What is a theory of everything, and will we ever have one?

What are the four forces, and which have been joined already?

CERN’s guide to the Standard Model lists four basic forces: the strong force, the weak force, the electromagnetic force and gravity. They differ in range and strength. Gravity is the weakest, but it reaches across the whole universe. The strong force is the strongest, but it works only over very short distances, at the level of particles smaller than atoms.

Joining forces has worked before. In the 1860s, James Clerk Maxwell showed that electricity and magnetism are two sides of one force, electromagnetism. About a century later, physicists linked electromagnetism with the weak force. In 1979, Sheldon Glashow, Abdus Salam and Steven Weinberg shared the Nobel Prize in Physics for that “unified weak and electromagnetic interaction.” CERN’s page on unified forces explains that the two act on equal terms only at the high energies reached in particle collisions.

The Standard Model grew from this work. Each force in it has a carrier particle: the gluon for the strong force, the photon for light and electromagnetism, and the W and Z bosons for the weak force. The model also needed a particle called the Higgs boson. CERN’s experiments found a particle matching it in 2012, and the 2013 Nobel Prize went to François Englert and Peter Higgs for the idea behind it. CERN calls the Standard Model a well-tested physics theory.

Why don’t gravity and quantum physics fit together?

Gravity has its own great theory, Einstein’s general relativity. In it, gravity is not a pull passed along by particles. It is the bending of space and time by mass and energy. The theory keeps passing tests. On 14 September 2015, the LIGO detectors in the U.S. caught gravitational waves from two colliding black holes, waves Einstein had predicted a century earlier. That work won the 2017 Nobel Prize.

They already fit together within a limited range. An effective field theory describes gravity’s small quantum effects at energies far below the scale where those effects become strong. A complete, tested theory for extreme conditions remains missing.

The difficulty is extending those calculations beyond their reliable range. At very high energies, terms normally small enough to ignore become important. More unknown ingredients enter, and this approximation loses its predictive power. A contrast between smooth space and tiny energy packets does not explain that problem by itself.

Could gravity turn out to be made of particles, the way light is? Possibly. A graviton would be one packet of a quantum gravitational field’s energy, comparable to a photon of light. This describes an energy exchange, not space built from little balls.

LIGO measures gravitational waves without counting individual gravitons. A 2024 proposal describes individual energy exchanges a detector might register. It reports no observed graviton. A classical gravitational wave could also produce that signal, the authors explain, so it would not prove gravity is quantum.

There are other gaps, too. CERN lists questions the Standard Model does not answer, such as what dark matter is and why there are three families of particles with very different masses. Hawking counted 24 numbers in the Standard Model that have to be measured, because the theory cannot work them out. A theory of everything would ideally explain them.

What are the main candidates, and why are they so hard to test?

The Stanford Encyclopedia of Philosophy names the two most popular approaches. String theory replaces point-like particles with tiny one-dimensional strings. Different ways a string vibrates show up as different particles, and one of those vibrations acts like the graviton, the particle expected to carry gravity. The versions that include matter particles need nine dimensions of space plus one of time. Loop quantum gravity takes another path. It tries to build a quantum version of general relativity directly, and it suggests that space itself may come in tiny, separate pieces.

Symmetry guides much of this work. In physics, a symmetry is a change that leaves the rules the same. The 2008 Nobel Prize honored Yoichiro Nambu “for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics.” CERN describes the related idea that one unified force may have ruled the first instants of the universe, before its parts separated out as the universe cooled. A proposed symmetry called supersymmetry would give every known particle a partner. CERN explains that, if it is right, the strengths of the strong, weak and electromagnetic forces could match exactly at very high energies.

The big problem is testing. The Stanford Encyclopedia says that most quantum gravity theories predict changes only at energies about ten million billion times higher than the proton collisions once made at Fermilab, a U.S. particle lab. It would take an accelerator the size of a galaxy to reach that directly. It also notes that string theory has a huge number of possible versions, with no clear rule for picking the one that matches our world. Loop quantum gravity’s tiny pieces of space are often called a concrete prediction, but the encyclopedia says it is still quite unclear how anyone could actually observe them.

Could there be no final theory at all?

This is the strongest objection to the whole dream. In his 2002 lecture, Hawking said he once thought a final theory might be found quite soon. Then he changed his mind. He wondered if the laws of the universe can be written down in a finite number of statements at all. He compared this to Gödel’s theorem. As the Stanford Encyclopedia explains, the logician Kurt Gödel published a proof in 1931 showing that any consistent system of math rules that can do basic arithmetic contains statements it can neither prove nor disprove.

Hawking ended on a hopeful note. He said he was now glad that our search for understanding will never come to an end.

Not everyone agrees that Gödel settles anything for physics. Gödel’s result is about math systems, and the Stanford Encyclopedia notes that attempts to apply it in other areas of philosophy are much more controversial. Hawking offered his idea as an analogy and a reasoned hunch, not a proof. Meanwhile, the search for quantum gravity goes on, as the Stanford Encyclopedia’s long survey of it shows. So the question stays open in both directions.

What do people often get wrong?

“String theory is already the theory of everything.” It is a leading candidate with rich math, but the Stanford Encyclopedia reports no consensus and no experiments in quantum gravity yet.

“Physics is broken.” It is not. The Standard Model and general relativity each pass their tests with great precision. The gap shows up mainly in extreme settings.

“If it can’t be tested now, it isn’t science.” This is a live debate. In a 2014 essay in Nature, the scientists George Ellis and Joe Silk argued that exempting speculative theories from experimental checks undermines science. On the other side, the Stanford Encyclopedia notes that in his 2014 book String Theory and the Scientific Method, R. Dawid defends a “no-alternatives” argument: that string theory is the only way to unify gravity with the other forces, which grounds physicists’ strong belief in it.

How can you check this yourself?

• Read CERN’s own page. Its short guide to the Standard Model spells out what the theory explains and what it leaves out.
• Ask “what does it predict?” When you hear a claim that someone has found the theory of everything, ask what new measurement it predicts that nobody has made yet.
• Look at the energy gap. Compare the energies of today’s colliders with the energies where quantum gravity is expected to show up. The gap explains why the question is so hard.
• Read the longer story. The longer story Is There a Theory of Everything? tells the history of the search in more depth.
• Read Hawking’s lecture. His 2002 talk, “Gödel and the End of Physics,” is online at his estate’s website and uses almost no math.

The answer stays the same after all this detail. We have superb theories for the forces, but not one theory for all of them, and nobody knows yet if a final one exists.