Will a Pear-Shaped Nucleus Ruin Time Travel?


Physicists have confirmed the existence of a new form of atomic nuclei, and the fact that it’s not symmetrical challenges the fundamental theories of physics that explain our Universe.

But that’s not as bad as it sounds, because the discovery could help scientists solve one of the biggest mysteries in theoretical physics – where is all the dark matter? – and could also explain why travelling backwards in time might actually be impossible.

“We’ve found these nuclei literally point towards a direction in space. This relates to a direction in time, proving there’s a well-defined direction in time and we will always travel from past to present,” Marcus Scheck from the University of the West of Scotland told Kenneth MacDonald at BBC News.

So let’s back up here, because to understand this new form of atomic nuclei, you have to get to know the old ones first. Until recently, it was established that the nuclei of atoms could be one of just three shapes – spherical, discus, or rugby ball.

These shapes are formed by the distribution of electrical charge within a nucleus, and are dictated by the specific combinations of protons and neutrons in a certain type of atom, whether it’s a hydrogen atom, a zinc atom, or a complex isotope created in a lab.

The common factor across all three shapes is their symmetry, and this marries nicely with a theory in particle physics known as CP-Symmetry. CP-symmetry is the combination of two symmetries that are thought to exist in the Universe: C-Symmtery and P-Symmetry.

C-Symmetry, also known as charge symmetry, states that if you flip an atomic charge to its opposite, the physics of that atom should still be the same. So if we take a hydrogen atom and an anti-hydrogen atom and mess with them, both should respond in identical ways, even though they have opposite charges.

P-Symmetry, also known as Parity, states that the the spatial coordinates describing a system can be inverted through the point at the origin, so that x, y, and z are replaced with −x, −y, and −z.

“Your left hand and your right hand exhibit P-Symmetry from one another: if you point your thumb up and curl your fingers, your left and right hands mirror one another,” Ethan Siegel from It Starts With a Bang explains.

CP-Symmetry is a combination of both of these assumptions. “In particle physics, if you have a particle spinning clockwise and decaying upwards, its antiparticle should spin counterclockwise and decay upwards 100 percent of the time if CP is conserved,” says Siegel. “If not, CP is violated.”

The possibility that the Universe could actually violate both C-Symmetry and CP-Symmetry is one of the conditions that have been proposed to explain the mystery of antimatter in the Universe. But proving that would mean the Standard Model of Physics needs a serious rethink.

According to the laws of physics, at the time of the Big Bang, equal amounts of matter and antimatter had to have been created, but now, billions of years later, we’re surrounded by heaps of matter (solid, liquid, gas, and plasma), but there appears to be almost no naturally occurring antimatter.

“This is a puzzling feature, as the theory of relativistic quantum mechanics suggests we should have equal amounts of the two,” mathematician Gianluca Sarri from Queen’s University Belfast in the UK writes for The Conversation. “In fact, no current model of physics can explain the discrepancy.”

Okay, so back to our atomic nuclei shapes. Most of our fundamental theories of physics are based on symmetry, so when physicists at CERN discovered an asymmetrical pear-shaped nucleus in the isotope Radium-224 back in 2013, it was a bit of a shock, because it showed that nuclei could have more mass at one end than the other.

Now, three years later, the find has been confirmed by a second study, which has shown that the nucleus of the isotope Barium-144 is also asymmetrical and pear-shaped.
“[T]he protons enrich in the bump of the pear and create a specific charge distribution in the nucleus,” Scheck told the BBC. “This violates the theory of mirror symmetry and relates to the violation shown in the distribution of matter and antimatter in our Universe.”

While physicists have suspected that Barium-144 has a pear-shaped nucleus for some time now, Scheck and his team finally figured out how to directly observe that, and it turns out its distortion is even more pronounced than predicted.

So what does all of this have to do with time travel? It’s a pretty out-there hypothesis, but Scheck says that this uneven distribition of mass and charge causes Barium-144’s nucleus to ‘point’ in a certain direction in spacetime, and this bias could explain why time seems to only want to go from past to present, and not backwards, even if the laws of physics don’t care which way it goes.

Of course, there’s no way of proving that without further evidence, but the discovery is yet another indication that the Universe might not be as symmetrical as the Standard Model of Physics needs it to be, and proving that could usher us into a whole new era of theoretical physics.

The study has been published in Physical Review Letters, and can be accessed for free at arXiv.org.

This article first appeared in Science Alert


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