Can photons or other particles form a wave-function again after an "observation" or "measurement"?

[Dagl1]

Can photons or other particles form a wave-function again after an "observation" or "measurement" and what determines when or how that happens. So for example, we attempt the double slit experiment, where we have a measurement device at/before the slits, and have thus collapsed the wave-function, leading to 2 lines and not an interference pattern. So if we put the screen further away, then add new splits, would we at some see an interference pattern again? 

I hope that my question itself is clear, in case there are technical reasons why my above experiment wouldn't work.

Thanks!

[swansont]

They always have a wave function. The collapse is a loss of superposition of states, not the loss of having a wave function.

You can send particles through multiple sets of slits and you will have interference, as long as you create a new superposition.

[Sensei]

Quote

Can photons or other particles form a wave-function again after an "observation" or "measurement"?

The photon ceases to exist after it is absorbed by matter.

The same with proton capture, neutron capture, neutrino capture, electron capture etc. etc.

 

[Strange]

1 hour ago, Sensei said:

The photon ceases to exist after it is absorbed by matter.

The same with proton capture, neutron capture, neutrino capture, electron capture etc. etc.

 

Although it is possible to make measurements of the photon and hence "collapse the wave function" without destroying it (without interacting with it at all). Typically by using an entangled partner.

[studiot]

16 hours ago, Dagl1 said:

So if we put the screen further away, then add new splits, would we at some see an interference pattern again? 

Well even in classical physics no screen is 100% opaque however thick the screen, which means some photons will pass right through.

The problem with moving the screen back and adding new slits in front is that you need the screen to observe interference.

Also the wave approaching the first set of slits approximates a plane wave, whereas the wave after the slits approximates a spherical wave approaching the second set.
So you have already changed the conditions of the experiment.

Further in a quantum experiment you have the added complication that the quantum wave function extends to infinity beyond the screen, even if the probability is very very tiny.
This is, of course, fully compatible with swanson't earlier comment that it is the state not the wavefunction itself that disappears.

Edited November 27, 2019 by studiot

[swansont]

You can put slits where you would expect an interference maximum to occur, so light going through must have interfered to get there. 

[Dagl1]

Alright so it seems my question boils down to: how does the particle/wave function form new superpositions, and what determines the time frame before this happens?

[studiot]

14 minutes ago, Dagl1 said:

Alright so it seems my question boils down to: how does the particle/wave function form new superpositions, and what determines the time frame before this happens?

I'm not sure about boiling down but to answer this question,

Superpositions are combinations of possible solutions to the quantum wave equation.
Because this equation is a partial differential equation its solutions contain arbitrary functions (not constants like an ordinary differential equation).
Possible solutions depend upon the boundary conditions as well as the equation itself.
The equation itself does not change so the boundary conditions must change.

[Dagl1]

After reading the thread again, I think I didn't absorb the information that much yesterday, but also realise this goes a little over my head for now.

So @studiot regarding your first message; I now understand the issue regarding planar vs spherical waves, so (please correct me if I am wrong) at first the wavefunction is only expanding "forward" (again my apologies for the lack of the right terminology, I don't know if either "expanding" or "forward" or the right words to use) towards the slits, but afterwards is expanding like a sound wave would, but that would only happen if there is no measurement at the first slits (in the double slit experiment, if we observe through which slit the wave travelled through, we see 2 lines instead of an interference pattern, right?)? But if we do no measurement, placing new slits after the first slit would not capture all of the spherical wavefunction (thereby completing changing the experiment)? 

For my understanding, if I would, lets say put a doubleslit at x distance (lets say, 5, 10 or 15cm away),  so that the new slits are directly in the path of light that goes through 1 of the slits. What kind of pattern (at all) would a screen behind this second pair of slits capture, if we have observed/measured at the first slits. And this is in a perfect vacuum with as little interference as possible from radiation(and whatever else is needed for such a thing to experimentally work). 

My thought process: As we observe the wavefunction at the first slits, we would expect the waves to produce 2 straight lines, so the wavefunctions are still travelling in a straight line, and have not become an interference pattern, so from the perspective of the second slits, this is just the light that has gone through 1 of the slits and since we aren't measuring here, I expect that this would then produce a normal interference pattern. But only if the wavefunction is not collapsed anymore because of the measurement at the first slits, so I was wondering, how would such a wavefunction "re-form" the state of superpositions in the time traveling to the next slit? 

I hope there is at least some part of this stuff that I understand, but I realise most of my assumptions or thought process will most likely be wrong or just filling in information, so thank you all for your help!

