Notices
Results 1 to 21 of 21
Like Tree1Likes
  • 1 Post By Flick Montana

Thread: Can you help me understand the double slit experiment

  1. #1 Can you help me understand the double slit experiment 
    New Member
    Join Date
    Sep 2012
    Posts
    1
    Niels Bohr said this: "Anyone who is not shocked by quantum theory has not understood it".

    I guess I don't understand it then.

    I remember reading about the double slit experiment years ago and my mind was completely blown. I then forgot all about it and today I have picked up some books to remind myself how the experiment worked, but I'm feeling underwhelmed.
    It would be good if someone more knowledgeable on this subject could point out what I may be misunderstanding, or if there's a part of it that I have missed.

    I'm going to go lay out my understanding of the experiment first, so that I can be corrected if I've got anything about it incorrect.

    ================
    To start off, we have a light source, which is aimed at a screen with two slits, these two slits can be open or closed. Additionally we have a second screen, which sits behind the first, which registers the light that hits it after passing through the first screen.

    So when there's only one slit open, a single band of light passes through the first screen and is visible on the second screen, consistent with the idea that light behaves as a wave (but at this point it's also fairly consistent with the idea that light behaves as a particle, ie, passing straight through).

    When there's two slits open, an interference pattern appears, which is also consistent with the idea that light behaves as a wave. The two separate waves passing the through the two separate slits interfere with each other and create the pattern on the second screen. All gravy so far.

    It seems to get interesting when individual photons (one after the other) are fired at the first screen instead of a beam of light. In theory, they should just act as particles and go straight through the slits hitting the second screen in a single band.

    So during the experiment, when individual photons are fired at the first screen with one slit open, a single band appears on the second screen. Ok, that makes sense, the photons appear to be acting as we would expect, as an individual piece of matter passing through a slit and directly hitting the second screen.

    We would expect that if two slits were open, the individual photons would pass through one slit or the other, and after many had been fired you would end up with two distinct bands on the second screen.

    But...that's not what happens...when individual photons are fired at the first screen with two slits open, an interference pattern appears...suggesting that the photons that were fired individually are somehow creating the effect of a wave, this obviously makes no sense.

    So it would seem that the only explanation is that the photon must be passing through both slits at the same time, and then somehow interfering with itself and creating the interference pattern.

    Additionally, if a detector is set up at the first screen to try and discover if the photon actually does pass through both slits at the same time, the photon then suddenly stops acting like a wave and then starts acting like a single photon. Ie, it stops causing an interference pattern on the second screen and instead it just creates two single bands, exactly what you would expect if light behaved as a particle and passed through only one slit.
    Turn the detector off, and it starts acting like a wave again.

    ================


    So to recap on the findings:
    1. It seems that a single particle passes through both slits at the same time in order to create an interference pattern?
    2. It seems that particles only act as a wave if you are not watching them, as soon as a detector is involved, they start acting as simple particles?

    But I'm not feeling blown away by this as I feel I should, I can think of fairly mundane explanations for the two points above, such as...

    1. An individual particle has a wave-like property bound to it (OK that would be pretty cool in itself but nothing insanely spectacular). So a particle passes through only one slit, but the wave-like property of that particle passes through both, thus creating the interference pattern.
    2. The presence of a detector simply interrupts/collapses the wave-like property of the particle so only the particle itself passes through, therefore behaving as a simple particle and not creating an interference pattern.
    1/2. OR maybe this wave is something that is always present. Maybe the individual particle just follows this natural wave and gets caught up in it's interference between the two slits. The presence of a detector interrupts this natural wave and then the particle just carries on separately and behaves as you would ordinarily expect...


    There's obviously more to it than that...what am I missing?




    Edit: Apologies for my use of the words 'photon' and 'particle' if I've misused them...


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Freshman
    Join Date
    Sep 2012
    Posts
    60
    No need to be blown away, sonny boy. It's as fundamental as a falling apple.

    You see, everything in this universe is a particle whose existence is described by a probability wavefunction.

