Physics – from falling apples to relativity and beyond

LOOps or strings?

General relativity was just the first step to understanding how gravity works. Jessie Hammond explores the way forward.

Over 100 years ago, Einstein rocked the physics community’s conception of how gravity works. His theory of general relativity usurped Newton’s theory which had been taken as fact for centuries beforehand. However, it isn’t the end of the story. We are still a long way off from finding a theory which fully describes how gravity behaves, especially when it comes to the bizarre phenomena of quantum mechanics mixing with gravity (known as quantum gravity). Whilst the theory of general relativity has been superb at explaining observational evidence thus far, it seems to not be compatible where small, dense areas are concerned.

Knowledge about the overlap of quantum mechanics and gravity is particularly valuable when studying objects which are extremely small but also have a lot of mass, such as black holes or neutron stars (a star remnant that is entirely made up of neutrons). The good news is that there are many candidates ready to take up the challenge of explaining quantum gravity. To start you off on your exploration of quantum gravity, I have narrowed the field down to two of the best chances we have right now: string theory and loop quantum gravity. At the end, you can decide which gets your vote as the most likely to describe our reality.

Space is just granules?

Image by Kier… in Sight courtesy of Unsplash

The first theory we will explore of the current favourites is called loop quantum gravity (LQG). Much beloved by Carlo Rovelli, a theoretical physicist considered as the founder of LQG, the theory states that space is not a continuous field, as once was thought, but made up of granules. From the early days of science with Aristotle and Newton, space has been thought of as a fixed entity on which objects interact. However, Einstein came along and changed that thinking so that now space, or more specifically spacetime, was a field which everything else interacted with. This spacetime field was also how gravity worked, forming the gravitational field. It is not a fixed entity in that get down to it, these are known as Faraday’s lines of force for the electromagnetic field (specifically the electric field in this case as we assume no magnetism) and are basically quantum excitations of the field, much like a ripple on a lake. In the absence of charge, these lines close around themselves to form loops. The theory of loop quantum gravity suggests that a similar thing happens with quantum excitations in the gravitational field.

There is a lot more to the theory than just that, but all the other mathematics and predictions come from this thought process. These loops interact with particles, so the particles travel in straight lines, and the loops also describe space as

spacetime affects the motion of matter, and matter in turn affects how spacetime curves. This means that there is no longer any background space; this is what loop quantum gravity accepts and uses to its advantage.

Imagine a point with a charge (you can decide whether it is negative or positive) which sits in the electromagnetic field, a field which describes how light travels (see diagram). The point charge has various lines of electric charge coming into (if negative) or going out of it (if positive). Now, what would happen if we take away this point charge? Do the lines disappear along with it? Not necessarily. When you

Image by Alina Grubnyak courtesy of Unsplash consisting of finite elements. In other words, space is granular! And from this theory spill out many other fascinating predictions. In the 1970s, Stephen Hawking formulated a theory in which black holes can radiate particles due to quantum mechanical effects at the black holes boundary (the closest any object can get to a black hole without being sucked into it). These particles, particularly when talking about photons, would have an associated heat energy and temperature. Hawking found that if you knew the black holes temperature and mass, you could find its entropy. Entropy is a measure of how disordered a system is; the more disorder, the higher the object's entropy. A success of loop quantum gravity is that, assuming LQG holds true, Hawking's entropy around a black hole can be derived.

Some of the dynamics of the Big Bang can also be worked out using loop quantum gravity, potentially getting rid of the fact the universe initially started out as a singularity (a point where it is infinitely small and dense). Instead, it suggests there was a “Big Bounce” whereby a previous, older universe “bounced” (or quantum tunnelled) into our universe.

However, the most important shortcoming of this theory is that none of this has been experimentally tested yet. For a scientific theory to have some credence, it must also be backed by observational evidence, of which there is none for LQG, whereas theories such as general relativity can boast a wide sleuth of experimental successes. Furthermore, there are still several areas of the theory that require further work, so it cannot be said for certain that this is the theory for us.

Strings, strings, strings

Another very popular theory of quantum gravity that has been enchanting physicists for decades is string theory. Starting in the 1970s, string theory has been developed by generations of physicists over the years to become one of the most mathematically refined theories of quantum gravity we have. Unlike loop quantum gravity’s granular structure of space, string theory suggests the universe is made up of infinitesimal one-dimensional strings. These strings can fold, twist, vibrate, or generally move, and we see these motions as observations in experiments in all fields, from particle physics to gravitational effects. As an example of this, some of the strings’ vibrational states produce the point particles that make up everything we see. More recently, a new and adapted version of the theory, M-theory, has added branes to what makes up our universe as well. These are also fundamental objects of the universe, but contain higher dimensions than strings.

So, our universe is made of unobservable strings and branes, governing all phenomena we observe. All good so far. Well, there are a few tricky things at play here still. For example, in order to explain this theory fully, we need 11 dimensions, six of which are only visible from the perspective of the strings. On top of this, it also relies on supersymmetry being valid. Supersymmetry states that the particles which are connected to a force (for example, photons or W bosons) are equivalent to those that we see in matter (for example, electrons, neutrinos, and quarks that make up protons which make up atoms). This has yet to be tested and confirmed, and it is looking less and less likely to be a possibility as even the theory’s most ardent supporters say experiments like the LHC aren’t powerful enough. Without supersymmetry, string theory would need a minimum of 26 dimensions, arguably less believable than the 11 needed for M-theory.

Even though the strings are too small to be observed, the theory has had many successes, including being able to explain fundamental constants such as the electron mass. It has also been able to describe Hawking’s black hole entropy by describing the internal structure of the black hole as its source. Additionally, it includes quantum gravity already built in as one of the many vibrational modes of the strings (or the ways the strings can vibrate) must be linked to the graviton, the “gravity particle”, and thus quantum gravity is merely a consequence of this theory.

Image by meriç tuna courtesy of Unsplash

Your choice: loop quantum gravity, string theory, or something else?

In either of these theories, a potentially fatal obstacle is the lack of experimental evidence to back them up. Science is based on a theory being proposed and only being truly accepted when its observational predictions are confirmed. It is extremely hard to gather data sensitive enough to rule out theories or constrain the ideas that physicists have kept coming up with, and it is a hurdle that quantum gravity will have to leap over someday in order to progress any further. There is still hope, however, for upcoming surveys and experiments with higher sensitivity to shed more light on this area of research.

For now, we can only ponder and debate: which theory strikes you as being able to explain our weird and wonderful universe better? Or is it all just made-up fantasy and some other explanation is much more likely?

Jessie Hammond (she/her) is a fifthyear MPhys Physics student and the head sub-editor at EUSci.