The double slit experiment

[Moonshed]

I have just been watching a program about quantum mechanics and the double slit experiment. What I keep hearing is that a conscious observer forces the  photon to go through one or other of the slits as a particle and hit the screen as a bullet would, but if not observed it will behave as a wave, go through both slits and go on to form an interference pattern. Okay, got it. Now here is my question. Is it not a fact that the detectors fitted at the slits are what determines which slit the photon goes through, not the conscious observer? It seems to me that the conscious observer plays no part in this as they obviously cannot observe a photon. This all seems so obvious that I feel I really must be missing something and showing my ignorance on the subject. Can someone please explain to me about the conscious observer’s role in this. I understand how it works with Schrödinger’s cat in the box experiment because we can all see if a cat is dead or alive when we open the box, but the double slit is different. Any input would be appreciated. I feel I should mention that in an alternate reality I am a world famous theoretical particle physicist. 

Edited October 19 by Moonshed

[AstroKeith]

Where did you read the bit about 'conscious'. That I think is a red herring. I've never the heard that word used.

If you just look at the screen (eyes, camera, etc) , you get an interference pattern.
If you put detectors at the slits to see which one the wave/photon goes through, you destroy the interference pattern.

An interesting effect, verified by experiment. If you wait until after the wave/photon has passed through the slits, but then try to measure which one it went through. It still destroys the interference pattern. 

[Moonshed]

AstroKeith 
 Where did you read the bit about 'conscious'. That I think is a red herring. I've never the heard that wordused.
 

From Wikipedia 

A notable example of the observer effect occurs in quantum mechanics, as demonstrated by the double-slit experiment. Physicists have found that observation of quantum phenomena by a detector or an instrument can change the measured results of this experiment. Despite the "observer effect" in the double-slit experiment being caused by the presence of an electronic detector, the experiment's results have been interpreted by some to suggest that a conscious mind can directly affect reality.[3] However, the need for the "observer" to be conscious is not supported by scientific research, and has been pointed out as a misconception rooted in a poor understanding of the quantum wave function ψ and the quantum measurement process.[4][5][6]

[TiffsAndAstro]

Polarised light and filters worry me even more than double slit implications.

Roger Penrose has written some quite provocative things about consciousness requiring quantum effects. I'm not sure anyone serious has posited the opposite, but I could be wrong.

Still, I'm optimistic someone, or some group, with far bigger brains than me can connect the dots at some not too distant point in the future.

I don't think we'll like the result or its implications, however

Edited October 19 by TiffsAndAstro

[Ouroboros]

The idea that somehow  reality snaps into existence (or the wave function collapses) because an observer looks at it strikes me as a somewhat ludicrous idea and one not supported by experiment, or by philosophy. 

I always thought that Heisenberg had it more or less right when he said that the equations of quantum mechanics simply tell us what we can know about a quantum system - nothing more.

The photon is the kind of thing that when traversing a double slit behaves according to the equations of quantum mechanics that describe its behaviour. That’s it. 

[Moonshed]

I did have a go at describing Quantum Mechanics in my website 

http://www.thekeyboard.org.uk/Quantum mechanics.htm

and later in my book. This was back in the late 90’s /early 00’s at which time the conscious observer was at the very heart of the problem. That view seems to have changed over time yet there is still no adequate explanation for the double slit experiment, let alone what Einstein famously described as “spooky action at a distance”.

[saac]

2 hours ago, Moonshed said:

I have just been watching a program about quantum mechanics and the double slit experiment. What I keep hearing is that a conscious observer forces the  photon to go through one or other of the slits as a particle and hit the screen as a bullet would, but if not observed it will behave as a wave, go through both slits and go on to form an interference pattern. Okay, got it. Now here is my question. Is it not a fact that the detectors fitted at the slits are what determines which slit the photon goes through, not the conscious observer? It seems to me that the conscious observer plays no part in this as they obviously cannot observe a photon. This all seems so obvious that I feel I really must be missing something and showing my ignorance on the subject. Can someone please explain to me about the conscious observer’s role in this. I understand how it works with Schrödinger’s cat in the box experiment because we can all see if a cat is dead or alive when we open the box, but the double slit is different. Any input would be appreciated. I feel I should mention that in an alternate reality I am a world famous theoretical particle physicist. 

I'm sure Einstein's quote "spooky action" was a comment on entanglement and not the double slit principle. The wiki article you quoted is playing fast and loose with language as happens in many commentaries regarding quantum physics. Replacing "conscious observer" with "act of measurement" would avoid a lot of the misconceptions. Woo woo managed. 

