Think you understand gravity? Think again. The everyday force is a constant source of questions for physicists - and some are looking to replace the theory
Gravity is everywhere, and you can see the effects everywhere you look. It’s the reason the planet orbits around the Sun, the Moon orbits Earth and your feet stay planted firmly on the ground. It’s one of the first aspects of physics we encounter at school, for example, with the tale of Newton and his apple falling off his head. Yet we know surprisingly little about this 'invisible force'.
Gravity belongs to a family of four forces that between them can answer pretty much any question along the lines of ‘why does that happen?’ when it comes to physics. Along with the strong force, the weak force and the electromagnetic force, the gravitational force governs the way everything interacts with each other in the known universe.
Gravity has been a source of wonder for humans since 1687 when Newton realised a force was needed to ‘keep the Moon in her Orb’. Because of this long history, it would be understandable to assume gravity is the most well-understood of the forces, but it’s not that simple.
Some of the greatest scientists to have lived, notably including Einstein, have dedicated their lives to understanding the force of gravity. Newton described gravity as a force, but Einstein went further, showing gravity to be the result of how mass and energy distort space and time. As a result, we are clued up on how it works on a classical scale in moderate gravitational fields – think of how satellites orbit the Earth, or how planets orbit the Sun, all of which can be described by Newton’s equations of motion.
The exception was Mercury’s orbit around the Sun, which Newton could not explain. Mercury, being so close to our nearest star, experiences a stronger gravitational field and it took Einstein and his General Theory of Relativity in 1915 to explain how that stronger gravity influences the swift world's path.
What is more, Einstein’s theories predict how massive objects like stars distort space-time to the extent that they bend light around them, magnifying it in a process known as gravitational lensing. Predictions Einstein made in equations almost 100 years ago are still being tested in the most extreme gravitational environments that we know of, such as around black holes and binary neutron stars, and those equations are turning out to hold true now. For example, the first evidence showing how the motions of stars orbiting a supermassive black hole are affected in accordance with general relativity was published in August of last year.
But there are a few key problems with gravity that still need to be explained.
“There are many mysteries about gravity” says Paul Sutter, astronomer at Ohio State University.
“For one, why is it so weak? It's far, far weaker than any of the other forces.”
Gravity might not feel feeble when you are walking up a steep hill, or falling over. Yet the fact that you can overcome the force of gravity from a mass as large as the Earth’s and stand up straight highlights how weak it is, comparatively. Gravity is around 1040 times weaker than the other forces, including the electromagnetic force that holds atoms together. It’s even weaker than the ‘weak force’ that causes radioactive decay. Maybe gravity has been given the wrong name.
There are a multitude of theories as to why gravity is so weak, but no clear leading contender. “I don't think any current options look very promising right now,” says Sutter. “We're floundering when it comes to extending our knowledge of physics, and we're only now beginning to design what we hope to be the right kinds of experiments to give us a clue as to what nature is thinking.”
However, the answer could lie in solving the problem that not only eluded Einstein, but every physicist since – the unification of gravity, which describes things on very larger scales, with quantum physics, which is famous for describing behaviour on very tiny scales.
If gravity is quantised on the smallest scales, like light is, then it must have a particle that carries its force through quantum fields. For light, this particle is the photon. Gravity’s force carrier would be the graviton, but it remains merely hypothetical at this point in time.
Two of the most popular attempts to derive a quantum theory of gravity are String Theory and Loop Quantum Gravity. String Theory proposes that all of matter is made up of tiny vibrating strings, and that one of these vibrational states represents that of a graviton.
String Theory also requires extra dimensions beyond the four of space and time that we are familiar with, and some scenarios suggest that the gravity we experience is weak because it is leaking in to our four-dimensional space-time from another dimension that is curled up so tight that we can’t otherwise detect it.
Meanwhile, Loop Quantum Gravity tackles the problem from a different angle by saying that space itself is quantised - that is, it can be broken down into the smallest possible pieces called quantum loops of gravity. However, it’s still a fairly undeveloped theory, since it does not predict the graviton and it hasn’t been shown that it is related to general relativity.
Like electromagnetism, gravity has an infinite range, which means it only gets weaker and weaker the further the two objects are apart, and never goes completely to zero. This is unlike the weak and strong forces, which only work within a specific range. This itself is not a problem when it comes to understanding gravity, but it does make gravity a strong driver of the universe around us.
Gravity is an attractive force only, and because of this it cannot be cancelled out. This
“Einstein threw a spanner in the works when he described gravity as not a force, but a consequence of the structure of space-time”
makes it very different to electromagnetism, which can attract and repel. The fact it is only attractive means gravity permeates across the vast distances in the universe with nothing getting in the way. But this is also another reason it makes no sense, according to some scientists.
Martin Tajmar, Professor of Physics at the Technical University Dresden, in Germany, says the strangest thing about gravity is “that there is only positive mass around us with no negative counterpart as in electromagnetism”.
