# I Why are there so few experiments in the foundation of physics? Or am I wrong?

#### ftr

Sabine Hossenfelder has a recent thread on her blog about experiments in physics, talking about how much money is spent on dark matter and what not. I actually wanted to open a thread even before that. My observation is that the field of experimental physics in foundation seem to be so thin and confusing (not even clear if they are about foundation). And here we are people spend endless time on the issue with endless papers. For example, lately we were discussing an interpretation called "thermal interpretation" which claimed that the energy we measure is not an eigenvalue but an expectation value of the energy spectrum. I was surprised that this was very hard experiment to do to achieve distinction between the two. also the wavefunction(density matrix) for the hydrogen atom was the only experiment that I heard about, and even that was disputed as not a singe measurement. Even something as simple as Young experiment itself is so controversial since it is not known how the spot actually is created on the detector.
I post here four example papers for experiments and possible experiments. My question is why are there no concentrated effort in that direction which seem to me much more important that the 10 billion dollar telescopes ...etc. Is that lack of funds, difficulty of them, low importance or what? even textbooks seem to avoid the issue like a plague, not half a chapter discussing thing even in advanced textbooks. I have countless books on theory but only managed to by handful of experimental textbooks.
https://arxiv.org/abs/1803.10703

I hope this thread doesn't die out like the subject itself.

Related Quantum Physics News on Phys.org

#### vanhees71

Gold Member
My answer is that the experiments on the foundations of quantum theory are done using standard quantum theory and not some alternative interpretation of it, which is published just a few months ago. I cannot say anything about it since despite some effort, I couldn't understand what's behind this "thermal interpretation" in physical terms. It's not clear to me, whether it predicts different outcomes for real-world experiments (than it's more than an interpretation of QT but a new theory) or whether it just reinterprets the meaning of the QT formalism but leading to the same predictions about observational facts. If this was the case, you couldn't check it "against" QT since it just has the same physical content of QT and thus is just a reinterpretation of QT. Then there's nothing to be tested about it. Just testing QT and finding a result contradicting a correctly calculated prediction of QT would falsify QT in all its interpretations.

If the thermal interpretation is a new theory, then you can by definition test it experimentally against QT. For this purpose you'd first have to find a clear real-world experimental setup and make predictions in the new theory and QT, which contradict each other (and be it only in a minute quantitative detail for some observable). Then experiment could check, which of the two theories are correct (or neither of them ;-)). Whether or not you find some experimentalist willing to do the experiment, which includes of course in finding funds to develop and perform it, depends upon whether the new theory is convincing enough to be considered an intersting alternative to QT.

I don't undestand you complaints in the more general context. There's an entire plethora of quantum-foundational experiments by the AMO community (in a wide sense). Since Bell and Aspect you cannot blame the physics community anymore to avoid or ignore the foundational issues. To the contrary, the entire business starts to reach even the next step of "normality", namely making it from the pure-physicists lab to the applied-engineering development labs. Quantum cryptography is already applied and also the first quantum computers are constructed.

The entire business of the foundations, as I see it, is indeed finished from an academic point of view with the conclusion that QT is correct, and all you need is the minimal interpretation to now apply it to engineering problems with some great opportunities for new gadgets to appear soon. Quantum technologies are now considered also economically as so highly important that there are huge public funds being given to it:

I think this one European example proves you completely wrong! The foundational issues are resolved to an extent that they are considered common knowledge from the fundamental-physics point of view and thus now rather in the ripe state to be used in the applied sciences to develop new technology for the very near future!

