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What's wrong with Physics?


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28 minutes ago, andrew s said:

We can do it that way but don't have to hence Largrangian and Hamiltonian mechanics and principle of least action.

Both of these are deeply deterministic.

 

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@vlaiv we will have to agree to differ. I don't view the state function as real in the way you do. In your world reality lies in a potentially infinite rigged hilbert space. In mine it sits in 4d spacetime.

In many world all potential possibilities exist so you went to the pub in one world  and in another you did not. 😊

Regards Andrew 

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6 minutes ago, vlaiv said:

Both of these are deeply deterministic.

 

Yes but they are not of the initial conditions + differential equation form which you claimed were the basis of our physics.  That was the point I was make here.

Regards Andrew 

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2 minutes ago, andrew s said:

In many world all potential possibilities exist so you went to the pub in one world  and in another you did not.

Yes, but, like I said - I don't think that many worlds is the solution.

I still haven't found answer to my objection to it, and it is fairly simple one. If we prepare electron with spin 1 degree of vertical and then measure it in vertical direction - we have achieved following setup:

experiment with only two outcomes has probability that is not 50:50. Not sure how many worlds can explain that we end up being a copy with spin up majority of the time. It also implies that opposite copy (or rather all other copies for repeated experiment) have different rules of physics.

I wonder how come that our branch always follows same set of physics rules (or has in past 100 years of knowledge of QM).

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1 hour ago, Macavity said:

I feel Science need all the support it can get.
If I can "LOVE" Brian Cox, so should you! 🤣

Has Chris just proven that we do not live in a deterministic universe!  

Jim 

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3 minutes ago, andrew s said:

Yes but they are not of the initial conditions + differential equation form which you claimed were the basis of our physics.  That was the point I was make here.

Regards Andrew 

They are however equivalent.

Initial conditions + transition function is equally deterministic as having two states and path between them that minimizes some quantity (after all - there is one and only one path that minimizes some quantity - otherwise it would not be minimal).

After all - that is why two formulations are equivalent as well.

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Just now, saac said:

Has Chris just proven that we do not live in a deterministic universe!  

Jim 

I think that it is obvious that we don't live in deterministic universe.

Problem is that our equations depict one.

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13 minutes ago, vlaiv said:

experiment with only two outcomes has probability that is not 50:50. Not sure how many worlds can explain that we end up being a copy with spin up majority of the time. It also implies that opposite copy (or rather all other copies for repeated experiment) have different rules of physics.

So I prepare a dice with 6 up. No matter how often I measure it is stays at 6 (unless I disturbed it). This seems quite normal to me.

As I understand it branching only occurs when the probability is not 100% 

Regards Andrew 

Edited by andrew s
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4 minutes ago, vlaiv said:

I think that it is obvious that we don't live in deterministic universe.

Problem is that our equations depict one.

Yes in classical mechanics no in QM. There are no deterministic equations for the disintegration of a radioactive nucleus or for the spontaneous emission of a photon from an excited atom.

Regards Andrew 

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1 minute ago, andrew s said:

As I understand it brancing only occurs when the probability is not 100% 

From what I understand - there is not really branching as such.

It seems that popular explanation fail to convey essence of this interpretation.

Many worlds just says there is universal wave function - and it can - like any other wave function be in superposition of states (like single electron can be in superposition of spin up and spin down).

"Branching" (in the lack of better term) - happens when entangled states loose phase and can no longer interfere.

So we have electron that is both spin up and spin down (super position) - and we measure it. It gets entangled with measuring device and new state is superposition of two states spin_up and measured spin up, and spin down and measured spin down - but because of decoherence - these two no longer interfere.

They are part of same wave function - but we have "branch" of sorts as these are now separate parts that no longer interfere.

In this view, when you say that you prepared dice 6 up and measured it - then you did not start in superpositioned state and did not further entangle dice with environment (it was already entangled by preparation and decohered) - and in that sense - there is no branching.

What is interesting is that when there is branching - like when spin of electron is slightly misaligned with direction of measurement - we have branching to two distinct new states - but these states don't have equal probability.

Now, in order to verify that probability - "our branch" must end up in respective state on each branch - and that can never happen for some cases (irrational probability), or is generally very small probability of it happening.

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Sabine has a blog page and the comments can get quite interesting 

http://backreaction.blogspot.com/2019/03/particle-physicists-continue-to-spread.html?m=1

”I don't have time for this nonsense. That Berners-Lee happened to work at CERN when he invented the www is the most stupid of all stupid arguments for building a next larger collider.” 🤣

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8 minutes ago, andrew s said:

There are no deterministic equations for the disintegration of a radioactive nucleus or for the spontaneous emission of a photon from an excited atom.

QED?

 

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2 hours ago, andrew s said:

Not if you believe in the "block" Universe.  It all exists but we just experience it a moment at a time.

I'm attracted to this as a resolution of the apparent conflict between determinism and free will. The universe (past, present, future) already "exists" as a completed entity, but we cannot access this perspective from our position within the universe. From our viewpoint, we appear to have agency, but it vanishes when seen in totality. Though I'm not sure if that theory is any more than untestable metaphysics.

