What is the Basics of Quantum Physics?

[ugur0072]

I am going to have about quantum physics next year in my physics class, but it would be nice to know something about it before.

Therefore am I wondering about what is Quantum physic. You don't have to tell everything about it, but the basic about it. :D

 

[Fyzix]

I'm pretty sure this will get shut down, so I'll just tell you: google...
I'm sure you can just copy paste the phrase "what is quantum mechanics" into google and you'll find a layman guide instantly.

 

[ugur0072]

I know. I have searched for it on the internet, but every article is very complicated with difficult language. I would just ask for somebody to tell me short and understandable about quantum physics :D

 

[DaveC426913]

I know. I have searched for it on the internet, but every article is very complicated with difficult language. I would just ask for somebody to tell me short and understandable about quantum physics :D

Well, the first thing to understand is that it is very complicated with difficult language, and there really isn't a short, understandable version.

The reasons for this are two-fold:
1] It is unlike anything we encounter in our daily lives. Just to begin talking about requires a textbook of primer material. Imagine Tarzan asking about Calculus. First we'll have to teach him about arithmetic and then geometry...
2] Because it's unlike anything we're used to, we don't have conceptual models for it. Most of it is simply formulae, which is why that's mostly what you're encountering.

All that being said, arguably the cornerstone of quantum physics - that from which all other weirdness extends - is http://en.wikipedia.org/wiki/Uncertainty_principle" . In a nutshell: you cannot know both the position and the momentum of a particle simultaneously.

The first weirdness that comes from that is that, if you constrain a particle carefully enough to know its momentum (its mass/velocity), the particle will smear out into a blob whose position is no longer definable. The atoms of which all matter is made are slippery, illusory gremlins.

 

[vanhees71]

I must contradict the previous poster! Quantum physics is something we experience everyday around us. At least, there's nothing known that contradicts this claim.

So my answer to the original question is that quantum theory is the most comprehensive scientific description of the natural world around us that we have today. It describes the behavior of matter reaching from the smallest building blocks, so far discovered, i.e., the quarks, leptons, gluons, photons, and W- and Z-bosons to hadrons, among them the protons and neutrons, which themselves build atomic nuclei. Together with the electrons, these are the building blocks of atoms, which again form molecules and finally the macroscopic matter around us.

Quantum theory allows us to describe the behavior of everyday matter in terms of its elementary building blocks (quarks, leptons, gauge bosons) and the interactions between them.

It is one of the most fascinating questions, why then we experience the "classical behavior" of macroscopic matter, which under usual circumstances behaves indeed according to classical (even non-relativistic) mechanics and classical electrodynamics.

Of course, learning quantum theory is not easy since usually one deals first with microscopic entities like electrons and atomic nuclei (building atoms and molecules), which are indeed not familiar to us since they are bound into the macroscopic bodies of everyday matter. So it is no wonder that their behavior is quite strange to us, and one has get used to the specific thinking about them, particularly in the probabilistic nature of the theory and the fact that not all observables (e.g., position and momentum of an electron) can take definite values at the same time, but as stressed above this doesn't mean that quantum theory is something unrelated to the behavior of every-day matter. Its very stable existence as we know it could not be understood without quantum theory and, last but not least, learning quantum theory is great fun and one of the greatest adventures of the human mind :-).

 

[DaveC426913]

I must contradict the previous poster! Quantum physics is something we experience everyday around us.

In my experience, a tennis ball that I catch, thus determining its momentum, does not begin smearing out so that I can no longer determine its position.

Perhaps you could list some peculiarly quantum mechancial phenomena that are analagous to every day experiences .

 

[Varon]

I am going to have about quantum physics next year in my physics class, but it would be nice to know something about it before.