-Dagl

[Strange]

1 hour ago, Dagl1 said:

So @studiot regarding your first message; I now understand the issue regarding planar vs spherical waves, so (please correct me if I am wrong) at first the wavefunction is only expanding "forward" (again my apologies for the lack of the right terminology, I don't know if either "expanding" or "forward" or the right words to use) towards the slits, but afterwards is expanding like a sound wave would, but that would only happen if there is no measurement at the first slits (in the double slit experiment, if we observe through which slit the wave travelled through, we see 2 lines instead of an interference pattern, right?)? But if we do no measurement, placing new slits after the first slit would not capture all of the spherical wavefunction (thereby completing changing the experiment)? 

I think the discussion of plane and spherical wave fronts only applies to the classical view (where light is a continuous wave). 

I don't think you can interpret the (quantum) wave function in that way. If you want to get an understanding of how the quantum view reproduces classical results, I recommend the Feyman lectures on QED (to a lay audience) - available online as videos (from the Vega Trust) or, for the old codgers among us, as a book.

[Dagl1]

1 minute ago, Strange said:

I think the discussion of plane and spherical wave fronts only applies to the classical view (where light is a continuous wave). 

I don't think you can interpret the (quantum) wave function in that way. If you want to get an understanding of how the quantum view reproduces classical results, I recommend the Feyman lectures on QED (to a lay audience) - available online as videos (from the Vega Trust) or, for the old codgers among us, as a book.

Ye will do, I also realise that I should, just like many here are recommended, understand the math behind this stuff, so I suppose I will be taking a look at the "scary" part of physics;p

[Strange]

4 minutes ago, Dagl1 said:

Ye will do, I also realise that I should, just like many here are recommended, understand the math behind this stuff, so I suppose I will be taking a look at the "scary" part of physics;p

There is some pretty complex math involved. I can just about stagger around the foothills and look admiringly at those climbing into the swirling mists around the peaks (this relates to a great extended metaphor I read once).

There is a great site from Prof. Matt Strassler which has good articles on lots of topics in quantum theory. Including an introduction to the math: https://profmattstrassler.com/articles-and-posts/particle-physics-basics/fields-and-their-particles-with-math/

Nd the Feynman lectures: http://www.vega.org.uk/video/subseries/8

[studiot]

55 minutes ago, Dagl1 said:

Ye will do, I also realise that I should, just like many here are recommended, understand the math behind this stuff, so I suppose I will be taking a look at the "scary" part of physics;p

 

59 minutes ago, Strange said:

I think the discussion of plane and spherical wave fronts only applies to the classical view (where light is a continuous wave). 

I don't think you can interpret the (quantum) wave function in that way. If you want to get an understanding of how the quantum view reproduces classical results, I recommend the Feyman lectures on QED (to a lay audience) - available online as videos (from the Vega Trust) or, for the old codgers among us, as a book.

 

Yes there are several differences between the the quantum wave function and classical travelling waves.

Understanding the Maths is good but also needed is a mental picture of the Physics and making sure they correlate.

 

So classical waves are distributed in space and time so that they interfere at a point in space when two interfering waves meet at the same place at the same time.
Two plane waves can interfere, as happens on the surface of the oceans, and you get diagonal patterns depending upon the angle of incidence of one wave to the other.
These appear as diagonal stripes on an intersecting surface. They do not build up in time as a quantum wave does (see below)

Two circular waves can also interfere and you get segmented radial and circumferential patterns which intersect a screen in the familiar 'two slits interference pattern'
Again these are static in time.

 

Quantum 'waves' on the other hand are not travelling waves insofar as they extend to the boundaries (or infinity) at all times.
It's just that the amplitude of the 'wave' may be very tiny indeed in some places.
Travelling particles, such as electrons may be modelled by neoclassical 'pilot waves' in some versions.
In any event they are travelling disturbances in the quantum field which exists all around
Interference happens slightly differently in that you need a 'timed exposure plate' to capture the hit pattern of say electrons fired through two slots.
This is what I mean by the quantum interference pattern builds up in time as a result of many electrons passing through.
One or two electrons will not give you an interference pattern, (I know there are some experiments where the observer can look for quantum or look for particle solutions),
but many electrons will.

[Dagl1]

2 minutes ago, studiot said:

Quantum 'waves' on the other hand are not travelling waves insofar as they extend to the boundaries (or infinity) at all times.
It's just that the amplitude of the 'wave' may be very tiny indeed in some places.
Travelling particles, such as electrons may be modelled by neoclassical 'pilot waves' in some versions.
In any event they are travelling disturbances in the quantum field which exists all around
Interference happens slightly differently in that you need a 'timed exposure plate' to capture the hit pattern of say electrons fired through two slots.
This is what I mean by the quantum interference pattern builds up in time as a result of many electrons passing through.
One or two electrons will not give you an interference pattern, (I know there are some experiments where the observer can look for quantum or look for particle solutions),
but many electrons will.