    A single photon has a wave function. It interacts with the wave functions of both slits, and the resulting wave can form an interference pattern.


    As I said, it's a probability wave. Max amplitude at certain position = High probability of being there = More photons ending up there = Bright fringe.

    Zero amplitude at certain position = Zero probability of being there = No photons ending up there = Dark fringe


    Simple right? It also applies to electrons, your entire body, your laptop etc etc.

    And the waves can also form a wavepacket. So the particle has a high probability of being around one location, and behaves a like a solid lump.


    What about the detector? In order to detect, the detector has to interact with the photon. Due to the large mass of the detector, the wavefunction "collapses".


    You managed to nail most of the points. The big question is, what underlies the different fundamental particles?


    Last edited by Naggy Doggy; September 8th, 2012 at 10:59 AM.
    Reply With Quote  
     

  4. #3  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,242
    Quote Originally Posted by ErenVae View Post
    Niels Bohr said this: "Anyone who is not shocked by quantum theory has not understood it".

    I guess I don't understand it then.

    I remember reading about the double slit experiment years ago and my mind was completely blown. I then forgot all about it and today I have picked up some books to remind myself how the experiment worked, but I'm feeling underwhelmed.
    It would be good if someone more knowledgeable on this subject could point out what I may be misunderstanding, or if there's a part of it that I have missed.

    I'm going to go lay out my understanding of the experiment first, so that I can be corrected if I've got anything about it incorrect.

    ================
    To start off, we have a light source, which is aimed at a screen with two slits, these two slits can be open or closed. Additionally we have a second screen, which sits behind the first, which registers the light that hits it after passing through the first screen.

    So when there's only one slit open, a single band of light passes through the first screen and is visible on the second screen, consistent with the idea that light behaves as a wave (but at this point it's also fairly consistent with the idea that light behaves as a particle, ie, passing straight through).

    When there's two slits open, an interference pattern appears, which is also consistent with the idea that light behaves as a wave. The two separate waves passing the through the two separate slits interfere with each other and create the pattern on the second screen. All gravy so far.

    It seems to get interesting when individual photons (one after the other) are fired at the first screen instead of a beam of light. In theory, they should just act as particles and go straight through the slits hitting the second screen in a single band.

    So during the experiment, when individual photons are fired at the first screen with one slit open, a single band appears on the second screen. Ok, that makes sense, the photons appear to be acting as we would expect, as an individual piece of matter passing through a slit and directly hitting the second screen.

    We would expect that if two slits were open, the individual photons would pass through one slit or the other, and after many had been fired you would end up with two distinct bands on the second screen.

    But...that's not what happens...when individual photons are fired at the first screen with two slits open, an interference pattern appears...suggesting that the photons that were fired individually are somehow creating the effect of a wave, this obviously makes no sense.

    So it would seem that the only explanation is that the photon must be passing through both slits at the same time, and then somehow interfering with itself and creating the interference pattern.

    Additionally, if a detector is set up at the first screen to try and discover if the photon actually does pass through both slits at the same time, the photon then suddenly stops acting like a wave and then starts acting like a single photon. Ie, it stops causing an interference pattern on the second screen and instead it just creates two single bands, exactly what you would expect if light behaved as a particle and passed through only one slit.
    Turn the detector off, and it starts acting like a wave again.

    ================


    So to recap on the findings:
    1. It seems that a single particle passes through both slits at the same time in order to create an interference pattern?
    2. It seems that particles only act as a wave if you are not watching them, as soon as a detector is involved, they start acting as simple particles?

    But I'm not feeling blown away by this as I feel I should, I can think of fairly mundane explanations for the two points above, such as...

    1. An individual particle has a wave-like property bound to it (OK that would be pretty cool in itself but nothing insanely spectacular). So a particle passes through only one slit, but the wave-like property of that particle passes through both, thus creating the interference pattern.
    2. The presence of a detector simply interrupts/collapses the wave-like property of the particle so only the particle itself passes through, therefore behaving as a simple particle and not creating an interference pattern.
    1/2. OR maybe this wave is something that is always present. Maybe the individual particle just follows this natural wave and gets caught up in it's interference between the two slits. The presence of a detector interrupts this natural wave and then the particle just carries on separately and behaves as you would ordinarily expect...