#### ftr

I was not really talking about TI specifically, just as an example. My point is there seem to be not enough( or appropriate) experiments to understand what is really behind QM which point to unlimited interpretations and interpretations of interpretations. I have said this a hundred times before WE believe that QM is basically correct, this issue is closed. But the simple hydrogen wavefunction experiment is a clear indication at least some of the things I am saying , I have brought this example so many time maybe I should tattoo it on my forearm although I not a tattoo man. I have also listed some papers on the top of the thread that was spun of on QTT which nobody could even figure out what the hell it is. And I am certainly not talking about applications although I do Hope some useful experiments in foundation that could actually shed some CLEAR light on the subject and not money for the "business" as Sabine would put it. I have also opened a thread about any experiment that proves the position probability, but I got two meager replies which seem to be irrelevant anyway.

#### vanhees71

Gold Member
What's the issue with the hydrogen wave function? The hydrogen spectrum today is among the prime examples for high-precision physics. It's hard to conceive that anything is not understood about it yet. So could you tell me, what you consider not understood?

For me it's also a mytery why people still think there must be "something more behind QT" than the theory as it is used by working physicists. Philosophers often have problems with mundane facts. So maybe that's the case for QT too. QT is just the best description we have about the behavior of matter as far as we can observe it (and we can observe a lot more than just everyday matter around us, making experiments with single atoms, molecules, photons, looking at neutron stars with both electromagnetic and just recently gravitational waves,... everywhere QT describes all observed effects right.

I've no clue, what you mean by "position probability". If it's about $|\psi(\vec{x})|^2$ in non-relativistic QT, also there are quite some experiments showing that it's correct, where it applies (even old ones by Davisson and Germer about electron interference, highly precise neutron interference experiments, also some quite exotic ones like the double-slit experiment with buckyball molecules by Zeilinger et al showing that also for such pretty large objects QT intereference effects are clearly measurable and how the emission of just a few IR photons destroy the coherence and provide some "which-way information").

Don't get me wrong. It's great that all these experiments are done and one should test all fundamental theories to the extreme accuracy possible with present technology just to see whether they finally turn out to be wrong. This would mean some real progress of science.

#### Aidyan

I don't think that the limited interest in funding research on the foundations of QP is particularly surprising. It is a small niche of interest even among physicist. It is something more for philosophers of science and people who ponder on the ontology and meaning of the world and the interpretations of QM, which are highly controversial and probably will remain as such for a long time to go and, more importantly, it did not and probably will not find practical applications. We live in a society that is obsessed with a strictly utilitarian science. What does not potentially produce money or at least lead to some practical application is considered of no interest. Period.
One might then ask why so much has been invested in particle colliders like the LHC or a space telescope that has become, financially speaking, a bottomless pit? My take on this (and I know that you all will certainly disagree .... but that's what I'm quite convinced of after seeing for years how the internal funding policies work) is that due to political reasons and the necessary media resonance science nowadays needs, big science projects are much more likely to get funded. That is, as paradoxically that might sound, projects that involve hundreds or even thousands of scientist worldwide and cost billions are more likely to be funded than little and cheaper enterprises. I think that if the LHC would have costed only few hundreds of mio \$ instead of dozens of billions and would have a staff of only say 100 people involved, it would have never found its way. And the research on the foundations of QP is precisely that sort of thing: it is unlikely it will produce anything with which you can make money AND is not big enough to become a big science project.

#### vanhees71

Gold Member
I don't understand how you come to the impression that research on the foundations of quantum physics is not funded. That was indeed the case before the groundbreaking work by Bell (theory) and Aspect (experiment). It was even a career killer for physicists working in this field. It's a sad chapter in the history of 20th century physics, because it indeed hindered progress in the field.

On the other hand the aversion of physicists against the subject is only too understandable since the subject was plagued (in still partially is) by non-scientific approaches to the problems. Rather than providing a clearly stated scientific argument, i.e., an argument which can be investigated by objective observations and measurements in the lab, some philosophical gibberish has been envoked (most infamously in the unfortunate EPR paper, which even was a failure in the view of Einstein himself, who made is quibbles much clearer in a later paper in 1948), and this is useless for physicists.

