laws that describe the motion of bodies.

According to Newton, gravity is a force between two bodies. All objects with weight attract each other. The extent of the attraction depends on the masses of the objects and the distances between them, according to the theory, which, Newton insisted, had to apply to all bodies in the entire universe, and so he named it the law of universal gravitation.

Thanks to Newton’s equations, it had finally become possible to calculate planetary orbits in the Solar System and the Moon’s orbit around Earth extremely accurately. Newton could even explain tides and Earth’s shape. Tides are caused by the attraction of the Moon and the Sun, and as a result of Earth’s rotation around its own axis, the world must be flat at the poles, Newton proved theoretically. Since then, the assertion has been fully confirmed by a wealth of measurements, photos from space, and radar and satellite data.

Newton has proved to be just as durable, when it comes to determining the orbits of planets and comets. By means of Newton’s formulas, astronomers can calculate the motions of heavenly bodies thousands of years into the future or back in time and predict future solar eclipses or state the time of past ones very accurately.

Newton’s law of gravity can also explain why Galileo’s two balls fell at the same speed, although one was heavier than the other. According to his gravity equation, the attraction which Earth exercises on the heavy ball is greater than its attraction on the light one. On the other hand, it takes more force to move the heavy ball as far as the light one, so the two factors cancel each other out.

Planet affects Uranus

According to Newton’s theory, gravity exists throughout the universe, and this very assumption was some camel to swallow for contemporary scholars. The fact that the forces of attraction can be exercised over millions of km and reach all the way from the Sun to Earth seemed completely contrary to nature to them.

Newton was claimed to work with occult forces, but in 1846, the criticism ceased once and for all. Up until then, all planets had been discovered by accident, but based solely on Newton’s theories, two astronomers, John Couch Adams and Urbain Le Verrier, independently predicted the existence of an unknown planet, Neptune. Both had noticed irregularities of Uranus’ orbit, which, they concluded, had to be due to the gravitational pull of an unknown planet outside Uranus’ orbit. The analysis proved correct. In the position which the two astronomers had predicted using pen and paper, Johann Galle of Germany in 1846 used his telescope to observe the planet of Neptune.

But although Isaac Newton was the "father" of the law of gravity, he did not believe that he had found an explanation of the nature of gravity – he had not discovered how it works, he had only found the formula.

“That one body may act upon another at a distance through a vacuum, without the mediation of anything else . . . is to me so great an absurdity, that I believe no man, who has in philosophical matters a competent faculty of thinking, can ever fall into it,” Newton wrote about his discovery in a letter to a friend in the 1690s.

Consequently, he passed the task of discovering the "soul" of gravity on to his descendants – specifically, it turned out, a German by the name of Albert Einstein, who in the early 1900s worked as a clerk in a patent office in Bern, Switzerland.

Are we on Earth or inside a spacecraft?

Space is bent, the man with the unruly locks and the vivid gaze claimed – and the planet of Mercury proves that Einstein was right, when it came to his epoch-making recognition.

From the mid-1800s, it was clear that Newton’s law of gravity could not explain Mercury’s orbit around the Sun. For every orbit, the elliptical orbit shifts slightly, which is inconsistent with Newton’s theory. The discovery turned physicists' hair grey and triggered a large-scale search for an unknown planet, which was able to influence Mercury’s orbit. But in spite of persistent efforts, the planet was never discovered. For very good reasons, as it does not exist.

In 1905, the young clerk Albert Einstein introduced his special relativity theory, according to which time and distance are relative factors that depend on how fast the observer is moving. Space and time cannot be seen as separate phenomena, but must be considered as one: spacetime. The special relativity theory can explain a lot about the universe, but not gravity. One autumn day in 1907, as Einstein was sitting in his office in Bern, staring out the window, he had his “best idea ever”. He thought that if a man falls from a roof, he will not feel gravity in the free fall – he will be weightless. The man will not feel that he accelerates, for if he drops his hammer or something else, it will accelerate at the exact same speed beside him.

In a moment of clear-sightedness, Einstein realized that there had to be a connection between gravity and acceleration. It is impossible to make an experiment that determines, if you are on the surface of Earth or in a spacecraft accelerating at a speed of 9.8 m/s2 – the acceleration of objects in a free fall at Earth’s surface, also known as the acceleration of free fall. In practice, acceleration and gravity are the same.

