Is There a Theory of Everything?

Not yet. Physicists have joined several forces of nature into single theories, and the Standard Model of particle physics describes three of the four known forces with great success. But gravity, described by Einstein’s general relativity, has never been joined with quantum physics, the rules of the very small. Serious candidates exist, including string theory and loop quantum gravity. None has passed an experimental test that could confirm it.

So a “theory of everything” is a goal, not a finished result. Whether nature even has one final theory is an open question.

What are the four forces?

As far as we know, everything in the universe is built from a few kinds of basic particles, pushed and pulled by four fundamental forces:

• Gravity is by far the weakest force, but it reaches across any distance. It rules at the scale of our bodies and of planets.
• Electromagnetism covers electricity, magnetism and light. It also reaches across any distance, and it is far stronger than gravity.
• The strong force is the strongest of the four, but it acts only across tiny, subatomic distances.
• The weak force also acts only at subatomic distances. It can change one kind of particle into another, which is part of what lets the Sun burn.

In the Standard Model, three of these forces are carried by particles: the photon for electromagnetism, gluons for the strong force, and the W and Z bosons for the weak force. A carrier for gravity, called the graviton, has been proposed but never found.

Has unifying forces worked before?

Yes, and that is why physicists keep trying. In the 1860s, James Clerk Maxwell showed that electricity and magnetism are two sides of one electromagnetic force. About a century later, Sheldon Glashow, Abdus Salam and Steven Weinberg linked electromagnetism with the weak force. Their “electroweak” theory made a bold prediction: a new kind of weak interaction, called a neutral current. In 1973, the Gargamelle bubble chamber at CERN found the first direct evidence of it. In 1983, CERN experiments discovered the W and Z particles the theory required. In 2012, CERN experiments found the Higgs boson, an essential part of the Standard Model.

Think of ice, water and steam. They look very different, but they are one substance at different temperatures. Physicists suspect the forces work in a similar way. At everyday energies, electromagnetism and the weak force look different. At the high energies reached in particle collisions, they begin to act on equal terms. The strong force also gets weaker at higher energies. That hints that three forces might merge at energies at least a thousand million times beyond any accelerator. Conditions like that existed only in the first tiny fraction of a second after the Big Bang. Ideas that merge these three forces are called grand unified theories.

Why does gravity refuse to join?

General relativity describes gravity as the curving of space and time. Quantum physics describes the world of particles, where energy comes in tiny packets and outcomes are matters of chance. Each works beautifully in its own domain. But as CERN puts it, no one has yet managed to make the two mathematically compatible.

Most of the time this does not matter. Between single particles, gravity is so weak it can be ignored. The trouble comes where both matter at once: at the very beginning of the universe, and deep inside black holes. There, gravity is strong and distances are tiny. Physicists expect a quantum theory of gravity to take over at the “Planck scale,” a length of about 10⁻³³ centimeters. Written out, that is a decimal point followed by 32 zeros and then a 1. It is so far beyond our instruments that direct tests have long seemed nearly impossible.

What are the leading candidates?

String theory replaces point-like particles with tiny vibrating strings. Different vibrations would show up as different particles, and one of them behaves like the graviton. That is why string theorists see it as a possible single theory of all four forces. It has a price: the math needs nine dimensions of space plus one of time. A major problem, the Stanford Encyclopedia of Philosophy notes, is a lack of testable predictions. The theory allows a huge number of possible “ground states,” each like a different version of the universe, with no agreed way to pick out ours.

Loop quantum gravity is more modest. It does not try to unite all the forces. It tries to make space and time themselves quantum. Its supporters say it predicts that area and volume come in tiny, separate chunks, like the pixels of a screen. That could, in principle, be tested.

Both approaches can reproduce a famous calculation about the hidden disorder, or entropy, of black holes. That is encouraging, but the calculation rests on Hawking radiation, which has never been observed. Other ideas, such as supersymmetry, predict a partner particle for every known particle. But in 2022, a decade after the Higgs discovery, CERN reported no statistically significant hints of new particles at the Large Hadron Collider.

Extra dimensions turn up in other ideas, too. The book behind this magazine explores one: author Ricardo Maldonado’s HD-Blast hypothesis, his own unconfirmed proposal that our hot early universe may be the local aftermath of a brief higher-dimensional blast. It is not a candidate theory of everything, and it has not been confirmed. Our explainer on how to test an extra dimension shows what checking ideas like this involves.

What do we still not know?

We do not know which approach, if any, is right. We do not even know whether one final theory exists. Researchers do not all mean the same thing by the goal. String theorists seek one theory for all four forces. Others seek only to make gravity quantum. And the Standard Model leaves other big questions open, including what dark matter is and why there is more matter than antimatter.

Even a complete theory would describe the basic rules of nature, not everything a person cares about. Questions of meaning, value and belief involve philosophy and personal conviction as well as science.

What would a real test look like?

A real test is a risky prediction, made before looking, that could turn out wrong. Here are the kinds of tests physicists run:

• Protons that decay. Grand unified theories predict that protons are not quite eternal. The simplest version predicted decay fast enough to see. Searches in the 1980s found none, and Japan’s Super-Kamiokande detector later ruled that version out. By 2016, Super-Kamiokande had shown that protons last, on average, at least 10³⁴ years. That is a 1 followed by 34 zeros. Seeing even one proton decay would be huge news.
• Light from distant explosions. Some quantum gravity ideas allow the speed of light in empty space to depend very slightly on its energy. NASA’s Fermi telescope timed gamma rays from bright bursts in distant galaxies. High- and low-energy rays from one 2009 burst arrived so close together that the results disfavor a whole class of those models.
• New particles or new laws. Partner particles at colliders, or small changes in gravity at short distances, would point toward specific theories.

The encyclopedia also notes recent proposals for tabletop laboratory experiments that could test whether gravity behaves in a quantum way. Any result like these would help. A candidate that only fits old data, without risking a new prediction, has not yet earned the same confidence. We explore that difference in fitting data versus making a prediction.

For now, the dream of one theory remains a guide, not a destination. Which piece of the puzzle would you most like to see tested next?