    There's obviously more to it than that...what am I missing?

    You need to consider what happens when you send the photon's through one at a time. The individual photon will not produce an interference pattern. What it does is produce a single dot on the screen. It isn't until after a number of photons have passed through that the individual dots on the screen fill in to form the interference pattern with two slits.
    Then you have to consider what makes the difference between a "detector" and the slot itself. Why doesn't passing through the slot(which in itself has an effect on the photon is determining the interference pattern) collapse the wave function?

    Also, there is the "delayed choice" experiment. In this experiment you have two telescopes behind the screen, each focused on a slit. The screen can be removed after the photon has passed through the slit, allowing the telescopes to detect which slit the photon passed through without have a detector at the slit itself. It determined that even when such a detection is made after the fact, the detection prevents the formation of a interference pattern.

    Then there is the quantum eraser experiment, where the information as to which slit the photon passed through is "erased" after it passes through the slit. If the info is not erased no interference pattern forms, but if it is, one does form.
    Last edited by Janus; September 8th, 2012 at 01:56 PM.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  5. #4  
    Forum Freshman Teralek's Avatar
    Join Date
    Jul 2013
    Posts
    15
    I am in Shock! I always been curious about science, I have a degree in biotechnology and Shrodinger story has been a fun story to tell friends.
    But only now, very recently, I was able to grasp this all subject about the double slit experiment and measurement problem!

    I am having an existential crysis! I am beggining to believe that reality is made of information... but that would open more questions that it can solve!
    Reply With Quote  
     

  6. #5  
    Malignant Pimple shlunka's Avatar
    Join Date
    Jan 2013
    Location
    Dogbox in front of Dywyddyr's house.
    Posts
    1,786
    The second slit was a toll road, so the light chooses the first slit to be more economical.
    "MODERATOR NOTE : We don't entertain trolls here, not even in the trash can. Banned." -Markus Hanke
    Reply With Quote  
     

  7. #6  
    Anti-Crank AlexG's Avatar
    Join Date
    Mar 2012
    Posts
    2,809
    ErenVae, you've got a pretty good handle on the two slit.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
    Reply With Quote  
     

  8. #7  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,730
    Quote Originally Posted by Janus View Post
    Then there is the quantum eraser experiment, where the information as to which slit the photon passed through is "erased" after it passes through the slit. If the info is not erased no interference pattern forms, but if it is, one does form.
    I think you omitted the most remarkable part which is that the photon whose which-slit information is being measured is not the same photon that is striking the screen to form or not form the interference pattern, but the other photon of a quantum-entangled pair.
    Reply With Quote  
     

  9. #8  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,730
    Quote Originally Posted by ErenVae View Post
    So when there's only one slit open, a single band of light passes through the first screen and is visible on the second screen, consistent with the idea that light behaves as a wave (but at this point it's also fairly consistent with the idea that light behaves as a particle, ie, passing straight through).
    One point that is not usually mentioned in discussions of the double-slit experiment is that a single slit also produces diffraction of the photon and a single-slit interference pattern. I mention this to point out that the photon is not just behaving as a particle that is passing straight through the slit. The photons passing through both slits will overlap whether there is a double-slit interference pattern or not. The difference between a double-slit interference pattern and no double-slit interference pattern is what is added together at the screen. In the case of a double-slit interference pattern, it is the fields from both slits that are added, which may cancel completely, whereas for no double-slit interference pattern, it is the intensities from both slits that are added, which cannot cancel.


