What is quantum physics, and what could you tell me about it?

Question

im very curious, please tell me in a way that i can acctually understand.

im 12 so i need you to use SMALL words lol.

just kidding.

but still what are some things you can tell me about it

Answer 1

About a hundred years ago, physicists thought that they understood just about everything physics had to say about the universe.  From the basics set down by Isaac Newton a couple centuries before, and the work of a lot of smart people, they'd come up with mathematical theories and explanations for just about everything they could observe; there were just a few details to iron out, and then, the physicists said, physics would become the first science to come to an end.  Fortunately, those physicists, which we now call "classical" physicists, were wrong.

Those few unanswered problems ended up leading to a completely new understanding of the nature of the universe.  Two in particular led to what is now called quantum physics: black body radiation, which is the spectrum of light emitted by a hot object, like a space heater element or a light bulb filament; and the photoelectric effect, which happens when light shining on metal knocks off electrons (you can look at the wikipedia links below if you want to go into the specifics).  One of the goals of physics is to use mathematics to describe physical phenomena, and no one could come up with equations based on known classical physics that explained either of these effects.

Here's where quantum physics comes in.  Max Planck (very famous) answered the first question by making a huge leap: he assumed that light (and I mean ALL of what we call "electromagnetic radiation": the light we can see with our eyes, radio waves, microwaves, X-rays...) couldn't have just ANY energy; frequency, he argued, is "quantized."  What does this mean?  The "frequency" of light is determined by its energy; our brain perceives light frequency as color.  Everyone thought that light could be any "color"; if you show me light with frequency X, I can show you light with frequency X+.000(lots of zeroes)1, as many zeroes as I want.  Planck said that frequencies went up in steps; this means that there are frequencies that don't exist.  One effect of this is that there are colors that do not exist in the universe; pretty weird.  To get a sense of how weird this is, imagine if position was quantized; it would be like living on a checkerboard, where you could move one square at a time but not stand between squares.

Einstein fixed the second problem and won his Nobel Prize for it.  You've maybe heard that light is a wave.  In explaining the photoelectric effect, Einstein proposed that light travels in little packets, which he called "quanta."  Now we call these "photons."  This means that, in some ways, light is like a particle: a object with a beginning and an end, like a car or a baseball.  For a quantum physicist, it makes sense to say "the lightbulb emitted a single photon; if I turn down the power, there will be no more light."  For a classical physicist, this is ridiculous; he would say, "You're wrong!  If you turn down the power, the light will get fainter, but you can always make it dimmer!"

We can test this with a simple experiment (simple to think about; actually doing it is pretty hard).  One well-known quality of waves is that they "interfere" with each other.  If you shine a light at a piece of cardboard with two little slits cut into it, the pattern on a screen on the other side will be a bunch of bright and dark lines (http://en.wikipedia.org/wiki/Interferenc...  Each slit acts like a separate light source, and when waves from each slit meet at the screen they can cancel each other out or make a really bright spot.  I know that this isn't something you see every day, but all waves do this: light, sound, even waves on the ocean.  Particles don't do this.  If I take a concrete wall with two slits in it and spray a machine gun at it randomly, a wall behind it will have a bunch of bullet holes in the middle and less and less on the sides (that analogy is from Richard Feynman, a very good physicist and teacher of physics).

Say I take my light bulb and slits, and instead of a screen I put some camera film; when light touches it, it's exposed and changes color.  Now I fire one light particle (photon) at a time; there is nothing for it to interfere with, so I might expect the photographic film to look like the wall behind the machine gun, with a bright spot in the middle right behind the slits that fades to dark at the edges.  But this isn't what happens!  At first, I'll see a random collection of dots, but if I leave my (really dim) light bulb on for a long time, the same old pattern of light and dark stripes emerges!  Somehow, the single photon "knows" where to go!

