What interference pattern of double-slit experiment does mean

vijayantv
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I have a doubt about the interference wave pattern of double slit experiment with light beam.
does it mean the interference of electromagnetic wave (which tells about the frequency causing color) or Spatial probability wave (which was explained by Max Born).

I am not good at math. can someone please explain with math-free manner.
 
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Welcome to PF;
There is no totally math-free manner - the short, non-math, answer is "both" - sort-of.

The distribution of photons is described using the spatial wavefunction.
The spatial probability amplitudes and EM waves are closely related to each other - so either approach is valid.
 
Simon Bridge said:
Welcome to PF;
The spatial probability amplitudes and EM waves are closely related to each other - so either approach is valid.

Thanks for the information.

double slit with single photon takes long time to get accumulated and shows interference pattern. but stream of continues photons shows interference immediately. Are both different phenomenon? or how both can be related if both are same phenomenon?.
 
Both are the same phenomenon.
The fact that it works for the single photon indicates that the result is not caused by EM field interference.
Rather the EM field is an emergent phenomenon of the statistics.

On the scale of individual particles, Nature is statistical.
Over many particles, individual effects get averaged out to give you the classical behavior.
 
ok. so the interference is caused by two waves regardless of intensity and time, fine.

light has photons of so many different polarizations. but we are seeing the interference with only two wave. can we consider that polarization doesn't matter in this interference pattern?
 
ok. so the interference is caused by two waves regardless of intensity and time, fine.
Nope - not what I said.
There is only one wavefunction - this is why the better descriptions have math.
But the wavefunction does not depend on intensity.

light has photons of so many different polarizations. but we are seeing the interference with only two wave. can we consider that polarization doesn't matter in this interference pattern?
The effect of different polarizations on the interference pattern is well documented - it's something you can look up.

But the statistics works at an individual-particle level.
You should probably see these lectures: http://vega.org.uk/video/subseries/8
 
Simon Bridge said:
On the scale of individual particles, Nature is statistical.
Over many particles, individual effects get averaged out to give you the classical behavior.

What is classical behaviour how it differs from statistical?
 
What is classical behavior how it differs from statistical?
Classical behavior is basically anything that follows Newton's Laws and the rules in his Principia.
Statistical behavior is anything that needs probability math to describe.

For examples - watch the lecture series I gave you in post #6.

But: if you really don't know what "classical physics" or "statistical" means, then I cannot help you because the answers require a much higher education level than you have in order to understand them. It would take years.
 
I know something about Classical and quantum physics

So, if the light has very low intensity (1 photon or 2 photons or 3 photons...) then the double-slit will show probability wave interference. on the other hand the high-intensity light is not probability wave under double slit and it is the EM wave (of classical) interference.

Double slit exhibits different wave behaviour based on the light intensity.

is that correct?
 
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  • #10
vijayantv said:
So, if the light has very low intensity (1 photon or 2 photons or 3 photons...) then the double-slit will show probability wave interference. on the other hand the high-intensity light is not probability wave under double slit and it is the EM wave (of classical).

Double slit exhibits different wave behaviour based on the light intensity.

is that correct?

No. There is no appreciable difference between the two.

Higher intensity simply means that there are more photons present at once. The pattern is always a build-up (sum) of the individual photons over time. With higher intensity, the number of photons needed to see the pattern (perhaps around 200* on the low side) will arrive in a much shorter time span. The pattern becomes more clear as the number of photons detected goes up.


*There is no exact minimum. Some people see the pattern earlier, some later.
 
  • #11
DrChinese said:
No. There is no appreciable difference between the two.
...
*There is no exact minimum. Some people see the pattern earlier, some later.

so did you mean that both the single-photon and stream-of-photons (Light beam) shows only the interference of spatial-probability-wave(of Max Born) and Not EM wave (of Maxwell, which tells wave frequency causing color)?
 
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  • #12
vijayantv said:
so did you mean that both the single-photon and stream-of-photons (Light beam) shows only the interference of spatial-probability-wave(of Max Born) and Not EM wave (of Maxwell, which tells wave frequency causing color)?
No. He is saying that there is no difference between the two.
The interference pattern you calculate using probability-amplitude waves is exactly the same as the pattern you calculate using EM waves, and both agree staggeringly well with the results of experiments. When it comes to the interference pattern, there is no difference between the theories.