It seems like you are saying that without a plate or screen there is no interference pattern? and I assumed in an experiment one would shoot many electrons (or photons) after each other so they can only interfere with themselves (that is one of the things the double slit experiment shows right?). In case you need a plate or screen at the end, I fail to see how this works, but is this in the same realm as the delayed quantum choice experiment? I will have to rewatch that as well I suppose.

[swansont]

If it’s with light and you’re observing the interference pattern, you’re destroying the photons. 

[Dagl1]

1 hour ago, swansont said:

If it’s with light and you’re observing the interference pattern, you’re destroying the photons. 

Even when observing which slit a photon goes through (then I suppose those experiments aren't done with photons but electrons or other mass-carrying particles?)

[swansont]

16 minutes ago, Dagl1 said:

Even when observing which slit a photon goes through (then I suppose those experiments aren't done with photons but electrons or other mass-carrying particles?)

If you do a which-path experiment you don’t get interference

[Dagl1]

Yes, so one would get 2 diagonal beams right (if you would have a screen after it), if now we place another 2 slits in the path of 1 of those beams, do we get another interference pattern from that second double slit? 

[Enthalpy]

About the original question:

Many experiments are made with photons, which are fragile particles, and this biasses the understanding and most explanations about QM.

Atomic Force Microscopes make observations and measurements of valence electrons (call them wavefunctions if you wish) without destroying the electrons, which go back to the original state afterwards. Interestingly, this microscope observes all the time the same electron pair, using one single electron pair, as opposed to a tunnel effect microscope, or to experiments with photons. Meditating that should debunk many misconceptions about observations, perturbation, observability, statistical nature of the wavefunction or not.

And the pictures are nice! There
science.sciencemag.org
and by googling
Pentacene AFM
Other pictures show also Lumo, no just Homo.

It should be clear enough that
- A measurement is not always destructive
- It doesn't always change or "collapse" the wavefunction
- The wavefunction can be observable and observed.

Edited December 5, 2019 by Enthalpy

[Mordred]

What does this have to do with the OPS question ?  Perhaps you should stick with the quantum physics in particular wave particle duality.  All particles are described by their wavefunction states under QM and QFT. 

They exhibit both pointlike and wavelike characteristics. The wavefunction is the probability of finding the characteristics we associate with a given particle at a particular location at a particular time. 

Edited December 7, 2019 by Mordred

[Edgard Neuman]

On 11/27/2019 at 12:51 PM, swansont said:

They always have a wave function. The collapse is a loss of superposition of states, not the loss of having a wave function.

You can send particles through multiple sets of slits and you will have interference, as long as you create a new superposition.

Hi,
I'm going to loose some other reputation points....  (I invoke the 1st amendment)
Everything is made of particles, the human, the brain, the air, the photomultiplier..
So maybe the only way of seeing the difference between multiple states and single states is relative to the present.. all states of photon hits the screen, maybe its only when it is multiplied into macroscopic world than states are selected...
Imagining that reality as a big tree of consequences made of various states branching into multiple new states at each moment.
- suppose simultaneous realities can always only differ to a certain extant (so the universe doesn't split into parallele stories).. (that is not limited by distance, allowing all the entangled things)
- but still at each point in time, there's always multiple state for each particles.. 
- stories always divides into several branches, but some branches constantly dies... (That's really what need to be solved in my opinion)
- so from the present, when we look far enough in the past (relatively to the scale of what we are looking at), what remains is only the common ancestors of all current states.. the states who branched into the reality that remains when all the others died.

In that views, (which I suppose is what the "quantum darwinism" is about), there's no need for "decoherence"

Edited December 12, 2019 by Edgard Neuman

[swansont]

1 hour ago, Edgard Neuman said:

Hi,
I'm going to loose some other reputation points....  (I invoke the 1st amendment)

This being science, and not a government entity, your rights pertaining to speech or religion aren’t relevant.

