How Does Counterfactual Computation Work?

[Kea]

I hope this news isn't moved to another forum - many of our readers would be interested in this:

Quantum computer solves problem, without running
http://www.news.uiuc.edu/news/06/0222quantum.html

"It seems absolutely bizarre that counterfactual computation – using information that is counter to what must have actually happened – could find an answer without running the entire quantum computer," said Kwiat, a John Bardeen Professor of Electrical and Computer Engineering and Physics at Illinois. "But the nature of quantum interrogation makes this amazing feat possible."

 

[rtharbaugh1]

Hi Kea

I suspect this is like the following analogy. You have four envelopes and three dollars. You put the three dollars each in an envelope, leaving one empty. You send them to your friend in Minnesota (Thanks!)

I open one, then another, then another. The last one is unopened, but I know what should be in it, even though I don't break the seal.

How is things in NZ? I am longing for Spring. I wonder if Marcus is tickling the peach blossums yet?

Hope you are well...

Richard

 

[Kea]

How is things in NZ? I am longing for Spring. I wonder if Marcus is tickling the peach blossums yet?

Hi Richard

Nice story, but the quantum world is even weirder than that! If you had intercepted all the mail by robbing the LA post centre, you would still come to the same (classical) conclusions.

We are enjoying a nice summer here. Balmy days full of sunshine. Sorry, I'm too poor to send you donations, but I might potentially be sending you money!
Kea

 

[rtharbaugh1]

Hey, that is great! I have to suppose that it will be some of those quantum bucks? The ones where you don't have to rob the LA post centre to get what is in there?

Actually, despite my poor-me philosophy, I am really very comfortable. My cabin is warm, the night is mild, and there is tea and roast potatoes.

Did you see that FQX is getting ready to publish their request for proposals? They now say they will have it on the table by Monday 27 February. I suppose they mean this year.

BTW, I see that Nature 23 Feb has stories on this same topic, quantum computing I mean, and the new findings. I don't have a subscription but I get the content alerts.

R.

 

[Kea]

Did you see that FQX is getting ready to publish their request for proposals?

Ah, you mean http://www.fqxi.org/about.html . Yes, I see. Not sure I want to apply.

 

[Careful]

I see the popularity of ghost like interpretations has no limits...
If people cannot come up with a straight, well done, experiment which demonstrates the necessity of non-locality once and for all, it is of course indispensable to launch even more irrational theories to convince the public.

 

[selfAdjoint]

I see the popularity of ghost like interpretations has no limits...
If people cannot come up with a straight, well done, experiment which demonstrates the necessity of non-locality once and for all, it is of course indispensable to launch even more irrational theories to convince the public.

Emoticons apart, are you denying the content of the experiment?

 

[Careful]

Emoticons apart, are you denying the content of the experiment?

I do not know all the details of the experiment; but what I am saying is that if somehow the computer is running without pushing the button, then a more rational *explanation* can certainly be found. I was just making jokes about how far people are willing to accept a certain type of explanation. No wonder consciousness crackpots claim a legitimate existence in the physics community; banning them would mean that you try to define something like common sense crazyness. The latter being an impossible task.

 

[selfAdjoint]

I do not know all the details of the experiment; but what I am saying is that if somehow the computer is running without pushing the button, then a more rational *explanation* can certainly be found. I was just making jokes about how far people are willing to accept a certain type of explanation. No wonder consciousness crackpots claim a legitimate existence in the physics community; banning them would mean that you try to define something like common sense crazyness. The latter being an impossible task.

That explanation is in a popular article, so it might have been a bit over the top. But the scientific paper is in this week's Nature. I haven't had access to that but I would bet the explanation there (a) supports at least generally what the physicists told the reporters, and (b) Uses standard QM to do it. I don't know if you consider that a "rational explanation" but if you don't, if you're selling some alternative to standard QM, then you should be up front about it.


(Added) By standard QM I mean the formal structure, not any particular interpretation of it, except that "quantum superposition and uncertainty are features of the world."

 

[Chronos]

I thought the experimenters were showing the particle-wave state of the algorithm itself could be used to predict certain computational outputs. Because quantum states are dynamical, quantum algorithms are always resident in a quantum computer. They therefore have a signature that can be used to predict the outcome of certain computations they are programmed to solve. . . at least I think that was the point of the article.

 

[Careful]

**That explanation is in a popular article, so it might have been a bit over the top. But the scientific paper is in this week's Nature. I haven't had access to that but I would bet the explanation there (a) supports at least generally what the physicists told the reporters, and (b) Uses standard QM to do it. **

In nature there also appear papers about how the existence of six extra dimensions could be shown to be true. I am humbly waiting for a detailed paper on the arxiv.

``

Well, this IMO is somehow nonsense: it is impossible to expect one single person to be on top of everything, and moroever, consider his/her alternative ideas to QM only viable if and only if he/she reproduces eighty years of QM results. For example: I was really willing to comment upon the Larsson papers (at least there is some original idea into it), which are on my laptop in the to read list, but I did not find any time since then and the list is growing faster than my reading of it is.


For example, let me once and for all ask you this question: suppose someone would show that the full, stable atomic structure can be recovered from an entire classical theory, would you consider this the end of QM or not? Or, would you consider this a viable reason to investigate this path vigorously or not?

 

[rtharbaugh1]

let me once and for all ask you this question: suppose someone would show that the full, stable atomic structure can be recovered from an entire classical theory, would you consider this the end of QM or not? Or, would you consider this a viable reason to investigate this path vigorously or not?

Suppose someone would show that the moon is made of green cheese. Would you consider this the end of astronomy, or not? Or, would you consider this a viable reason to send cheese miners to the moon, so that we can feed our starving multitudes, or not?

