# QM in one sentence

1. Dec 9, 2008

### tgt

Would the most penetrating answer be "QM views all fundamental entities in the universe as both waves and discrete units of entity (i.e particles although a photon isn't a particle)"? Since all QM properties can be derived from this sentence.

In fact if this description of QM is correct then it shouldn't come as a surprise that no one understands it since within this description lies a paradox. In other words, the foundation of QM is paradoxical.

Last edited: Dec 9, 2008
2. Dec 9, 2008

### Dmitry67

No, the foundation of QM (or, digging deeper, the TOE) is pure mathematics (idea of Max Tegmark)

Paradoxes appear when you try to understand it and it conflicts with an our "common reasoning".

3. Dec 9, 2008

Dmitry .. can you list any of the of the Paradoxes you refer to above .. i dont think there would be allowed any more paradoxes neccesary than there would be found in Mathematics .

4. Dec 9, 2008

### Dmitry67

5. Dec 9, 2008

I have a philosophical aversion to that scary cat story . :)

And too i agree there need be no necessary paradoxes .

6. Dec 9, 2008

### shaun_o_kane

Since we are being a bit ?... there is no gaurantee mathematics is consistent. And what qualifies as a paradox depends on which interpretation you pick.

I wouldn't say the foundations of QM is pure mathematics - they were in another building if I remember correctly. The "great leaps forward" have come when we looks at the physical world, IMHO, and QM without physical interpretation is just PDE, linear algebra and other bits and pieces. ;-)

7. Dec 9, 2008

### Gerenuk

That's nice to say to perplex people, but the idea of duality is rubbish. QM has a single model where everything moves according to complex probability waves and sometimes the waves can collapse into a different state. OK, that's again a really ugly picture and a lot of more understandable interpretations have been discussed. I'll also make up my own when I find some time...

8. Dec 9, 2008

### Dmitry67

Did anyone read Max Tegmark's "The mathematical universe"?
He explains *everything*

In the bird's view laws are deterministic, and there are waves. From the frog's perspective, nature is random and there are particles.

9. Dec 9, 2008

### mn4j

I think this excerpt describes the situation succinctly:

"... But the most tangled area in present physical science
is surely the standard old 1927 vintage quantum theory, where the conceptual problems of the
Copenhagen interpretation" refuse to go away, but are brought up for renewed discussion by every
new generation of physicists much to the puzzlement, we suspect, of the older generation who
thought these problems were all solved. Starting with the debates between Bohr and Einstein
over sixty years ago, different ways of looking at quantum theory persist in making some see deep
mysteries and contradictions in need of resolution, while others insist that there is no difficulty.

How can scientists of unquestioned competence be in sharp disagreement about such things? It
must be that we have different unstated premises hidden assumptions in the back of our minds. If
so, then until we bring out into the open just what those premises are, there would be no possibility
of resolving the issue.

Defenders of the Copenhagen interpretation have displayed a supreme self confidence in the
correctness of their position, but this has not enabled them to give the rest of us any rational
explanations of why there is no difficulty. Richard Feynman, while defending the QM formalism
on grounds of its practical success, at least had the honesty to admit: Nobody knows how it can
be that way."

While we doubters have not shown so much self confidence, nevertheless for all these years it
has seemed obvious to me for the same reasons that it did to Einstein and Schrodinger that the
Copenhagen interpretation is a mass of contradictions and irrationality and that, while theoretical
physics can of course continue to make progress in the mathematical details and computational
techniques, there is no hope of any further progress in our basic understanding of Nature until this
conceptual mess is cleared up.

Because this position seems to arouse fierce controversy, let me stress our motivation: if quan-
tum theory were not successful pragmatically, we would have no interest in its interpretation. It is
precisely because of the enormous success of the QM mathematical formalism that it becomes cru-
cially important to learn what that mathematics means. To find a rational physical interpretation
of the QM formalism ought to be considered the top priority research problem of theoretical physics;
until this is accomplished, all other theoretical results can only be provisional and temporary.

...

The failure of quantum theorists to distinguish in calculations between several quite different
meanings of probability', between expectation values and actual values, makes us do things that
don't need to be done; and to fail to do things that do need to be done. We fail to distinguish
in our verbiage between prediction and measurement. For example, the famous vague phrases:
It is impossible to specify . . . '; or It is impossible to define . . . ' can be interpreted equally well
as statements about prediction or statements about measurement. Thus the demonstrably correct
statement that the present formalism cannot predict something becomes perverted into the logically
unjustified and almost certainly false claim that the experimentalist cannot measure it!

We routinely commit the Mind Projection Fallacy: supposing that creations of our own imag-
ination are real properties of Nature, or that our own ignorance signifies some indecision on the
part of Nature. It is then impossible to agree on the proper place of information in physics. This
muddying up of the distinction between reality and our knowledge of reality is carried to the point
where we find some otherwise rational physicists, on the basis of the Bell inequality experiments,
asserting the objective reality of probabilities, while denying the objective reality of atoms! These
sloppy habits of language have tricked us into mystical, pre scientific standards of logic, and leave
the meaning of any QM result ambiguous. Yet from decades of trial and error we have managed
to learn how to calculate with enough art and tact so that we come out with the right numbers!

