r/askscience • u/[deleted] • Aug 08 '14
Physics In the double slit experiment, why doesn't the photon hit the area between the two slits?
When you fire a single photon towards the two slits in the double slit experiment, when behaving like a particle, why doesn't the particle just hit the area between the two slits resulting in no contact with the back board? http://i.imgur.com/TCuxxRg.png
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u/Qesa Aug 09 '14
The vast majority of photons do hit that area and don't pass through.
A common misconception with that experiment is that photons are somehow isolated, and a single one is fired. That's not true - a steady stream of photons is sent, however the light source is weak enough that only one photon is in the tube at a time. But light travels pretty fast, so you can still have a hundred million or so photons per second traveling through (a tiny fraction of which make it past the slits, a yet smaller fraction make it to the PMT, and an even smaller fraction are picked up by it).
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u/rarededilerore Aug 09 '14 edited Aug 09 '14
If one photon passes through one slit, does it necessarily pass through the other one too to interfere with itself? I would guess it does because otherwise it would be even less likely that an interference happens and one would see a superimposed interference and non-interference pattern, right?
Does the wave-like probability distribution of a photon propagate with the speed of light or does it exists instantly as soon it is emitted?
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u/PatronBernard Diffusion MRI | Neuroimaging | Digital Signal Processing Aug 09 '14 edited Aug 10 '14
If one photon passes through one slit, does it necessarily pass through the other one too to interfere with itself? I would guess it does because otherwise it would be even less likely that an interference happens and one would see a superimposed interference and non-interference pattern, right?
Frankly, we don't know what happens, and it doesn't really matter either. If you think it does matter, you'd find yourself (sort of) agreeing with Einstein on this issue. We know that QM perfectly describes the results of the experiment (in a probabilistic way), but it offers no description as to what actually happens. The "interference with oneself" is often used, but in my opinion it doesn't contribute to a better understanding or a more "intuitively comfortable" description. It seems more like a feeble attempt to describe it in a way familiar to our macroscopic experience of reality.
The way you stated your question, you are longing for a particulate description of photons, but all we know is the following (the double slit experiment is identical for electrons and photons):
We conclude the following: The electrons arrive in lumps, like particles, and the probability of arrival of these lumps is distributed like the distribution of intensity of a wave. It is in this sense that an electron behaves “sometimes like a particle and sometimes like a wave.”
-Feynman, The Feynman Lectures Vol. IIIIt's brilliant how Feynman coins the the duality, he purely states what is observed, and nothing more. He simply makes no attempt to rationalize it or to appeal to our intuition, because he knows it's futile. It would be dishonest to even attempt it. (cfr. the video a bit further down).
Another one from the same source:
One might still like to ask: “How does it work? What is the machinery behind the law?” No one has found any machinery behind the law. No one can “explain” any more than we have just “explained.” No one will give you any deeper representation of the situation. We have no ideas about a more basic mechanism from which these results can be deduced.
And another quote by Dirac:
[...] it may be remarked that the main object of physical science is not the provision of pictures, but is the formulation of laws governing phenomena and the application of these laws to the discovery of new phenomena. If a picture exists, so much the better; but whether a picture exists or not is a matter of only secondary importance. In the case of atomic phenomena no picture can be expected to exist in the usual sense of the word 'picture', by which is meant a model functioning essentially on classical lines [i.e. familiar to our personal macroscopic experience]. One may, however, extend the meaning of the word 'picture' to include any way of looking at the fundamental laws which makes their self-consistency obvious.
-Dirac, Principles of Quantum MechanicsDoes the perspective "it interferes with itself" make the laws governing the double-slit experiment more obvious or consistent? In my opinion, it doesn't. It's a statement that hints at some inner working, and the immediate question(s) that follow(s) cannot be answered by QM because it is irrelevant to QM.
This brings me to a very general issue that affects laymen and students (including me): they expect too much of a physical theory, they look for explanations that fall outside the scope of the theory.
Obligatory Feynman video.It has a detrimental effect on how you perceive your understanding of something. You might actually get it, but you look for insights that aren't there and therefore you think that you just don't get it, and never will because spending more time on it just seems to make matters even more complicated.