Therefore am I wondering about what is Quantum physic. You don't have to tell everything about it, but the basic about it. :D

Maybe the following passage from Scientific American (June 2011) I just purchased a while ago can give you a clue of the basis of the gist what is quantum physics:

"Thus, the fact that quantum mechanics applies on all scales forces us to confront the theory’s deepest mysteries. We cannot simply write them off as mere details that matter only on the very smallest scales. For instance, space and time are two of the most fundamental classical concepts, but according to quantum mechanics they are secondary. The entanglements are primary. They interconnect quantum systems without reference to space and time. If there were a dividing line between the quantum and the classical worlds, we could use the space and time of the classical world to provide a framework for describing quantum processes. But without such a dividing line—and, indeed, without a truly classical world—we lose this framework. We must explain space and time as somehow emerging from fundamentally spaceless and timeless physics."

 

[vanhees71]

In my experience, a tennis ball that I catch, thus determining its momentum, does not begin smearing out so that I can no longer determine its position.

Perhaps you could list some peculiarly quantum mechancial phenomena that are analagous to every day experiences .

Of course, for any practical purposes to describe a tennis ball in everyday life, classical Newtonian mechanics is a very precise approximation. You take into accound all the forces (gravity, approximated as a constant force, air resistance etc.) and treat the tennis ball as a spinning solid object (when interacting with the racket, for sure you have to treat it more carefully as an elastic body).

From the point of view of quantum theory classical mechanics is an approximation, and this works in this case for two reasons: first of all you concentrate only on the relevant effects for your physics problem at hand, namely the collective motion of a many-body system, i.e., the location and velocity of its center of mass and some classical spin degrees of freedom to describe rotation against the air to take into account air resistance (Magnus effect!). Such collective observables are given quantum mechanically by averaging over a huge number of microscopical degrees of freedom. Already this coarse graining leads to almost classical behavior of these collective macroscopical observables. Second the ball interacts with the environment, and this leads very efficiently to decoherence. So you'll have a very hard time to do, say, double-slit interference experiments with a tennis ball. As far as I know the largest objects ever used successfully in such an experiments are bucky balls (socker-ball shaped bound states of 60 C atoms), and already there it's tough to prepare them such as to get interference effects. If they are a few degrees above 0K their intrinsic excitations and the radiation of black-body radiation photons is enough for decoherence, and the bucky balls behave classically.

Now, if you look more carefully at your tennis ball, it becomes totally ununderstandable within classical physics, why it exists as a nice stable solid object at all. If you look into its microscopic details, it's enough to go to the level of atomic physics, i.e., you describe it as a bound state of atomic nuclei (which you can treat as Coulomb centers) with the electrons swirling around them. Then you already are in a dilemma within classical physics. Within classical electrodynamics, which has to be applied here to this set of charged particles, you can not have static bound-state solutions. Thus the electrons must move around the nuclei, and thereby they must be accelerated due to Newtons 2nd Law since there are the Coulomb forces from the nuclei and among the electrons themselves. The electrons thus, again according to classical electrodynamics, radiate em. waves and the whole tennis ball would collapse within tiny fractions of a second into a cloud of electrons and nuclei in clear contradiction to the fact that you can nicely play tennis with such a ball. As is well known, this problem is solved by quantum theory since it admits nice stable static solutions, which you usually learn to calculate first in the quantum-theory lecture (potential pots, harmonic oscillator, the hydrogen atom etc. etc. are nice applications of quantum mechanics 1).

Set aside the stability problem, one should also be aware that the tennis ball consists to a large extent of "empty" space (or not quite empty but with the electromagnetic field of the charges present). So why can you take the ball without your fingers slipping simply through this empty space? The answer is again a quantum effect, namely the Pauli principle: The electrons in your fingers interact with the electrons in the tennis ball, but they cannot simply penetrate into the tennis ball since there are not so many empty states left. So not only repell the electrons in your fingers those of the ball but they cannot push them away to easily, and thus the tennis ball appears as a solid (elastic) object and not as something glibbery which falls appart when you touch it.

