Seeing without actually seeing is a real thing.
We can't very well see with our eyes closed or otherwise covered, and that seems more or less fundamental. But what if it were possible to see without that usual interaction between matter, light, and photosensitive surface? According to new research in quantum physics, that's possible—to a tiny degree, sure, but a measurable one.
Photography, put as simply as possible, is the practice of capturing light. For a fraction of a second, a photosensitive element is exposed to some slice of the world, ideally a slice flooded by photons, the packets/particles tasked with carrying electromagnetic force (featuring: visible light). Photons crash into "things" and those things in turn kick out their own photons at different wavelengths.
And so we are able to see (and photograph) the world because the world radiates energy via these photons. Our visual sense of everything depends on the particles emitted by that everything interacting with our eyes (or the "eyes" of a camera).
knowledge can be extracted by, and about, a photon that is never detected
What if we could skip a step? I don't mean this in the sense of plugging directly into neurons in the brain, but more literally: an image transmitted from object to eye, but without the direct photonic courier.
This is possible thanks to a peculiar quirk of quantum physics allowing, in a very limited sense, the seeming transmission of information between separated elements of the same physical system, which we would call entangled elements. According to work published this week in the journal Nature, this entanglement is exploitable such that it's possible to derive information from a photon that's never interacted with the object to be imaged, so long as it's in an entangled state with one that has.
To start, imagine a setup with two pathways down which photons may travel. One features the object to be photographed and the other doesn't.
In the weirdo scheme of quantum physics, it holds that if a particle is sent down a pathway with some split in it, with the particle able to take one of two possible routes before the two branches recombine, we don't have to say that the particle ever "chose" one of those routes.
So long as no measurement is performed on the particle during its journey, it will have effectively taken both routes. This is the literal version of Schrödinger's dead/alive cat.
In the first path, one photon in the pair passes through the object to be imaged, and the other does not. The photon that passed through the object is then recombined with its other 'possible self'—which travelled down the second path and not through the object — and is thrown away. The remaining photon from the second path is also reunited with itself from the first path and directed towards a camera, where it is used to build the image, despite having never interacted with the object.
One might imagine this in an advanced medical or biological imaging system. As the Nature post notes, such a system enables something to be probed with very low-energy photons, with the results being reflected in much higher energy particles at the viewing end.
"This enables the probe wavelength to be chosen in a range for which suitable detectors are not available," the paper says.
Imagine, for example, a camera loaded with some super-slow speed film, the sort best suited for very bright conditions. In this scheme, we might be able to take a decent photo of some dim room because we no longer need to depend on the room providing higher-energy particles to match the film.
Because if, in the paper's words, "knowledge can be extracted by, and about, a photon that is never detected," all sorts of bizarre things might become possible. Or, rather, they are already possible. The current study isn't a description of some theorized possibility, but a description of an actual prototype. In homage to Erwin Schrödinger, the object first photographed using the new technique was, naturally, a cat.