Have you watched the lecture series in post #6 yet?
It is essential viewing for anyone contemplating this stuff.

I know something about Classical and quantum physics
... what does that mean?
What education level are you doing this at?

If we don't have a good idea of your current state of knowledge, we cannot help you very well.
If you won't follow suggestions, we cannot help you at all.
 
  • #13
I just saw some videos about Quantum Mechanics and I am not Physics student.

I saw that video, and hard to understand.

Simon Bridge said:
No. He is saying that there is no difference between the two.

This is the place I get confused.

if both are same then why are you saying that low intensity light(1, photon , 2 photos 3 photons) is statistical and high intensity light (multiple-particles stream) are classical.

I meant statistical that the interference pattern is caused by two Spatial-probability wave. and I meant classical that the interference patterns is caused by two electromagnetic wave.

does intensity determine whether it is statistical or classical?.
 
  • #14
vijayantv said:
does intensity determine whether it is statistical or classical?.

Its just that by the law of large numbers probabilistic things can behave in a deterministic way if the number of observations is so large you don't even notice that are separate observations. In this case it means the photons strike the screen so frequently it looks predictable and continuous.

Thanks
Bill
 
  • #15
I just saw some videos about Quantum Mechanics and I am not Physics student.
Ahah ... I had a feeling this would be the case.
It would still help to know where your understanding is at - you realize that the questions you are asking involve some quite deep concepts?

The trouble is, there is a limit to what we can get you to appreciate if you don't have the background.
It's like trying to show the beauty of fibonnachi numbers to someone who has yet to learn to add or subtract.

So where is your science and math at generally - besides what you see in videos (this was online right?).
Don't be imbarrassed if it's quite basic - that's just normal. It's us uber-educated folks who are the weird ones.

This is the place I get confused.

if both are same then why are you saying that low intensity light(1, photon , 2 photos 3 photons) is statistical and high intensity light (multiple-particles stream) are classical.
I did not intend to give that impression.
I was making a generalization between quantum mechanics and classical physics ... classical physics is what happens on average.

In the interference example - while there is no difference in the pattern, there is a difference in the details of how we do the predictions at different scales.

i.e. for many particles, we can work out the intensity of different parts of the pattern by multiplying the probability by the total number of particles. This does not make sense for just one particle - which either gets detected in it's entirety or not at all.

The first approach is really just an average - but it works out so well because there are a great many particles so the variation from the average is usually too small to measure - like you cannot hear someone shouting backstage during a rock concert. It is this first approach that is indistinguishable from the EM approach.

So you can think of it like this: it's all statistics and probability.
 
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  • #16
bhobba said:
large numbers probabilistic things can behave in a deterministic way

as we are seeing wave in both single-photon and photon-stream scenarios how light can be a deterministic?

can you agree that both are probability waves.

Sorry I am a non physics student, I am a software engineer. somehow get interested in quantum mechanics. it is very hard to me. please don't mind.
 
  • #17
Simon Bridge said:
So you can think of it like this: it's all statistics and probability.

Ok. thanks. then my assumed logic will be matched. clear now.
 
  • #18
Simon Bridge.
I need to clarify some doubts in Quantum mechanics. can I use this amed thread?
 
  • #20
bhobba said:
Yes indeed they are probability waves - but a funny sort of probability:

clear now. thanks
 
  • #21
I am extending My queries.

Assume a photon is passing double-slit. Here the probability wave is splitted into two and interfere with each other. So we are seeing interference pattern over the time period. It is fine and clear.

Assume two photons are passing double slit, simultaniously.
Photn1 and photn2.

so 4 probability waves will be generated after the double-slit.

Wave1-Photon1 , Wave2-Photo1 , wave1-Photon2 and Wave2-Photon2.

Wave1-Photon1 and Wave2-photon1 are colliding each other and show interference pattern this is fine.

Definitely Wave1-Photon2 and Wave2-photon2 are also colliding each other and show interference pattern this is fine.

are Wave1-Photon1 and Wave1-Photon-2 colliding here?
by the same way, are Wave1-Photon1 and Wave2-Photon2 colliding here?

Does probability wave of one photon interfere with the probability wave of other photon?
 
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  • #22
vijayantv said:
Does probability wave of one photon interfere with the probability wave of other photon?

