What is the hardest question to ask a quantum physicist?

[jostpuur]

One of the most surprising things I was forced to learn during my QM studies, is that physicists do not yet know how to compute spatial probability densities of quantum particles in order to predict interference patterns and other patterns you encounter if you shoot particles to a detector wall or film.

So I would ask a following question: Suppose we have a source approximately at origo (0,0,0), a detector plane orthogonal to the z-axis, at location z=L (means a plane (x,y,L)_{x,y\in\mathbb{R}}), and suppose we know the time evolution of the wave function \Psi(x,y,z,t), which starts at t=0 approximately at origo, and then proceeds as a wave packet towards the detector wall. What is the probability density \rho(x,y) for the particle's observation point on the wall?

IMO this is an unknown unknown so it qualifies as a question to the realblonde who didn't want known unknowns.

 

[conway]

People continue to scoff at the purported claim for all the wrong reasons. Why is it unreasonable to claim that one can "answer any question" about quantum mechanics?
Obvioulsy this is out of the question if we interpret it as meaning that you could calculate anything to any desired accuracy. But how unreasonable would it be to say you can "explain any phenomenon" involving quantum physics?

Let's turn it around and consider what this claim might mean if you applied it to classical physics. Because I would venture that most of us think we basically understand all of classical physics. But to take an example, does that mean we can all calculate the velocity distribution of molecules in an ideal gas, as Maxwell did 150 years ago? Probably very few people could. And yet there is no deep mystery as to how the equilibrium comes into being, once you have thought about the collision of two billiard balls essentially creating a new, random distribution of velocities under the constraints of energy and momentum conservation. Apply this to millions of particles and it is easy to believe that eventually a velocity distribution will be created which is in a sense more random than all other possible results. In this way you can reasonably claim to understand, and even "explain" what is happening, without being able to do the actual calculation.

What is really different about quantum mechanics is that even for relatively straightforward questions, like the energy states of the Helium atom, there are very few people that feel they have a solid idea of what exactly is going on physically, regardless of the actual calculation. In fact, for a lot of people who have acheived some proficiency, there are probably more problems for which they can do the detailed calculations that problems for which they can actually "explain" what is going on. This is the opposite of the situation with classical physics.

 

[marthkiki]

My friend thinks he can answer any question related to Quantum physics (although he claims he won't be able to answer known unknowns i.e 'the mass of the Higgs-Boson particle'). However, I would like to challenge him with a series of the hardest questions anyone on this forum can put to him.

I promise to post his answers up on this blog for you to see what he comes back with.

Many Thanks!

Unify quantum mechanics and relativistic physics in a perfect theory.

 

[Count Iblis]

How to do computations in http://insti.physics.sunysb.edu/~siegel/parodies/atchoo.html" ?

 

[Phyisab****]

A good one is to prove mathematically that gauge theories with spontaneous symmetry are renormalizable.

Clearly the best of all the questions posed. So many ill posed/philosophical/unknown questions here it makes my head hurt! Is the moon there if you don't look at it? The moon isn't even a quantum particle, and it's not a reasonable question if it was.

 

[heldervelez]

How to derive the value of Mass property of particles?

 

[Demystifier]

The moon isn't even a quantum particle,

But it is made of quantum particles, isn't it?

 

[amir11]

a bit late but I think very simply ask him what the photon is.
probably every one knows the famous answer by einstien!

 

[humanino]

a bit late but I think very simply ask him what the photon is.

It's a quantum of the photon field. You may as well say "but what is the photon field ?", however one can play this game with any question. Same game could be played with "but what is a quantum, really ?". My point is : within QED, a photon is a well-defined mathematical concept, and if you want to go into "but what is it really ?" the only answer you will get is that QED works fine as far as experiment is concerned.

 

[Count Iblis]

It's a quantum of the photon field. You may as well say "but what is the photon field ?", however one can play this game with any question. Same game could be played with "but what is a quantum, really ?". My point is : within QED, a photon is a well-defined mathematical concept, and if you want to go into "but what is it really ?" the only answer you will get is that QED works fine as far as experiment is concerned.

That's why all physicists should believe in Tegmark's ideas about reality being purely mathematical in nature: All that exists is only abstract math, and thus our universe is the mathematical model that describes it and nothing more.