Jim 

[Moonshed]

14 minutes ago, saac said:

I'm sure Einstein's quote "spooky action" was a comment on entanglement and not the double slit principle. The wiki article you quoted is playing fast and loose with language as happens in many commentaries regarding quantum physics. Replacing "conscious observer" with "act of measurement" would avoid a lot of the misconceptions. Woo woo managed. 

Jim 

Yes, of course “spooky action at a distance” was a reference to quantum entanglement and I thought I had made it clear that it was not related to the double slit experiment.  As for woo woo managed we do not have an explanation for quantum entanglement's instantaneous communication over any distance thus beating the speed of light limit. Plus this point as raised by AstroKeith “An interesting effect, verified by experiment. If you wait until after the wave/photon has passed through the slits, but then try to measure which one it went through. It still destroys the interference pattern.” Interesting. 

[saac]

7 minutes ago, Moonshed said:

Yes, of course “spooky action at a distance” was a reference to quantum entanglement and I thought I had made it clear that it was not related to the double slit experiment.  As for woo woo managed we do not have an explanation for quantum entanglement's instantaneous communication over any distance thus beating the speed of light limit. Plus this point as raised by AstroKeith “An interesting effect, verified by experiment. If you wait until after the wave/photon has passed through the slits, but then try to measure which one it went through. It still destroys the interference pattern.” Interesting. 

When I said "Woo woo managed" it was in reference to the "conscious mind" comment and not that we had a full explanation for the double slit interaction. Too much of physics is blighted by overly poetic language which sets hares running.  

Jim  

[saac]

When I said "Woo woo managed" it was in reference to the "conscious mind" comment and not that we had a full explanation for the double slit interaction. Too much of physics is blighted by overly poetic language which sets hares running. The double slit outcome is certainly a well studied and enduring mystery  

Edited October 19 by saac

[PeterC65]

I thought it was because of quantum entanglement that the photon(s) in the double slit experiment act as they do.

I remember watching the double slit experiment being performed on a Royal Institution Christmas Lecture and seeing the diffraction pattern dissappear into lines as soon as the slot detectors were enabled. It's certainly spooky.

Strikes me that with this, and with dark matter, we are missing some significant things in our understanding of why things are as they are.

I'd love to know the answer in my lifetime.

 

[saac]

7 minutes ago, PeterC65 said:

Strikes me that with this, and with dark matter, we are missing some significant things in our understanding of why things are as they are.

I'd love to know the answer in my lifetime.

 

We're certainly missing a lot and that's the beauty of physics. As we develop an interpretation the understanding it brings opens other levels to explore. It would be sad in a way if we ever come to a complete understanding.  

Jim  

[AstroKeith]

There is an excellent book (IMO) 'On the Origin of Time' by Thomas Hertog who describes the last work he & Stephen Hawking were doing. Discusses quite a few areas where QM and Relativity are getting closer together.

An interesting thought experiment ...

Ask a photon emitted at the Big Bang, 13.8 Billion years ago, how it found the journey and the time that has passed. It wouldn't understand because for the photon, no time has passed (Relativity).

So if two photons travel away from each other, at any future time (measured by us) from their point of view, they are both at the start and end of their journeys. This goes someway towards quantum entanglement.

[Moonshed]

38 minutes ago, AstroKeith said:

So if two photons travel away from each other, at any future time (measured by us) from their point of view, they are both at the start and end of their journeys. This goes someway towards quantum entanglement

Yes, from the photons point of view there is no such thing as the passage of time. I think that our inability to understand time, we see it as flowing in only one direction, is holding back our understanding of the physics of the universe. Time is a puzzling. How long does the present last? Where does it go? Does the past still exist? Does the future exist? Is time continuous or come in discrete quanta? If it does it has to be less than a femtosecond because we have recorded chemical changes taking place at that incredibly small fraction of a second. Time is a real mystery. 

Edited October 19 by Moonshed

[Zermelo]

My understanding was that the physics of Schrodinger/Heisenberg/Dirac describes how the wave develops over time, but doesn't describe how it collapses to a single state when an observation is performed. I think that bit was contributed by Max Born, but it still doesn't explain what determines an "observation", a question that is seen (at least by those in the "shut up and calculate" school) to be within the realms of metaphysics. Hence the philosophical gap being filled (or at least attempted) by Penrose, etc.

[Moonshed]

There are of course many books covering quantum mechanics and I’m sure most of us could recommend a few. One that I particularly like is “Schrödinger’s kittens and the search for reality” by John Gribbon. Although I have a bookcase full of such books and trying to pick out just one is almost impossible but this is a good one. 

[saac]

Regarding time, I'm pretty sure there is mainstream acceptance now within the physicist community that our biological experience of time certainly obscures a deeper and altogether non intuitive nature. But perhaps that peculiar experience of time is the very thing that brings meaning to life and what it means to live. I for one am glad to experience fleeting moments, the sense of memories from yesterdays I can no longer visit and the promise and hopes of things to come. Have I a need for eternity? I don't know.