That's why it's no surprise that with Bell's work this changed: He provided precisely a clear physical question about the foundations. In some sense he translated the philosophical gibberish of EPR to physics: Based on very general assumptions he constructed a class of "local realistic hidden-variable theories", which make predictions in terms of his famous inequality about certain correlation functions that clearly contradict the predictions of QT, which (at his time only in principle) could be tested in the lab. Subsequently Aspect and others took up the challenge to realize these ideas in real-world experiments and with great success.

Since then the field has exploded rather than being ignored and a lot of specialization related to it resulted from this: quantum optics, nano physics, cavity QED, quantum informatics. You can hardly claim that these fields lack funding at all!

#### phyzguy

I think part of the answer to your question is that the differences between interpretations are by nature not answerable by experiment. All of the different interpretations of QM give the same experimental results, so how could you define an experiment that would decide between them?

#### StevieTNZ

I think part of the answer to your question is that the differences between interpretations are by nature not answerable by experiment. All of the different interpretations of QM give the same experimental results, so how could you define an experiment that would decide between them?
I think each interpretation gives a different result in a given experiment, it is simply at present just hard to differentiate between them. See for example the thought experiment on macro superposition in 'Sneaking a Look at God's Cards'.

#### PeterDonis

Mentor
I think each interpretation gives a different result in a given experiment
No, they don't. They all use the same underlying theory and math of QM.

If you have something that predicts a different result from standard QM in some experiment, that isn't an interpretation of QM. It's a different theory.

Gold Member

#### Demystifier

2018 Award
Experiments testing predictions of alternatives to standard QM exist (see http://de.arxiv.org/abs/quant-ph/0701071 for a review), but there is no much talk about them for two reasons:

1. Practically all such tests performed so far gave only negative results.

2. In comparison with LHC experiments (which also produce mostly negative results), such experiments are relatively cheap and do not involve such big collaborations.

ftr

#### StevieTNZ

No, they don't. They all use the same underlying theory and math of QM.

If you have something that predicts a different result from standard QM in some experiment, that isn't an interpretation of QM. It's a different theory.
Again, see the thought experiment in the book I mention. Also Albert, chapter 7 I believe, 'Quantum Mechanics and Experience'.

#### ftr

So could you tell me, what you consider not understood?
I did not say anything about "understanding". I was just talking about experiments that clearly preferred one interpretation to another because clearly the measurement axiom(Born rule) is an interpretation entangled axiom ( I know I am going to be stoned now). OR experiments that really could enforce the view that it is an objective machinery that which highly disputed in many interpretations, let alone it being the ugliest thing you can encounter in a science where we believe strongly that this existence has nothing to do with us, despite some "new age thinking".

The experiment I was taking about is this

which is very recent. I was hesitant to write this reply because I have a book that I wanted to read about Born's postulate( history) which everybody believed without sufficient experiments at the time and even later with crude technology which even now we seem to have hard time, now I can call the man supergenius for his conclusion (more like godly intuition) if he was/is truly correct.

Sure all the experiments that you point at CONFIRMS the basic QM but many many many questions are left and I don't understand why no nobody is doing anything about it except to write some papers in a bashful way.

#### phyzguy

Sure all the experiments that you point at CONFIRMS the basic QM but many many many questions are left and I don't understand why no nobody is doing anything about it except to write some papers in a bashful way.
Can you list one of the "many many many" questions which could be decided by an experiment? I think most of these questions are interpretation related, and are hence philosophical. These are not decidable by an experiment. As an example, can you suggest an experiment which can decide whether the wave function is "real" or not.

#### ftr

Can you list one of the "many many many" questions which could be decided by an experiment? I think most of these questions are interpretation related, and are hence philosophical. These are not decidable by an experiment. As an example, can you suggest an experiment which can decide whether the wave function is "real" or not.
Well, just for start BM claims that particles do have trajectories, there is nothing philosophical about that. And TI says the energy we measure is not an eigenvalue. Ultimately we want physics to tell us why we (universe ) exist, if it is just about some experiment that CAN be done and we create a model for that, I feel very sad for the human race, now not only they have limited their knowledge but also their spirit is broken and lost. I believe such understanding is pivotal in our future well being as a race and not making colonies on this moon and that planet... etc.

