How can physicists unify different theories? Photo credit: Andrew George via Unsplash
The single greatest aim that physicists work towards is unification. As science continually uncovers natural phenomena, the language of mathematics can be used to fluently describe and link it all together. This could imply that all of science is underpinned by a singular theory. All physicists chase these Grand Unified Theories or GUTs, but the unification of quantum physics and gravity seems to hit a dead end. Quantisation is the process of applying quantum physics to classical theories resulting in discretely defined physical systems. So far, gravity has proved to be impossible to break up into bits, unless it can be strung together by string theory.
So far, gravity has proved to be impossible to break up into bits, unless it can be strung together by string theory.
Once as elusive as the current task of unifying quantum physics and gravity, the most well-known unification was that of electromagnetism. Electricity was first identified in 1752 when Benjamin Franklin flew a kite in a thunderstorm, supposedly shocking him into identifying the flow of charge. This discovery provided a basis for other great minds of the time such as Alessandro Volta, and later Thomas Edison, to create the voltaic cell and carbon filament lamp respectively, powered by this idea of current. Magnetism was regarded as a separate phenomenon, first observed as an intrinsic material property. When the natural philosopher Thales of Miletus stumbled upon the magnetism of materials such as iron ore in Ancient Greece, he proclaimed that they must have a soul in order to exhibit such life-like attractive properties. Fast forward to 1820, when in separate experiments conducted by Hans Christian Oersted and Andre-Marie Ampere, they noticed that current carrying wires had an effect on each other and any nearby compasses. This link was developed mathematically by James Clerk Maxwell who built a theory of electromagnetic fields. His equations describe the propagation of electromagnetic waves, crucially at the speed of light, through this medium. It took a village of physicists to unify electricity and magnetism and declare them a force.
A widely distributed yet incorrect representation of Franklin’s discovery. The kite was attached to a key that stored the collected charge, so Franklin would not have held the kite itself, or he would have grounded it and discharged the entire experiment.
The four fundamental forces of physics as they stand are electromagnetism, gravity, and two nuclear forces known as the strong force and the weak force. Unification didn’t end with electricity and magnetism, and in the late 20th century, was extended to the weak nuclear force. In 1979 Sheldon Glashow, Steven Weinberg, and Abdus Salam won a Nobel prize for developing the electroweak theory. At low energies, the electroweak splits into the electromagnetic force, which describes charged particles and the weak interaction which governs radioactive decay. At high energies, these distinct forces become unified. This electroweak theory, along with the strong nuclear theory, is mediated by a force carrier, meaning that although the individual carriers (referred to as “bosons”) differ, mathematically the three quantizable forces can be expressed in a similar manner . Gravity, despite affecting every particle with mass in the entire universe, is the odd one out because it has no carrier. It is also the only force that is always attractive, whereas the other three can be repulsive.
The four fundamental forces of physics as they stand are electromagnetism, gravity, and two nuclear forces known as the strong force and the weak force.
Aside from gravity, each of the four fundamental forces has a quantum interpretation. The carriers of the forces are given names and quantum properties, such as photons, which are packets of energy that transmit the electromagnetic force. An equivalent carrier for gravity called the graviton has been theorised but never observed. The discrepancy occurs when comparing general relativity, which describes gravity, and quantum mechanics, which describes everything else. Both are complex theories, but the fundamental difference is that quantum physics comprises a series of effects that are experienced by particles relative to the background of spacetime, whereas gravity is spacetime itself. Applying quantum physics to gravity alters its very geometry and removes the fixed background against which quantum mechanics unfolds.
The defining feature of quantum mechanics is that the properties particles possess, such as their energy, momentum, and even position, become a matter of probability.
The defining feature of quantum mechanics is that the properties particles possess, such as their energy, momentum, and even position, become a matter of probability. Particles become smeared out into clouds of likelihood. This gives rise to all manner of weird and wonderful effects. Consider an electron, which is sitting next to a wall. The largest probability is that it will be found on the original wall, but if the wall is small enough, or low enough in energy, there is a non-zero probability that it will exist on the other side of the wall. This is a phenomenon known as quantum tunnelling which allows particles to exist where they shouldn’t. One of the first papers on this, published in 1928, described quantum particles stuck in a ‘valley’ as having ‘a small but finite chance of slipping through the mountain and escaping from the valley’. In real world examples of tunnelling, the ‘mountain’ or ‘wall’ could be an insulator separating two conductors, or protons overcoming electrostatic repulsion during nuclear fusion in our sun.
