Can you give examples of physical phenomena that are counter-intuitive?

Question

I mean stuff that happens that you would never expect based on "common sense", your intuition, or whatever. Give specific examples. The most original, striking - and best described-submission wins.

Answer 1

Yes, and it was the spookiest, scariest thing that I ever participated in, because I was literally WATCHING quantum physics happen.   We did the double-slit experiment in college physics class.  My single act of looking at the results, was actually affecting matter on a subatomic scale.  It...blew...my...mind.

Here is a video of it below but in cartoon form.  Frankly, it is much easier to swallow in cartoon form.  If someone just "told" you what would happen, you wouldn't actually believe it.

http://tinyurl.com/3a7stq

I can not think of anything more counter-intuitive in all of...everything.

Answer 2

I saw this done in a physics class.  it blew me away completely.  just watch the video below:  

http://www.youtube.com/watch?v=P014jvaB3...

I don't completely understand it.

Edit: I did some more research on it and have a slightly better understanding now.  

because angular momentum on the wheel must be conserved, when a torque or moment is applied to the wheel that would cause rotation perpendicular to it's current rotational axis, the torque causes a net change in angular momentum that causes a phenomenon known as precession.  

what it amounts to is that because the angular momentum must be conserved, the wheel's axis of rotation rotates perpendicular to the direction the wheel would fall if no angular momentum existed.

Answer 3

Cosmologist Leonard Susskind successfully argued with Stephen Hawking on whether or not black holes can "extinguish" information completely, resulting in what is now called "Black Hole Complimentarity".  The implications of this complimentarity principle is probably as mind blowing physics can get.  According to General Relativty, if someone falls into a black hole and passes through the event horizon, he would simply experience floating past the horizon on his way to the black hole singularity---there is no immediate destruction of his existence.  However, according to quantum mechanics, from the point of view of an observer outside the black hole, the person falling into the black hole will see him being destroyed at the event horizon, ripped into a hot plasma of particles.  In other words, to the person falling into the black hole, he does not die at the event horizon, but to the outside observer, the falling person does die a violent death.  Turns out that both versions are true---it depends on the observer, the person falling into the black hole, and the observer outside.  We've heard about Schrodinger's Cat being both dead and alive, and we've heard about the concept of simultaneity being shattered by Special Relativity, but this Black Hole Complimentarity principle takes this to an new extreme that I think is just mind bending.  Yet, Stephen Hawkings was eventually persuaded that Susskind was correct.

Answer 4

The double slit experiment already referenced above is probably the most counter-intuitive I've ever run across, and is so easily reproduced that it's great for blowing the mind.  A few others come to mind:

- The speed of light is constant, no matter how fast you are moving.  For example, let's say you are traveling at 90% the speed of light, and I am at rest.  At the exact moment you pass me, I turn on a laser.  Classical physics would say that you would observe the light traveling a lot slower than I would, but that doesn't happen.  We both would say that it is traveling at c.

- Time and distance dilation.  If I take a trip to the nearest galaxy and back with a constant acceleration of 9.8 m/s, many thousands of years will pass on Earth, while I will have aged less than 50.  Doubly spooky since not only did I essentially travel into Earth's future, but I reached that galaxy far faster than it should have been possible given that it's a couple million light years away.

-   Quantum entanglement.  This still blows my mind.  When two quantum particles are entangled, and I measure the spin of one, the spin of the other is instantly determined, no matter how far apart they are.  This seems to violate the notion that information cannot be transmitted faster than the speed of light. It doesn't really allow information to be passed at faster than c, but that's too long an explanation for this.  Entanglement is used practically in quantum encryption devices.