"
From, Jaynes, E. T., 1990, Probability in Quantum Theory,' (200Kb) in Complexity, Entropy, and the Physics of Information, W. H. Zurek (ed.), Addison-Wesley, Redwood City, CA, p. 381
http://bayes.wustl.edu/etj/articles/prob.in.qm.ps.gz

10. Dec 9, 2008

### mn4j

One paradox would be the "Measurement paradox" which goes to the heart of the problem with QM.

11. Dec 9, 2008

### Fredrik

Staff Emeritus
I looked around a bit and found this, and this much shorter article which seems to be making the same point.

I read the shorter article today and I've read the longer one before. It's interesting stuff, but it certainly doesn't explaining everything.

12. Dec 9, 2008

### Dmitry67

mn4j - I believe most of the progress in this area (Quantum Decoherence) had been donce AFTER 1990, and the current view of the QM does not assign any special role to a measurement. ANyway, measurement device is just a collection of QM particles.

Frederick - and what questions are not answered? just curious

13. Dec 9, 2008

### shaun_o_kane

Dmitry67 - You are striding with supreme confidence and faith across an area which many great physicists have express anxiety and uncertainty. I suggest that you regard Max Tegmark's work as one possible approach. Max Tegmark is not a "Great" physicist (as far as I'm aware). Personally I think that Bohr was right. Particles are nonsense, and no more real than waves. The world is very subtle. I'm also with Feynman - if you are not shocked by QM, you haven't understood it.

14. Dec 9, 2008

### Dmitry67

Of course there are too many ideas in the Max Tegmark's article, so it is better to focus on only one of them.

The Quantum decoherence - it is not Max's idea. It is a well-defined theory:
http://en.wikipedia.org/wiki/Quantum_decoherence

This is different from the early (1950) Many-Worlds (Everett) because at that time the 'splitting' was just postulated without any mathematical proof

Also, in the decoherence approach there are no Many Worlds! Wavefuction is common to all branches, and branches can interact, this interaction is evident for QM objects but almost (but not completely) dissapear for the macroscopic objects.

So, if we have such theory, do we need a 'collapse'? Collapse should be eliminated by the Occams razor. There are still some open questions, but I think it is a really great progress which is ignored by the public (new particle is much more interesting and easy to explain to any John Doe then that weird stuff with decoherence and cats)

15. Dec 9, 2008

### shaun_o_kane

Wikipedia quite rightly stamps "All or part of this article may be confusing or unclear" on many of the QM pages. I would never quote any of them.

Decoherence as an independent interpretation is questionable. Splitting in MWI still has no proof. (Is that even possible?) I don't know what "this interaction is visible for QM objects" means. What does collapse even mean? What collapses? Anyway...I didn't understand the point of the original post, but I'm sure we have gone past QM in one sentence. ;-)

16. Dec 9, 2008

### f95toli

I'd say most of the "practical" work on decoherence (meaning work that is actually relevant when interpreting experiments and not just "philosophical") is actually even newer than that; I was sorting through my collection of papers today (working on a grant application:grumpy:) and the amount of published work has grown very rapidly over the past 10 years (since about 1999-1998 when experimental quantum computing staring to take off); we are now at a point where there are new papers appearing on the arXiv almost on a daily basis and there are a few papers in just about every issue of PRL/PRB/PRA.

17. Dec 9, 2008

### jms5631

Tegmark's "many worlds" solution is entirely mataphysical and thus in principle unobservable. I don't see how this can be a scientific resolution of the problem. Until further mathamatical and experimental progress is made, quantum decoherence doesn't seem to solve anything, other then preventing a macroscopic Schrodinger's cat type situation in the environment. As a condensed matter experimentalist student, maybe I look at situations differently then theorists. I don't see the measurement problem as necessitating these mystical resolutions, and philosohical angst. It's an emergent phenomona resluting from exploiting the symmetry breaking charecteristic of the solid state. I think the conflict arises because of confusion over what is meant be measurement and observation, but this is not necessary.

18. Dec 9, 2008

### tgt

No one has given a description of QM in one sentence yet. The one I gave in the OP is wrong. Electrons are particles, not waves.

QM it seems is a tool used for calculation and without a mechanism. Just like Newton's theory of gravity. In this way, interpretations of QM are unnecessary. i.e Actions at a distance is just as spooky as prob waves. As Lee Smolin puts it, QM is not a final theory. So one shouldn't be spooked about QM at all. I'm not an expert so how do people think of my idea?

19. Dec 9, 2008

I actually dont completely understand your point of reference , tgt .
Because , Surely Electrons are more certainly waves than particles - aren't they ?

Have you seen the mention that as soon as the particle model is dropped in favor of the Wave model a whole lot of complex problems (paradoxes) as regards conversion of state , simply disappear ?

In a wave dominant model , there are liberated wave packets and bound wave packets - that's not a paradox is it ?

I'd imagine that reference to QM being the study of the behavior and interaction of quantum articles (most usefully seen as wave packets) , would be a better description .

20. Dec 10, 2008

### Fredrik

Staff Emeritus
He says it isn't, and I don't see anything wrong with his argument.