The end of space and time
To better understand the universe, we may need to kill off Einstein's famous idea
As in history, revolutions are the lifeblood of science. Bubbling undercurrents of disquiet boil over until a new regime emerges to seize power. Then attention turns to toppling the new ruler. The king is dead, long live the king. This has happened many times in the history of physics and astronomy. First we thought the Earth was at the centre of the Solar System – an idea that stood for over a thousand years. Then Copernicus stuck his neck out to say we are just another planet orbiting the Sun. Despite much initial opposition, the old geocentric picture buckled under the weight of evidence from the newly invented telescope.
Then Newton came along to explain that gravity is why the planets orbit the Sun. He said all objects with mass have a gravitational attraction towards each other. According to his ideas we orbit the
Sun because it is pulling on us, and the Moon orbits Earth because we are pulling on it. Newton ruled for two-and-a-half centuries before Albert Einstein turned up in 1915 to usurp him with his general theory of relativity. This new picture neatly explained inconsistencies in Mercury’s orbit, and was famously confirmed by observations of a solar eclipse off the coast of Africa in 1919.
Instead of a pull, Einstein saw gravity as the result of curved space. He said that all objects in the universe sit in a smooth, four-dimensional fabric called space-time. Massive objects such as the Sun warp the space-time around them, and so Earth’s orbit is simply the result of our planet following this curvature. To us that looks like a Newtonian gravitational pull. This space-time picture has now been on the throne for over 100 years and has so far vanquished all pretenders to its crown. The discovery of gravitational waves in 2015 was a decisive victory, but like its predecessors, it too might be about to fall. That’s because it is fundamentally incompatible with quantum theory.
The quantum world is notoriously weird. Single particles can be in two places at once, for example. Only by making an observation do we force it to ‘choose’. Before an observation we can only assign probabilities to the likely outcomes. In the 1930s Erwin Schrödinger devised a famous way to expose how perverse this idea is. He imagined a cat in a sealed box accompanied by a vial of poison attached to a hammer. The hammer is hooked up to a device that measures the quantum state of a particle. Whether or not the hammer smashes the vial and kills the cat hinges on that measurement, but quantum physics says that until such a measurement is made the particle is simultaneously in both states, which means the vial is both broken and unbroken and the cat is alive and dead.
Such a picture cannot be reconciled with a smooth, continuous fabric of space-time. “A gravitational field cannot be in two places at once,” says Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies. According to Einstein, space-time is warped by matter and energy, but quantum physics says matter and energy exist in multiple states simultaneously – they can be both here and over there. “So where is the gravitational field?” asks Hossenfelder. “Nobody has an answer to that question. It’s kind of embarrassing,” she says.
Try and use general relativity and quantum theory together and it doesn’t work. “Above a certain energy you get probabilities that are larger than one,” says Hossenfelder. One is the highest probability possible – it means an outcome is certain. You can’t be more certain than certain. Equally, calculations sometimes give you the answer infinity, which has no real physical meaning. The two theories are therefore mathematically inconsistent.
So, like many monarchs throughout
history, physicists are seeking a marriage between rival factions to secure peace. They’re searching for a theory of quantum gravity – the ultimate diplomatic exercise in getting these two rivals to share the throne. This has seen theorists turn to some outlandish possibilities.
Arguably the most famous is string theory.
It’s the idea that subatomic particles such as electrons and quarks are made from tiny vibrating strings. Just as you can play strings on a musical instrument to create different notes, string theorists argue that different combinations of strings create different particles. The attraction of the theory is that it can reconcile general relativity and quantum physics, at least on paper. However, to pull that particular rabbit out of the hat, the strings have to vibrate across eleven dimensions – seven more than the four in Einstein’s space-time fabric. As yet there is no experimental evidence that these extra dimensions really exist. “It might be interesting mathematics, but whether it describes the spacetime in which we live, we don’t really know until there is an experiment,” says Jorma Louko from the University of Nottingham.
Other physicists have turned to an alternative called loop quantum gravity (LQG). They can get the two theories to play nicely if they do away with one of the central tenets of general relativity: that space-time is a smooth, continuous fabric. Instead, they argue, space-time is made up of a series of interwoven loops – it has structure at the smallest scales. This is a bit like a length of cloth. At first glance it looks like one smooth fabric. Look closely, however, and you’ll see it is really made of a network of stitches. Alternatively, think of it like a photograph on a computer screen: zoom in and you’ll see it is really made of individual pixels.
The trouble is that when LQG physicists say small, they mean really small. These defects in space-time would only be apparent on the level of the Planck scale – around a trillionth of a trillionth of a trillionth of a metre. That’s so tiny that there would be more loops in a cubic centimetre of space
“If we understand the quantum structure of space-time better, that will have an impact on future technologies”
Sabine Hossenfelder
than cubic centimetres in the entire observable universe. “If space-time only differs on the Planck scale then this would be difficult to test in any particle accelerator,” says Louko. You’d need an atom smasher 1,000 trillion-times more powerful than the Large Hadron Collider (LHC) at CERN. How can you detect space-time defects that small? The answer is to look across a large area of space.
Light arriving here from the furthest reaches of the universe has travelled through billions of light years of space-time along the way. While the effect of each space-time defect would be tiny, over those distances interactions with multiple defects might well add up to a potentially observable effect. For the last decade astronomers have been using light from far-off gamma-ray bursts to look for evidence. These cosmic flashes are the result of massive stars collapsing at the ends of their lives, and there is something about these distant detonations we currently cannot explain. “Their spectrum has a systematic distortion to it,” says Hossenfelder. But no one knows if that is something that happens on the way here or if it’s something to do with the source of the bursts themselves. The jury is still out.
To make progress we might have to go a step further than saying space-time isn’t the smooth,