    Describing the double-slit experiment in terms of waves is easy. In fact it was the double-slit experiment performed by Thomas Young at the beginning of the nineteenth century that led to the conclusion that light is a wave. It is describing the double-slit experiment in terms of particles that is the tricky part. If the notion that an individual particle can travel through both slits doesn't surprise you, then well and good, but it certainly isn't classical behaviour.
    Reply With Quote  
     

  10. #9  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    Quote Originally Posted by ErenVae View Post
    So when there's only one slit open, a single band of light passes through the first screen and is visible on the second screen, consistent with the idea that light behaves as a wave (but at this point it's also fairly consistent with the idea that light behaves as a particle, ie, passing straight through).
    One often sees this statement in a discussion about the double slit experiment, but a single slit also makes a diffraction pattern.
    Fraunhofer Single Slit Diffraction
    Just thought I'd point that out, even though it doesn't answer your question.

    ETA: Oops, KJW beat me to it.
    Reply With Quote  
     

  11. #10  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by ErenVae View Post
    Niels Bohr said this: "Anyone who is not shocked by quantum theory has not understood it".
    Note that Bohr said this before most of us were born. Those of us who have grown up with these ideas as part of the zeitgeist may not be so shocked by it.

    (Note to everyone else: ErenVae posted this about a year ago and doesn't seem to have been back since. But interesting discussion anyway.)
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  12. #11  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,730
    Quote Originally Posted by ErenVae View Post
    1. It seems that a single particle passes through both slits at the same time in order to create an interference pattern?
    A single photon will create a single point on the screen. Only with the accumulation of many photons will the interference pattern be seen or not.


    Quote Originally Posted by ErenVae View Post
    2. It seems that particles only act as a wave if you are not watching them, as soon as a detector is involved, they start acting as simple particles?

    Not true. The sinusoidal wave is a consequence of the photon having a definite momentum. It is best to dispense with the notion of a classical particle entirely. All quantum particles are waveforms of some shape. They can be of any shape, including a pulse, giving the appearance of a point particle. In the double-slit experiment, the photon is a double-pulse as it passes through the slits. A waveform can be decomposed as the sum of a set of waveforms with the special property called "orthogonality" (for example, the sinusoidal waveforms of Fourier (frequency) analysis, but there are many other sets of orthogonal waveforms). A measurement (any measurement, not just position) is effectively the random selection of a particular member from a particular set of orthogonal waveforms, the particular set being determined by the observable that is being measured. For example, if the position is measured, the selected waveform will be a pulse, which is why the measurement of position always gives a definite position, and if the momentum is measured, the selected waveform will be a sinusoidal wave of a definite wavelength. The Schrödinger equation is nothing more than a non-relativistic description of a waveform corresponding to a definite energy. The Heisenberg uncertainty principle is nothing more than a relationship between a waveform and its Fourier transform.

    Quote Originally Posted by ErenVae View Post
    2. The presence of a detector simply interrupts/collapses the wave-like property of the particle so only the particle itself passes through, therefore behaving as a simple particle and not creating an interference pattern.

    It is incorrect to view the detector as disturbing the photon. Experiments have been performed where the photon passing through the slits have been converted to an entangled pair of photons, with one being measured for an interference pattern and the other being measured for the slit the photon passed through. Measuring the second photon destroys the interference pattern of the first photon, even though the first photon was not disturbed in any way.
    Reply With Quote  
     

  13. #12  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,730
    The crucial property of a set of waveforms associated with an observable is the mutual orthogonality of its members. Orthogonal waveforms do not interfere with each other due to Pythagoras' theorem (in the Hilbert space). When the which-slit information is measured, the two waveforms corresponding to each slit become orthogonal because this is the observable that is being measured, and therefore do not interfere with each other. This is why there is no interference pattern. But when the which-slit information does not exist, the components of the waveform associated with each slit are not orthogonal and the amount of parallelism or antiparallelism between them produces the interference pattern.