 

2 hours ago, saac said:

Do these existential crisis only afflict Physics, I've never seen such applied to Chemistry or Biology!  What is it about Physics that seems to attract such statements

Because, as everyone knows, physics is second only to mathematics in approaching the fundamental essence of reality:

purity.png.d2a697747e682f883cab2ee4416f656d.png

https://xkcd.com/435/

 

1 hour ago, vlaiv said:

If something is deterministic - then it is predictable. Can we predict it with 100% certainty? That really depends if:

1) we can determine initial conditions with sufficient precision

2) we can determine physical laws with sufficient precision

Yes, I think we may be discussing more than one level here.

On the one hand, there's a question as to the nature of reality, irrespective of our (or any other) attempt to measure and predict it; that's ontology rather than epistemology. If we can't be sure about that, we can never predict with an arbitrary degree of accuracy. The (quantum) uncertainty in the initial measurement is of that kind.

Then, even if you take a realist/Platonist view on the universe (i.e. it has objective, quantified existence), it still might be the case that it can't be modelled and predicted, for purely practical reasons. The uncertainty described by chaos theory is, I believe, of that sort: the nature of certain processes is such that the calculated future values of certain quantities do not converge, however accurately you know their initial values.
I would add another example from maths: there are real numbers (in fact, "almost all" real numbers are of this kind) that can be defined unambiguously, yet whose values cannot be computed to arbitrary accuracy, even in principle. Some practitioners (intuitionists, etc.) have taken this as evidence against the existence of the real numbers and, in some cases, by extension, against reality itself. But I think it's equally possible to view it as being compatible the alternative perspective: the universe exists objectively but is unknowable in arbitrary detail.

 

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10 minutes ago, andrew s said:

No for the photon you can just calculate the probability of emissions and similarly for the nucleus and QCD. Regards Andrew 

https://physics.stackexchange.com/questions/258256/how-can-quantum-tunnelling-lead-to-spontaneous-decay

https://en.wikipedia.org/wiki/Spontaneous_emission#Theory

What happens is that given initial state of atom - it evolves into superposition of decayed + not decayed states (depending on surrounding field vacuum states). There is time evolution of this process and from that you can calculate probability of finding atom in decayed state at some time. It is wave function that is used to calculate probability.

It is also foundation for Schrodinger's cat thing - atom is both decayed and not decayed which leads to cat dead and alive thing.

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2 hours ago, vlaiv said:

https://physics.stackexchange.com/questions/258256/how-can-quantum-tunnelling-lead-to-spontaneous-decay

https://en.wikipedia.org/wiki/Spontaneous_emission#Theory

What happens is that given initial state of atom - it evolves into superposition of decayed + not decayed states (depending on surrounding field vacuum states). There is time evolution of this process and from that you can calculate probability of finding atom in decayed state at some time. It is wave function that is used to calculate probability.

It is also foundation for Schrodinger's cat thing - atom is both decayed and not decayed which leads to cat dead and alive thing.

Which supports what I said it only predicts probabilities.  To be deterministic it has to predict when it will decay. 

As mentioned previously not all quantum systems have a wave function and so Schrodingers equation can't  be used you need a more general theory. The Pauli matrices for electron spin are an example.  

Schrodingers cat is a nonsense no macroscopic object can be in such a superposition the interaction with the air it needs to breathe will almost instantaneously decohere it to one state or the other. (Or failing that the CMB will do it just as quickly.)  Schrodinger proposed it because it was a nonsense in his view.

Regards Andrew 

PS all the interesting things in QM happen at the point of "measurement " when Schrodingers equation no longer applies. It is still a mystery - the measurement problem is one outstanding problem in the foundations of physics. 

 

Edited by andrew s
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3 hours ago, vlaiv said:

They are however equivalent.

Initial conditions + transition function is equally deterministic as having two states and path between them that minimizes some quantity (after all - there is one and only one path that minimizes some quantity - otherwise it would not be minimal).

After all - that is why two formulations are equivalent as well.

Lol it was a little joke vlaiv: I was pulling Chris' leg about his new found admiration of a certain celebrity Physicist :)  

Jim 

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Posted (edited)
9 minutes ago, saac said:

Lol it was a little joke vlaiv: I was pulling Chris' leg about
his new found admiration of a certain celebrity Physicist :)

I think the whole world has now been PUT OFF Particle Physics... 🤣
But I hope there are still keen/robust scientific youngsters etc. lol.

Edited by Macavity
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3 minutes ago, andrew s said:

Which supports what I said it only predicts probabilities.  To be deterministic it has to predict when it will decay.

Thing that we measure is not complete picture of reality. It is only part of underlying reality.

Similarly you can say for particle that it's motion is not predictable (as we can't be sure of its momentum and position) and hence not deterministic - but it is deterministic in the way wave function evolves.