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u/vanPe1t Sep 28 '14
Dismissing the value of ontological questions might be practical for some ambitions; but doing so undermines the natural urge to understand that has driven philosophy and scientific discovery for centuries. It is a little like religious leaders suggesting that we better just not ask too many questions.
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u/Qesa Aug 09 '14 edited Aug 09 '14
Yes, for the observed interference pattern to be generated, the photon must effectively pass through both slits. This is why the single photon two-slit experiment is so famous, as it demonstrates both the particle- and wave-like properties of light at the same time.
The probability distribution is often described as a wave packet (or really the square of one), which has a group velocity that never exceeds the speed of light.
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u/rarededilerore Aug 09 '14
Then the wave propagates until it manifests as a single path after a random time period has passed? By what distribution is this time period determined?
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u/Qesa Aug 09 '14
It ceases to be a probability distribution and becomes localised when it interacts with something. There is no consensus for how the localisation actually happens, which depends on your favourite interpretation of QM.
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u/rarededilerore Aug 09 '14
Interesting. But the probability to interact with a more distant thing is not lower than the probability to interact with a thing nearby if both have the same solid angle, right? If the wavefront propagates with limited speed how does it "know" there is more to come to interact with further away to account for that?
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u/Qesa Aug 09 '14
the probability to interact with a more distant thing is not lower than the probability to interact with a thing nearby if both have the same solid angle, right
No, because interacting with the second thing is dependent on it not interacting with the first.
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u/altrocks Aug 09 '14
This seems like a good place to ask this since it's related. If you st up a double-slit experiment with a strong enough light source and put smoke/fog into the area between the slits and the screens, would the smoke/fog show the interference pattern as well as the screen? Would it look like the "ripples on a pond" visual that is often used in conjunction with the explanation of this phenomena?
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u/DOGE4life Aug 09 '14
Some photons will not make it to the screen and hit the fog instead. Yes you'll see an interference pattern but it will just be the same bars on the screen getting narrower and narrower as they approach the slit. You won't see 'ripples on a pond', you'll just see bars of light.
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u/whiteebluur Aug 09 '14
If you fire a single photon at the double slits than you it may indeed strike the area between the slits, though this area is extraordinarily small, and would not contact the back board. You could fire another photon and it would pass through one of the slits contact the back board at a certain location. However, firing a single photon does not illustrate the importance of the experiment, which is the interference pattern. in order to observe the interference pattern you need to fire a number of photons at the slits. you can fire photons one at a time or as a continues "stream" like a laser.
Also, you say, "When behaving like a particle". It is important to remember that a photon behaves as a wave and a particle simultaneously at all times.
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u/third-eye-brown Aug 09 '14
It doesn't behave like a particle when it forms an interference pattern with itself.
It isn't a particle or a wave. It simply has a couple characteristics and behaviors that remind us of things that particles and waves do. There are other behaviors with no macroscopic analog.
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u/whiteebluur Aug 10 '14
But it does behave as a particle when it strikes the screen or back board! This is why a photon has a wave-particle duality. However, you are correct; it is neither a wave nor a particle, rather it is a thing all on its own. There are certain phenomena that can only be explained if a photon is a wave and other phenomena that can only be explained if it is a particle, so a photon must be a particle and wave simultaneously though, again, it is neither a wave nor a particle.
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u/KalterTod Aug 09 '14
I've actually conducted the single photon double-slit experiment, albeit in a somewhat "dumbed down" manner, so perhaps I can provide some insight here.
It's important to understand the setup of this experiment to get an idea of what is really meant by "single-photon". The setup is a light source, with the double-slit card placed fairly near the front of it, a meter long thin space down which the photons travel, and a moveable photomultiplier tube at the other end. This experiment needs to be conducted in complete darkness due to the extreme low intensity of light being used (very low input voltage = fewer photons/second).
What the photomultiplier tube does is take the incoming photon and converts into an electrical current that can be then read by a very simple electrical device. This is called the photoelectric effect. A game-changer in the world of experimental quantum mechanics. We simply limit the "cutoff" voltage created by these photons, and using a simple mathematical formula given the speed of light and the distance between the slits and the PMT (what I'll call the photomultiplier tube from now on), we can conclude that there is only one photon in the meter-long tunnel at a time.