    If the polarisation of the photons passing through one of the slits is rotated by 90°, then the components of the waveform corresponding to each slit will become orthogonal due to this and there will be no interference pattern. Interestingly, the which-slit information can then be obtained by measuring the polarisation of the photons, but even if this is not done, there will still be no interference pattern.
    Last edited by KJW; July 15th, 2013 at 12:12 PM.
    Reply With Quote  
     

  14. #13  
    Life-Size Nanoputian Flick Montana's Avatar
    Join Date
    Jul 2012
    Location
    Flatland
    Posts
    5,438
    Quote Originally Posted by Teralek View Post
    I am beggining to believe that reality is made of information...
    If you start talking about qubits, I'm leaving.
    SpeedFreek likes this.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
    Reply With Quote  
     

  15. #14  
    Forum Freshman Teralek's Avatar
    Join Date
    Jul 2013
    Posts
    15
    Quote Originally Posted by Flick Montana View Post
    Quote Originally Posted by Teralek View Post
    I am beggining to believe that reality is made of information...
    If you start talking about qubits, I'm leaving.
    I'm not educated enough to talk about that. But nothing else makes sense to me, not even qbits. How and why the observation collapses the wave function is a mystery to me and one of the greatest in science.
    Reply With Quote  
     

  16. #15  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Teralek View Post
    How and why the observation collapses the wave function is a mystery to me and one of the greatest in science.
    Maybe that is the definition of observation.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  17. #16  
    Forum Freshman Teralek's Avatar
    Join Date
    Jul 2013
    Posts
    15
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Teralek View Post
    How and why the observation collapses the wave function is a mystery to me and one of the greatest in science.
    Maybe that is the definition of observation.
    Hmmm, interesting... so what is observation, or better yet, who or what can make one?! Is it the scientist looking at the data? Shrodinger's cat? Availability of information to a conscious being? anything interacting with anything can make a mesurement?
    Reply With Quote  
     

  18. #17  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Any interaction counts as an "observation"; and this is what causes the state to collapse to a well defined state. (As I understand it.)
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  19. #18  
    Forum Freshman Teralek's Avatar
    Join Date
    Jul 2013
    Posts
    15
    Quote Originally Posted by Strange View Post
    Any interaction counts as an "observation"; and this is what causes the state to collapse to a well defined state. (As I understand it.)
    The reason I don't accept that is because everything is always interacting with everything else. So the wave function would never be formed in the first place... And we would have no problems, the world would behave coherently, and this would all make sense.

    The only consistent thing in the experiments seems to be if the measurement produces available information or not.

    Well, I don't mind mystery in the age of all knowing people... but this one is sometimes too daunting.
    Reply With Quote  
     

  20. #19  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Markus Hanke pointed me at this interpretation of quantum mechanics the other day. It might help make sense of things (it certainly makes more sense than the Copenhagen or Many Worlds interpretation).
    Relational quantum mechanics - Wikipedia, the free encyclopedia
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  21. #20  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Teralek View Post
    The reason I don't accept that is because everything is always interacting with everything else.
    Not quite. The state is unknown/undefined when it is not interacting. So, for example, while the photon travels from the source to the slits, and then from the slits to the detector. QM says we cannot say anything about how the photon gets from A to B, just what happens when it gets to B. This seems to fit perfectly with the relational interpretation.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  22. #21  
    Forum Bachelors Degree Kerling's Avatar
    Join Date
    Jul 2012
    Location
    Copenhagen
    Posts
    440
    If you want to be shocked, read up upon the: "Wheeler's Delayed-Choice experiment"
    In the information age ignorance is a choice.
    Reply With Quote  
     

Similar Threads

  1. The Double Slit Experiment
    By Andy Swainson in forum Physics
    Replies: 8
    Last Post: September 15th, 2010, 03:41 PM
  2. the double slit experiment in the '50's
    By Tantric in forum Physics
    Replies: 8
    Last Post: March 5th, 2009, 08:45 AM
  3. Double slit experiment
    By leohopkins in forum Physics
    Replies: 3
    Last Post: December 8th, 2007, 06:23 PM
  4. Double Slit Experiment
    By Development in forum Physics
    Replies: 13
    Last Post: October 27th, 2007, 08:28 AM
  5. The double slit experiment
    By leohopkins in forum Physics
    Replies: 0
    Last Post: January 17th, 2007, 05:01 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •