Tonight I feel like indulging in a little Quantum physics 101. There’s something in quantum physics which is always fascinating: the double-slit experiment.

Take a light source, like a light bulb. Across the room is photosensitive paper which will get dark if exposed to light. Between them is an opaque wall. In the wall there is a tall, thin slit that can be opened and closed.

You open the slit. As expected, it will expose a section of the photosensitive paper behind it and you’ll get a tall, thin exposure there.

Now, imagine there’s a second slit. Do the experiment again, but this time open both slits at the same time. You might expect that you will expose two slits on the photosensitive paper, one for each opening in the wall. But in reality, you get a series of exposures on the photosensitive paper, as if there were many slits.

This is because when energy travels, it does so as a wave, like waves in a pond. If you drop two stones into a pond, waves go forth, and when the two sets of waves meet, they form an interference pattern. Same thing with the light through the two slits; the light pouring through each slit comes through in a wave form, like the waves from the stones in the pond; the waves interfere with each other, and so form the extra exposures.

Okay, so far we’ve only determined that energy travels as a wave. You’ve heard about “radio waves” your whole life, so this is not too impressive. But when further experiments are done, weird things begin to happen.

The idea of the interference pattern is that large numbers of energy packets are flowing through both slits, so obviously (or so one would think) the interference pattern comes from different light waves coming through each slit. Light wave A travels through one slit, and light wave B travels through the other slit; A and B meet on the other side and interfere with each other.

So someone decided to carry out this experiment with a variation: instead of using a light bulb, a different energy source was used: one that would emit just one packet of energy at a time, a packet of energy that could not be divided. Each packet would have to travel through one slit or the other, with no other packets to interfere along the way. After letting this experiment run its course and enough of packets of energy to pass through both slits, one would expect to see only two exposures, one for each slit, on the photosensitive paper on the far side of the wall with slits.

Except that’s not what happens. Instead, you get the same multiple-exposure interference pattern. And that’s where the weirdness comes in: if only one packet of energy, only one “wave” of light, is issued at one time, what could be interfering with it?

The answer: the energy is interfering with itself.

According to the theory, when you issue a packet of energy and let it go flying across the room, it does not simply follow a single straight path across the room. It takes every path across the room, simultaneously. In the case of the double-slit experiment, the energy packet does not just pass through slit A or slit B, it passes through both slit A and slit B.

But how can one piece of energy go through two slits? Here’s where it gets hairy: every possible path the light could possibly take is taken by a ghost-like probability of the energy packet, each probability nonetheless being “real” enough to interfere with other probabilities. It is as if a near-infinite number of probable versions of that energy packet take off across the room, interfering with each other all along the way. When the packet reaches the photosensitive paper and is forced to make an observable record of its location, the multitudinous probabilities collapse into the one most likely probability, and the impact of the energy packet on the photosensitive paper is recorded.

What’s more weird is what makes the probabilities collapse: observation. Place a detector at each slit that can observe the energy packet going through the slit–before it has a chance to interfere with itself on the other side–and the multiple exposures disappear, leaving only two exposures on the photosensitive paper. Even a detector over just one slit is enough to collapse the probabilities into single, definite paths–somehow the energy packets going through the unobserved slit “know” that observation is going on at the other slit, and behave accordingly.

You’ve probably heard of Schrödinger’s Cat; this thought experiment expresses what is explained above. I prefer to simplify it as a coin-in-a-box. Flip a coin in the air so it will land in an opaque, soundproofed box. When it drops below the upper rim of the box, slap a cover onto the box. You cannot hear where the coin went, how long it bounced around, and being human, you can’t predict which side will come up, heads or tails, just from watching the trajectory and the rate of spin of the coin going into the box. To make absolutely sure, flip the coin in the dark, so you can’t possibly predict how it landed.

The common-sense expectation is that the coin has landed in the box, and is either heads-up or tails-up, and it has a specific location in the box. We assume these things are decided just as if we were watching them, except in this case, we simply haven’t seen the results yet.

But the quantum theory expectation is far different. Once we stop observing the coin, it no longer exists as an object with a specific location. Instead, it exists as a cloud of probable-coins. Half of those are heads-up, and half are tails-up, aside from a small number of probable-coins that rest on their sides along the edges of the box. Probable-coins exist in all possible locations within the box. It’s really just one coin, expressed as a cloud of probable outcomes. And it stays that way until we open the box and observe it–at which time the probable-coins collapse into the most likely probability, which we observe.

That idea really messes with your head. It gives a whole new interpretation to the question, “if a tree falls in the forest and no one is there to hear it, does it make a sound?” According to this theory, if no one is there to hear it, the tree doesn’t even fall. Just probable-versions of the tree fall, and what sound they do or do not make, I won’t venture to guess.

Einstein didn’t like this–probably for many different reasons, but at least because the theory implies that different probable-versions of a particle somehow instantaneously communicate with each other. That is, when the probable-versions collapse, they don’t all shout “Olly Olly Oxen-Free!” to each other. When an observation is made, they simply all collapse, regardless of distance, and that violates classical expectations of how things work.

Einstein tried to disprove this “spooky action at a distance,” but in fact, an experiment performed in Switzerland has shown it to be real, where two “twinned” photons shared information instantaneously at a distance of ten kilometers. We now know that two intertwined particles can share information without paying attention to the speed of light. However, because of the way things work, we cannot use this as a means of faster-than-light communication.

This one effect aside, the whole situation raises bigger questions: is a conscious observer required, and if so, then is consciousness tied in with the fabric of the universe? Is nothing real until it is observed by life forms capable of being observers?

Those into New-Age spiritualism will often might conclude that this is proof of the soul. Scientists will tell you that these are abstractions created by mathematical attempts to explain observable phenomena and do not necessarily translate into real-world conclusions that New-Agers might subscribe to. Science fiction writers use this as a launching point to travel to all sorts of possible explanations, fiction made more fascinating because of its “grounding” in real physics.

In the end, we don’t know what the hell is going on. But it is fun as hell to speculate, to travel to those possible realities, and to know that–to paraphrase J. B. S. Haldane–the universe is not only stranger than we imagine, but it is stranger than we can imagine.