Quantum mechanics isn’t usually a breakfast table or coffee at Starbucks kind of a discussion. For many people, science fiction and the world of superheroes are more likely the first introduction points into this weird world, which is actually responsible for how we perceive everyday reality.
Suppose I said to you that you could change the outcome of something that technically had already happened? I wish I was taking about the lottery, but I’m not. I’m talking about the path a photon of light travels down. While not having such a spectacular outcome of changing a lottery result, it would still have a significant impact on how we view the universe, albeit at a quantum level. And indeed it has, because this isn’t a thought experiment, it’s real and referred to as a delayed-choice experiment in quantum mechanics.
It’s essential to understand that in the quantum world, a photon (a single ‘packet’ of electromagnetic energy - the same stuff that light bulbs push out), can behave as if they were a single ‘particle’, like a bullet from a gun, or as a wave, like the waves that propagate as ripples in a pond. Until you actually try to measure the photon, it exists in a curious state of superposition - that is to say, it exists in both states at the same time. This is one reason why the average person takes one look at quantum mechanics and walks away - it just sounds plain crazy. But, as we shall see, this is really how the universe works at a fundamental level.
In the experimental setup shown in diagram 1, a photon is sent to a splitter (1). A splitter is essentially a mirror that only reflects 50% of the light reaching it. This sends ‘half’ of the photon to one mirror and half to the other. It’s fairly easy to understand that if you shone a torch as a such a splitter, you’d see two beams emerge, each half the brightness of the source. If you fired just a single photo at the splitter, it’s hard to understand how half of the ‘bullet’ can go one way and half the other. Well, that’s because at this point, the photon is still a superposition of the two, and will remain so until it has been measured. So, half the ‘wave’ goes one way and half the other. Each beam is then directed to another splitter, which in this instance acts to combine the beams again with each other. This being the case, what you expect each detector to ‘see’?
Before I answer that question, there’s a bit of basic quantum physics you need to appreciate. It’s called the double-slit experiment and is shown in Diag 2.
It’s clear that the waves in this instance could be anything from light to water, the effect is the same. The wavelets generated by each slit interfere with each other and create areas of constructive and destructive interference - a classic interference pattern. When viewed on a screen with light, this appears as bands of light and dark. In the case of the experiment with photos, the slits are just a few microns apart.
So, easy so far? What happens then, if you fire just one photon of light at the setup? Quite naturally, you’d probably think that the ‘single’ photon would go through one or the other, right? Wrong. Even though it’s a single photon, it still manages to behave as it’s a wave, and so it still passes through both and creates the interference pattern (if a continual stream of single photons is used - a bit like firing a machine gun).
If, however, you tried to detect which slit the photon goes through, by perhaps putting some simple measuring device on one slit, the interference pattern disappears and you end up with a series of dots at the points the photons arrive on the screen. As more come through, the dots just get brighter. In short, the photon ceases to act as a wave.
Let’s get back to the original experiment, now that we know what happens to photos when they interfere with each other, or indeed, with itself as is the case of a single photon.
In this experiment, the slits have been replaced by splitters. In splitter 2, the photons are recombined into two beams - A+B and B+A. If these beams have travelled exactly the same distance, they’ll produce exactly the same interference pattern within the detectors. If you were to now increase the length of the path to detector 2 - for argument's sake, a million miles, and now seek to measure, say, the photon’s phase (position X), the interference pattern at detector two would collapse, just as it did in the double slit experiment. However, it would also collapse at detector 1. Think about that for a minute. The photon being measured (half of which went to the other detector), was measured after the other half arrived it its detector because it had to travel further. Put it another way; it’s as if the event in the future reached back in time to change the result on the other detector in the past.
First proposed by John Wheeler back in the '70s, this isn't a thought experiment anymore, as it's been verified many times in the lab.
At the heart of this is something called Quantum Entanglement. The two halves of the photon were entangled, as they originated from the same photon. The other aspect of this experiment, if the apparent time travel part wasn’t enough for you, is that the collapse of the interference pattern is instantaneous - in other words, it’s as if either the information passed to tell the ‘earlier’ photon its property (phase) travelled faster than the speed of light or, the two parts experience no distance separation. Alternatively, as the mere act of placing the measuring equipment in the path of the longer leg photon also collapses the pattern, you could suggest that the other photon knew about it already - i.e. a form of precognition. Either way, to entangled quantum mechanical particles, it seems as though either time or space doesn’t exist to them. An intriguing proposition, no?
There’s a huge amount of detail left out of this description and to say what’s written here is an oversimplification, is an oversimplification. As I said originally, this isn’t supposed to be a detailed treatise.
One last thought: At the point the universe came into existence, all the matter that currently exists in today's universe originated from the same (point?) source and is therefore fundamentally ENTANGLED. That means every single particle in your body is entangled at some level with everything else in this universe.