R

 

[Careful]

Suppose someone would show that the moon is made of green cheese. Would you consider this the end of astronomy, or not? Or, would you consider this a viable reason to send cheese miners to the moon, so that we can feed our starving multitudes, or not?

R

I always love it when quantum amateurs come up with an idiotic phrase like yours You clearly do not understand how you just might do that, despite of the several references/hints I have dropped once in a while. Physics is not about citation out of a book, but about a constant exploration to UNDERSTAND nature. Now, QM tells us that if we want to get the right results at the microscopic level, then we have to believe in magic. Therefore, any classical explanation for the very reasons why science impovered in the 1920 ties (atomic spectra, black body radiation) is IMO of major interest. As I said repeatedly and gave references accordingly, there exist hopeful - partial - results in this direction.

 

[Chronos]

Don't be too hard on rt, careful, he was just making a light hearted analogy. The most interesting part of the subject article was the simulation of a virtual quantum computer using laser interference patterns. I was more intrigued by the methodology than the result.

 

[Hurkyl]

If people cannot come up with a straight, well done, experiment which demonstrates the necessity of non-locality once and for all, it is of course indispensable to launch even more irrational theories to convince the public.

They weren't trying to demonstrate non-locality, or validate QM to anybody. They're trying to do quantum computing.

(Or... trying not to do quantum computing, I suppose. )

 

[selfAdjoint]

Cosmic Variance has a wonderful description of QM ideas underpinning the Quantum Zeno Effect at http://cosmicvariance.com/2006/02/27/quantum-interrogation/trackback/. Careful, what do you think of this?

 

[rtharbaugh1]

a visit to fairyland, and the circus

I always love it when quantum amateurs come up with an idiotic phrase like yours You clearly do not understand how you just might do that, despite of the several references/hints I have dropped once in a while. Physics is not about citation out of a book, but about a constant exploration to UNDERSTAND nature. Now, QM tells us that if we want to get the right results at the microscopic level, then we have to believe in magic. Therefore, any classical explanation for the very reasons why science impovered in the 1920 ties (atomic spectra, black body radiation) is IMO of major interest. As I said repeatedly and gave references accordingly, there exist hopeful - partial - results in this direction.

I apologise, Careful, I know you are serious and don't deserve jibes. There is no reason for anyone to attempt to be humorous at your expense. But really, if you would, what is the difference between your statement and mine? Of course, if someone shows that the known features of particle physics can be demonstrated in a classical model, then it would be less than amature-ish to insist on maintaining quantum puzzlement.

However, the facts seem to show otherwise. I cannot explain quantum effects such as are shown in benchtop experiments by classical means. Maybe we need a list so that we can consider them one by one, but starting from the early days, for example, there is the two slit experiment demonstrating wave-particle duality. I have not seen any classical treatment that shows how a single event involving a single photon can behave in that odd way.

Now I will admit to you as part of my apology that the idea of magic is not entirely disreputable in my estimation. You see I am giving you stones to throw at me if you wish. Here: I have in the past and hope again in the future to visit undespoiled wilderness places where elves and other fair critters still reside.

But even close to home, where magic is more often just a street trick, designed with the sole purpose of transfering wealth from the gullible to the crafty, there is a certain pedantic value to it. It is the mystery. How did they do that? Motivation to investigate and perhaps discover.

It happens I am re-reading Penrose, The Road to Reality, in his discussion of imaginary numbers, in which he finds the word "magic" to be useful. Magic, in this sense, and in the sense which I prefer, means the demonstation of verifiable actions which seem to result in consequences which are forbidden under certain rules of behavior which we ordinarily find reliable. Something must be going on which we do not yet see.

Now, I have given up this personal revelation just to show my honesty in that there are lots of things I do not understand, and I am aware that some of them at least are presented in a way meant to decieve me to my loss. I give you this because what I want from you is that you stop using the word "magic" as if it were something entirely reprehensible, in order that we may see what it is that you find reprehensible about the findings you slur as magical.

I don't have a subscription to Nature and have not had an opportunity to closely examine the claims that are made at the top of this thread. I am vaguely aware of the idea of quantum computation and the seemingly magical things that photons can do. I put up the idea of the un-opened letter in my first post in this thread hopeing someone would be able to tell me how this experiment was more than that, but no one here who has read the actual report seems to be forthcoming on that topic. Probably it isn't really very interesting if you have better access to the research than I currently have. No one has taken time to show me how entanglement is different from the unopened letter trick either.

However, I continue in my perhaps naive belief that there is a natural, physical explanation for these events, and faith that I can understand it if I apply myself. I may be wrong on both counts, or either, but I do not find it productive or interesting to brand the events with a slur and so justify my ignorence. If these researchers have found yet another way to uncover the quantum mystery in a bench-top apparatus, I would gladly pay my quarter to enter their sideshow tent.

And if you, dear Careful, have found out the secret of their machinations I would be grateful to you if you presented it here so that I can spend my nickles on soda pop instead. Rather, you seem to me to be a spoil-sport at the circus, muttering darkly that it is all a trick and that anyone who buys a ticket is a fool. Sadly I acknowlege that you are almost certainly right, but I should still like to experience the trick myself, and if possible, figure out how it is done, or, failing that, have someone wiser explain it to my understanding.

Meanwhile, I am having what passes for fun under my thinning canvas, and I hope you are having fun also. And there is always the remote chance that we will discover something interesting.

Thanks for your comments.

Richard

 

[Hans de Vries]

If people cannot come up with a straight, well done, experiment which demonstrates the necessity of non-locality once and for all

In QFT there is an argument that a measurement at position A cannot
influence a measurement at position B if it's outside the lightcone, see for
instance Peshkin & Schroeder paragraph 2.4 at page 27. Causality requires
every particle to have an anti-particle for the commutator to vanish.

Regards, Hans.