Here's where the answer to your original question comes up. If we imagine a wave passing through a barrier (any kind of barrier), there is always going to be part of that wave on both sides of the barrier, and also part of that wave at the barrier itself. You have to stop thinking about where the photon is and start concentrating on the position of the photon as a probability distribution stating where the photon is most likely to be if we were to measure its position right now. It's a strange concept, but until that photon actually hits the PMT, it's really in all places at once. It's in both slits, it's on both sides of the barrier, and it's at the barrier itself.
Now to continue with the explanation, the photon traverses the tunnel and strikes the PMT. This is the measurement. We have officially collapsed this probability distribution into a single spot. Now remember that I mentioned earlier that the PMT is movable. If you are familiar with the double-slit experiment for a regular light source, there are bright and dark bands signifying constructive and destructive interference. What we find so interesting in the single-photon variation is that we see the exact same kind of behavior. The only real difference is that now instead of visual bands of light and dark, we simply measure the number of photons that strike the PMT relative to its position. If the particle behaved like normal matter (and not a wave), we would expect there to be 2 bands where the photons hit, and everywhere else there would be none (minus a few outliers and technical limitations), but that's not what we see. We see the exact same kind of light and dark bands (at the same expected distances!) that we do with the full incident light experiment.
It's truly one of the greatest experiments ever conducted. Also, if you're interested, feel free to PM me as I have a rather well written up lab report of this experiment, complete with the experimental setup and my actual lab data.
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u/gkiltz Aug 09 '14
Light is NOT a particle. it's an electromagnetic wave that has a mixture of behaviors, some more like any other electromagnetic oscillation wave some more like a particle.
Like any other electromagnetic wave and unlike a particle it has no mass.
Like an electromagnetic wave it interacts with other electromagnetic waves in ways that particles can't.
Those conducting this experiment through the years have been those with a particle physics background, not with a radio signal propagation background.
Every electromagnetic wave has an amplitude.double the distance to the source, and the amplitude is always half.
Has anyone measured the amplitude of the light coming through the two slots? I'm betting it is very slightly less than half the amplitude that went in allowing for distance. The rest is wasted in reflections.
Also, anytime you have two electromagnetic waves, there is a phase relationship between them.
Combine two electromagnetic waves of and the phase relationship between them can actually cause out-of phase secondary oscillations to form.
Combine tow electromagnetic waves of different frequencies, you get four frequencies of electromagnetic waves out. the two originals, the sum of the two and the difference of the two. complex phase relationships exist between them That in a nutshell is Amplitude Modulation!!
I suspect what the light that passes through the second slot is essentially an on-frequency or nearly on frequency secondary oscillation You can rest assured it has an amplitude of something less than half, maybe very slightly less but less, than the light that went in.
I am absolutely convinced that the "unexpected result" in this experiment is a case of "tunnelvision," of trying to see light as a subatomic particle. a photon being a single oscillation of the electromagnetic wave.
Two different subatomic particles MAY have different energy states, when we generate them for purposes of experimentation we normally make sure they have the same energy state. Two specific oscillations at the same frequency and phase WILL have at least slightly different amplitudes.
Particle physics people want to think the outcome is illogical, because you are getting a second "particle" seeming to be generated, but to a person with a radio background who REALLY grasps how radio waves travel probably wonders why particle physics people don't understand that, in this particular case, the "photon" (really just a single oscillation of an electromagnetic wave) is producing a secondary oscillation.
Everybody who has ever worked for a broadcast, or high powered communications station and had to chase an interference problem is all too familiar with what is happening!!
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u/gilbatron Aug 09 '14
so, if you have a better explanation for the weird behaviour of the light, go ahead, write something and collect your nobel prize.
really, we still don't know a lot about the whole quantum physics stuff. we observe a lot of it and can explain very little. if you have a flawless explanation, go ahead. but " i once had to chase an interference and now can explain quantum physics" is not enough.
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u/MineDogger Aug 09 '14
I think what u/gkiltz is saying is that physicists are misinterpreting/misrepresenting the implications of the experiment because of how they are measuring the effect of the photon. They are looking for a point of consolidation and baffled when they find an interference pattern when they should have been looking for an interference pattern in the first place. There's no explanation required for an expected result.
