Quantum physics deals with a realm we can't measure with comfortable instruments like tape measures or stopwatches. Most of it isn't even physical, or at least what the ordinary definition of physical is, and all we can define it with is mathematics. And the math can be very strange.
In the quantum realm, all sorts of oddities take place and are perfectly normal and one of the oddest experiments up to this point is something called the quantum eraser experiment.
Before we go any farther, you have to understand that one of the basic principles of quantum mechanics is that variables come in pairs that compliment each other. However there is a certain amount of unpredictability built in. The more precise your knowledge of one member of the pairs is, the more unpredictable the other member is.
As an example, a pair might be the position and momentum of a particle. The better we define the position of the particle, the less we know about it's actual momentum.
This never works in the world we inhabit, this macro-realm. If you want to know the position and momentum of a car, you're never prevented. You can map out exactly where the thing is and exactly what it's momentum is with no problem. GPS units do it all the time.
But particles in the quantum realm don't play by our rules. It's almost as if there's a law in effect that says the act of observation is disruptive, and in order to keep order from breaking things, the more order is forced in one spot, the more chaotic it'll become in another.
A classic experiment with light concerns a screen with 2 slits in it. When the light passes through the screen, wave-like interference patterns occur. This odd behavior, which happens with all particles, not just protons, happens even when it's just a single particle that is sent through the slit. The particle winds up in 2 places, going through 2 slits at the same time, still making the wave-like interference pattern as if it were interfering with itself. Wrap your mind around that one.
Knowledge of what path the particle is going to travel is the compliment to the appearance of the wave interference and there's a very complicated mathematical formula that explains it. Enter the uncertainty principle which says that nothing can be measured so carefully that it won't disturb the thing it's measuring, thus throwing off the results. The upshot though, is that the more carefully the particle is observed going through the slit, the less of the wave-like pattern of interference there is.
You might think about that the next time you stretch a tape measure across a board and mark where you're going to cut. Are you SURE that line's in the right spot?
Anyway, back to the realm of quantum physics and the odd things that go on there.
In the eraser experiment, scientists managed to find out which way the particles were going without disturbing the wave interference pattern that they were making. Particles come in sets which are tangled up and even when they're moved apart, what one does affects what the other does. The eraser experiment showed that if the observation/measurement information was erased from the system after it happened, then the particles acted as if they'd never been observed.
No? Then think about this:
Let's bring the idea of a particle up to our normal level. Now, instead of a blip of energy, you're holding a baseball. You throw it and watch where it goes, where it hits, how fast it traveled. Then you time jump back, and stop yourself from watching it and... I can see your eyes crossing now :) How can you possibly erase the observation of something after you do it?
We have to get a little more technical here so stay with me. In order to measure what is going on, we have to have some sort of apparatus to measure with and something to measure. In the original experiment, scientists used beams of atoms. Subsequent versions of the experiment used various beams of light. A tightly focused laser shot photons through the double-slit curtain where they impacted a beta-barium borate crystal, one crystal for each slit. This sort of crystal has a special optical property - when it absorbs a photon, it re-emits from the exact same point a PAIR of entangled photons going in opposite directions. This allowed the scientists to figure out what the path of the original photon was without measuring IT.
All of this is interesting, of course, but the real question is:
What is it that's going on in the quantum realm that:
1. Allows a particle to interfere with itself and be in two places at the same time?
2. That forces an increase of chaos in one place to make up for order being imposed in another?
Scientists may never know, but they're certainly trying to figure it out.