This intrinsically probabilistic nature of quantum particles applies to their unobserved states too, in an effect named superposition, or more colloquially known through Schrödinger’s cat. This phenomenon allows particles, or indeed cats, to exist as an unknown combination of two states that collapse upon observation. The cat exists as both dead and alive until the box is opened, and is then observed to be one or the other. In the last decade, the application of quantum effects has exponentially increased our computing power. While quantum computers are not yet fully realised, they stand as a rapidly emerging branch of physics that will revolutionise technology as we know it.
… your feet are ageing slightly slower than your head because they are closer to the centre of the earth.
General relativity, on the other hand, is an utterly different and much older theory, proposed by Albert Einstein in 1915. It describes the geometry of space-time itself, stating that heavy bodies distort its fabric. Much like a ball on a trampoline, space-time is warped and develops contours that not even light is immune from falling into. Einstein’s novel idea was that this distortion is in fact the force known as gravity. This theory was described at the time by fellow physicist Max Born as ‘the greatest feat of human thinking about nature’ and still stands as almost unanimously accepted. A critical feature of Einstein’s curved universe is that space and time are unified, and so time is no longer constant. As counter-intuitive as it sounds, time slows down near to heavier bodies. For example, your feet are ageing slightly slower than your head because they are closer to the centre of the earth. Time dilation impacts daily life in hidden ways such as within the calibration of GPS satellites. Without corrective calculations directly utilising the equations of general relativity, every position a GPS system ever outputted would be wrong because of the time dilations between Earth and the satellite.
Artist’s representation of the curvature of space-time
General relativity also managed to predict gravitational waves a century before they were detected. When incredibly heavy bodies move, the distortion of space-time changes, sending ripples reverberating through the universe. These ripples were detected after years of waiting at the laser interferometer gravitational-wave observatory (LIGO) in September 2015. The waves came from the collision of two massive black holes so far away that their collision must have happened 1.3 billion years ago. In order to measure such a distant event, LIGO uses detection equipment that is sensitive enough to record when their receptors move even a hair’s breadth. Einstein had predicted that gravitational waves would never be detected but modern science has since disproved him with exceptionally sensitive measurement apparatus.
Combining contrasting interpretations such as quantum mechanics and general relativity requires innovative thinking. Physicists have thought quite literally out of the box and have ended up with a theory that goes beyond its original three dimensions. String theory attempts to unify all forces and particles by reframing matter in a completely different configuration and set of dimensions. All particles are visualised as one-dimensional strings that can join together or break apart as they propagate through space. Particular vibrations of a string mean it is observed as different particles or force carriers, such as a photon or even an electron. String theory says a graviton, the carrier for gravity, is a string vibrating at the lowest possible frequency. The fact that this theory is able to describe a graviton means it is describing a quantised theory of gravity. String theory diverges from the standard model of particle physics into a wacky world of ten dimensions. The issue with this is that string theory estimates there are innumerable ways these extra dimensions could be hidden. Instead of a unique solution, billions of theories spawn from these extra dimensions. Physicists tend to call string theory a landscape, because of the span of different solutions and behaviours it predicts.
String theory diverges from the standard model of particle physics into a wacky world of ten dimensions.
Quantum mechanics and gravity take on very different but vital roles within physics. Quantum particles are like actors whereas gravity is the stage they perform on. Unifying the two is not as simple as the unification of electricity and magnetism or even the Grand Unification Theory that subsequently brought electromagnetism and the weak force together. Quantum physics requires an entirely different framework, and the theories proposed in the last few decades are far from perfect. Though string theory comes with a built-in quantisation of gravity, its dimensions generate a multitude of possibilities, none of which are experimentally provable. A graviton is a little piece of gravity that would, upon detection, immediately prove that the quantisation of gravity is possible. Quantum gravity is the last great unification problem in physics and is still determinedly believed to be possible. The philosophical impulse to find cohesion in our perception of the universe still points towards an underlying theory of everything. Whether this is tied up in strings remains an open question.