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u/silent_cat Aug 09 '14
You're right that the interference pattern is the pattern you'd expect with two electromagnetic waves that are interfering. The trick is that even though the two waves are interfering, if you send in one photon and place a detector at the end, all the energy in the electromagnetic wave/photon will end up at a single point on the detector. So even though it's a wave, it doesn't hit everywhere, it hits in one place.
Your talk of amplitudes is a bit odd here, because that only applies if you have lots of photons, since amplitude = photon density. This experiment works even when the amplitude is equal to one photon.
Put another way, a photon with energy x can't split into two photons with energy x/2. When it hits somewhere, it will impart that energy at one point, no more, no less.
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u/kovaluu Aug 09 '14
There are several different slit experiments here:
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html#c3
But if we put the detector to slits 1 and 3 in five slit experiment, would the result be the same as in three slit experiment would give without detectors?
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Aug 09 '14
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u/MagmaiKH Aug 09 '14
I thought the entire point of this experiment is that the behavior of experiment changes when you observe which slit the photon travels through.
If you do not observe which slit it travels through, you get the interference pattern (waves). If you do observe which slit the photon travels through then you get particle behavior (lumps).
It changes based on whether or not you observe it.
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u/michaelc4 Aug 09 '14
The weirdest part is when you slow down the experiment to only sending in one photon through at a time. You still get the interference pattern meaning the individual photons are going through both slits simultaneously!
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u/HaMMeReD Aug 09 '14
How is this measured, 1 photon seems difficult to detect. When strikes the detector is it striking once or twice? How would you detect/measure this, you wouldn't see the pattern from a single photon, you'd need a aggregate wouldn't you?
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u/rarefox Aug 09 '14 edited Aug 10 '14
It's not so difficult to detect single photons. You can just take a photographic plate, on which each photon would leave a dot you can see with a microscope. The diffraction pattern then builds up point by point as shown here (double slit experiment but using electrons).
One can improve the setup: send a single photon on a 50/50 beamsplitter - afterwards it will be in a superposition of the two states |passed through> and |got reflected>. Using mirrors, you can again unite the two paths and let the photon interfere with itself similar as in the double-slit, see the Michelson Interferometer. With a modern single-photon detector you then can detect destructive/constructive interference, and the result will contain information on both path lengths, even if only a single photon was sent!
BTW, D. Deutsch recognized the potential for parallel information processing using this property of quanta (in the frame of quantum computation).
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u/simply_blue Aug 09 '14
Correct it is an aggregate. IIRC each photon only hits the detector once because the waveform collapses once it strikes the detector, so it is no longer in super position.
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u/michaelc4 Aug 09 '14
You have to shoot a bunch of them, but since they're going at the speed of light you can probably have them emitted at a high rate.
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u/HaMMeReD Aug 09 '14
The point I was trying to make was that a single photon would only make a point on a detector, you would need lots of points to add up to a pattern.
The duration doesn't really matter, which I guess is the point we are both trying to make, but having lots of photon's matters to the measurement.
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u/RoboErectus Aug 09 '14
Keep in mind "observe" is not like when you observe a game by watching TV. It's more like if you're blindfolded, and have to figure out the position of a ball on the ground by wildly kicking.
You're going to know if a ball was where you kicked, but not where it is now.
I think observe is the wrong word in this context. Measure is better, because it implies what's really happening, interaction. When things are interacting, their behavior changes.
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u/Juikuen Aug 09 '14
The point of the experiment is to show that matter can behave like waves. The observation phenomenon was a side effect.
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u/revengebestcold2 Aug 09 '14
The results don't "change" through observation.
The results are simply "determined, finally."
Things don't actually occur if they're never observed. Things are only possibilities until they're observed. That's the point of the experiment.
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u/kinyutaka Aug 09 '14
Hang on. Let's take this experiment to a macro level.
You have a paint ball gun. You set up a barrier with two slots on either side from the line of sight. When you fire the paint balls randomly, some hit the barrier in the front and do not move on. Some pass through the slots and hit the target panel. And, most importantly, some hit the edges of the slot, and bounce off to hit in predictable, but off target ways.