This insight put Einstein on the track of a new, groundbreaking theory, the general relativity theory, which he introduced in 1915. According to his special relativity theory from 1905, differences of speed make space and time change. Acceleration is a change of speed, and as acceleration and gravity are basically the same, it is clear that spacetime changes around all objects with a mass. In his general relativity theory, Einstein determines that gravity is simply spacetime bends. The heavier an object, the greater the bend around it. Spacetime can be compared to a rubber sheet, on which the Sun, etc., is lying like a heavy iron ball. The weight of the ball makes the rubber sheet give in, wearing it down to

completed We have this landmark experiment testing Einstein's universe, and Einstein survives. ASTRONOMER FRANK DYSON at a meeting of the Royal Society and Royal Astronomical Society in London on 6 November 1919

form a kind of funnel, and when a lighter ball such as Earth rolls across the sheet, it is forced to change direction.

Solar eclipse puts Einstein to the test

Newton had understood gravity as an enigmatic force between two bodies, but in his general relativity theory, Einstein claimed that gravity is a characteristic of space itself – and with his ground-breaking theory, he was able to solve the old mystery of Mercury’s strange orbit.

Mercury is maintained in its orbit around the Sun, because the Sun’s powerful gravitational field causes a bowl-shaped bend of space, in which the small planet is rolling about like a ball in a game of roulette. This means that for every orbit, the angle of the path changes relative to the Sun. Mercury is the Solar System planet which is orbiting the closest to the Sun, and so, it is subjected to the most powerful gravitational pull. In the case of such strong gravitational fields, Newton’s law of gravity is inadequate.

The decisive test of Einstein’s relativity theory came during a total solar eclipse in 1919. Einstin had predicted that the light from a remote star passing closely by the Sun would be bent by the star’s bend of space, and now, his prediction would be put to the test.

During the solar eclipse of 29 May 1919, British astronomer Arthur Eddington photographed a star close to the Sun, and at a meeting of the Royal Society and Royal Astronomical Society in London on 6 November of the same year, the tension was finally relieved:

“We have completed this landmark experiment testing Einstein's universe, and Einstein survives,” astronomer Frank Dyson said at the meeting.

The Sun had indeed bent the light of the star. And Einstein had "defeated" Newton with his general relativity theory, which made the headlines of newspapers throughout the world in the days that followed:

“Scientific revolution. New theory about the universe. Newton’s ideas have been defeated,” it said on the front page of the Times of London. “The light is off course in the sky,” the New York Times wrote, adding: “Scientists are more or less beside themselves due to eclipse observations. Einstein’s theory wins.”

Satellite measures Earth’s bend of space

Einstein’s general relativity theory is now the best concerning gravity, but Newton’s law of gravity still functions quite well, when it comes to calculating rocket paths as they are launched from Earth, where the bend of space is minimal, and Albert Einstein himself doubted that it was really possible to measure the effect of Earth’s relatively weak gravity on space. But in 2011, like an echo from 1919, NASA scientists declared, that Albert Einstein’s theory also holds water in this respect.

When we are in a free fall, such as on a steep roller coaster, we are weightless and do not feel that we are accelerating, because everything within our reach is accelerating at the same pace. If we drop a ball, it will "fly" in the air next to us. This made Albert Einstein realize that it is impossible to differ between acceleration and gravity.

By means of four ultra-accurate gyroscopes – i.e. devices used to measure direction – the Gravity Probe B satellite had tested Einstein’s theories in its orbit 640 km above Earth. The measurements consisted of following the axes of rotation of the four gyroscopes inside the probe, whose telescope was aimed at one single star, IM Pegasi. As the direction to the star was fixed, tiny changes of the gyroscopes’ axes of rotation could be measured by magnetic quantum detectors. According to Einstein, the axes of rotation of Gravity Probe B’s gyroscopes were to gradually change due to Earth’s mass and rotation, and when the scientists analysed the measurement results, they found an angle change of the gyroscopes’ orientations. In other words, the data definitively revealed that Earth’s gravitational field bends space in the same way as a ball wears down the rubber sheet of a trampoline.

“By means of this ground-breaking experiment, we have tested Einstein’s universe, and Einstein holds water,” one of the scientists, Francis Everitt from the Stanford University, said during a press conference on 4 May 2011.

Five years later, in February 2016, Einstein’s idea of the bend of spacetime was once again confirmed. Physicists from the American Laser Interferometer GravitationalWave Observatory sensationally published that they had measured waves in spacetime, i.e. gravitational waves, that ripple through space, spreading like rings on the water. The ripples of time and space came from two black holes that had collided, causing – just as Einstein had predicted – waves in spacetime.

Is gravity carried by a particle?