Bell's inequality shows that underlying wave function is in fact element of reality - despite the fact that we don't know if particle is spin up or spin down (it is both at the same time and that is fine).

12 minutes ago, andrew s said:

As mentioned previously not all quantum systems have a wave function and so Schrodingers equation can't  be used you need a more general theory. The Pauli matrices for electron spin are an example.  

https://www.youtube.com/watch?v=4nxnoTVbNGM

(Time evolution of electron spin state)

12 minutes ago, andrew s said:

Schrodingers cat is a nonsense no macroscopic object can be in such a superposition the interaction with the air it needs to breathe will almost instantaneously decohere it to one state or the other. (Or failing that the CMB will do it just as quickly.)  Schrodinger proposed it because it was a nonsense in his view.

I agree - but it shows time evolution of system starting with decay as it evolves into superposition of states (this is used as starting part of argument to point out that final superposition never happens).

Similarly - if you look at tunneling - we can say that particle tunnels at some random point - but what really happens is that wave function (or vibration in quantum field of QFT) splits into two parts - one in potential well and one outside.

At that time we can't say that particle is either in potential well nor that it is outside - best description is simply that it is both - or rather super position of positions and will stay that way until (something magical happens) we determine its actual position - either still in potential well or tunneled outside.

All of that does not exclude time evolution of wave function

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18 minutes ago, vlaiv said:

Bell's inequality shows that underlying wave function is in fact element of reality - despite the fact that we don't know if particle is spin up or spin down (it is both at the same time and that is fine).

QM is a generalised probability theory. The simplest is the one where the "amplitudes " or probabilities sum to one. The next is where the sum of the squares of the "amplitudes " sum to one. This is QM. What Bell showed is that it can't be replicated by a local hidden variable theory with the first type of probability.  

An example would be a red and green sock each put in a box and separated. On opening one you box know instantly what the result of opening the other would be. You can't do this for QM systems. There are not hidden variables like sock colour. 

I am not arguing time evolution does not happen in a continuous way. All I am maintaining is that predictions of probability is not the same as being deterministic. 

Saying a coin will land 50/50 heads/tails is not being deterministic.  Being correct 100% of the time in saying the next tossing will give a head and it does or tail and it does is.

Regards Andrew 

Edited by andrew s
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56 minutes ago, Macavity said:

I think the whole world has now been PUT OFF Particle Physics... 🤣
But I hope there are still keen/robust scientific youngsters etc. lol.

I find these sort of discussions a double edge sword Chris. Of course there is a hidden beauty of the true nature of reality that frustratingly alludes our understanding; maybe that is actually a good thing given our (humanity) tract record.  But I sometimes wonder if we overlook the more beautiful reality, that of our biological or lived experience.  I like to think that whatever we are (life) we are raised above the quantum fields, energy, and wave functions in terms of true reality.  It is such a specialist area of Physics,  obscured by mathematics and statistics, that it is all too easy to dismiss with an ignorant "so what" on hearing of the latest particle (pentaquark etc). It can be a hard sell.  A hard sell, especially to teenagers who have pretty much nailed the art of "so what" :) 

Jim 

Edited by saac
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Posted (edited)
13 hours ago, saac said:

I find these sort of discussions a double edge sword Chris. Of course there is a hidden beauty of the true nature of reality that frustratingly alludes our understanding; maybe that is actually a good thing given our (humanity) tract record.  But I sometimes wonder if we overlook the more beautiful reality, that of our biological or lived experience.  I like to think that whatever we are (life) we are raised above the quantum fields, energy, and wave functions in terms of true reality.  It is such a specialist area of Physics,  obscured by mathematics and statistics, that it is all too easy to dismiss with an ignorant "so what" on hearing of the latest particle (pentaquark etc). It can be a hard sell.  A hard sell, especially to teenagers who have pretty much nailed the art of "so what" :) 

Jim 

Most "Particle Physicists" (sic!) must (openly or secretly) lament the notion that we have apparently entered
the "particle discovery desert" that was predicted - Many years ago. But, back then, most had not imagined
the charmed quark (discovery), the intermediate vector boson (discovery), top quark, and Higgs (discovery). 😎

It is fortunate (unfortunate?) that the main stages of my "science career" coincided with many of the above.
There was a lot of *beauty* (symmetry) in the "Eightfold Way"... Charmed quark discovery... the spectroscopy
of particles etc. Such things were *relatively easy* to understand (In some ways, I remain Chemist at heart!). 🙃

Equally though, I can empathise with Sabine's concern about Physics and it's current "beauty" obsession?!?
But then, I'm not really a "Wunderz of Science" type! When I was doing my "Video Astronomy", aside from
a cursory glance skywards (to see if there were ANY Stars!) that was it. But once or twice, on nights of good
seeing, I didn't even *bother* to "set up"... So I am not entirely immune to the "trancendental" experience! 🤪

(Apparently) Sabine can sell her "Chat with a Theorist" at *50 Bucks* for 20 mins!
Ever think you were in the *wrong* profession? Or maybe a job opportunity... 🥳

Edited by Macavity
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