 

[Careful]

In QFT there is an argument that a measurement at position A cannot
influence a measurement at position B if it's outside the lightcone, see for
instance Peshkin & Schroeder paragraph 2.4 at page 27. Causality requires
every particle to have an anti-particle for the commutator to vanish.

Regards, Hans.

Obviously: some call QFT dynamically local (precisely for this reason) and kinematically non local (because of entanglement of states) - these are just words (and you have to give up local realism anyway)! Anyway, that was NOT the point: I was talking about an unambiguous EXPERIMENT which demonstrates this action at a distance once and for all and not about the sick theory itself.

Careful

 

[Careful]

Hi R,

Well, classical explanations are bound to be more involved but are nevertheless possible IMO. Now, I am not going to boast around here that I know how it all works since I am a serious person and the calculational work involved is tremendous (actually dr Chinese was once making unjustified fun about this). What I am trying to tell people is that IF you want to have a chance at solving problems, you better start out from a deep CLASSICAL understanding of it. It is very amusing for me to notice (and hard work to go through all of this), that understanding self interactions (as well electromagnetic and gravitational) classically is still a very hard partially open problem (the Lorentz dirac equation for the radiation back reaction is by no means correct - it relies upon some ad hoc assumptions about how the internal particle pressures are to behave). To give you some idea about this ``simple topic´´, here is some *useful* literature:

gr-qc/9912045
math-ph/0505042
gr-qc/0508123
gr-qc/0512111
gr-qc/0306052

Just to whet your appetite. Look: QM is useful (as I repeated many times) and is *by construction* bound to be correct on microscales; however it has a part which gives rise to magic on macroscopic scales (EPR type of experiments). It is precisely there where any sensible person believes QM to be wrong. The fact that nobody has convincingly demonstrated the MAGIC up to date despite of serious efforts strengthens me in that belief. So, what do you have to do? You have to understand the core reasons for the very construction of QM deeply from a CLASSICAL perspective. It turns out that the calculation of radiation back reaction and related issues is the very first oustanding issue in this quest (many smart people have gone that road before - with partial succes). Doing this you will also learn again why QED is still a ill behaved theory (this is already crystal clear at the level of the Lorentz Dirac equation) - Barut has written some illuminating stuff about that.

This is why I despise this good news show about MACROSCOPIC ENTANGLEMENT so much: (a) it is not convincingly demonstrated (b) it is not at all at the heart of quantum mechanics; quantum mechanics = (sub)atomic microphysics which cannot explain the macroworld unless you go to some weird MWI stuff. It almost seems as if this show is an excuse to cover up the fact that we still do not understand classical Einstein Maxwell theory properly.

Concerning my dark muttering and me being a spoil sport. This is kind of funny: you prefer monkeys in the circus (but then go to a real circus) above anyone who tells you that real insight has to come from hard work/ asking deeper questions about nature. The point I try to make is that you better figure first out what an elementary particle is before you start wondering about exotic many particle systems.

Cheers,

Careful

 

[Hurkyl]

however it has a part which gives rise to magic on macroscopic scales (EPR type of experiments). It is precisely there where any sensible person believes QM to be wrong.

A sensible person would judge QM on its merit -- and would not summarily reject its consequences simply because he doesn't like them.

If you feel the need to lace your responses with loaded comments like this, then maybe your position isn't as solid as you think it is.

 

[Careful]

**A sensible person would judge QM on its merit -- and would not summarily reject its consequences simply because he doesn't like them. **

But I do exactly that: I judge QM on its *merits* (atomic structure, superconductivity, superfluidity). In this spirit you should not mind *at all* that I say one should *not* see nonlocality as the main aspect of QM since that has *not* been confirmed. It seems you are mixing up your sentiment about QM with reality here.

**
If you feel the need to lace your responses with loaded comments like this, then maybe your position isn't as solid as you think it is. ¨**

Oh, but my latest response is actually very realistic and humble. My position is not solid at all, but certainly not more shaky than yours. I actually realize something is deeply wrong, you still have to come to that point.

But let's reverse the situation here: you seem to defend the classical statement that radiation problems are well understood and that there exists no way to extract atomic stability from classical EM - otherwise you would not comment in the way you did. To say that, you must have a full knowledge of the above papers, I invite you to comment on them. You see: it is extremely easy to simply repeat what your texbook says, but mind that at least you have to be responsable for a legitimate *scientific* defense of that piece of knowledge.

Cheers,

Careful

 

[Hurkyl]

I would be just as grumpy if someone appeared to be dismissing classical mechanics in favor of QM on the basis that he didn't like the conclusions of classical mechanics.

 

[Kea]

Although we disagree on the meaning of QM, whatever that is, Careful has a good point here. I must admit I'm always uneasy about talk of cats and puppies as if they are just like photons.

 

[rtharbaugh1]

Yeah, Kea, I read the puppy dog tale too. I think we are grown up enough here to talk about the actual results, despite my belief in Santa and his Elves. But what are they? What was the actual experiment? I only know it has something to do with lasers, and I suppose interacting vrs noninteracting spin states?

Doesn't anyone here have a subscription to Nature? Please take a few moments from your busy schedule to fill us in on some details? Otherwise we are all really no better than dogs barking in the darkness.

Careful, don't be gruff. I like you just as well as I like the boys with lasers. In fact I like you better. I talk to you, don't I? But speaking of monkeys, I saw a video clip from a movie made back in the WWII era, sort of a rah-rah thing for IQ testing, in which I saw a monkey do a neat trick.

There was a room with a bunch of bananas eight feet off the floor, and a six foot long stick lying beneath it. A small child was left alone in the room, and soon poked at the bananas until one fell of the bunch. Oh well, something to do I guess.