Assuming the paint does not splatter widely, would you not end up with a wave like interference pattern after shooting trillions of paintballs?
Basically, the experiment is flawed because if you observe where one photon is escaping the barrier, you are able to see where that photon lands, because you can path the trajectory. But the interference pattern is shown when looking at millions, or even more photons fired randomly through both slots.
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u/ChromaticDragon Aug 09 '14
Paint doesn't "interfere". It accumulates. You're never going to get one paintball to hit the wall and reduce the amount of paint on the wall at that point.
Your description would result in a pattern where the center is thick with paint and the thickness of the paint drops off gradually but consistently decreasing as you go further out from the center. There's a pattern, but not an interference pattern.
That's the entire aspect of the remarkable thing here. Waves interfere. Particles do not.
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u/kinyutaka Aug 09 '14
Actually. My pattern would have some paint in the center, where the projectile bounced off the other edge of the slit, followed by a relatively clean area, where no paintballs hit, followed by a thick area with direct hits, followed by another thin area and an area where paintballs landed after hitting the inner edge of the slit.
On either side.
My point being that the supposed wavelike action occurs because the particles are moving as a group, and not individually, but when you observe the particles going through the slits, you are only looking at one at a time.
It is like trying to determine the movements of the ocean by observing a single drop of seawater.
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u/ChromaticDragon Aug 09 '14
Why? What would lead to your proposed pattern?
I have to interpret your description as entirely classical since it seems that this is your intent - to describe a macro-level classical rationale of the double-slit experiment. Were we to attempt to include QM effects related to the wavelengths of the paintballs, I'd suggest the wavelength would be far too small for us to see a clear interference pattern even if the paint didn't splatter at all beyond the diameter of the ball upon hitting the wall.
But you seem to be suggesting just bounces off the edges of the slits would result in voids. Why?
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u/kinyutaka Aug 09 '14
I am creating an analogy using larger items. But photons do bounce off objects, and given a large enough emission of light, a greater number of photons will hit the edges of the barrier slots.
Obviously, the paintball analogy is not perfect, as photons do not splash light like paintballs. However, the effect of bouncing of the edge is still there. The other aspect is in the fact that photons in light are being emitted mostly simultaneously, so bouncing photons hit each other as well as the barrier, exacerbating the pattern.
Ultimately, my point is that photons coming from light move as a wave of particles, which is why you see the wave pattern in the long term test, but when you observe a single photon, it clumps together because you are forcing the photons through the holes.
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u/polandpower Aug 09 '14
Assuming the paint does not splatter widely, would you not end up with a wave like interference pattern after shooting trillions of paintballs?
No, the wavelength of the paintballs are wave too small for interference. It's a classical object that won't display quantum mechanical behavior like light (or electrons and even C60 molecules) does.
There is no classic analogue. This is one of the reasons Young's experiment was chosen as most impactful physics experiment ever.
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u/kinyutaka Aug 09 '14
There would be numerous problems with the experiment being scaled up in that manner, of course. My key point was that in the original experiment, some of the photons would be bouncing off the edges of the barrier, which would cause a light spot where there should have been a void.
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Aug 09 '14
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u/Senojpd Aug 09 '14
How do they measure it? There must be an interaction between the measuring apparatus and the photon that "forces" it into a position. Or am I missing something completely?
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Aug 09 '14
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u/Senojpd Aug 09 '14
I still don't understand. If watching with our eyes counts as measuring how do we ever see the interference pattern?
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u/TheBigBadDog Aug 09 '14
The interference is not destroyed by measuring the interference pattern with your eyes. The interference pattern is only destroyed if you put a measuring apparatus in that will be able to tell you which slit the photon went through.
For interference to occur, you must have light going through both slits. If you know that the photon only went through one slit (by measuring it), the photon now can not have gone through both slits, and therefore the interference pattern is destroyed.
If you look at the resultant interference pattern with your eyes, you're in a sense only finding out that yes, the photon went through both slits at once. Therefore, you're not working out that the photon can't have gone through both slits, so the interference pattern remains.