This is a complicated question. Early QM held that photons do not interfere with each other. Over time, it seems that position has changed with a host of opinions one way or the other. The answer also somewhat depends on interpretational issues.

I have not seen a lot of literature on this. If I remember correctly, I believe Glauber comes down on the side that 2 photons can interfere. I believe part of the problem is that to test, you need photons of the same phase. Here is an article, sorry it is abstract only:

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=785438

"We have performed several original experiments, in order to investigate the nature of optical interference. In some contexts, the assumption that light beams suffer perturbations during their interaction is the most plausible. In others, the assumption that they do not is more appealing. Yet, the observable outcomes of both models are compatible with each other, in theory as well as in experiment. We conclude that they work equally well for the purpose of making physical predictions, and that each of them is logically valid. However, their interpretive value is not equal. If we assume that light beams cross each other unperturbed (even at the microscopic level), then we run into theoretical complications and even paradoxes that are not otherwise present."

To the others out there, I welcome your clarification and comments on this.
 
  • #23
DrChinese said:
In some contexts, the assumption that light beams suffer perturbations during their interaction is the most plausible.

I think it tells about two separate light beam. I am asking about the probability wave of photons in same light beam in double-slit experiment.

if we assume the probability waves of two different photon can collide with each other then why we can not see the interference without double-slit?

so can we assume that the probability wave of one photon can not collide with other photon's probability wave?
 
  • #24
vijayantv said:
if we assume the probability waves of two different photon can collide with each other then why we can not see the interference without double-slit?

Assuming I understand your question correctly: with a single slit, everything simply reinforces (adds). There is no pattern being formed.
 
  • #25
DrChinese said:
with a single slit, everything simply reinforces (adds). There is no pattern being formed.

ok let me ask the question in different way.

individual photons are having their own spatial-probability wave. So anyone of these (bellow) two different phenomenons must be true.

1). One photon's probability wave can collide with another photon's probability wave. the two-slit interference pattern is generated by all photons probability waves are colliding with each others.

2). One photon's probability wave can not collide with another photon's probability wave. the two-slit interference pattern is generated by the sum/average of all individual photons' s wave.

let us assume 1st one is true. that means that "Can collide". then the question is that "why they can collide with others and showing interference only after two-slit".

if they (probability waves) are able to collide with each others, the they also could collide and generate interference pattern, without two-slit. right?

so can we consider 2nd is true?
 
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  • #26
vijayantv said:
individual photons are having their own spatial-probability wave. Any one of these (bellow) two different phenomenons must be true.

1). One photon's probability wave can collide with another photon's probability wave. the two-slit interference pattern is generated by all photons probability waves are colliding with each others.

2). One photon's probability wave can not collide with another photon's probability wave. the two-slit interference pattern is generated by the sum/average of all individual photons' s wave.

Putting the question of two photon interference (which is most important if the relative phase of two photons/beams is better defined than the individual phase of the single beams like for entangled photons), this depends on the seemingly trivial question what really qualifies as "individual" photons.

Coherence is intimately related to indistinguishability. Whether you treat the problem classically or in a quantum way, you always add up all the fields/probability amplitudes leading to detection events at some position and get the modulus squared of them. By definition, interference means that some non-trivial pattern survives after averaging over many repetitions. This corresponds to some cross-product terms between fields surviving. So only fields with well defined relative phase in the average can give some contribution here. However, that also means that it is meaningless to attribute the corresponding detection event to just one of the fields. Both fields contributed to the detection event, so this situation automatically also results in indistinguishability. You cannot in principle trace the detected photon back to one of the initial fields, so talking about individual photons and their individual wave functions is somewhat dangerous. As a rule of thumb, all photons contained within a coherence volume (the volume inside which the phase of fields is so well defined that interference takes place) are indistinguishable.

It may be beneficial to visualize this as a single state of the field occupied by more than just one particle, rather than many individual photons. In the limit of many indistinguishable photons, that is usually well described by a coherent state with large photon number, which is the closest thing to a classical light beam that qm has to offer.
 
  • #27
Cthugha said:
Putting the question of two photon interference (which is most important if the relative phase of two photons/beams is better defined than the individual phase of the single beams like for entangled photons), this depends on the seemingly trivial question what really qualifies as "individual" photons.

I am lost. Actually I asked about two photons from same light source and not entangled. let me make in more detail.