Then the child went away and a monkey was let in the room. Would the monkey be smart enough to use the stick to knock loose a banana? No. Stupid monkey. Instead, he climbed up the stick like a pole vaulter and grabbed one!

OK, it is more work to climb up the stick than it is to poke at the bananas from the floor. But I never would have thought of that solution! And climbing the stick looked easy for the monkey. In fact, it looked like fun. Plus, I bet the monkey didn't end up with bruised fruit. My opinion? Smart monkey. The child? A poor developmentally disabled primate.

Careful, it isn't any fun teasing you if you take things so personally. That just makes it entirely too easy. This discussion board isn't really about personalities, but about ideas. The personalities make it more interesting, but that has to be a side show. I agree with Kea that you make a good point, if I understand it, that we should look for classical explanations carefully before leaping into the quantum foam. And I would be very happy to hear one. The maths for quantum are very difficult. I don't get it yet. So, are you saying that in your opinion I am wasting my time trying to understand quantum theory? Because that is the feeling I sometimes get when reading your posts.

Anyway, for my part, I offer you the Pax Quanta. It works. We probably will never get the banana, and figure out just exactly why it works. But let's have fun trying, OK?

R

 

[Hurkyl]

The puppy thing could be translated into a more mathematical depiction, if that's what you'd like to see. I will still use the suggestive labels from the story though -- of course they could be replaced with whatever you want.


Suppose we have an unknown particle whose state space has the following basis:
|no>
|asleep>
|awake>

And, for whatever reason, we know that it is in one of the first two basis states.

We have a particle under our control whose state space has the following basis:
|no>
|salad>
|steak>

And we have a device that implements the following transformation:

|steak, no> --> |steak, no>
|steak, asleep> --> |no, awake>
|steak, awake> --> |no, awake>
|salad, x> --> |salad, x>
|no, x> --> |no, x>


We also will be applying the rotation:
|salad> --> (1/sqrt(2)) [ |salad> + |steak> ]
|steak> --> (1/sqrt(2)) [ -|salad> + |steak> ]
|no> --> |no>

So our experiment is as follows:
1. Prepare our particle in the |salad> state.
2. Rotate it.
3. Apply the measuring device.
4. Rotate it.
5. Observe our particle.


Observe what happens to the |no> state of the hidden particle:
1:: |salad, no>
2:: (1/sqrt(2)) [ |salad, no> + |steak, no> ]
3:: (1/sqrt(2)) [ |salad, no> + |steak, no> ]
4:: |steak, no>
5:: 100% chance of observing steak.

Now, observe what happens to the |asleep> state of the hidden particle:
1:: |salad, asleep>
2:: (1/sqrt(2)) [ |salad, asleep> + |steak, asleep> ]
3:: (1/sqrt(2)) [ |salad, asleep> + |no, awake> ]
4:: (1/2) [ |salad, asleep> + |steak, asleep> ] + (1/sqrt(2)) |no, awake>
5::
25% chance of observing salad, and leaving hidden particle unchanged.
25% chance of observing steak, and leaving hidden particle unchanged.
50% chance of observing no, and having altered the hidden particle.



So, we have given ourselves some ability to distingush between the |no> and |asleep> states without disturbing the particle. And with a more clever approach, you can arbitrarily improve this method.

I haven't yet worked out how to map this into the quantum computer case, but I think this at least conveys the idea about what's going on.

 

[Kea]

Kwiat has made a few clarifying remarks over at CosmicVariance:
http://cosmicvariance.com/2006/02/28/paul-kwiat-on-quantum-computation/

I don't have a subscription to Nature.

 

[Careful]

I would be just as grumpy if someone appeared to be dismissing classical mechanics in favor of QM on the basis that he didn't like the conclusions of classical mechanics.

hehe, exactly what I thougt , the problem being that we don't know what the conclusions of classical mechanics are in the first place. But I see you are more interested in magic devices which operate on steaks and salads, counting up complex amplitudes of nonlocal quantities and magically taking the square if the ``experiment is done´´

Cheers,

Careful

 

[Chronos]

Perhaps you misunderstood what has been posted here, careful. I think many of us have a deep understanding of classical mechanics. Perhaps it would be illuminating to clarify your point. It was obscure to me.

 

[Careful]

Perhaps you misunderstood what has been posted here, careful. I think many of us have a deep understanding of classical mechanics. Perhaps it would be illuminating to clarify your point. It was obscure to me.

My god! Generic classical chaotic dynamics in three spatial dimensions is only understood at a fairly elementary level and moreover only in some special cases. Of course everyone knows Newtons laws, knows what torque is, understands to some point GR, but very few deeply understand their consequences. Why do you think that it happens fairly often that a full analysis of a non linear problem reveals surprising, counterintuitive results ? I think my point of view is CRYSTAL clear, as should be obvious from the papers I quoted above.

Cheers,

Careful

 

[jgraber]

Photons, not puppies

Richard and Kea, well, I do have a subscription to Nature, and the real objects in the actual performed experiment are photons and Pockels cells, of course, not puppies. They even mention "single 670nm photons conditionally prepared by downconversion".They do discuss how to do a similar experiment with trapped ions. They also discuss, but have not performed, or at least do not report, the more advanced experiments of chained, as opposed to single, quantum Zeno effectand of error correction. They do give a diagram of a complex, but apparently feasible, optical setup that would perform the chained quantum Zeno effect with Grovers algorithm.

Careful et al, Quantum experts sometimes say QM is signal-local but not Einstein-local. I interpret this to mean that if you understand locallity as signal-locality, no experiment has or will demonstrate non-locality. If you interpret locality as Einstein-locality, then the EPR-Aspect type experiments have already demonstrated it up to some very quibbly loopholes. (Closing these loopholes will be hard, but I expect it to happen eventually. Proving this impossible would be a big revolution.)