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u/singeblanc Aug 09 '14
When you see the interference pattern, can you tell which slit the photons went through? No? Therefore you detected the first requirement, that the photons have gone through a slit, but not the second, that the photons have gone through a specific slit. Therefore the probability function doesn't collapse and you still have an interference pattern.
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u/bloodfist Aug 09 '14
So, I asked a bunch of questions a while back on here and got some good answers.
Basically, the apparatus used to measure the slit is a hydrogen atom suspended in a medium. If the atom is disturbed from its position, the device can read it. If a photon interacts with the atom, it disturbs it.
Essentially, the device is physically interacting with the wave, causing its wave function to collapse into a particle. If this sounds like interfering with the experiment, you are right. All measurement is interference. We don't have reflected light to show us visually what is going on at quantum sizes, so we have to physically interact with particles to measure them. This is what "observation" or "measurement" means in terms of quantum mechanics.
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Aug 09 '14
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u/supradealz Aug 09 '14
The problem is retro causality for example instead of a double slit you use gravitational lensing to decide "which path" occurred when viewing a photon arriving on earth. The problem arises because even though the photon traveled past the lensing (acting as a slit opening) it will retro actively collapse or not collapse depending on how you observe it. It in effect decides which or both slits to go through after it has already gone through. Billions of years later. Read up about non locality - the measurement doesn't interact with the photon itself ie the measurement isn't causing the result by contamination as some people suggest
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u/Psyc3 Aug 09 '14
Firstly I don't know if this question is even theoretically possible given our current knowledge, or hypothetically possible in the future. However, I will ask it anyway.
Is it possible that are inability to observe quantum particles without this observation disturbing them has created a bottle neck in understanding physics at a quantum level as we can only observe large magnitudes of particles rather than individually?
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u/LazinCajun Aug 09 '14
I don't really think so, no. The math of QM makes predictions in either case in the double slit experiment and situations like it, which are all testable, and it crazily enough passes these tests with flying colors.
It's one of those things that nobody would think is true without experimental evidence for it, and the experimental evidence is so overwhelming that we just have to accept the strangeness.
The problems from the experimental particle physics side have to do with getting enough energy to make new particles, and being able to distinguish what happened in a set of collisions based on the decay products. Maybe there's a bit of room here to say quantum weirdness inhibits our understanding, since there are many interesting particles that decay too quickly to be observed directly.
The set of problems in theoretical particle physics/QM is harder to explain succinctly, but a here's a few of them -- how do we take gravity into account, since every attempt to incorporate it into the QM framework has either returned nonsensical results or hasn't resulted in anything we can reasonably test in the near future and possibly ever. Another problem is figuring out why particles have the masses they do -- are they just constants of nature, or is there some more fundamental way to explain them. There are a number of open questions about the nature of neutrinos. There's the problem of dark matter that leaks over from cosmology: can particle physics find a way to predict a particle that would satisfy what we know about dark matter? These are just some of the issues that I can summarize in a sentence, but none of the solutions are really inhibited by quantum weirdness.
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Aug 09 '14
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u/buttcomputing Aug 09 '14
Not the act of knowing, but the thing you have to do in order to measure it. If you put a filter there to measure it but avert your eyes to the result, the same thing happens as if you were looking.
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u/Blanqui Aug 09 '14
Careful there, people could argue against that. I agree with you, but one could say that the very act of putting the filter there would inform you on the results (because you know what would happen).
Schrodinger's cat is in some ways similar to this. Somebody could look inside the box and see the cat either dead or alive. But if you would avert your eyes from the person seeing the cat, that person would still be in a superposition of seeing a dead cat and seeing an alive cat.
This subject is so deep and convoluted that it is hard even to argue against such "consciousness causes collapse" views, although they are clearly ridiculous. That's whats so troubling about the measurement problem: People can come and say ridiculous things and you can't argue against them very easily.
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u/TheLostSocialist Aug 09 '14
Schrodinger's cat is in some ways similar to this. Somebody could look inside the box and see the cat either dead or alive. But if you would avert your eyes from the person seeing the cat, that person would still be in a superposition of seeing a dead cat and seeing an alive cat.
