Laymen's definition of Quantum Physics, I'm old?

Very short answer:

At the sub-atomic level:

The universe is probabilistic ===> We cannot know every thing for certain, we can only assign a probability.

Any thing is possible (Walk through walls for example. There is a small probability that all the atoms and molecules in your body will align with all the atoms and molecules in the wall and you could pass through. Very unlikely, but the probability does exist.)

We are not sure what sub-atomic things are. Sometimes they act like waves. Sometimes like particles. (The rest is long and you may have no real interest. So, stop here if you wish. PS: I'm 67)

WARNING! I am a heretic. According to the Physics Majors, the following is “misleading” or “nonsense.”

Yours is a very profound question. I can tell from your question that you are ready to be let in on the "Dirty Little Secret" of Theoretical Physics: We have NOT yet figured everything out. The universe is governed by whole sets of "Laws" that do NOT agree and are mutually exclusive or illogical. This is a great embarrassment. We assume we live in an orderly, rational universe that makes sense. Perhaps when Quantum Mechanics and General Relativity are unified we will have a better answer to your question.

For now:

We need the concept of alternating electromagnetic fields (waves) to explain certain physical phenomena, like the interference pattern in the 2 slit experiment. So we keep that. We must somehow explain how a particle orders of magnitude smaller than the distance between the slits somehow passes through both slits and interferes with itself. Problem, we cannot explain this well using "quanta" (Particles, photons). Wave mechanics gives a simple easy to understand explanation.

We need the concept of quanta (particles, photons) to explain other phenomena, like the Photoelectric Effect. So we keep that too. But, this means we use 2 different, mutually exclusive systems of mechanics to explain electromagnetic radiation.

What to do? We cheat! We say that electromagnetic radiation has a dual nature and choose the system of mechanics that works best for the problem at hand.

Quantum Mechanics gets out of this mess by introducing the Uncertainty Principle, Indeterminacy, and the Copenhagen Interpretation of QM. If we do NOT know which slit the particle went through, then the particle is "smeared out in equal parts" and goes through both slits. It turns into a fog. As long as it is a fog, it can pass through both slits. That is: The particle occupies a volume of space with some probability. QM says that so long as the position is not known, the particle occupies the entire volume. If we learn its position, the fog condenses into that location and the particle goes through one slit. (The problem with this is: Fog does not form interference, waves do. Sooner or later, in those problems where appropriate, you must give up particles and fogs and start cranking through the equations of Wave Mechanics) But, Quantum Mechanists prefer “fog” to, "The particle turns into a wave and goes through which ever slits are open. One slit, no pattern. Two slits, pattern. “ Take your choice of which mental picture you form.

Rule of Thumb that got me through QM: If its’ position is known, it is a particle. If its’ position is unknown, it is a wave. This works because the equations of Wave Mechanics work, if the position is unknown. If you don’t tell anybody, no one will know the mental picture you formed to solve the problem. (This may not always work, but I do not remember a case where it failed.)

It is not uncommon for engineers to accept the reality of phenomena that

are not yet understood, as it is very common for physicists to disbelieve

the reality of phenomena that seem to contradict contemporary beliefs of

physics - H. Bauer

First of all, the basics of quantum physics which were around in the 60's are still valid today. So it depends how far you got in Physics (and Mathematics) in your original courses.

The books by Stephen Hawking do not concentrate on Quantum Physics, but mixes in General and Special Relativity as well. As well, he simplifies so much so as to sometimes oversimplify: for example, he gives into the temptation to "prove" the Uncertainty Principle by referring to the "Observer Effect", which is incorrect. (Google the Observer Effect.) Many popular authors do that, because the proof of the Uncertainty Principle is a little bit trickier. Also, it is rarely explained in the popularizations what "uncertainty" the Principle refers to. Here is a proper statement of that basic principle, without the notation: if you prepare a large number of particles in identical states, and measure the momenta of some of them, you will find the measurements following a probability distribution with a standard deviation M. If you now measure the position of the rest of them, again you will find the measurements following a probability distribution with a standard deviation P. The product M times P will be greater than or equal to a certain constant number. In fact, there are lots of pairs of quantities that follow this idea: Energy and time, for example, or almost any quantity and the change in that quantity. Quantum physics is the ramifications of this fact, along with the facts that mass and energy are equivalent, and that energy only comes in discrete amounts, not continuous ones. The fourth cornerstone of quantum physics is Schrödinger's equation, which basically says that the probability distributions mentioned in the Uncertainty Principle change in a deterministic way, according to the energy that the system receives or gives out. This is also often stated in terms of information, as "changes in Energy states" and "Information" are equivalent.

To go further, I would suggest starting by going to the Wikipedia article on each of these four pillars, and following as many of the blue links as you can before it gets bogged down in Mathematics. You won't get a perfect idea, but you will get a better one. Beware of the many pseudo-scientific explanations that are circulated in the press, silly films, and the like.

Once you understand these, you can then understand why, for example, an electron is a probability wave that becomes determined into a particle only at the instant of measurement. How the measurement does this is still a matter of debate, called "The Measurement Problem."

Yes, it has been shown to be correct in many, many experiments. It is now very important not only in physics but also in chemistry. Chemical reactions are now known to be quantum processes. Or the tunneling which allows fusion in the sun or in nuclear reactors is a quantum process. Superconductivity, superfluids, and many other useful processes are only explainable with quantum physics. There are large numbers of applications.

(By the way, Asimov, not Assimov.)

How did it originate? The history is well documented, and involves names such as Einstein, Planck, Bohr, Schrödinger, Dirac, and many others.

Good luck. Have fun. I'm about your age, and I still have fun with it.

Quantum Physics is the physical laws that apply at the sub-atomic level.

It is not a theory or an equation of anything, it is simply the study of the physical laws at the sub-atomic level.

Yes, I think there might be a QM book for dummies. Mr. Hawking published two books (which can be bought combined at Barnes & Noble) which offers a look at the differences. It was called a Brief History of Time, and The Universe in a Nutshell.

Mr. Hawking starts out by stating that everything we heard about is just them guessing - that they could very well be just full of crap.

When looking for definitions, the best source is the primary source... the dictionary. Or in the following quote... dictionaries, turns out to be quite a few definitions, not all of them agree. "From the above definitions of Quantum Physics, we can glean the following facts: Quantum physics deals with the very small objects that are not seen by the naked eye. These microscopic objects behave both like particles and like waves (wave-particle duality). Particles are defined as localized phenomena (restricted to a specific place or area) that transport both mass and energy while moving from one place to another. Wave, on the other hand, tend to spread out in space; they carry energy but they don’t transport mass. For example, the grains of sand are particles, while the ripples on a body of water are waves. These particles move randomly or unpredictably, meaning, we cannot predict their position or momentum at the same time. It’s like these particles have a mind of their own and know they’re being studied and, just to torture the observer, they move around freely while seem to be saying, “Catch me if you can.” The reality in the atomic level as observed in Quantum Physics is unlike the reality we see around us. This particular truth offered by Quantum Physics is used to interpret the power of the mind to influence events." [See source.]

If you're interested in quantum physics I suggest you watch the Feynman Lectures. If your interest still holds after watching these videos, then I suggest you buy a few books written by him as well.