1). Photon1 and photon2 are from same light source, not entangled and travelling. And have Wave1 and Wave2 respectively..
2). These are passing two-slit simultaneously.
3). now 4 waves are generated from the 2 waves.
4). Wave1 has splitted into WaveP1-1 and WaveP1-2.
5). Wave2 has splitted into WaveP2-1 and Wavep2-2.
6). we are seeing interference pattern in the screen.

which one of the bellow is true?

I).
All 4 waves are interfered with each others (6 combinations). multiple probability waves are generated by Probability waves collision. So these multiple combinations are resulting this interference pattern.

II)
WaveP1-1 is summed with WaveP2-1 resulting waveS1.
WaveP1-2 is summed with WaveP2-2 resulting waveS2.
only two probability wave can cause this interference pattern. So waveS1 and waveS2 are interfered and showing interference pattern.
 
  • #28
vijayantv said:
I am extending My queries.

Assume a photon is passing double-slit. Here the probability wave is splitted into two and interfere with each other.
No. That is not what happens.
The probability wave does not split in two nor does it interfere with itself in the manner you seem to be thinking.

Note: the statistical theory does not say anything about what physically causes the interference pattern. It just predicts one if you make a certain kind of measurement.
If you turn the same theory onto what happens at the slits - it predicts a different pattern depending on how measurements are performed.

This is where you really need the math - this sort of qualitative approach will not work in detail.

Assume two photons are passing double slit, simultaniously.
Photn1 and photn2.

so 4 probability waves will be generated after the double-slit.
No - just one probability wave before the slit and one probability wave after the slit. One photon arrives on one part of the screen and the other ends up somewhere else.

The system is properly described by a 2-particle wavefunction, which can be decomposed into two single-particle wavefunctions and an interaction term. Photons are normally very weakly interacting with each other so the two single-photon terms dominate ...

The bits about individual wavefunctions are just ways to do the math and should not be confused with a physical process.

Just like if you wanted to work out how big 1/8 th of a cake was, you may find it easier to do the math by halving the size of the whole cake, then again, then again ... but that does not mean you are going to cut the cake in half three times (you could, but you don't have to), you may just use the answer you got in math to measure the cake and cut once, or maybe the cake is already cut into 16ths and you just pick out two of them?

Just because we talk about two wavefunctions, one for each photon, does not mean that there are physically two wavefunctions... there might be, but how would we tell the difference?

The best qualitative description of the process I've seen comes from Richard Feynman.
http://vega.org.uk/video/subseries/8
... unfortunately it is rather long because you need the basic concepts first.
It is possible you need more background than that - i.e. probability theory - but it does cover all your questions - like why we need the slits to be able to see an interference pattern.

Feynamns description uses an infinity of terms for each particle. It's just maths.

tldr: the reason we only see a two-slit interference pattern in the presence of two slits is because it is a two slit interference pattern. Any other situation and we see the pattern appropriate to that situation according to how the wavefunction changes.

Do give it a go though.
 
  • #29
Somehow it is making sense to me. Thanks.

but one doubt in your answer.

Simon Bridge said:
No - just one probability wave before the slit and one probability wave after the slit. One photon arrives on one part of the screen and the other ends up somewhere else.

assume single photon scenario.

one probability wave before slit is fine.
but after the two slit it is splitted into two wave and and then the collision of two is causing interference pattern. is that not correct?
 
  • #30
vijayantv said:
but after the two slit it is splitted into two wave and and then the collision of two is causing interference pattern. is that not correct?
No - that is not a useful way of thinking about it at this stage.
For instance, there is nothing to "collide" - it's just maths, probability amplitudes, no substance there at all.
In the maths, which you want to avoid, the probability amplitude is a complex number - it involves the square-root of minus one.

Perhaps we need to be careful...
The setup is as follows:

We define a x-y coordinate system in a plane.
We put a pair of slits width a at the origin so one slit is centered on y=+b/2 and one slit is centered at y=-b/2. We need b>a to get two distinct slits.
The source is placed on the -x axis at position x=-S
A detector may be placed anywhere - D=(x,y).

The theory tells us the probability that the detector will go off (detecting a photon) given the properties of the source along with the type and (x,y) position of the detector.