(Understanding the difference between signal-locality and Einstein-locality is sort of a mind blower all by itself, especially that it is really possible to be consistently one without the other.)

Nevertheless, I think signal-locality is closer to what is ordinarily understood by locality than Einstein-locality. Thus you can still say QM is "local". In one variation of this form of language you say EPR-Aspect prove that QM is "contextual" instead of "non-local". Understanding what "contextual" means in this context is another mind-blower. However, it appears that many quantum computer people appear to prefer to say QM is "non-local", but of course they know it is "signal-local". Of course, they use the term "superposition" which can be interpreted to imply non-locality when it is a "simultaneous" superposition of spatially separated states. Take a look at Dave Bacons course and all the mentions of "entanglement and nonlocality".
http://dabacon.org/pontiff/

By the way, I just found Scott Aaronson's blog has a post objecting to the popular press reports. http://www.scottaaronson.com/blog/

At a quick reading, the Nature article never mentions "locality" or "contextuality", but talks a lot about "counterfactual" computing. Of course, they use the term "superposition" which can be interpreted to imply non-locality when it is a "simultaneous" superposition of spatially separated states.

"I'll stop here for now and perhaps post more later about the difference between photons and puppies, and maybe also whether counterfactuality is related to nonlocality.


Jim

 

[rtharbaugh1]

In QFT there is an argument that a measurement at position A cannot
influence a measurement at position B if it's outside the lightcone, see for
instance Peshkin & Schroeder paragraph 2.4 at page 27. Causality requires
every particle to have an anti-particle for the commutator to vanish.

Regards, Hans.

Hi Hans.

So, if we descend the lightcone to the very tip, that is, consider extremely short event times, does it reach a quantum limit in which it resumes the shape of a full sphere? In other words, given the Planck quantum, might there be influence between A and B even if the angle of measurment does not fall within the lightcone, when A and B are extremely close together?

I am trying out the idea that gravity at these short lengths and times is spread out into areas which at slightly longer lengths become inaccessible. I'd like to know what you think about this approach.


Thanks,

Richard

 

[rtharbaugh1]

My god! Generic classical chaotic dynamics in three spatial dimensions is only understood at a fairly elementary level and moreover only in some special cases. Of course everyone knows Newtons laws, knows what torque is, understands to some point GR, but very few deeply understand their consequences. Why do you think that it happens fairly often that a full analysis of a non linear problem reveals surprising, counterintuitive results ? I think my point of view is CRYSTAL clear, as should be obvious from the papers I quoted above.

Cheers,

Careful

Hi Careful

I hope to have time to look at those papers today. You know, it seems to me that the main objection to your posts is that you seem to like to imply that everyone else here is an idiot. That rankles, and makes it difficult to get past the personality thing to the ideas.

I clown around because I want it to be clear that if I offend someone by calling one of their foundational ideas idiotic, it is forgivable because I admit that I am an idiot about many things myself. Whoa, I think I just found another way for someone else to make a million bucks. How about a twelve step program for egotists?

Hello, everyone, my name is Richard and I think very highly of myself, but it is OK because I know from long experience that I am only a human, and humans are very silly creatures.

Thanks for the links.

R.

 

[Hurkyl]

Well, classical explanations are bound to be more involved but are nevertheless possible IMO.

(emphasis mine)

What is your point? It sounds as if you are saying that we don't know absolutely everything, and therefore we should have faith that all will eventually be explained classically. Is this accurate?

 

[Careful]

**
Careful et al, Quantum experts sometimes say QM is signal-local but not Einstein-local. I interpret this to mean that if you understand locallity as signal-locality, no experiment has or will demonstrate non-locality. If you interpret locality as Einstein-locality, then the EPR-Aspect type experiments have already demonstrated it up to some very quibbly loopholes. (Closing these loopholes will be hard, but I expect it to happen eventually. Proving this impossible would be a big revolution. **

Thanks for repeating what I know for around 12 years now: as I said in my reply to Hans, these are just words; quantum mechanics violates local realism (what you associate to Einstein locality - but it is actually not quite the same) in a clear way. Your comment about the loopholes is irrelevant, 25 years have passed by and the lucrative business of EPR experiments has not succeeded in closing them and they have succesfully been exploited in offering realist explanations. At such a point, reasonable doubt is justified, how long do we need to wait : 100 years ?? By the way, local realists do not have to prove that these loopholes are impossible to close at all: local realism is just fine as long as it predicts the unbiased experimental outcome.


** (Understanding the difference between signal-locality and Einstein-locality is sort of a mind blower all by itself, especially that it is really possible to be consistently one without the other.) **

It is the kind of pervertedness which you can expect from giving up realism - there is nothing ``deep´´ about it; the rest of your message falls in that category. I would like to remind you that I do not know of any 3+1 dimensional realistic (interacting) QFT in which signal locality is a proven fact (apart from the free theories and the ill defined QFT's in the interaction picture), so please do not attribute desirable features to QFT which are by far not known to be true.

Actually, if one thinks about the measurement problem and the fictition of quantum gravity for long enough then consistently insisting upon local realism seems like most logical strategy to go for. Actually, I could go on for hours about the shortcomings of QM as a physical theory (not as an effective scheme under the appropriate circumstances), but I have experienced my efforts to be in vain.

Cheers,

Careful

 

[Careful]

(emphasis mine)
What is your point? It sounds as if you are saying that we don't know absolutely everything, and therefore we should have faith that all will eventually be explained classically. Is this accurate?