Although several astronomical observations have confirmed Einstein’s relativity theory, scientists still get blank expressions, when they are to explain, how gravity works. They now know that gravity exists, as space bends. But how the force is carried – how masses attract each other – they cannot say. Presently, the most likely explanation is that gravity is carried by a special – so far only imaginary – particle know as a graviton. And the assumption does not come out of the blue, as the exact same principle applies to the other forces of nature.

Gravity is one of four fundamental forces of nature which govern our world. If atoms are the building blocks of the universe, the forces of nature are the glue and mortar that do not only hold the atoms together, but also tell materials how to behave. Two of the forces, gravity and the electromagnetic force, have eternal reaches. All mass in the universe attracts other mass via gravity, and the electromagnetic force can be observed even from remote galaxies in the shape of light. The two other forces of nature, the strong and the weak nuclear forces, only apply inside atoms, where the first one holds the atomic nucleus together, whereas the other one is responsible for radioactive decay.

Of the four forces of nature, gravity is the one which scientists know the least about, which could seem to be a paradox, as we feel its effect anywhere. However, the problem is that gravity is extremely much weaker than the other forces of nature – even a fridge magnet will easily overcome Earth’s gravity to pick up a needle from the floor.

In experiments, physicists have proved the existence of the particles that carry force in the cases of both the electromagnetic force and the weak and strong nuclear forces. Small packets of energy are sent and received, which physicists have named quanta. The best known examples are light quanta, called photons, which carry the electromagnetic force. When this applies to the three other forces of nature, why would the last force, gravity, not also function by means of quanta, they argue.

However, the problem is that all physicist efforts to find the imaginary gravity particles have been in vain. But at the European Organization for Nuclear Research, CERN, in Switzerland, scientists are working hard to find it. In the future, physicists hope to be able to detect the graviton in experiments in the world’s largest particle accelerator, the 27-km-long, underground Large Hadron Collider. In the accelerator, protons are fired at speeds close to that of the light, and when they collide, particles result, which do not exist under normal circumstances.

The force is hiding in invisible dimensions

If they do one day confirm the existence of the graviton, physicists will have come a giant step closer to one of the greatest aims of science: a theory of everything. The theory is to explain both the largest and the tiniest of phenomena in the universe – from stars and galaxies to atoms and molecules – and hence solve the greatest of all mysteries: what caused the Big Bang, the explosive birth of space about 13.7 billion years ago, and what happened during the period immediately after the Big Bang?

In the search for a theory which can explain all phenomena, scientists have, throughout history, been on the lookout for simple laws of nature to describe a complex world. But gravity is the eternal problem and the only one of the four forces of nature that cannot be explained by means of quantum mechanics – the theory which describes nature at the smallest scales of energy levels of atoms and subatomic particles – but only by means of Einstein’s relativity theory.

“Our problem in physics is that everything is based on these two different theories, and when we combine them, we get nonsense.”

These words were said by American physicist Edward Witten. The formulas of quantum mechanics and the relativity theory are mathematically incompatible, but Witten represents the so far most promising theory of everything – a theory that could combine Einstein’s general relativity theory and quantum mechanics. Witten, who has been named the most gifted physicist of his generation, has been working on the string theory since 1975. The theory aims for a coherent understanding of matter and forces of nature, and the essence of the theory is that everything in the universe – all matter and all four forces of nature – were

Our problem in physics is that everything is based on these two different theories and when we combine them, we get nonsense. EDWARD WITTEN, the physicist behind the string theory about the incompatibility of quantum mechanics and the relativity theory.

produced from incredibly tiny, vibrating strings, which are the tiniest building blocks of the universe. The strings are to be understood as threads of energy that vibrate in no less than 11 dimensions: the three spacial ones and the time dimension plus seven other dimensions that are curled up so we cannot see them.

According to the superstring theory, gravity is not weaker than the other forces of nature, although that seems to be the case – we just do not feel its full effect, as it is spread across the extra dimensions.

The superstring theory lives up to all physicists’ requirements for the long sought theory about everything – but fails big time, when it comes to documentation. So far, the theory is only a mathematical construction and pure imagination. The strings and the extra dimensions are so tiny that we can never spot them. So, the theory cannot immediately be tested – unless the Large Hadron Collider produces a miracle.

If the particle accelerator detectors suddenly spot an unexpected guest in the shape of an unknown particle, it might prove to be the long sought graviton, which shows signs of life, before it disappears into the invisible dimensions. If it happens, scientists will, in spite of gravity, have major difficulties keeping their feet on the ground.