We can get cute and work out the probability p(x,y) that the detector will go off, sometime during the experiment, given that a single particle has been sent out from the source. This is worked out using the single particle wavefunction - and some other maths - and depends on the detector position (and the exact setup).

If n non-interacting particles are sent out, then the probability pn(x,y) that at least one will hit the detector can be worked out from the single-particle case in the usual way for the probability of independent events.

The average number expected at the detector is np(x,y).

The probability function for such a setup does not change with time - it is static.

Now try to ask your questions within that framework.
A lot of them stop making sense. i.e. it makes no sense to talk about a probability wave passing through the slits because the probability function does not move about. You could picture it as standing waves if you like - that's close. We can decompose standing waves into two interfering traveling waves if we like ...

Things can get tricky for other situations - it depends on how much we control the photons that get sent out by the source and exactly how we build the slits. (This statement included because I know there are people here who have had a major cringe-reaction at the simplifications I just made.) Let's not deal with that right now - no entanglement, no funny polarizations, nothing like that. Let's get you familiar with just the basics.

I have left out the details of how we find the probability distribution before and after the slits ... that's maths. Instead I've concentrated on how we shift from the single particle case to the multi-particle case - for the same setup.
 
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  • #31
this is the problem to me.

I am lack of this kind of math. I came to know that single photon is showing interference pattern in two slit experiment by traveling into both slit at a time. and it causes two wave from each slit and these two waves are interfering and showing interference pattern.

if that is wrong, then my entire assumption is wrong.

ok thanks for the help.
 
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  • #32
vijayantv said:
this is the problem to me.

I am lack of this kind of math. I came to know that single photon is showing interference pattern in two slit experiment by traveling into both slit at a time.
Then you "came to know" something that is not true.
That is a very common misunderstanding though.
 
  • #33
vijayantv said:
I am lost. Actually I asked about two photons from same light source and not entangled.

My comment was not about entangled light. Is is just important to note that your typical light source does not emit well defined individual, unentangled photons. Only highly specialized light sources in few physics labs around the world can do that.

vijayantv said:
I am lack of this kind of math. I came to know that single photon is showing interference pattern in two slit experiment by traveling into both slit at a time. and it causes two wave from each slit and these two waves are interfering and showing interference pattern.

Well, if you just have one wave moving outward radially and damp it everywhere except at two points, what is left over is just a small part of the initial wave. Putting a double slit basically does that.
 
  • #34
Simon Bridge said:
Then you "came to know" something that is not true.
That is a very common misunderstanding though.

If we let one photon pass through two-slit, photon will be detected some location at screen. if we continue the same for long time and sending one photon again and again, we can see the interference patterns in the detector screen.

how it is happening? how single photon is showing interference pattern over the time period?
 
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  • #35
Cthugha said:
Well, if you just have one wave moving outward radially and damp it everywhere except at two points, what is left over is just a small part of the initial wave. Putting a double slit basically does that.

Are we then in some sense preselecting the photons that get through?
 
  • #36
vijayantv said:
how it is happening? how single photon is showing interference pattern over the time period?

Because QM is a theory about what happens when something is observed, not about when its not being observed.

Here is the conceptual core of QM - forget about stuff you have read elsewhere - this is its core:
http://www.scottaaronson.com/democritus/lec9.html
'So, what is quantum mechanics? Even though it was discovered by physicists, it's not a physical theory in the same sense as electromagnetism or general relativity. In the usual "hierarchy of sciences" -- with biology at the top, then chemistry, then physics, then math -- quantum mechanics sits at a level between math and physics that I don't know a good name for. Basically, quantum mechanics is the operating system that other physical theories run on as application software (with the exception of general relativity, which hasn't yet been successfully ported to this particular OS). There's even a word for taking a physical theory and porting it to this OS: "to quantize."

Normally the double slit experiment is used to motivate the QM formalism, but really it should be the other way around - QM should explain it - and it does:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Of course the answer is expressed in the language of mathematics - sorry but physics is about mathematical models.

Basically QM is a variant on standard probability theory that allows continuous transformations between so called pure states:
http://arxiv.org/pdf/quant-ph/0101012.pdf

Consider flipping a coin. Probability theory describes the frequency of outcomes - but not what causes each outcome. Same with QM - it describes the frequency of outcomes - but not what causes any outcome. We simply do not know what that is - or even if there is a cause - nature may simply be like that. But regardless the QM formalism is silent about it. We have interpretations that speculate about it - but until there is some way to decide experimentally they are simply conjectures.