What I want to say boils down to this: in the beginning of th 20'th century, the consequences of Maxwell theory were not well known yet - the famous dirac radiation formula for the electron being ``derived´´ only in 1938 (correction!) for the first time (actually CLASSICAL micromagnetism is an active research area now and reveals many new results still); the known classical phenomena basically being limited to some very special class of solutions to simple (celestial) problems and a whole class of linear problems and first order corrections to suitable asymptotic linearizations of simple non-linear ODE's (or PDE's). It is rather obvious that in such times the phenomena observed at the microscale (double split and black body radiation) came as a shock ! Another example (out of many) is the claim made by Feynman that the gyromagnetic factor of 2 for the electron could only be predicted by QM while actually in 1970 or even before, Wheeler, Carter and others realized that the Kerr solutions in GR gave exactly this figure too (actually it was thought that this could not be because the naive calculation with a circular current in the plane gave the factor of one ).

Now QM really was like an emergency solution, a relatively easy formula which was made to fit observations. It's consequences however, non-locality, the probability interpretation and so on, led its inventers to claim that the scheme would probably be replaced soon (Einstein, Schroedinger, Dirac, de Broglie ...). Of course, it does not lead to any of the following insights: (a) it does not explain why charge is quantized (ii) it does not explain *how* a particle decays... all it did was introducing a new *adaptive* force which allows for equilibrium to settle in (but this could have been achieved by classical means - though involving much more difficult computations !).

My suggestion is that if you really want to understand what is going on at the microlevel then you need to look for a realistic model concerning the structure of elementary particles (actually one of the references I gave above shows that this substructure can significantly influence the motion of the centre of mass coordinate in the radiation back reaction process). So in that sense, QM did not offer any understanding, on the other hand QM did offer the realization that equilibrium can be reached through interactions with a vacuum field - which is the starting point of SED. But many other options are possible...

So, nonlocality so far seems a problem for QM, since I am quite optimistic that (sub) atomic quantum physics can be retrieved without giving up local realism. QM in that sense is like thermodynamics of microscopic phenomena. What I want to tell you, is that this is an exciting time for discovering the next range of phenomena classical physics offers us, this thanks to the computational power of the computer.

Cheers,

Careful

 

[Hans de Vries]

Hi Hans.

So, if we descend the lightcone to the very tip, that is, consider extremely short event times, does it reach a quantum limit in which it resumes the shape of a full sphere?

Actually, The QFT propagator leaks a little bit out of the lightcone,
but only in the range of the Compton radius of the particle and expo-
nentially decreasing further out.

This is not that strange since the Compton Radius is about the smallest
range to which you can confine a particle according to HUP

The measurement statement you quote relies on an argument with
vanishing commutators. Sometimes you see this argument 'attacked'
by proponents of non-locallity, see for instance by Zeh here:

http://www.rzuser.uni-heidelberg.de/~as3/nonlocality.html

(last paragraph)

In other words, given the Planck quantum, might there be influence between A and B even if the angle of measurment does not fall within the lightcone, when A and B are extremely close together?

I am trying out the idea that gravity at these short lengths and times is spread out into areas which at slightly longer lengths become inaccessible. I'd like to know what you think about this approach.

What is happening at Planck's scale is anybody's guess Richard. Regards, Hans

 

[Hans de Vries]

Kwiat Links:

Quantum Zeno Effect:
http://www.physics.uiuc.edu/People/Faculty/profiles/Kwiat/Interaction-Free-Measurements.htm


Grover’s search algorithm : an optical approach:
http://www.physics.uiuc.edu/Research/QI/Photonics/papers/kwiat-jmo-47-257.pdf


Regards, Hans

 

[setAI]

David Deutsch has definitively shown [for me anyway] that quantum computing requires a many-worlds interpretation of QM- non-locality is a perceptual error of trying to interpret entanglement across universes in terms of classical mechanics and getting paradoxes

 

[rtharbaugh1]

Actually, The QFT propagator leaks a little bit out of the lightcone,
but only in the range of the Compton radius of the particle and expo-
nentially decreasing further out.

This is not that strange since the Compton Radius is about the smallest
range to which you can confine a particle according to HUP

The measurement statement you quote relies on an argument with
vanishing commutators. Sometimes you see this argument 'attacked'
by proponents of non-locallity, see for instance by Zeh here:

http://www.rzuser.uni-heidelberg.de/~as3/nonlocality.html

(last paragraph)



What is happening at Planck's scale is anybody's guess Richard.


Regards, Hans

Thanks Hans. I wasn't sure how small the length would need to be, but placing it at the Compton wavelength makes it seem more accessible.

So perhaps a window to the other parts of the multiverse would be available for quantum gravity to leak through, and so the argument that other branches of the multiverse could have no affect on our branch is weakened.

I did scan the Mitchison-Jozsa paper, 9907007, and picked this quote:

"Hence the two worlds or branches will never again interfere in practice - re-interference would require an enormous correlated effort to undo the widspread interaction in eq. 2 and is extremely improbable." (page 5)

I just wonder. Eq. 2 just associates a given ket with a long string of other kets with which it interacts. But each ket is also an action, so takes time. Seems to me that the probablilty of interaction will be very high locally. In other words single actions would likely interfere with each other a lot at first, and interactions would only become rare at some distance from the separation event. At what scale would we expect these interactions to be strong?

Thanks,

R.

 

[rtharbaugh1]

gr-qc/9912045
math-ph/0505042
gr-qc/0508123
gr-qc/0512111
gr-qc/0306052

Careful

Downloaded and had a glance at these. I will have time tomorrow to look at them. Thanks, Careful

monkey

 

[Careful]

** Actually, The QFT propagator leaks a little bit out of the lightcone,
but only in the range of the Compton radius of the particle and expo-
nentially decreasing further out.