#### phyzguy

Well, just for start BM claims that particles do have trajectories, there is nothing philosophical about that. And TI says the energy we measure is not an eigenvalue. Untimely we want physics to tell us why we (universe ) exist, if it is just about some experiment that CAN be done and we create a model for that, I feel very sad for the human race, now not only they have limited their knowledge but also their spirit is broken and lost. I believe such understanding is pivotal in our future well being as a race and not making colonies on this moon and that planet... etc.
Whether I agree with you or not, what you are saying is just philosophy. Can you define an experiment to tell us whether particles have trajectories or not? When I measure an energy, is there an experiment that you can do that will tell you whether or not it is an eigenvalue?

#### ftr

Whether I agree with you or not, what you are saying is just philosophy. Can you define an experiment to tell us whether particles have trajectories or not? When I measure an energy, is there an experiment that you can do that will tell you whether or not it is an eigenvalue?
That is the whole point of my question. I have worked for 40 years in many field (mostly engineering, we love experiments) and many countries and I am the master of understanding how organizations succeed and fail and all the constraints. I just feel something is wrong, and just wanted other people opinions. I am certainly not responsible for thousands upon thousands of physics institutions around the world.

There has been attempts to answer your question, and I have listed some in my OP I am sure some googling will give you some information and I think I have given enough examples. But I was not about to research the subject here, but I will try to give maybe better and clear examples.

#### PeterDonis

Mentor
see the thought experiment in the book I mention. Also Albert, chapter 7 I believe, 'Quantum Mechanics and Experience'.
I don't have the first book. I do have the second but it's been a while since I've read it and my copy is not handy.

In any case, neither of these books are textbooks or peer-reviewed papers. They might describe things they call "interpretations" that make different predictions from standard QM; but if so then they are not using the word "interpretation" the way it is usually used in QM.

#### f95toli

Gold Member
That is the whole point of my question. I have worked for 40 years in many field (mostly engineering, we love experiments) and many countries and I am the master of understanding how organizations succeed and fail and all the constraints. I just feel something is wrong, and just wanted other people opinions. I am certainly not responsible for thousands upon thousands of physics institutions around the world.
A couple of points. First of all, as has already been pointed out above: Different interpretation predict by definition the same outcome of experiments. If this was not the case they would be different theories. This is certainly true for all the "mainstream" theories including Bohm's

Secondly, whenever someone has managed to come up with realistic idea for experiments that could test the "foundations" these have quite quickly been realised. Bell tests have already been mentioned, but you also have the work on Macroscopic quantum tunnelling (a la Legget) in the 80s and 90s or say more recent work on tests of (non)realistic-(non)local tests (which have become possible because of technology developed for QKD). The main reason you don't here about this work is that so far all experiments have shown that "conventional" QM works just fine an can predict the outcomes extremely well; i.e. no surprises so no new theories are needed.

Last but not least it important to understand that many applications that are now coming online are themselves to some extent telling us quite a bit about nature. It wasn't that long ago that people were still wondering about whether QM was applicable to macroscopic objects; today people are building quantum processors, optical transducers based on MEMS resonators (in a superposition), or even large QKD networks.
A few decades ago no one really knew if this would be possible even in principle.

#### kith

What you seem to miss is that there are physical reasons for the difficulty to give a realistic account of what happens in the quantum realm.

In the range of validity of classical physics, measurements can be considered to simply read out the state of a system. The influence of the measurement apparatus is negligible. The quantum regime is characterized by effects which are so small that the interaction between the measurement apparatus and the system can't be ignored anymore. This is what leads to the problems with simply continuing the paradigm of classical realism. It isn't that humans "have limited their knowledge" - Nature has.