If an electromagnetic force is attracting an object, applying an opposite force would balance the original out, meaning the net force on the object would be zero. In what would potentially be the equivalent of this phenomenon but for gravity, Tajmar is looking into ways to ‘counter’ gravity, an ‘anti-gravity’ of sorts. He hopes come up with an opposite kind of force that would balance the gravitational pull of an object.
The idea of countering gravity is not only important for making gravity behave like all the other forces. It may have practical applications too. “It would be a game-changing technology like the use of electromagnetism that created the modern world” says Tajmar. “Perhaps, it could open up new ways for space travel too.”
But in order to come up with a way to counter gravity, we need to go beyond the theories we currently have.
“Together with my students, I’m looking into theories that predict deviations from our current understanding of gravity” he says. “We are performing experiments to test and verify those very concepts.”
For an alternative theory of gravity to be taken seriously, it must be possible to reduce it to general relativity in certain scenarios. Because of this, it is necessary to test the limits of the general relativity to see where it deviates from reality and into the territory of a completely new theory.
Some of these boundaries involve strange links between gravity and other phenomena in physics, like superconductivity. In 2003, Tajmar wrote a paper proposing gravitational effects could be responsible for a difference between the measured mass of pairs of electrons at low temperatures, found in superconductors, compared to the theoretical value. This theory is known to physicists as gravitomagnetism.
“Gravitomagnetism is an approximation to Einstein’s theory of general relativity for low velocities in flat space-time” says Tajmar. “It allows us to easily illustrate the effect of space-time ‘framedragging’, for example by the rotating Earth.”
Frame-dragging occurs when a massive rotating object drags nearby space-time – and any objects, such as satellites, in that space-time – around with it. In the theory of gravitomagnetism, when the Earth rotates it generates a field similar to the magnetic field generated by a rotating charge. This, in turn, affects the motion of masses similar to the way electrons are deflected if they move at right angle to a magnetic field.
This has been measured, says Tajmar. “Satellites in polar orbit around the Earth are slightly deflected by the rotating Earth in line with Einstein’s theory,” says Tajmar. “However, the effect [of this framedragging] is so small that there are no practical applications so far.” There is much more research needed before it can be used as a way to start countering gravity, it seems, or even explaining why gravity only attracts and does not repel.
Gravity is especially important when it comes to the structure of the universe. It is responsible for the formation and evolution of galaxies and black holes and, it also acts against the expansion of the universe, trying to slow it down in the face of dark energy, which is accelerating the expansion. Whether gravity or dark energy wins will decide the fate of the cosmos.
Gravitational lenses are a pretty handy tool for measuring the structure and distribution of matter in the universe. The more mass there is in a star, or a galaxy, or a cluster of galaxies, the greater the space-time distortion and the more powerful the gravitational lens. Therefore, the mass, and the distribution of that mass, can be calculated based on the amount of lensing. Yet when astronomers look at gravitational lenses formed by huge galaxy
“Gravity is responsible for the structure of galaxies and black holes and slowing down the expansion of the universe”
clusters, the find that the lensing effect is greater than the mass of the visible galaxies can account for. The implication is that there is some unseen matter - dark matter - that is providing the extra gravity for the lenses. We also see the influence of this extra gravity in the motions of the galaxies on the outskirts of clusters, and stars on the edges of galaxies, which are moving faster than they should be based on the amount of visible matter.
What if, rather than there being dark matter to provide extra gravity, it is gravity itself, or rather our understanding of it, that is wrong? This is the theory put forward by Israeli scientist Mordehai Milgrom in 1983. He called it Modified Newtonian Dynamics, or MOND for short, and it argues that in very low acceleration environments, such as those experienced on the outskirts of galaxies and galaxy cluster, gravity does not follow the traditional theories set out by Newton and Einstein.
Practitioners of MOND are few and far between, since dark matter is by far the most popular theory, but the level of popularity doesn’t necessarily dictate that a theory is correct or not. MOND has had some successes, particularly in explaining the rotation of stars around galaxies, but efforts to develop a cosmological theory of MOND, in which it co-exists in the Big Bang and Inflation theories, have so far failed. Nor does MOND have a theoretical framework from which it can be derived, leaving many scientists sceptical about MOND. However, the theory of dark matter has its own problems: nobody knows what dark matter is made of, and searches for dark matter have so far drawn a blank.
MOND does not seek to replace Newton and Einstein’s theories, but to complement them, since they operate in stronger gravitational fields to MOND. At the moment, the General Theory of Relativity is our best explanation for gravity, but we know it is not the final answer. For now quantum gravity remains mysterious, but whoever can solve that puzzle which links the gravity of the very large to the gravity of the very small could change physics and our understanding of gravity forever.
Gravitational lensing has beenused to map the distribution of mass around stars, galaxiesand clusters of galaxies
Gravity is crucial for life on Earth; without it we wouldnot even have a planet The attraction of gravity holds together huge galaxies like this one, with its bright blue gas at the centre Gravitational waves, ripples in space-time, were predicted by Einstein and first found in 2015
Albert Einstein described gravity not as a force, but as a consequence of the shape of space-time
This digital collage contains a highly stylised rendition of our Solar System