This is not that strange since the Compton Radius is about the smallest
range to which you can confine a particle according to HUP

The measurement statement you quote relies on an argument with
vanishing commutators. Sometimes you see this argument 'attacked'
by proponents of non-locallity, see for instance by Zeh here:

**

I am confused now, the REAL propagator (and that is the only one used) in QFT has support within the lightcone. It is true that the COMPLEX propagator coincides with the real one inside the lightcone and becomes purely imaginary outside it, with an exponential decay indeed. Real solutions actually do not allow you to take the complex square root (ie. do not satisfy a Schroedinger type equation - therefore, no ``energy´´ eigenstates, those being all complex) the latter being the textbook reason to conclude that first relativistic quantization does not work and second quantization for HERMITIAN (ie. real) fields is necessary. Of course one could try to come up with alternatives to the KG equation which do satisfy causality without introducing the operational machinery of second quantization (or one could not bother about this in the first place which is what you seem to do).

Cheers,

Careful

ADDENDUM: again, Hans, you do not need to go down to the Planck scale to discover ``new´´ physics or to cure the old deseases... the Compton scale is sufficient for these purposes.

 

[rtharbaugh1]

Hi Careful

I was the one who suggested the Planck scale, actually, and Hans corrected me by pointing out that the Compton scale is deep enough.

Careful, I know you do not like MWI. I am curious about what you think of the idea of virtual particles. You seem to want to be the march warden of things at the classical limit. Are virtual particles just more magic, in your opinion?

Thanks

R.

ps computer problems have kept me from looking at those papers, but I promise to do it today if the keyboard doesn't freeze-up again. Thanks, R

 

[selfAdjoint]

What I want to say boils down to this: in the beginning of th 20'th century, the consequences of Maxwell theory were not well known yet - the famous dirac radiation formula for the electron being ``derived´´ only in 1938 (correction!) for the first time (actually CLASSICAL micromagnetism is an active research area now and reveals many new results still); the known classical phenomena basically being limited to some very special class of solutions to simple (celestial) problems and a whole class of linear problems and first order corrections to suitable asymptotic linearizations of simple non-linear ODE's (or PDE's). It is rather obvious that in such times the phenomena observed at the microscale (double split and black body radiation) came as a shock ! Another example (out of many) is the claim made by Feynman that the gyromagnetic factor of 2 for the electron could only be predicted by QM while actually in 1970 or even before, Wheeler, Carter and others realized that the Kerr solutions in GR gave exactly this figure too (actually it was thought that this could not be because the naive calculation with a circular current in the plane gave the factor of one ).

Now QM really was like an emergency solution, a relatively easy formula which was made to fit observations. It's consequences however, non-locality, the probability interpretation and so on, led its inventers to claim that the scheme would probably be replaced soon (Einstein, Schroedinger, Dirac, de Broglie ...). Of course, it does not lead to any of the following insights: (a) it does not explain why charge is quantized (ii) it does not explain *how* a particle decays... all it did was introducing a new *adaptive* force which allows for equilibrium to settle in (but this could have been achieved by classical means - though involving much more difficult computations !).

My suggestion is that if you really want to understand what is going on at the microlevel then you need to look for a realistic model concerning the structure of elementary particles (actually one of the references I gave above shows that this substructure can significantly influence the motion of the centre of mass coordinate in the radiation back reaction process). So in that sense, QM did not offer any understanding, on the other hand QM did offer the realization that equilibrium can be reached through interactions with a vacuum field - which is the starting point of SED. But many other options are possible...

So, nonlocality so far seems a problem for QM, since I am quite optimistic that (sub) atomic quantum physics can be retrieved without giving up local realism. QM in that sense is like thermodynamics of microscopic phenomena. What I want to tell you, is that this is an exciting time for discovering the next range of phenomena classical physics offers us, this thanks to the computational power of the computer.

Cheers,

Careful

This seems rather a hand-waving argument since a quick scan of the papers you linked to supports your point about active research in microlevel classical EM, but none of them provides any account that I could see of double slit, or any other characteristic quantum phenomena.

All this resarch has had nearly 70 years (since your date of 1938) to give at least a glimpse of these things, and what is there to show? If we criticize string physics for 35 years without a testible prediction, shouldn't we also back off micro-EM as a QM substitute for nearly twice as long coming up empty?

My take is that solving the measurement problem will take more quantum, not less. But maybe we should take this discussion to another thread, if not another board.

 

[Careful]

Hi Careful

I was the one who suggested the Planck scale, actually, and Hans corrected me by pointing out that the Compton scale is deep enough.

Careful, I know you do not like MWI. I am curious about what you think of the idea of virtual particles. You seem to want to be the march warden of things at the classical limit. Are virtual particles just more magic, in your opinion?

Thanks

R.

ps computer problems have kept me from looking at those papers, but I promise to do it today if the keyboard doesn't freeze-up again. Thanks, R

Virtual particles are just a way to diagramatically bookkeep the Feynman series and have no direct physical significance at all, so why should I be against that? Moreover, they correspond to unnormalizable plane waves (eigenstates of the momentum operator), so...

Cheers,

Careful

 

[Careful]

**This seems rather a hand-waving argument since a quick scan of the papers you linked to supports your point about active research in microlevel classical EM, but none of them provides any account that I could see of double slit, or any other characteristic quantum phenomena.**

Let me just tell you that interference patters for particles moving in an eather background have even been calculated by Antoon Lorentz as far as I remember. Now, I presume it is feasible to show that a (radiating) electron moving in a suitable microscopic background radiation field is going to resonate with the Fourier component determined by the Compton frequency of the particle, hence resulting in your double slit. I guess this idea must have been worked out already somewhere, but I do not know of any references...