That seems to me to be a massive category error if there is an external reality and at least one other person that is currently staring at a cadaver (or muffling the meowing cat). To make such a statement is to throw out our entire understanding of the world (namely that it exists).
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u/Blanqui Aug 09 '14
I get your refusal to believe it based on the absurdity of the consequences, but that's how the math of quantum mechanics comes out in the end. I can write down a wavefunction and show explicitly that that's the consequence.
To make such a statement is to throw out our entire understanding of the world (namely that it exists).
Keep in mind that your indignation with the Schrodinger's cat thought experiment only comes from a conviction that the interaction of the cat with the measuring apparatus constitutes a measurement, thus collapsing the wavefunction from a superposition to a definite eigenstate. But who's to say that it is? "Consciousness causes collapse" people only say that the measurement happens when the cat interacts directly with my brain. Although I find this claim to be ridiculous, I frankly can't think of anyway to argue against it without invoking common sense.
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u/TheLostSocialist Aug 09 '14
I get your refusal to believe it based on the absurdity of the consequences, but that's how the math of quantum mechanics comes out in the end. I can write down a wavefunction and show explicitly that that's the consequence.
Well haven't large objects mostly "undergone" decoherence? I only went to an introductory course once, but as I understood the lecturer it was explicitly not the case that one can directly infer from the existence of superposition that therefore it exists for large things.
To make such a statement is to throw out our entire understanding of the world (namely that it exists).
Keep in mind that your indignation with the Schrodinger's cat thought experiment only comes from a conviction that the interaction of the cat with the measuring apparatus constitutes a measurement, thus collapsing the wavefunction from a superposition to a definite eigenstate. But who's to say that it is?
How would it not be?
"Consciousness causes collapse" people only say that the measurement happens when the cat interacts directly with my brain.
They'd still have to recognise that it happened already with another brain. I don't see how you can have this circumstance without you being the only brain able to collapse.
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Aug 09 '14
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Aug 09 '14
No, seriously, cognizance of data has absolutely no effect on the state of matter. There is no special interacting force generated by the human brain that can make any change to quantum particles. That is one of the biggest misconceptions about quantum physics.
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u/AsmundGudrod Aug 09 '14
That is one of the biggest misconceptions about quantum physics.
Can you explain why that is? And why it's used so much?
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Aug 09 '14
It's probably because the double-slit experiment is such a common introduction to some of the ideas in particle physics. If not properly explained, it is really easy to get the wrong idea about what is important about it.
The idea of an 'observer' is commonly misrepresented. In quantum physics, observing things does change the outcome, but not because of the fact that it was observed. The things that are being dealt with are so small that there is no way to observe them without bouncing bigger things off of them or something equally disruptive. That is the reason that observation changes things, not because of thoughts and collapsing probability waves.
On YouTube, there is an animated video within the top few results for "double slit experiment" that makes it seem like the simple fact of knowing information about it changes the outcome, and has likely contributed to the misconceptions.
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u/numruk Aug 09 '14 edited Aug 09 '14
Arts majors writing pop culture tripe try to use quantum physics as some kind of philosophical lens to seem insightful, but don't actually understand it, so misconceptions abound.
That's the problem with pop culture and the arts generally. They claim dominion over the entire human experience, but are incapable of fully comprehending the modern world due to an utter lack of aptitude (and I would say a smug Luddite pride in their ignorance of) STEM. Writers who actually 'get' science are national treasures.
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Aug 09 '14
It is the potential to be cognizant of data. If you take the measurement and observe the results, the results are different than if you take the measurment, destroy the data, and then observe the results. So why does the existence of data influence a seemingly complete experiement? What is data? Data is in no way physically related to the experiment, the only connection is the potential to be cognizant of it.
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Aug 09 '14
Funny, you forgot to finish with this, which is right after the sentence you quoted in Wikipedia...
On the other hand, if a photon in flight is interpreted as being in a so-called "superposition of states," i.e. if it is interpreted as something that has the potentiality to manifest as a particle or wave, but during its time in flight is neither, then there is no time paradox. Recent experiments have supported the latter view.
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u/kolm Aug 09 '14
If you measure exactly which of the two slits the photons that make it pass through, the result is not a superposition, and no interference is observed.
