3 Ideas at the Weird End of Physics (and What They Mean in Practice)

Imagine being interested in science in the late 18th century. You might have come across the experiments of Luigi Galvani, running electricity through dead frogs and causing them, briefly, to animate and twitch. Or perhaps the work of Benjamin Franklin, flying a kite into a storm to charge a Leyden jar. Undoubtedly, the workings of electricity would seem weird and mysterious to you, and you might well conclude that it was good for party tricks and not much else.
Or move forward 50 years. You might encounter the work of Charles Babbage and Ada Lovelace, creating calculating machines that could replicate the sums that were done by human clerks and accountants, though slowly and at considerable expense. Impressive stuff, you might think, but paying clerks works out a lot cheaper. (One similar machine, the ‘Mechanical Turk’, which played chess, turned out to be just that – a person in a box doing the work and pretending to be a machine). These machines were fun curiosities, complex to understand but without much other value.
Of course, if you had the chance to time-travel further, either from the 1780s or the 1830s to the present day, you would be shocked by how much electricity and ‘calculating machines’ had changed and improved human lives, enabling us to live longer, communicate across incredible distances, and even travel to the Moon. The frontier of modern physics looks a little bit like electricity might have done in the 18th century. There are some very weird ideas that are hugely complicated to understand, and it’s easy to imagine they might not have any real-world application. But if that’s your conclusion, you would be very much mistaken. Here are three of the strangest ideas in modern physics – and how they could change our understanding of the world as we know it.

1. Multiverse theory

Could our universe be just one of many?

Multiverse theory is familiar to many of us from science fiction – it’s the idea that the universe we live in is not the only one that exists. Multiverse theory doesn’t suggest just a handful of universes, though; it suggests that there may be infinite universes, in which every possible version of reality exists, from universes that are utterly different to our own with alternative laws of physics, to universes that are identical to our own except for the placement of a single atom.
One view of this is that they would all exist side-by-side in higher dimensions that we can’t understand or perceive. To see how that might work, imagine a table in a house. For twenty years, it’s in the same place. Then the family decide to rearrange their furniture and move the table, putting a rocking chair in its place. The table and the rocking chair occupy the same dimensions in space, but not in time; if you couldn’t perceive time as a dimension, you would think they were both in the same place, and not be able to understand how that could be possible. It’s argued that the same is true for humans, who can only perceive length, height, depth (the spatial dimensions) and the fourth dimension of time, when considering the multiverse. We can only perceive one universe, but that doesn’t mean other universes don’t exist.
Another approach argues in favour of bubble universes. This is based on the theory of cosmic inflation, where a tiny piece of space blows up at incredible speed – in our universe, we call that the Big Bang. The multiverse theory of bubble universes suggests that this hasn’t just happened once, with our own universe, but countless times, creating countless other universes alongside ours, all constantly expanding and pushing one another away. We are trapped within our own bubble, but we might still be able to detect the effects of the bubbles brushing up against our own.
Imagine our universe as a single bubble among multitudes.

The existence of the multiverse could solve several problems in physics. One is that if space-time is infinite, then it must begin to repeat itself, because there are only so many different ways that atoms can be arranged. Infinitive space-time effectively requires a multiverse, because our own universe does not provide infinite variation. Another is to provide an explanation – albeit an unsatisfying one – about why the laws of physics in our universe are what they are; namely, that it’s all by chance, and in other universes they might be quite different. The laws of physics in our universe seem ‘lucky’ in that they allow matter and life to exist, but in a multiverse, this is purely down to chance. It just so happens that in our universe, the laws of physics work in such a way as to allow physicists to study them; there are other universes with other laws, but with no physicists to lament their bad luck.
Because we cannot leave our own universe, multiverse theory is exceptionally hard to prove or disprove (for which reason it has been criticised as unscientific). But were it to be proven, it would have significant ramifications for physics, and perhaps for how we as humans see our place in the universe. The holy grail in physics is a “theory of everything” that produces a full theoretical framework of all physical aspects of the universe. But in a multiverse, that theory becomes pointless, because the nature of our universe would be purely a matter of chance. And seeing our existence as a matter of chance is not something that many humans find appealing, either.

2. Quantum mechanics

Very few theories in Physics come with such adorable metaphors.