**All this resarch has had nearly 70 years (since your date of 1938) to give at least a glimpse of these things, and what is there to show? If we criticize string physics for 35 years without a testible prediction, shouldn't we also back off micro-EM as a QM substitute for nearly twice as long coming up empty? **

You are funny, the point I wanted to make is that the first reasonable equation for a radiating electron (there existed the lorentz equation before but that one had serious limitations - very slowly varying fields) was developed 20 years after the majority of the physics community was convinced that classical mechanics would predict it to fall on the nucleus Actually, one of the other papers about the Elizer theorem - which came a decade later - shows that this is not necessarily the case for reasonable types of Coulomb fields (even without a background radiation field! ). Then, the other papers show that if you take into account internal structure, then subtantial corrections on the centre of mass motion could be expected. These latest papers nota bene date from 2005 (!) so the topic being very old is far from well understood.

** My take is that solving the measurement problem will take more quantum, not less. But maybe we should take this discussion to another thread, if not another board. **

Oh, if you are prepared to violate Occam's razor by going over excentric constructions such as MWI zombies and alike, go ahead you know.

The point I made is that we better be a bit more humble and try to understand more basic problems: how can one make a definite statement about the radiation behaviour of an electron when we do not even know what the reasonable backreaction *equation* is (let along what an electron is ! ) ? It is for sure not the Lorentz - Dirac equation because of preacceleration and running away phenomena.

And yes, if the same investment would have been made in this kind of reasoning as has been given to string theory, then very likely we would not speak about QM anymore - successes have actually been reached in similar programs as was discussed already at the QM forum.

Cheers,

Careful

 

[Hurkyl]

The point I made is that we better be a bit more humble and try to understand more basic problems: how can one make a definite statement about the radiation behaviour of an electron when we do not even know what the reasonable backreaction *equation* is (let along what an electron is ! ) ?

By using an argument that doesn't require use of the backreaction equation, nor knowledge of the precise nature of an electron.

Oh, if you are prepared to violate Occam's razor by going over excentric constructions such as MWI zombies and alike, go ahead you know.

A violation of your sensibilities does not constitute a violation of Occam's razor.

 

[Careful]

**By using an argument that doesn't require use of the backreaction equation, nor knowledge of the precise nature of an electron. **

Funny: you also have to take into account backreaction effects in QM (of course this is not mentioned in textbooks) for the double slit as well as for the atomic structure (I mentioned that recently in another thread - you know the existence of atoms in QM is not an old RESULT - and for heavy nuclei it is still open in the relativistic case AFAIK ). In both cases, you are obliged to make the full computation in QED which as you know is not renormalizable (it is perturbatively) - so you have to make a cutoff, break Lorentz invariance and so on... So, the same problems come back to you over there. What do you think, that QM allows you to forget about radiation ? By the way, it seems you have what it takes to become an expert in quantum mechanics: you are only remotely interested in the precise nature of things... blurr it up and it will be ok and certainly affirm that we cannot do any better.

**
A violation of your sensibilities does not constitute a violation of Occam's razor. [/QUOTE] **

You should open a restaurant with interfering steaks bien cuit and a point ... , so that the costumers can project it down to their favorite taste without being asked a priori.

Cheers,

Careful

 

[Hurkyl]

Funny: you also have to take into account backreaction effects in QM ...

So what? This is irrelevant to whatever arguments we might be making about classical EM.

The classical textbook proof that you can't have a stable atom doesn't involve at all the forces acting on the electron, nor its true nature. Accelerating charge radiates electromagnetic waves which carries energy. A stable atom is therefore a source of free energy.

All of your arguments that we don't know everything about the classical stuff is irrelevant -- we knew enough that there is a serious problem. In order to reconcile a stable atom with classical EM, you have to tweak something in the theory so that the textbook argument is invalid. Arguments that we don't have complete knowledge, and progress in finding the forces acting on and in an electron does not do that.

I reject your arguments because they continually have the form "We don't know everything. Look, we're still learning new stuff. Have faith and all will be explained", and you don't seem to make any attempt to address the traditional questions that are considered to be killing blows.

(P.S. as I understand it, QM "smears" the electron throughout the entire orbital, and as a result, the system has stationary charge and current distributions -- a configuration that does not radiate, thus resolving the problems with a stable atom)

 

[Careful]

**So what? This is irrelevant to whatever arguments we might be making about classical EM. **

No, it is not, that is the whole point.

** The classical textbook proof that you can't have a stable atom doesn't involve at all the forces acting on the electron, nor its true nature. Accelerating charge radiates electromagnetic waves which carries energy. A stable atom is therefore a source of free energy. **

Nothing says that this energy is free for the taken... [Actually, if you calculate the EM field at large distances from the slow moving source, you get an outward directed Poynting vector (in lowest order) for the radiation - however close to the source, the Poynting vector is ZERO (in first order)...] If you would take the effort to look at the recent papers of Cole (who builds further on the work of Boyer and Marshall) I quoted before (on another thread), you will actually see that an equilibrium between radiation and matter could be formed (provided the existence of zero point radiation) - there are pleanty of papers about this (starting from the 1960'ties) which are all published in Phys. Rev. which is not so easy for realist papers.

**
All of your arguments that we don't know everything about the classical stuff is irrelevant -- we knew enough that there is a serious problem. **

Difficult problems are there to be solved rationally.

** In order to reconcile a stable atom with classical EM, you have to tweak something in the theory so that the textbook argument is invalid. Arguments that we don't have complete knowledge, and progress in finding the forces acting on and in an electron does not do that. **

But a zero point radiation field seemingly does that.

** I reject your arguments because they continually have the form "We don't know everything. Look, we're still learning new stuff. Have faith and all will be explained", and you don't seem to make any attempt to address the traditional questions that are considered to be killing blows. **

But people (and I) do that (Marshall, Santos, Barut...), it takes a while you know


**
(P.S. as I understand it, QM "smears" the electron throughout the entire orbital, and as a result, the system has stationary charge and current distributions -- a configuration that does not radiate, thus resolving the problems with a stable atom)[/QUOTE] **

The static spherical ground state does not radiate (classically), but the orbitals with angular momentum do.