Finally, we can apply quantum mechanics.  A quantum physicist will say, "It's quite simple.  The photon goes through BOTH slits and interferes with itself."  In some way that we cannot observe, the photon occupies both places at once.  "But," you may say, "can't we test that?  What if we put a sensor in one the slits?  Then we'd know which one the photon went through!"  So we put some sensor on and turn on the light bulb; when a photon goes through the sensor slit, it beeps.  If our light source is good, our sensor beeps about half the time; the photon is equally likely to go through each slit.  Now look at the film.  The pattern is gone!  It looks just like the machine-gunned wall, with a ton of dots in the middle and less and less as you go to the edges.  No dark spots.  Somehow, knowing which slit the photon went through ruins the interference pattern.

Now, you might say our sensor altered the path of the photon somehow.  You'd be right.  You may suggest we build a better sensor, one that doesn't affect the photon in any way.  Here's the weirdest part: it doesn't matter how clever your sensor is, OUR KNOWING WHICH SLIT THE PARTICLE WENT THROUGH IS ENOUGH TO DESTROY THE INTERFERENCE.  Observation changes the result of the experiment!  Quantum physicists talk about "collapse of the wave function"; that sounds like jargon, but what they mean is that OBSERVING SOMETHING FUNDAMENTALLY CHANGES IT.  This is probably the biggest difference between quantum mechanics and everything that came before;.

This is not limited to light.  Once people accepted that light "waves" also acted like particles, they started to wonder if matter "particles" could act like waves.  They repeated the double slit experiment, but instead of shooting photons, they shot electrons: the tiniest bits of matter they could make in a lab.  They got the same result!  In the mathematics of quantum mechanics (and believe me, there is a lot of mathematics in quantum mechanics), EVERYTHING, from a beam of light to a baseball, is described as a wave.  This doesn't mean that when you throw a baseball it wiggles through the air; matter "waves" don't wiggle like the waves you make on a jump rope or slinky.  The truth is, when a physicist talks about "wave-particle duality" and says that everything is both a particle and a wave, he has no idea what a wave-particle looks like.  For a particle to act like a wave, it has to be tiny; even atoms are too big to exhibit significant wave properties like interference.  In the macroscopic world, the world that we can experience with our senses, there are things that act like waves and there are things that act like particles, but there's nothing we can see that acts the way photons and tiny particles do.  We don't really have the mental machinery to fit these "wave-particles" into our everyday experience; we can think of a baseball making tiny zig-zags as it flies, but that's not really what's happening.  Instead, we make experiments that demonstrate those effects, and create a mathematical framework that predicts them.  Sorry if that's a bit disappointing; if you want the best explanation you can get, go to the library and pick up a copy of "The Feynman Lectures on Physics."

Answer 2

If you say you understand quantum physics, you don't understand quantum physics.

Answer 3

This is a good question. I recommend a small book by Richard Feynman called "QED", The Strange Theory of Light and Matter. It explains the idea and discusses the problems it presents.

Answer 4

In physics, we talk about all kinds of energy. The heat you feel from the sun, for instance, is thermal energy.

We used to think that when something (like the sun) was sending out energy, that it did so continuously, like a stream of water.

However, quantum theory suggests that the energy comes out in little packets, like tiny individual droplets of water. These "droplets" are called "quanta".

Quantum theory explains how many, many things work, and so we accept that it is basicly true.

It has some interesting implications, though. First of all, it suggests that any single quanta of energy or any single particle of matter doesn't just occupy one point in space. Rather, it has a probability of being found anywhere within a particular region. It's as if when you choose to look at the particle, someone, somewhere, rolls a die, and places the particle within that region at random!

This effect is still very mysterious to us, and we don't know exactly why or how it happens, only that it does, in fact, happen.

One application is this: imagine that for one tiny particle, the region it can be found in liles half on one side of a wall, and half on the other. Imagine further that this wall would normally stop the particle moving through it; no matter how fast it was moving, it could never burst through the wall.

If you observe that particle to be on one side of the wall, you might expect that you will -always- find it on that side afterwards. However, in actual laboratory testing, you would find that it will sometimes be on the other side of the impassable wall!

This effect is called "quantum tunneling", and is the basis for semi-conductor technology that is very important in all modern electronics.

I hope that helps get you started!

Answer 5

Start with wiki.

http://en.wikipedia.org/wiki/Introductio...