"It's a really exciting time," says Dr Marcella Bona. "We are ready."

She is 100 metres below the French-Swiss border inside the biggest scientific instrument ever. The Large Hadron Collider (LHC) fires photons around a 27km tunnel close to the speed of light. Scientists smash these particles together to unlock the secrets of the Universe.

After a major upgrade, the revamped particle accelerator has narrower beams and more sensitive detectors. It allows physicists to analyse more collisions in greater detail.

Quantum physics studies the behaviour of atoms and subatomic particles. For five decades, the Standard Model has explained how particles interact via three forces of nature. These are electromagnetismThe force that holds atoms and molecules together, without which matter could not exist., the weak forceThe force that causes radioactive substances to decay. Without it, stars could not exist. that causes radioactivity and the strong forceThe force that holds together protons and neutrons in the nucleus of every atom. that binds atoms together.

The model does not explain gravity. And cannot make sense of the dark matterA hypothetical substance that can not be detected by scientific instruments. Its existence is inferred from evidence that the universe consists of more mass than we can perceive. that makes up over 80% of the Universe. Or the dark energyAn unknown form of energy causing the expansion of the universe to accelerate. that causes it to keep expanding.

When the LHC begins breaking apart protons, physicists hope to discover particles unknown to science. Evidence of a fifth force of nature could lead to a "theory of everything".

The LHC has measured strange behaviour in decaying quarksExperiments suggest some unseen particle or force is affecting how these elementary particles break down in the particle accelerator..

Dr Sam Harper is a "little bit terrified". His 20-year hunt for a fifth force may be about to end.

Researchers must reproduce these experiments thousands of times. Ten years after the LHC discovered the Higgs bosonThe subatomic particle was first theorised in the 1960s, but observed in the LHC in 2012. Its existence does not challenge the Standard Model of particle physics. particle, the world is still waiting for the long-predicted scientific revolution.

"Let's be honest: It's disappointing," says physicist Sabine Hossenfelder. She argues that scientific predictions are "guesswork" and a huge amount of public money is being wasted.

Could it also be dangerous? When the LHC was switched on in 2008, headlines warned of black holes that would devour the Earth. This did not happen. Physicists are confident it is impossible.

CERN says the unlikely creation of "quantum black holes" at the LHC would be "thrilling", helping us understand how gravity works. But they would pose no threat.

The stakes are high as Dr Harper waits for the first results of the supercharged LHC. "This would upend the field. It would be the biggest discovery in particle physics since, since..."

He is at a loss for words.

Yes: Science is on the brink of something big. These giant colliders are throwing out results that current theories can't explain. The next few years will be a momentous time for the field of physics.

No: This is the end of physics. We have reached the limit of what science can tell us about the universe. Physicists offer theories of parallel universes and fifth forces, but can't provide any proof.

Or... The Universe may reveal its secrets when we least expect it. Physicists are confident they are on the right path, but many discoveries have been accidental, like X-rays, microwaves and radioactivity.