The limited knowledge isn't specific to non-realistic interpretations like the Copenhagen interpretation but is also present in hidden variable theories. It is the reason why the variables are called "hidden"and considered to be unnecessary artificial baggage by many physicists.

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#### StevieTNZ

I don't have the first book. I do have the second but it's been a while since I've read it and my copy is not handy.

In any case, neither of these books are textbooks or peer-reviewed papers. They might describe things they call "interpretations" that make different predictions from standard QM; but if so then they are not using the word "interpretation" the way it is usually used in QM.
They are indeed textbooks for courses. And most likely peer-reviewed, published by university presses. But that shouldn't deter from the fact they mention the answer I stand by.

#### PeterDonis

Mentor
They are indeed textbooks for courses.
Possibly for philosophy courses, at least in the case of Albert's book, which I have read. I have not the other book, but it is advertised as "for the general reader" on its Amazon page. Not for physics courses.

Having a book published by a university press is not the same as having a physics book or paper peer-reviewed by other physics experts in the field.

that shouldn't deter from the fact they mention the answer I stand by.
Nothing you have said changes my response in post #9.

#### ftr

What you seem to miss is that there are physical reasons for the difficulty to give a realistic account of what happens in the quantum realm.

In the range of validity of classical physics, measurements can be considered to simply read out the state of a system. The influence of the measurement apparatus is negligible. The quantum regime is characterized by effects which are so small that the interaction between the measurement apparatus and the system can't be ignored anymore. This is what leads to the problems with simply continuing the paradigm of classical realism. It isn't that humans "have limited their knowledge" - Nature has.

The limited knowledge isn't specific to non-realistic interpretations like the Copenhagen interpretation but is also present in hidden variable theories. It is the reason why the variables are called "hidden"and considered to be unnecessary artificial baggage by many physicists.
I certainly agree with you on your point, and I am not saying that there are no attempts but only that they are not enough. The paper listed by demystifier in post #11 is very much my concern.

"Quantum Mechanics (QM) represents nowadays one of the pillars of modern physics: so far a huge amount of theoretical predictions deriving from this theory has been confirmed by very accurate experimental data. No doubts can be raised on the validity of this theory. Nevertheless, even at one century since its birth, many problems related to the interpretation of this theory persist: non-local effects of entangled states; wave function reduction and the concept of measurement in Quantum Mechanics; the transition from a microscopic probabilistic world to a macroscopic deterministic world 1 perfectly described by classical mechanics (macro-objectivation) and so on. A possible way out from these problems would be if QM represents a statistical approximation of an unknown deterministic theory, where all observables have defined values fixed by unknown variables, the so called Hidden Variable Theories (HVT), a suggestion dating since the celebrated Einstein-Podolsky-Rosen paper of 1935. Even though, in the past decades, many interesting theoretical works were focused on the solution of this problem originating a large debate, nevertheless they could not lead to any conclusive solution [1,2,3,4]. However, in the last years the experimental know-how concerning creation and manipulation of quantum systems has hugely increased, permitting the realisation of several experiments originally thought as Gedanken Experiment and the conceiving of new ones. "

#### weirdoguy

Nevertheless, even at one century since its birth, many problems related to the interpretation of this theory persist
And as been pointed out a few milion times here and in other threads, interpretations are not distinguishable by experiments, so bringing them up in thread about experiments is kind of off-topic.

and I am not saying that there are no attempts but only that they are not enough
Well, apparently you are almost alone with this concern.

#### ftr

And as been pointed out a few milion times here and in other threads, interpretations are not distinguishable by experiments, so bringing them up in thread about experiments is kind of off-topic.
I only quoted what is in a paper that was written by apparently an expert in the field.

Well, apparently you are almost alone with this concern.
Maybe so. that does not make necessarily me wrong.

"Why are there so few experiments in the foundation of physics? Or am I wrong?"

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