If you look at the number of stars for long enough, you will probably start to wonder the same thing that scientists and philosophers have for years: if there are so many potential worlds out there, where is everybody, and why is nobody getting in touch?
Solutions to what is known as the Fermi Paradox range from the horrifying to the really horrifying. But a new pre-print paper (not yet peer-reviewed) by a quantum physicist at Imperial College London proposes a new possibility: what if alien messages are hidden right there the stars themselves?
Terry Rudolph suggests in his paper that if aliens should wish to communicate vast distances without alerting others to their messages, they might be able to do so in a way that is indistinguishable from thermal radiation for any other species listening to the skies. The basic principle is that species that have spread out among the stars could then send messages by entangling photons in separate stars, altering the light that is given off by one star by interfering with another. The receiver could then check in on their message by observing the second star, using linear optics.
A quick refresher from Science ABC.
"Photons can propagate billions of light-years and retain significant quantum coherence," Rudolph writes in the paper. "One consequence is therefore that a sufficiently advanced civilization can perform quantum non-demolition measurements of photon number on suitable modes of light being emitted from stars, in such a way that useful large-scale entanglement is distributed by the subsequent free-space propagation of that light through the universe."
The method, he believes, would be particularly appealing to paranoid aliens due to how difficult it would be to distinguish from normal thermal signatures.
"The upshot is that when we look to the stars and see only thermal radiation we typically conclude the universe is empty," he writes. "But perhaps, riding in the correlations of that radiation, the universe is actually bathed in alien chatter and other forms of distributed quantum information processing."
Though the (incredibly complicated) method he suggests is possible according to the laws of physics as we understand them, he is not suggesting that this is a way that aliens are communicating, just that it's a method by which they could communicate. In fact, disappointingly, the only way we may know that technically advanced aliens are using stars as an intergalactic WhatsApp is... if they tell us that.
"Unfortunately this is all fundamentally hidden from us if quantum theory is correct. And if it isn’t correct, then presumably the aliens know that and so are not using this method," Rudolph writes.
"It seems, therefore, that the only way to test this hypothesis is to wait for them to drop by (again?) and let us know which case pertains."