To say it short: The working of everyday life is inexplanable from a classical point of view; on a very basic level you need quantum theory to explain why for any practical perposes matter exists in the way we know it and the "classical behavior" of its macroscopic observables, relevant for handling them in every-day life as we are used to it!

 

[ueit]

In my experience, a tennis ball that I catch, thus determining its momentum, does not begin smearing out so that I can no longer determine its position.

This is also true for a particle, such as an electron. No electron has ever been seen "smearing out"

 

[DaveC426913]

This is also true for a particle, such as an electron. No electron has ever been seen "smearing out"

au contraire - they have indeed. It is the very basis of the two-slit experiment - the core of quantum mechanics.

They are now doing this with large molecules. But it does not happen at macroscopic levels, which is the points I'm making.

 

[ueit]

au contraire - they have indeed. It is the very basis of the two-slit experiment - the core of quantum mechanics.

They are now doing this with large molecules. But it does not happen at macroscopic levels, which is the points I'm making.

The fact that for many particles you see interference does not imply that each particle gets smeared out. It might be a possible explanation but it is far from proven.

 

[DaveC426913]

The fact that for many particles you see interference does not imply that each particle gets smeared out. It might be a possible explanation but it is far from proven.

There is no proven in physics. What there is is the standard model.

And it is not about smearing as some sort of model, it is simply an observation. This is what atoms and subatomic particles are observed to do. Atoms and subatomic particles are subject to HUP, this is very well understood. Electrons do indeed smear out, as do whole atoms.

Read up on Bose-Einstein condensates. As atoms are cooled to absolute zero (such that their momentum approaches zero), they do smear out into a cloud (such that their position becomes uncertain).

You can champion other models but they will all have to explian the observed effect of obeying HUP.

 

[Fredrik]

I know. I have searched for it on the internet, but every article is very complicated with difficult language. I would just ask for somebody to tell me short and understandable about quantum physics :D

It would be easier if you told us how much math and physics you know. In particular, have you studied linear algebra and Newtonian mechanics?

 

[ugur0072]

It would be easier if you told us how much math and physics you know. In particular, have you studied linear algebra and Newtonian mechanics?

First of all thank you to all of you who have tried to answer my question. Now I understand better :D. I have also watched some videos on youtube which helps me to understand it better with animation :P

My math is very good and I have finished my first year with Physics. I have learned about the basic of physics like the Newtons three laws and how suns is made, about energy and wave and more. I am going to have physics next year too, and a friend told me that we should learn about quantum physics and the general relavity next year and that's why I am asking you this question :D

 

[Fredrik]

QM is an assignment of probabilities to possible results of experiments. Each probability is to be interpreted as the relative frequency of a specific result in a long series of measurements using identical measuring devices on identically prepared systems. These probabilities are the theory's predictions. The accuracy of the predictions are tested by comparing the probabilities with the relative frequencies obtained in actual experiments.

The above can actually be said about all theories. The main difference between classical and quantum mechanics, is that CM only assigns trivial probabilities (i.e. 0 or 1) while QM assigns probabilities from the interval [0,1].

Another difference is that CM tells you what the results of experiments will be by describing in detail exactly what's going to happen (if we want to, we can interpret this as an assignment of probability 1 to that outcome, and 0 to all other outcomes), while QM doesn't even try to tell you what is "actually happening". It just tells you how to calculate probabilities of possible results of experiments.

I think it's important to give up the expectation that QM will be a description of reality as early as possible. You will often slip into old habits, and ask yourself "what is actually happening here?". When that happens, you should remind yourself that some guy with a Wolverine avatar once told you that QM will not tell you the answer.

I thought about writing a lot more, but I don't really have the time now.

 

[unusualname]

^^I think you meant to say QM assigns complex probabilities, not "real values between 0 and 1"

 

[Fredrik]

^^I think you meant to say QM assigns complex probabilities, not "real values between 0 and 1"

Nope.

QM assigns complex probability amplitudes to other things, but not to results of actual measurements.