Thanks
Bill
 
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  • #37
vijayantv said:
If we let one photon pass through two-slit, photon will be detected some location at screen. if we continue the same for long time and sending one photon again and again, we can see the interference patterns in the detector screen.

how it is happening? how single photon is showing interference pattern over the time period?

As Simon Bridge absolutely correctly pointed out already, the pattern you see is rather a quastion of geometry. You see a double slit pattern because it is a double slit. If you have a different double slit, you will get a different pattern. If you start from the position of your light source and draw a line to some slit and draw another line from the slit to the detection plane, you will get some total path length for every point. If you do the same for the other slit, you will get a second path length for every position. Now you more or less get a map of the path length difference via the two slits for every single point in the detection plane. This path length difference of course translates to a phase difference. This map of phase difference versus position is what defines your interference pattern. Please note that it is really just the difference that matters. Independent of the initial phase you assume, the phase difference just depends on geometry.

Jilang said:
Are we then in some sense preselecting the photons that get through?

Well, yes. Sure.
 
  • #38
Cthugha said:
Coherence is intimately related to indistinguishability. Whether you treat the problem classically or in a quantum way, you always add up all the fields/probability amplitudes leading to detection events at some position and get the modulus squared of them. By definition, interference means that some non-trivial pattern survives after averaging over many repetitions. This corresponds to some cross-product terms between fields surviving. So only fields with well defined relative phase in the average can give some contribution here. However, that also means that it is meaningless to attribute the corresponding detection event to just one of the fields. Both fields contributed to the detection event, so this situation automatically also results in indistinguishability. You cannot in principle trace the detected photon back to one of the initial fields, so talking about individual photons and their individual wave functions is somewhat dangerous. As a rule of thumb, all photons contained within a coherence volume (the volume inside which the phase of fields is so well defined that interference takes place) are indistinguishable.

It may be beneficial to visualize this as a single state of the field occupied by more than just one particle, rather than many individual photons. In the limit of many indistinguishable photons, that is usually well described by a coherent state with large photon number, which is the closest thing to a classical light beam that qm has to offer.

Does decoherence appear somewhere in the double slit experiment?
 
  • #39
naima said:
Does decoherence appear somewhere in the double slit experiment?

In the typical experiment with photons: No, they do not really interact with other stuff on their way. If you instead insist on doing the double slit with heavy molecules like C60 or want to do the experiment in a chamber full of incoherent scatterers, decoherence will matter.
 
  • #40
bhobba said:
Because QM is a theory about what happens when something is observed, not about when its not being observed.

ok, what is happening when we not observed? how interference pattern is generated while we are not observing?


bhobba said:
it doesn't say anything about double-slit.


bhobba said:
it doesn't tell about the single-particle interference over the time period. it tells only about single-photon is passing one time to two-slit and detected at the screen. (single time only)

one of ref. says that

" ----the maxima of this interference pattern are given by a formula familiar from wave optics."

and the conclusion: (it says about single photon is being sent one time).

"Quantum interference can occur only when a large number of identically prepared
particles are observed. These particles are detected at different locations, one at a time [9].
A single particle is always detected as a localized entity and no wave properties can be
discerned from it"




bhobba said:
it doesn't say anything about double-slit.
 
  • #41
vijayantv said:
ok, what is happening when we not observed? how interference pattern is generated while we are not observing?

...

it doesn't say anything about double-slit.

The answers being provided are absolutely correct. The issue is that your questions either don't specify important details, or you are not understanding some of the deeper issues involved. It can be very difficult to jump from general statements to the specific when talking about QM.

You said: "I am lack of this kind of math. I came to know that single photon is showing interference pattern in two slit experiment by traveling into both slit at a time. and it causes two wave from each slit and these two waves are interfering and showing interference pattern."

And the answer is: when you do the math, the formula correctly predicts a probability of detection consistent with an interference pattern. That is what QM tells us.

You then ask: "what is happening when we not observed?"

And the answer is: QM is silent on that point. You are free to visualize it any way you find convenient. It would be reasonable to picture it as if 2 waves are interfering, as that matches the detection pattern. However, that is just a convenient and helpful picture and should not be taken too literally.

