Ha! I couldn't resist. Actually, I was flipping through the book The Great Scientists by John Farndon when I came across one of my favourite principles: Heisenberg's Uncertainty Principle.
This principle, along with Bohr's Complementarity Principle, forms the basis of quantum mechanics. Quantum mechanics is a scientific field which tries to explain the relationship between energy & matter in the sub-atomic world. It's an incredibly fascinating but very mathematical branch of physics, so I'll try to keep things simple.
Before developing his Principle, Werner Heisenberg worked out complex mathematical equations to describe the atom & sub-atomic particles. He called this theory matrix mechanics since it used an obscure branch of mathematics using matrices - but since this wasn't well known at the time & was difficult to visualize, his colleagues didn't readily accept his theory. Schrödinger came up with a competing theory called wave mechanics which was essetially the exact same thing - but more "elegant" (in Schrödinger's own words). Two other scientists, Jordan & Dirac, combined these 2 theories into what they called the "transformation theory."
Confused yet? So were they. Heisenberg was studying this transformation theory when he noticed that he couldn't measure the position & velocity of a particle at the same time. It was one or the other. But this wasn't just a mistake in the equations - it was the nature of the particles they were measuring.
The Uncertainty Principle then states that the position & velocity (speed + direction) of any particle cannot be measured simultaneously. This is most obvious at the subatomic level where everything is extremely small (making the uncertainty larger) - but the principle applies in the "real world" too. It's only that the object are so large (and visible) that the uncertainty is very small.
If you think about it, it makes sense. If you're measuring the position & the velocity of a car, the position will be changing simply because the car is moving. So there is some small uncertainty of where EXACTLY the car was when it was going that exact speed. When it comes down to the subatomic level, this uncertainty becomes significant. Indeed, the very act of measuring the velocity of a particle changes it, making the simultaneous measurement of its position invalid. Instead, the probability that the particle will be in that position is measured.
This theory was rapidly accepted into the physics field - but there were a few people opposed to the idea. Einstein was actually one of the biggest opponents - he didn't like that it relied on probabilities. He also disliked that it says the observer will influence what he was observing, believing nature to be independent of the investigator. This really isn't the case though - I'm sure you've noticed yourself acting differently around certain people. If someone is watching you - or testing you - you're going to behave accordingly. It's harder to apply this to the world around you though but I think this (clichéd) question sums it up nicely: if a tree falls in a forest & no one is around to hear it, does it make a sound?
If no one is there to observe something, does it exist?
If you like these types of thought exercises, google/wiki Schrödinger's cat. Which could lead you into parallel universes...
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