A diagram to demonstrate parity conservation and parity violation. In parity violation, mirror symmetry is not observed, so the reflected figure does not raise its arm.

A special thanks to Dr. Maloy Das for making this article possible. Das is a consultant surgeon with the Saolta University Healthcare Group in the Republic of Ireland. He has previously worked as a doctor in the United Kingdom, Canada, and South Africa, and he has a lifelong interest in medical humanities, history, and writing

If I asked you to explain the difference between right and left to a friend, you would probably show them your hands and say which is which. In this instance, right and left can only be defined by their relation to each other, and you are showing them how they are different.

Let’s make things harder: now explain right and left to a friend using language alone and without using the words right and left. As you can only use language, you can’t show your hands or use pictures!

It’s tricky, isn’t it? The difference between right and left isn’t as straightforward as it seems. If we dig a little deeper, we will find that the science behind right and left is surprising, complex, and profound.

Kant, Explain It

How can two things be identical yet different at the same time? This was the question that puzzled one of humankind’s greatest thinkers, Immanuel Kant.

Many of the great debates of the Scientific Revolution during the 16th and 17th centuries concerned the nature of space. The English polymath Sir Isaac Newton proposed that space was absolute: space is an entity in itself and exists even without objects, matter, or living beings filling it.

In contrast, Gottfried Leibniz, Newton’s bitter rival, argued that space was relational: it only existed because of the relations between the objects that fill it. If objects do not exist, then space doesn’t either.

Meanwhile, Immanuel Kant used handedness to give his two cents. He asked us to imagine a solitary hand floating in an otherwise completely empty space. The hand must either be a right hand or a left hand, and this will be the case even in a space where no relationships between objects can be observed. Kant noted that our hands are geometrically and mathematically identical in every way possible, whether it be the lengths of the fingers or the angles between them. Yet, the one fundamental difference between them—that one is a right hand, and the other is a left hand—exists in itself; it is intrinsic to the hand and not related to any other object, similar to space itself. Space has an absolute property.

Ultimately, Kant’s theories of handedness were not foolproof and could not be used to prove that space is absolute. Indeed, Kant would switch between the Newtonian and Leibnizian schools of thought during his lifetime. However, Kant did show just how puzzling and difficult it is to explain why right hands and left hands are identical but different. That intrinsic quality of handedness is almost impossible to explain without showing, and this is the root of the Ozma Problem.

The Ozma Problem

In 1960, Project Ozma was launched in West Virginia. Named after the ruler of the fictional Land of Oz, Project Ozma was a huge telescope that listened for signals from space, signals that could be proof of extraterrestrial intelligence. Unfortunately, the project only ran for a few months, and it had no major success.

Let’s say the telescope had picked up these signals. How would we on Earth respond? We would need to convert their signals, after which we would send our own. Telescopes and computers use binary code. And directionality is crucial to understanding binary, as it is read left to right and decoded right to left. So, if we are sending binary signals to aliens, we need to be sure they understand which direction is left and which is right. How can we be sure they share our understanding of directions?

This is the Ozma Problem, a thought experiment first described by Martin Gardner in his 1964 book, The Ambidextrous Universe. In this book, Gardner pitched a number of solutions.

Before going into Gardner’s work, here’s a seemingly simple solution: lay your palms face down on a table and equally spaced from your body. The thumb that’s closer to your heart? That’s the left side. The right side is defined by the thumb farther away from the heart.

Another potential solution would be to use north and south as reference points: when facing north, everything towards east is the right side, and everything pointing west is the left side.

The problem with these solutions is that they both rely on a shared point of reference, like the direction of north-south-east-west and the location of the heart. In no way can we be certain that an alien species would share these!

Some of the solutions that Gardner proposed in his book use magnetic fields, planetary rotation, and the direction of current flow. And as we discussed before, they all fail because of the need for a shared point of reference.

So, after centuries of wondering whether we are alone in the universe, we finally make contact with an alien species, only to find that our inability to explain something as mundane as right and left precludes meaningful dialogue. The Ozma Problem demonstrates the limits of our language, and it challenges anthropocentrism, which is the notion that human beings and our experiences are the center of the universe.

The Ozma Problem Solved

Many thought problems are hypothetical and can’t be solved, but the Ozma Problem does have a solution. In fact, the solution already existed when Gardner first described it. But it’s not immediately associated with right-left asymmetry or aliens.

While we cannot be sure that aliens share our anatomy or our perception of north-south-east-west, if they inhabit the same universe as us, we can assume the fundamental forces of physics apply to them too.

There are four fundamental forces of physics: gravity, electromagnetism, strong nuclear forces (the force that binds atomic nuclei together), and weak nuclear forces (the force that causes atomic decay).

Up until 1956, it was assumed these fundamental forces all display parity. Parity is an important concept in physics, and it can be demonstrated visually by using a mirror. If we stand in front of a mirror holding an apple in our right hand and then drop it, the reflection will show it falling to the ground, but the apple will fall from your left hand. Gravity still works in the reflection. Likewise, if we look at the strong forces binding atomic nuclei and then observe them in a mirror, the images would be identical, just with right and left switched.

But in 1956, a physicist named Professor Chien-Shiung Wu conducted a ground breaking experiment. She was able to prove that the weak nuclear force—the decay of atoms—did not always demonstrate parity. The weak nuclear force does not adhere to mirror symmetry.

Professor Wu showed this by observing the decay of cobalt-60 atoms. When atoms decay, they spin out electrons. Up until then, scientists had always observed these electrons spinning out equally in all directions. But Professor Wu saw that cobalt-60 will always preferentially spin out electrons in a certain direction. In other words, the movement is asymmetric. For some reason, the decay of atoms is the one fundamental force that does not adhere to parity or mirror symmetry, thus showing that directionality is intrinsic to the universe, just as Kant had postulated in the 18th century.

For the first time in history, it was proven that nature can prefer one direction. Very soon after Wu’s findings, physicists were able to prove that elementary particles known as neutrinos always spin towards the left.

What does this mean for our communication with aliens? If the aliens can replicate Professor Wu’s experiment and visualize the spin of electrons while cobalt-60 decays, they can orient right and left!

Ironically, Professor Wu was not afforded any sort of parity herself during her working life. Other scientists were recognized for research that could not have been achieved without hers. Today, the weak force remains one of the most important and mysterious topics in physics today, thanks to Professor Wu.

Conclusion

So, if the only way to scientifically and definitively define the difference between right and left is to build a particle accelerator and observe the decay of cobalt-60, clearly the difference is not as straightforward as it may first seem! The Ozma Problem is proof that the most mundane concepts are sometimes directly linked to the cosmos and speak to the nature of existence itself.