In 1848, Louis Pasteur observed tiny grains of salt under a magnifying glass and was able to distinguish two crystalline shapes. They looked almost identical – but not quite. The two crystals had a symmetry of reflection: they were “to each other what an image is, in a mirror, to the real thing”.

It was suggested that Pasteur was attentive to this subtle symmetry because of his experience as an artist. As a teenager, he made lithographs in which an image engraved on a stone slab is transferred to paper, creating a mirror version. (At least once, he used a mirror to ensure that a portrait would be acceptable when inverted.) The visible difference between crystals, Pasteur reasoned, came from an invisible difference in their molecules.

Many molecules can exist in two reflective ways, each a strange twin of the other. Symmetry is similar to how a right hand and a left hand look similar to each other in a mirror but can never overlap. This “handiness” is known as the chirality of a molecule (from the Greek word for hand). Not all molecules are chiral: water and methanol, for example, are both symmetrical.

However, many of the key molecules of life exist only in a single chirality and assemble into structures that reveal this distinction, much like Pasteur crystals, in one of the most compelling evidence of the common origins of all life on earth. If we walked through the mirror into the world of mirrors, we would be able to know.

The DNA double helix, for example, is straight. (Think of it as a spiral staircase where going down the stairs means turning right.) In the mirror world, it would be left-handed. The inverted version does not exist in reality, although images of it appear in many popular science books or in company logos. A biologist at the National Institutes of Health in Maryland maintains a Hall of Fame for blatant images of impossible left-handed DNA, including one blanket of Natural biotechnology last year.

In the mirror world, your mirrored self would have mirror cells containing mirror DNA, coiled into left-handed helices. But we don’t need to travel to the mirror world to obtain mirror life. In principle, nothing in the laws of physics or chemistry seems to prevent us from constructing mirror organisms. We could apply Pasteur’s reasoning in reverse, starting from mirrored biomolecules and ending up with creatures that operated the widdershins.

It would be very difficult. But creating bizarre bacteria has become an essential part of biotechnology. recoding their genomes turning one species into another. From this point of view, a mirror bacteria constitutes an attractive challenge. Indeed, it is an idea that has already received funding: in 2019, the National Science Foundation in the United States gave $4 million to a research group that hoped to “design, build, and safely deploy mirror cells.” Other donors have contributed to similar efforts. None have yet succeeded, and the suite of technological developments this would require are likely decades away. But as an international consortium of scientists pointed out in a technical report and a article published in Science last week, there’s good reason to pause before rushing through the looking glass.

We know that mirror molecules cause differences that are not just visual. They can be trivial: a molecule the smells like caraway while its mirrored form smells like spearmint. But they can also be fatal: the drug thalidomide exists in two forms: one is a mild sedative but the other causes serious birth defects. We can’t be sure what would happen with a mirror bacteria, but it would likely be closer to the thalidomide end of the mirror hazard spectrum.

A mirror bacteria would not need mirror food. It would be able to survive on achiral molecules such as water and glycerol – and the human body contains plenty of them. But the worrying asymmetry is that our immune system is chiral, because it evolved to target real bacteria. As the report puts it, many immune mechanisms “therefore appear unlikely to function properly against mirror bacteria.” A mirror bacteria could pass through the net.

It would also not be affected by current vaccines and most antibiotics. We could probably create new ones, but that would take time and they could in turn interact in problematic ways with our own cells. A mirror bacteria would not need to be actively pathogenic to kill us: it could do so simply by growing out of control.

And not just us. While normal life must compete with an entire ecosystem, a mirror bacteria could behave like the only real thing in a world of ghostly reflections. Normal organisms are controlled by an ecological balance between their death rate and their growth rate. But without any predators, mirror bacteria escaped from a laboratory could grow exponentially, even at a lower growth rate than normal bacteria.

The technical report is necessarily speculative about how a yet uncreated entity would interact with the world we know. But, over two hundred pages, he goes into plausible and disturbing detail, with chapters not only on human infection but also on infections in animals and plants, as well as the difficulty of containment. Before reading it, I had never considered many of these risks. Neither do most of the scientists who wrote it, who confess that “several authors of this report have long considered the possibility of creating mirror cells.”

It is clear that they have changed their minds. Today, they believe that mirror organisms should never be created. “Although we were initially skeptical that mirror bacteria could pose major risks,” they write, “we became deeply concerned.” These words echo those of the MAUD committee scientists who wrote a report in 1941 on the possibility of an atomic bomb: “We approached this project with more skepticism than conviction… as we progressed, we became more more convinced.

Creating a mirrored life may remain impossible for unknown reasons. But in the coming decades, it is at least conceivable that we can achieve this if we try hard enough. The report makes a compelling argument for stopping these efforts. For more than four billion years, there has been no mirror life. The authors call its dangers “unprecedented.” An overused word – but here literally true.