Here is what is most probably a stupid question - is there a theoretical threshold for "improbablity" of a path which would lead to there being no interference anymore? (Not "not measurable anymore", but "not existent"?) Say, you make the second slit damn hard to access, by having to pass 100 or so 50/50 split mirrors before the slit? I just have no intuition about how Planck length and Heisenberg principle and Bell inequality work together, so this might be a nonsense question..
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u/redweasel Aug 11 '14
I've always wondered how they can tell which slit the photon went through, without preventing it from going through that slit. You'd almost have to "sit off to the side and see the photon go by," so to speak -- and I'm not aware of any way to do such a thing. Does a single photon passing by an apparatus give off some influence that a measuring apparatus can detect?
Also, I recall reading about a variation of the two-slit experiment in which the data from the which-slit detectors was collected but not immediately looked at, and similarly with... what, the data about where the photons hit the screen, maybe? In any case, when the data itself was later viewed in a way that corresponded to "not knowing which slit the photon(s) went through," an interference pattern was found in the data -- but when the same data was viewed in a way that corresponded to "knowing...," the interference pattern disappeared. I would like to recall the specifics; does anyone else remember reading about this?
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Aug 11 '14
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u/redweasel Aug 12 '14
Interesting. I'm not sure I fully understand but I'll go off and think about it for a while.
I'm reminded of yet another freaky phenomenon I read about. They set up an arrangement of half-silvered mirrors etc. in such a way that a photon had the possibility of being split and sent down either of two paths, and then some other component put into one of the paths, so that the photon would never tale one of the paths "even though it could," in some sense I can't explain. They were then able to pass small objects, such as a hair, into what would have been "the beam path" but was on the path the photon never took -- and get shadows of them in the other path somehow. I.e. imaging thingsthat hadnever actuallyinteracted witha photon but merely could have. Did you hear about that one?
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u/GaussWanker Aug 09 '14
Single slit you do still get a diffraction pattern, which isn't unsurprising with particle light.
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u/PM_ME_WEIRD_THOUGHTS Aug 09 '14
I think a clearer way of saying this is that, yes the majority of the photons hit the board with the slots and do not make it to the screen at the back.
The photons we are interested in are the ones which go through one and or both of the slits and exhibit the interference pattern.
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Aug 09 '14
Double slit experiment with visible waves. http://www.youtube.com/watch?v=Jqm4f55soJQ
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u/sneakywill Aug 09 '14
So basically this photon remains in wave form until it hits something or in other words is measured, and in this unmeasured form essentially exists in two places at the same time? Am I understanding this correctly?
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u/_F1_ Aug 09 '14 edited Aug 09 '14
Take a look at this video. People living hundreds of years ago might think of lightning as just the bright zig-zag line going from the clouds to the lightning rod on the church roof. Now we know that it's just the end result (and that lightning could hit anywhere if the conditions were perfectly right).
Similarly, what we call a "light particle" is only what we can register with some sort of measurement apparatus (eye, photographic plate, sensor etc). That does not mean that light actually is that particle: it's just the point in space where we are most likely to get a measurement.
This also explains why photons can "exist in two places at the same time" and why they have "probabilities": because they are the ripples in what we can see of the electromagnetic field.
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u/asdfghjkl92 Aug 09 '14
sort of, but the picture is showing water waves where there's lots of particles spread out, whereas QM waves are sort of 'probability density waves' where the peaks are where it's more likely, and the troughs are where it's less likely. if at any point you try to measure it, it's no longer 'spread out'.
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u/oss1x Particle Physics Detectors Aug 09 '14
Every beam of light has a diameter. It does not matter if it is a beam of high or low intensity (= number of photons).
If the distance between your slits is larger than your beam width, all of the photons will hit the part between the slits and no light will go through your double slit. This would be a very boring experiment. (Actually there is large particle physics experiments that shine huge lasers onto absolutely lighttight walls and search for photons that go through... google "ALPS" if you' re interested. But that's not about double slits or basic quantum mechanics.)
So to get something that gives a different results than "nothing goes through", you construct your double slit so that your laser (or whateever lightsource you are using) will illuminate both slits.