 

[Frame Dragger]

The same question parents and teachers have struggled with forever! "Where do baby Quantum Physicists come from?"

 

[torquil]

That's why all physicists should believe in Tegmark's ideas about reality being purely mathematical in nature: All that exists is only abstract math, and thus our universe is the mathematical model that describes it and nothing more.

Since I'm reading Penrose's "Road to Reality" at the moment, I've been exposed to a lot of discussion about the "nature of reality". However, I find the concept of "reality" to be much less well-defined than the physical theories that are used in practice.

For me it is not sufficiently well-defined to talk about the nature of reality.

What is great however, is that the ability of physics to describe/predict our observations of all kinds of phenomena is improving with time, independently of how one wishes to define "reality".

Torquil

 

[yoda jedi]

not only quantum physicists

Is the moon there when nobody looks?

i like it !
easy one !

"Which interpretation of QM is the correct one?"

i like it !

Your friend obviously knows very little, to have claimed he knows so much.

I would ask him to solve the hydrogen atom by path integral methods, and explain clearly why Feynman wasn't able to do it. If he can do this then he is arrogant and clever, rather than arrogant and stupid.

(This is not a known unknown - Kleinert did it in 1979 so it is certainly doable, but you can imagine that if it left Feynman stumped, it is not an easy problem even though the H-atom is almost the first bit of QM anyone studies.)

well said !

 

[amir11]

It's a quantum of the photon field. You may as well say "but what is the photon field ?", however one can play this game with any question. Same game could be played with "but what is a quantum, really ?". My point is : within QED, a photon is a well-defined mathematical concept, and if you want to go into "but what is it really ?" the only answer you will get is that QED works fine as far as experiment is concerned.

As far as I know a photon has not got any clear description even in terms of QED. I ment the quot by einstien
: "All the fifty years of conscious brooding have brought me no closer to the answer to the
question: what are light quanta? …..…… Of course, today every rascal thinks he knows the
answer, but he is deluding himself.”

 

[Living_Dog]

My friend thinks he can answer any question related to Quantum physics (although he claims he won't be able to answer known unknowns i.e 'the mass of the Higgs-Boson particle'). However, I would like to challenge him with a series of the hardest questions anyone on this forum can put to him.

I promise to post his answers up on this blog for you to see what he comes back with.

Many Thanks!

Is the cat alive?

 

[Frame Dragger]

Is the cat alive?

Do that one better and ask him about 'Wigner's Friend'.

EDIT: If you want to trip him up with something obvious, ask him what the difference is between the 'Sum Over Histories' for a particle, and the "Path Integral" for a particle. If he says anything other than, "same thing", shoot him, he's an alien.

 

[torquil]

As far as I know a photon has not got any clear description even in terms of QED. I ment the quot by einstien
: "All the fifty years of conscious brooding have brought me no closer to the answer to the
question: what are light quanta? …..…… Of course, today every rascal thinks he knows the
answer, but he is deluding himself.”

What was Einsteins objection to the description of the photon in QED? Is it simply related to his general objection to quantum physics and its non-local measurement aspects?

Or is it related to technical aspects of QED, e.g. regarding the definition of an asymptotically free single photon in/out state, in the presence of vacuum fluctuations? Or maybe problems with convergence of perturbation theory?

Btw, do these technical aspects pose any problem in pure QED?

Torquil

 

[humanino]

As far as I know a photon has not got any clear description even in terms of QED. I ment the quot by einstien
: "All the fifty years of conscious brooding have brought me no closer to the answer to the
question: what are light quanta? …..…… Of course, today every rascal thinks he knows the
answer, but he is deluding himself.”

Again, the photon is well defined mathematical concept in QED. It's actually quite funny that you decided to use a quote from Einstein to justify your attitude, because Einstein himself came up with the idea of a photon. So you might as well have used a quote from Planck rejecting Einstein's idea of the photon.

 

[jtbell]

As far as I know a photon has not got any clear description even in terms of QED. I ment the quot by einstien
: "All the fifty years of conscious brooding have brought me no closer to the answer to the
question: what are light quanta? …..…… Of course, today every rascal thinks he knows the
answer, but he is deluding himself.”

It's been more than fifty years since Einstein died! Don't you think there might be a possibility that we might have learned a few things about photons since then?

