Somebody explain Quantom Theory!!

So what you’re saying is that matter exists in both particle and wave format at the same time, yet that basically you actually cannot determine the location of any given particle, b/c it is in two places at the same time? Or can be? Is this basically why Einstein hated his night-job?

And where does the probability come into it? How can the location of a particle be dependant upon the probability of it existing? I must have this the wrong way round…

And can someone also clue me into to the supposedly “magical” stuff we can do with quantom theory? I know about quantom supercomputers and understand (in general terms) why they are so powerful, but I don’t understand why the exisiting computers and lasers we have today are dependant upon Quantom Theory.

Thank you for your wisdom, and good day/evening to you (depending where you live).

Hi Xavier,

I did a quick Google search on “Quantum mechanics beginner” and found this nice page of links

Unfortunately, it seems several of the links are dead. Take a look though, you might find one of the pages that works helpful.

Matter doesn’t exist as both a particle and a wave. It exists in a form we can’t be quite certain of. When we look at it one way it appears to be a particle. When we look at it a different way it appears to be a wave. Scientists merely pick and choose the form that makes the most sense to our minds and the experiment at hand.

In quantum theory particles do indeed seem to be able to be in more than one place at once. Only when the particle is measured (or observed) does it seem to take on a definite location and speed (known only within the boundaries of the Heisenberg Uncertainty Principle). The probability thing only creeps in when we wish to predict what is going on. Say we emit a particle. We can use probabilities to predict where it might be at a given time but we can only know for certain once a measurment is taken.

Of course, it is this issue with observing a system to cause it to ‘choose’ a location that much of the weird funkiness with QM arises from (e.g. Schrodinger’s Cat thought experiment). Nevertheless the math of QM works out really well and it is one of the most tested and accurate theories in the history of science despite its unsettling properties.

Technically everything is dependant upon QM. Keep going down far enough into the particles that compose everything you see around you and sooner or later you’ll have to pick up QM to describe what is going on. However, on a macro scale things aren’t so bad. You are sitting in front of your computer and not in two (or more) places at once. Regular computers and laser and such likewise don’t suffer in their operation from the weirdness at the quantum level (however, if you want to trace thepath of one particular photon in a laser beam and predict what it will do you have to use QM).

This best describes the theory:

Not it doesn’t.
It does.
Can’t picture it.
Doesn’t matter
No matter
Yes matter
doesn’t matter.
Got it?
Perfect! It makes sense then!
No it doesn’t!
Sure it does!
Oh, then you must be confused!
I am!
Perfect! You got it!
No I don’t!
That’s it!

The electromagnetic wave consists of a time varying electric field creating a magnetic field and vice versa. This is pretty basic stuff.

However consider this: The probability that a photon will be detected in any small volume centered on a given point in an EM wave is proportional to the square of the amplitude of the wave’s electric field vector.

So the electromagnetic wave is the probability wave for photons.

This “magical” stuff is called quantum cookery, and most people who “cook” with the principles of quantum physics don’t really need to contemplate the deeper meaning- they just need to use the equations to spit out answers. A good explanation of how lasers work can be found in In Search of Shrodinger’s Cat by John Gribbin:

If you have a problem with the concept of wave-particle duality, think of it this way: Am I a brother or an uncle? If you ask my sister, she’ll tell you that I’m a brother. On the other hand, if you ask her daughter, she’ll say I’m an uncle. Actually, she’ll probably say “gurgle blub ga ga”, but you get the point. What I am depends on who’s asking.

To extend the OP, or perhaps to answer it in some part, do I have this right?:

A quantum computer exists in all possible states it could take given the input until the output is measured. Thus a quantum computer is best modeled by a non-deterministic finite state machine.

For example, in order to factor a 30 million-digit composite number known to be the product of two primes, we need only write a program that randomly selects two numbers with digits totaling to 30 million, and then checks whether they multiply out to the 30 million digit number. The quantum computer will be sure to pick the right two numbers if they exist, for in a sense it instantly picks all the numbers so long as they satisfy the check.

So if a quantum computer is developed it will defeat the existing encryption technology?

This is probably simplistic and has many holes but it is the way I resolve the idea that a particle doesn’t exist, except as a probability, until we perform an experiment. That way I can sleep well at night.

The wave equation is interpreted as a probability function. I.e. the probability that you will find a particle at some place and the wave equation extends through all space. To me this is analogous to a macro, physical particle whose space position in time is specified by an equation in x,y,z and t. It can be in any postion in the x,y,z space and no position in the space has any unique meaning until I specify a time.

Things aren’t quite so simple. Quantum computers are different from nondeterministic state machines, in that each computation path has an associated probability. It will only pick the “right answer” with this probability. If you tried to factor a number by randomly picking factors and multiplying, you’d get no advantage over a classical computer.

The difference between a quantum computer and a probabilistic finite state machine is that different computational paths can interfere, either constructively or destructively. By clever choice of computation, you can reinforce certain final answers. This interference is not present in classical state machines and is usually thought of as the extra ingredient allowing fast integer factoring. It seems to require some clever work to apply, though, and doesn’t provide any obvious way of rapidly solving arbitrary problems in NP, though, so currently it’s only known how to use it to defeat encryption based on factoring (RSA) and related algorithms, like Diffie-Hellman. (There are quantum algorithms known to solve problems in NP more efficiently than a brute-force search, but they are not polynomial; they take about quadratically less time than the classical brute-force search, which is still prohibitive for large keyspaces.) In particular, most of the symmetric-key encryption schemes can’t be broken by factoring and so aren’t as vulnerable to attacks using a quantum computer, although the quadratic speedup implies that you might want to double your key length.

They weren’t dead until you clicked on them.

They’re not dead, they’re, uh, …, they’re resting!

Beautiful plumage on those links, eh?

Darn, I was hoping to be praised for making a quantum mechanics joke. It’s not like we’re drowning in them

Also, when I’m in a bookstore with friends and I spot a book on feng shui, I can’t resist moving it to another shelf and saying “this should really be over here.”

Maybe the links aren’t dead - maybe they simply have no spin

John Gribbin’s book “In Search of Schrodinger’s Cat: Quantum Physics and Reality” does a fairly good job of putting the basics of Quantum theory, physiscs, etc. in understandable terms. It’s been around for many years, since the late 70’s or early 80’s I think - luckily the basics haven’t changed all that much since then.