Jim 

Edited October 19 by saac

[Deadlake]

I can understand where the idea of consciousness causes the collapse of the wave function, it came from the  Neumann–Wigner interpretation of quantum mechanics however with sensors this idea of "consciousness" is required to collapse the wave function has been disproved. 

Any measurement of a system will cause the collapse of the wave function within the limitations of the Heisenberg uncertainty principle. 

Returning to the double slit experiment the measurement of the electron or photon at the slit is the trigger for the wave function is collapsing.

The word observation in this context  is confusing.

Now what happens if the double slit experiment is run backwards, as long as no system take's a peek at the photon it will return to its point of origin. This is a real weirdness to quantum mechanics.

With regards to entanglement, the first important note is that this does not make super luminous communication possible, the particles that  are entangled need to be together to exchange their state information, say spin up or spin down state. It's the measurement of state causes the collapse of the wave function and since we know the state of one particle we know the state of the other. How do both particles wave function collapse into a concrete state?

The current understanding is that worm holes connect the particles, hence the instantaneous collapses of the wave function . Ironically Einstein  wrote two papers in 1935 around spooky action at a distance and worm holes and did not realise both papers described the same phenomena, the so called EP = EPR papers.
This was released by Susskind and Juan Maldacena's (who also has the most cited ever high energy physics paper, AdS/CFT correspondence) much later who then used this idea to resolve the black hole information firewall paradox , this explanation also requires entanglement via worm holes to resolve this issue...

[Moonshed]

When we talk of quantum entanglement and wormholes, dark energy and dark matter, black holes and virtual particles, multiverses and multi- dimensions, it makes me realise how very little we really know about the universe in which we live. One thing I know for sure, at some point I will die, my atoms will eventually return to the universe from whence they came, I am as much an integral part of the universe as any star or planet and I will again be at one with the universe. Does this make me immortal? I don’t think so because the me that is me at this moment in time will cease to exist, only my already recycled atoms will survive, unless I have a soul, a consciousness that lives on after death, but I have no idea if that is true or not, although I tend to think not. Will we ever know? Will we ever fully comprehend the universe? I hope not, for what would be left to wonder about?

[iantaylor2uk]

The double slit experiment works even if only one photon is sent through the double slits. It is important to note that a photon only ever interferes with itself, not with other photons. If you send through single photons at a time the interference pattern will build up. There is a great popular science book by Feynmann called "QED: The strange theory of light and matter" which has a good discussion of the double slit experiment.

[Moonshed]

24 minutes ago, iantaylor2uk said:

The double slit experiment works even if only one photon is sent through the double slits. It is important to note that a photon only ever interferes with itself, not with other photons. If you send through single photons at a time the interference pattern will build up. There is a great popular science book by Feynmann called "QED: The strange theory of light and matter" which has a good discussion of the double slit experiment.

I have to agree with you, that Feynmann book is a very good read, lots of interesting information explained clearly. As you say, single photons fired through either slit unobserved will place themselves on the screen such that when enough have gone through they will have built up an interference pattern on the screen. A single photon interferes with itself! How does each photon know where to place itself on the screen in order to build up the interference pattern? It really is a mystery. 

Edited October 19 by Moonshed

[Ags]

Doesn't distance dilate as well. So the light from a distant star arrives instantly at my eye, but in addition it traversed no distance getting to me?

[iantaylor2uk]

49 minutes ago, Moonshed said:

I have to agree with you, that Feynmann book is a very good read, lots of interesting information explained clearly. As you say, single photons fired through either slit unobserved will place themselves on the screen such that when enough have gone through they will have built up an interference pattern on the screen. A single photon interferes with itself! How does each photon know where to place itself on the screen in order to build up the interference pattern? It really is a mystery. 

Any particular photon will be in a random position after passing through the slit and hitting the detector screen, but given enough photons, they will be distributed according to the probability distribution predicted by quantum mechanics.

[Moonshed]

35 minutes ago, Ags said:

Doesn't distance dilate as well. So the light from a distant star arrives instantly at my eye, but in addition it traversed no distance getting to me?

The light from a distant star arrives at your eye at the speed of light, so if it is 100 light years away it will take 100 years to reach us, from out perspective. From the perspective of the photons travelling from the star no time has passed, it left the star and instantaneous arrived at your eye. It still traveled 100 light years though. Well, that’s my understanding of it  though I am no expert and may be proven wrong due to the strangeness of Relativity. 

[ollypenrice]

If no conscious observer looks at the consequences of the double slit experiment when detectors are fitted, does the diffraction pattern appear or not? How would we know?

Olly