Your ghosts are harmless. Mine would end the world in a second

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Why would the Archaeological Survey of India, an organisation whose daily concerns are stone, dates and conservation budgets, put up a board at a ruined fort telling visitors not to go in after sunset?

The fort is Bhangarh, three hours from Delhi. The official reason is almost certainly the boring one: broken ground, no lights, and an easy place to turn an ankle. But it is the closest thing an Indian government office has ever come to a supernatural advisory, and nobody reads it any other way. Tourists photograph the board. Then they leave before dark.

I get asked about this. Somebody's uncle finds out there is a physicist at the table, works politely through the small talk, and then arrives at what he actually wanted to ask. Do you believe in ghosts? The honest answer is yes. I work with them. They have names, properties, and a fixed place in the calculation.

They are also, in their genuine form, so much worse than anything at Bhangarh that the comparison is almost unfair.

Start with the tame ones.

In the late 1960s two Soviet physicists, Ludvig Faddeev and Victor Popov, were stuck. The equations describing how particles shove each other around kept generating rubbish, extra wobbles that existed only because of the way the description had been set up. Double-entries in a badly kept ledger.

So they cheated, beautifully. They invented a particle whose entire job was to cancel the rubbish out.

It cannot exist. Its properties contradict each other: it behaves like a particle with no spin while obeying rules meant for things like the electron, and nothing in nature is permitted to be both. It has never turned up in an experiment and never will, because there is nothing there to find.

They called it a ghost. The name stuck. It appears in the middle of the working, quietly deletes the nonsense, and is gone before the answer comes out the other end. Every prediction our best theory of matter has ever got right, and it has got a great many right to more decimal places than almost anything else in science, passed through a stage where an imaginary particle was doing the tidying up.

That is the harmless kind. Now, the other kind. Everything we know says the universe has a floor. There is a lowest amount of energy a thing can have, and once it is there, it stops. This is why empty space stays empty, why a stone left alone stays on the ground, and why you are not, as you read this, spontaneously bursting into flame.

A real ghost carries negative energy. It takes the floor away. Once negative energy is available, empty space can produce a ghost and an ordinary particle in the same instant, the two energies cancelling to nothing. Nothing is stolen. No law is broken. And having managed it once, there is nothing to stop it happening again, everywhere, endlessly, because there is no shortage of nothing to make it from.

The universe would not slowly decay. It would go up like a match. So when a physicist says she does not believe in ghosts, she is not being tiresome about evidence. She is saying something far larger. One real ghost, anywhere, at any point in the last fourteen billion years, and there would be no galaxies, no Rajasthan, no fort, and nobody standing in it being frightened.

You being alive to read this is a better argument against ghosts than any night anyone has ever spent in a haunted building with a camera. And now the twist, which is my favourite thing in all of physics.

Negative energy is real. We have measured it. Take two metal plates and hold them a whisker apart in a vacuum. Empty space, it turns out, is not empty. It hums and fidgets constantly. But the gap between the plates is too narrow for all of that fidgeting to fit, so the space between them ends up holding less energy than properly empty space does.

Less than nothing. And the plates feel it: a real force pulling them together, measured in a laboratory in 1997, and now a headache for engineers building very small machines whose parts keep sticking to each other.

Stephen Hawking's famous result about black holes runs on the same fuel. A black hole shrinks because something with negative energy falls in. Every black hole in the sky is slowly dying on a diet of less-than-nothing.

Our gravitational wave detectors use it deliberately, in the form of specially prepared light, to hear black holes collide a billion light years away. So will the one being built at Hingoli, in Maharashtra.

So why hasn't everything exploded?

Because the universe lends, but it does not give. In the 1990s Larry Ford and Thomas Roman worked out the terms, and they read like the fine print of a moneylender with no interest whatsoever in your circumstances. You may go into the red. The deeper you go, the faster you must come out. And you always repay more than you borrowed.

People have spent thirty years looking for a loophole. Nobody has found one. Which is a real shame, and here is why.

If you ever want to open a wormhole, the shortcut across space that every film about reaching another star quietly assumes, you need negative energy to hold the thing open. Same for a warp drive. Same for very nearly every idea anyone has ever had about getting somewhere interesting within a human lifetime.

All of them need a ghost. We spend our careers scrubbing these things out of our equations, and then the moment we want to leave the solar system, they are the first thing we ask for. There is one last one, and it is the only haunting I know of with real evidence behind it.

Dark energy, the thing steadily pushing the universe apart, is assumed to be steady. If it turns out to be even slightly stronger than steady, then it is a ghost in the technical sense, and the expansion eventually pulls apart galaxies, then solar systems, then atoms. Recent survey data has hinted, uncomfortably, in that direction. Nobody is certain. Ask again in 10 years.

So, no. I would not worry about the fort. I would worry about the sky.

Nishant Sahdev is a physicist at the University of North Carolina at Chapel Hill. He chases ideas that refuse to stay in laboratories, for your favourite Sunday mid-day.