 

1 hour ago, Edgard Neuman said:

Everything is made of particles, the human, the brain, the air, the photomultiplier..
So maybe the only way of seeing the difference between multiple states and single states is relative to the present.. all states of photon hits the screen, maybe its only when it is multiplied into macroscopic world than states are selected...
Imagining that reality as a big tree of consequences made of various states branching into multiple new states at each moment.
- suppose simultaneous realities can always only differ to a certain extant (so the universe doesn't split into parallele stories).. (that is not limited by distance, allowing all the entangled things)
- but still at each point in time, there's always multiple state for each particles.. 
- stories always divides into several branches, but some branches constantly dies... (That's really what need to be solved in my opinion)
- so from the present, when we look far enough in the past (relatively to the scale of what we are looking at), what remains is only the common ancestors of all current states.. the states who branched into the reality that remains when all the others died.

In that views, (which I suppose is what the "quantum darwinism" is about), there's no need for "decoherence"

I prefer actual QM to supposition 

[Enthalpy]

On 12/7/2019 at 8:11 AM, Mordred said:

What does this have to do with the OPS question ?

It relates very much with the original question: "Can photons or other particles form a wave-function again after an observation". The pictures of pentacene show that the observation doesn't destroy the wavefunction.

[Strange]

5 minutes ago, Enthalpy said:

It relates very much with the original question: "Can photons or other particles form a wave-function again after an observation". The pictures of pentacene show that the observation doesn't destroy the wavefunction.

As pointed out at the start of the thread, the wavefunction is not destroyed by an observation (so you are making a strawman argument).

[Enthalpy]

On 12/7/2019 at 8:11 AM, Mordred said:

The wavefunction is the probability of finding the characteristics we associate with a given particle at a particular location at a particular time. 

In some cases, the wavefunction boils down to a probability density. Generally, it's more than that. As the wavefunction determines interference patterns or future distributions, the particle exists at all the possible positions, not only at one position where we have some chance to find it. The wavefunction has also a phase, which doesn't influence the local probability density, but is all-important to determine the momentum or the angular momentum among others.

Writing of the wavefunction can make it a function of other variables than the position, for instance a function of the momentum.

1 minute ago, Strange said:

As pointed out at the start of the thread, the wavefunction is not destroyed by an observation (so you are making a strawman argument).

True, that was already answered. What the pentacene pictures bring new is that the wavefunction can be observed and then released intact - in some cases.

On 12/12/2019 at 11:34 PM, Edgard Neuman said:

Everything is made of particles, the human, the brain, the air, the photomultiplier.

Sure. Observations are interactions, often with some sort of amplification, that end with an effect that we perceive by our senses. And I agree too that superposition may go on at the detecting particle(s), measuring device, observer. This was a debate question, and maybe it's still one.

On 12/12/2019 at 11:34 PM, Edgard Neuman said:

So maybe the only way of seeing the difference between multiple states and single states is relative to the present.. all states of photon hits the screen, maybe its only when it is multiplied into macroscopic world than states are selected...

I haven't grasped from you text where past/present/future makes a difference.

Yes, all the possible paths for the photon contribute to the electron's wavefunction at the screen (or wherever) to give for instance a probability density of detection there.

One interpretation common presently is indeed decoherence. The many stories exist simultaneously, there is no wavefunction collapse at all, the measurement instrument exists in all the states that result from the many photon histories, and the observer too. In a given state, the observer is not aware of the other states because decoherence let the other states (that have become en even huger number by the many microscopic interactions) add up to nearly zero.

A very nice advantage of this interpretation is that is supposes nearly nothing. It superposes states as we observe they do in other circumstances. Maybe this interpretation has drawbacks.

So you propose instead some limitation on the number of possibilities, histories, scenarios that coexist and may sometimes (generally at microscopic scale) produce stable and observable interferences - if I understand you properly. That's an interesting option, provided it answers some difficulties of the competing interpretations. I don't grasp why you got a neg eval, and I gave you a pos.

But is there a rationale, observations, re-interpretation of experiments... something that suggests this limitation or selection among the stories? Adding an arbitrary hypothesis isn't generally desired, it gets accepted (like the wavefunction collapse was or is) if it fits the observation or simplifies the understanding.

Cumulating and propagating all the possible states of a particle isn't a good interpretation - I botched that in an other thread, changed my mind meanwhile. Apparently this description suffices when the possible states are exclusive: spin up or down, photon detection at pixel 27 or 38... But for instance a photon's polarisation is not exclusive. Vertical, horizontal, biassed, right, left, elliptic... if one wants the photon to exist in all these possible states, he gets more than 1 for the total probability. Other problem, with two entangled photons he doesn't explain the observed correlation of the photons' polarisations. So some sort of "decision" must happen at the detection, being it a collapse of the wavefunction or something else.

I don't grasp when such a collapse or equivalent should happen or not, so I welcome alternative theories too.