 

[DrChinese]

The same question parents and teachers have struggled with forever! "Where do baby Quantum Physicists come from?"

LOL

 

[conway]

How about this one: How would the universe be different if there were no such thing as photons?

 

[tiny-tim]

How about this one: How would the universe be different if there were no such thing as photons?

"without form, and void"?

 

[Frame Dragger]

"without form, and void"?

A Biblical reference... wow. This is a gooooood thread.

I submit also that this could be taken as, "What is the hardest question [for YOU] to ask a quantum physicist?" in which case... we have a whole new challenge. Then perhaps the hardest question would be, "Why do you love that stupid Boson and not the children?!"

@DrChinese: What can I say, sometimes you must turn to the classics in all things.

 

[Tao-Fu]

That's why all physicists should believe in Tegmark's ideas about reality being purely mathematical in nature: All that exists is only abstract math, and thus our universe is the mathematical model that describes it and nothing more.

Hmm, I would stray more toward our models being purely mathematical. Science consists of models or descriptions. We can only ever make definitions and tabulate our observations. We can not ever address the question of what anything actually "is". For example, I can tabulate a set of observed properties for an electron but I can't really say what an electron IS.

Our descriptions correspond to mathematical models but I think it unfounded to claim that reality/the universe _is_ a mathematical model. The universe is not equivalent to our descriptions of it.

 

[ytuab]

If we can't say what "spin" actually is (not only as the "mathematical" models), we can't say what the electorns and the quantum phenomena actually are. (See this thread).
Because all the electrons always have "spin" in QM

If all phenomena in this world are caused by the quantum mechanics, mustn't we say what any phenomena around us actually are forever?

 

[yoda jedi]

Hmm, I would stray more toward our models being purely mathematical. Science consists of models or descriptions. We can only ever make definitions and tabulate our observations. We can not ever address the question of what anything actually "is". For example, I can tabulate a set of observed properties for an electron but I can't really say what an electron IS.

Our descriptions correspond to mathematical models but I think it unfounded to claim that reality/the universe _is_ a mathematical model. The universe is not equivalent to our descriptions of it.

i agree.
they just, mislead one for other.
(description for reality or reality by description)
REALITY is independent of explanations.

If we can't say what "spin" actually is (not only as the "mathematical" models), we can't say what the electorns and the quantum phenomena actually are. (See this thread).
Because all the electrons always have "spin" in QM

If all phenomena in this world are caused by the quantum mechanics, mustn't we say what any phenomena around us actually are forever?

i agree, and at the end in any case, spin are relative (i say, the orientation).

 

[kikushiyo]

Another question (for Unification people) : why perturbative gaussian fixed points are expected to drive the flow of from one theory to another ?

 

[torquil]

Regarding this debate about what "reality" really is, I will hereby proclaim in the spirit of Nietzsche:

Reality is dead!

According to Wikipedia, the historic Nietzsche quote about god is "Nietzsche's way of saying that the "God" of the times (religion and other such spirituality) is no longer a viable source of any received wisdom".

In the same manner, I believe that it does not give us any valuable wisdom by debating what is real and what is not.

Science however, is fortunately alive.

Torquil

 

[yoda jedi]

a bit late but I think very simply ask him what the photon is.

well...
maybe:

The photon is no strict particle and nonlocality is far from
being proven Fachverband Theoretische und Mathematische Grundlagen der Physik
2010.

Karl Otto Greulich.

Two aspects of philosophical discussions on physics are the wave particle
dualism and non locality including entanglement. However the
strict particle aspect of the photon, in the common sense view, has
never been proven. The accumulation time argument, the only experimental
verification of a strictly particle like photon, has so far not
yet been satisfied. Also, experiments thought to prove nonlocality
have loophole which have so far not yet been safely closed, and now
an even more serious loophole emerges. Thus, also nonlocality cannot
be seen as proven. This demands some fine tuning of philosophical
discussions on critical experiments in physics.


--------------
and apart

Single Molecule Experiments Challenge the Strict Wave-Particle Dualism of Light
http://www.mdpi.com/1422-0067/11/1/304/pdf

 

[Troponin]

Too easy...
A much more difficult problem would be finding a question on Quantum Physics that I can answer.