Of all aspects of physics, quantum mechanics has perhaps the coolest, most sci-fi sounding name. As theories go, it has the dubious distinction that a metaphor invented to explain part of it is perhaps now more famous than the theory itself: the thought experiment of Schrödinger’s cat.
Leaving cats and boxes aside for the moment, quantum mechanics is the theory of the interactions of the smallest things in nature, including subatomic particles and electromagnetic waves. Classical physics describes nature at a macroscopic scale, but its theories do not apply at the scale at which quantum mechanics operates. And when things get very, very small, they also get very, very weird. For instance, one feature of quantum mechanics is wave-particle duality, where the idea of a sharp distinction between waves and particles, seem in classical physics, breaks down. In 1905, Einstein identified that light exhibits this dual nature, and subsequent physicists have learned that the same is true for electrons.
One even weirder effect of this is quantum superposition. Multiple waves can exist in the same place; to understand this, some people find it useful to picture ripples crossing over one another on a pond – that’s an example of superposition at a macro scale. Where this gets strange is that because multiple waves can exist in the same place, quantum particles can exist in different places at the same time along a given wave. Only when the particle is observed can it be determined to be in a particular state and position. That’s what Schrödinger’s cat tries to explain – the cat, like a quantum particle in superposition, is both alive and dead. Only when you open the box to check (observing the particle) does it become one or the other.
Quantum computers not only sound cool, they also look cool.

So far, this sounds like a Just-So Story to enable the understanding of things that are, with the best will in the world, extremely hard to understand. Where it gets exciting is that quantum mechanics isn’t just a way to give physicists headaches. It has real-world implications that could change the nature of computing. In a conventional computer, data is stored as bits: binary digits that can be either 0 or 1. Those are the only options. But instead of bits, a quantum computer uses qubits; the quantum equivalent, which instead of being only 0 or 1, exists in quantum superposition where it could be in any number of states. This allows quantum computers to encode and process vastly more data at much greater speed than a conventional computer. In terms of speed and processing power, a quantum computer is to a conventional computer what a conventional computer is to an abacus.
Quantum computing is still in its infancy; IBM and others have built very small-scale quantum computers that prove the principle, but are nowhere near ready for larger-scale practical use. For instance, most prototypes can only be used at very low temperatures. One example of an existing quantum computer is D-Wave’s 2000Q, which has 2,000 qubits; for comparison, your PC processes billions of bits per second. Yet the 2000Q is already being used by scientists for problems that conventional computers struggle with. When quantum computing really takes off, this mind-bending area of physics could change the world.

3. String theory

String theory could help us understand what happens inside a black hole.

Remember how multiverse theory might ruin our hopes for a “theory of everything”? String theory is currently the closest thing we have to a coherent theory of everything – though it’s very far from complete. It rests on the idea that elementary particles such as electrons, photons and, crucially, gravitons – that’s the hypothetical particle that mediates the force of gravity – are in fact one-dimensional strings of energy that vibrate in different ways. At a larger scale, these strings look like particles. An elementary particle is a particle that is not composed of other particles, a category that includes matter particles, antimatter particles, and force particles. These can combine to form composite particles, such as neutrons.
Gravitons are crucial here because one of the problems that gives rise to string theory is the problem of describing gravity according to the principles of quantum mechanics. Currently, general relativity satisfactorily describes the effect of gravity on a non-quantum scale, and quantum mechanics describe the universe at a quantum scale – but marrying the two together to produce a theory that fits both is still a work in progress.
A visualisation of what it means for spacetime to be curved.

One of the issues is that general relativity models gravity as curvature of spacetime. Spacetime is the mathematical model that takes the three dimensions of space and fuses them with the fourth dimension of time to create a single continuum. Quantum field theory, meanwhile, models spacetime as flat, not curved. That means that the theories and calculations of general relativity don’t translate to the theories and calculations of quantum mechanics. Using equations from general relativity to understand what’s happening at a quantum level produces answers that are ridiculously, obviously wrong. This disconnect is what string theory attempts to address. After all, it isn’t plausible that the universe operates under two different sets of rules depending on scale; there must be some way that the two theories can be unified.
So how does string theory attempt to solve the problem? To some extent, it simplifies it. Instead of different particles that work in different ways, there are only strings, vibrating in different ways; the different vibrations correspond to the different particles. Every different force similarly results from different vibrations of strings. This sounds tremendously straightforward, but the process of scaling up this simple explanation and applying it to our universe as a whole is hugely complicated; and to make it worse, it’s still a work in progress.
The problem is that any predictions made by string theory are far beyond our current capacity to test experimentally. String theory has even been criticised as unscientific because the scientific method depends on the creation of hypotheses that can be proven or disproven; something that isn’t currently possible with string theory. That’s not to say, however, that it will never be possible, and if we can prove aspects of string theory, then that would represent an astonishing advance in physics – especially if it does come close to being that coveted “theory of everything”. It’s hoped that one day, string theory will help us answer questions from what’s happening inside a black hole, to what happened at the very beginning of the universe, and why.
Image credits: difference engine; ; bubbles; cat in a box; quantum computer; black hole; spacetime