# What is Quantum Physics?

1. Nov 8, 2007

### Lavace

I can't find a definite answer, and I need to know for a statement.

Am I correct saying that Quantum Physics, is the physics behind particles at a microscopic scale?

And that all the Physics I learnt on my course, modelling particles for real life situations (momentum, mass on a plane etc). Is nearly completely differerent when you get to a microscopic scale?

2. Nov 8, 2007

### ZapperZ

Staff Emeritus
Quantum physics isn't just one thing. It is not like an entity in which you can point to, the same way you can't pin down what "classical physics" is.

The best way to have a feel of what it is is to actually read a pop-science book. "In Search of the Schrodinger Cat" by Gribbin would be a good start.

Zz.

3. Nov 8, 2007

### country boy

ZapperZ is correct that you can't summarize it in a simple statement. However, you are on the right track. You might say that quantum physics covers the realm where Planck's constant is important.

4. Nov 8, 2007

### DaveC426913

Microscopic scale is too big. We're talking atomic scale.

I'd say Quantum Physcis is the physics that comes in to play at scales where matter can be distinguished as discrete entities rather than a continuum of material.

5. Nov 9, 2007

### ZapperZ

Staff Emeritus
Actually, you can't even pin down the "length scale" here. Recall that the whole issue with maintaining quantum effects has more to do with maintaining "coherence", rather than "size". It just happens that the larger an object is, the more separate components that it has, and thus, the easier it would be for decoherence to set in.

However, it does mean that if I can have a larger conglomerate of "stuff", but I can maintain coherence effects, then I should be able to still detect quantum effects. That is what we see with 10^11 particles in the Stony Brook/Delft SQUID experiment. In fact, if anyone recall the paper that I cited by Carver Mead on PNAS, he clearly indicated that superconductivity is the clearest manifestation of quantum effects at the macroscopic scale. Why? Because the entire supercurrent (all of those Cooper Pairs) maintain their long-range coherence with each other, despite of the "size" of such conglomeration.

So I think the issue of "size" for the applicability of quantum mechanics may not be a "strong" requirement here.

Zz.

Last edited: Nov 9, 2007
6. Nov 9, 2007

### staf9

Having just checked it out myself, the Quantum Mechanics section of wikipedia is extensive (if anything).

This is how I've always thought of quantum mechanics: If a single object acts more like a wave than a particle (exhibits wave-particle duality, which I think is first thing you're taught in a QM course), then it's quantized and it falls into the ream of quantum mechanics. Now I know that isn't 100% correct and it's a sweeping generalization too.

The real way to say what I'm trying to say is that all objects exhibit wavelike and particle-like properties. With big objects, velocity is usually small compared to their size and the wavelength is shorter. With small objects, they usually have a higher velocity compared to their size and a larger wavelength (smaller momentum for small objects, larger momentum for big objects, and wavelength is inversely related to momentum)

7. Nov 9, 2007

### DaveC426913

Technically true, but in the spirit of the OP's question, we do not make much use of quantum physics when dealing with macroscopic objects.

8. Nov 10, 2007

### vanesch

Staff Emeritus
I'll give my 2 cents. Quantum physics is a model of nature, or part of nature, that is based upon certain postulates, which are the postulates of quantum theory. When you use these kinds of models, or whether you experimentally inquire in properties which are (supposed to be) described by these models, then you are doing quantum physics.

In order to make this clear, let's ask the question for a different field: classical mechanics. What is classical mechanics ? Well, classical mechanics is the study of nature, or a parts of nature, which are described by a model of nature build upon the postulates of classical mechanics (Newton's laws). Given that, say, automobile engines, aerodynamics, planetary motion etc... are all well described by these kinds of models, we can say that studying them belongs to "classical mechanics".

So what's the area of quantum physics ? Well, it are these domains of nature which are quite well-described by models inspired by the postulates of quantum theory (and one usually assumes that they are NOT well described by purely classical models). It turns out that these phenomena usually manifest themselves on the atomic and sub-atomic scale, but there are also - as Zz pointed out - certain more "macroscopic" phenomena which use models inspired by the postulates of quantum theory.

So what's quantum physics ? It is that kind of physics for which you use quantum models preferentially over classical models.

Last edited: Nov 10, 2007
9. Nov 10, 2007

### DaveC426913

Hm. Well, Felgarcarb Physics is the physics in which you use Felgarcarb models over other models.

How much closer are you to a definition of Felgarcarb Physics?

10. Nov 12, 2007

Quantum physics applies to those realms of our existence for which we have no classical anology, no mechanical model to explain the observations of classical experiments. In one respect, Quantum physics begins with a contradiction, in that the phenomenon that it attempts to describe are built upon a classical framework, the experiment itself. The problem is the discontinuity between classical notions of what a particle should be and the results of the experiment itself. Perhaps, in that respect, particle-wave duality is an important cause of the discontinuity. Quantum theory attempts to project the nature of particles away from any concept of what they might be in a classical sense, since no model or inherent understanding of their internal nature yet exists, into a projection of what that entity as a whole accomplishes. While it begins in one sense with an uncertainty, it ends with a probability that something will occur, that a particle will follow a certain path of uncertainty to a position that we know as a fact, within uncertainty...

The necessity of quantum theory arose from the blackbody radiation, wherein the constant h was described by Planck. This is the first place where our classical ideas, mechanical models, failed to describe the nature of the observation. The work of Einstein in the photoelectric effect, showed that light was packaged, again, an idea with no classical foundation. Bohr and others attempted to use these concepts to explain the nature of the spectrum of light emitted by atoms, but every attempt that they made to use classical constructs led to failure. Eventually, it was realized that a successful approach might be reached if we disconnect ourselves from the need for classical analogy, interpretations of what is occuring in a modular sense, and just accept the mathematical treatment of quantum physics and the results it is able to reproduce.

From a classical perspective quantum physics is a mathematical projection of a realm of our existence that we cannot describe with a rational classical model.

11. Nov 12, 2007

### DaveC426913

Better, but isn't still boiling down to describing QM as what is not classical physics?

12. Nov 13, 2007

### vanesch

Staff Emeritus
I mentioned the basic postulates of quantum theory, which I assumed clear (statespace, superposition, linear time evolution, hilbert norm as probability of observation).

You can't say the same of Felgarcarb physics.

I would say, from the moment that you use the superposition principle (superposition of states), you do quantum physics.

Last edited: Nov 13, 2007
13. Nov 13, 2007

### vanesch

Staff Emeritus
Well, you can say the same about relativistic physics. You do relativistic physics from the moment that you use the postulates of relativity (including the invariance of the interval).

Maybe one day we'll have something else than quantum physics, but we will probably still do quantum physics, and it will be distinctive, in that we will use the postulates of quantum physics.