You can ask all you like as to "what is really happening?" but no one really claims to know exactly. All that QM provides is predictions for experiments you can set up and observe results. It does a great job at that. So you need to phrase your questions in that form if you want solid answers.
 
  • #42
bhobba said:
Because QM is a theory about what happens when something is observed, not about when its not being observed.

vijayantv said:
ok, what is happening when we not observed? how interference pattern is generated while we are not observing?

Bill answered this question, to the best of our knowledge, later in that post. I've put the key parts in boldface:

bhobba said:
Consider flipping a coin. Probability theory describes the frequency of outcomes - but not what causes each outcome. Same with QM - it describes the frequency of outcomes - but not what causes any outcome. We simply do not know what that is - or even if there is a cause - nature may simply be like that. But regardless the QM formalism is silent about it. We have interpretations that speculate about it - but until there is some way to decide experimentally they are simply conjectures.

On this forum, some people love to debate about these conjectures (interpretations). Look for things like "Bohmian mechanics", "many-worlds", etc. However, until someone can come up with a way to decide between them by experiment (which at this moment doesn't appear to be possible even in principle), arguments for and against them end up based on personal philosophical preferences about the way the universe "should" work.
 
  • #43
The question, what's the meaning of observation in the sense of quantum physics is not so trivial as it might seem. There are a lot of pretty "esoteric" claims even by very famous people. E.g., John von Neumann, who was the first to give a mathematically strict formulation of non-relativistic quantum mechanics, revealing the Hilbert-space structure, applying the spectral theorems for non-bound self-adjoint operators to make sense of continuous spectra, etc. had ideas about the "measurement problem" which sound very strange (at least to me). It's known as the Princeton interpretation of quantum mechanics, and I don't want to get into any detail about this. The upshot is that he thinks the result of a measurement is established only when a conscious mind has become aware of it. For further details have a look at

https://en.wikipedia.org/wiki/Von_Neumann–Wigner_interpretation

In my opinion that's nonsense. Nowadays the experimentalists, say at the LHC doing experiments in ultrarelativistic proton-proton or heavy-ion scatttering, take notice of the outcome of an experiments much later than it was done. The results of the measurement are the momenta of zillions of produced particles in all kinds of detectors in the big experiments (ATLAS, CMS, LHCb, ALICE) in form of data in huge computer files. Then the experimentalists uses these "raw data" from the computer file and analyzes them, making plots etc. Then we get aware of what's come out, e.g., that in certain invariant-mass distributions after careful background subtraction and statistical analysis "a Higgs-boson like particle is seen".

In my opinion, it's very mondane, when an observation is made: It's made at the moment when the observable of interest of the quantum system (say an elementary particle) is somehow registered by a macroscopic measurement device and this information is somehow irreversibly stored.

This is not very different as with any probabilistic experiment. Throwing a die leads with some chance to a number between 1,...,6 which is practically unpredictable and thus can be seen as a probability experiment. If the die is not manipulated the probability to get a certain result is 1/6, and this can be tested repeating many (uncorrelated) experiments, and we can give a significance in how far the prediction of the probability 1/6 for each possible outcome is true or not. The more often we repeat the experiment the better we can tell whether the probabilistic prediction is compatible with the experiment or not.

The difference to the quantum mechanical probability is just that according to quantum mechanics, even when we know everything about the quantum system, i.e., we have prepared it in some pure state, not all observables have a determined value. This is not due to some lack of knowledge about the state of the system but because, according to quantum theory, the observable has no determined value due to the preparation of the system in the pure state. Knowing the pure state only tells us the probability to find a certain value when an observable is measured, and again we can make a probability experiment by repeating the measurement of the observable many times, the more often the better. A certain value of the observable is known as soon it is measured by an appropriate (macroscopic) device and this information somehow stored so that we can take notice of it, as in the example of throwing dice. For the double-slit experiment the particle's position is established as soon as it hits the screen, and you can count the pixels on your photo plate (or CCD in a more modern lab) to figure out, whether the predictions of quantum theory are correct with regard to the position probability distribution of not, and the more particles you run through the apparatus the more precise becomes your statistical significance for success or failure of this prediction.

This is the socalled minimal statistical interpretation, which is the one making the least assumptions and which is, in my opinion, the most natural one. Of course, it's a bit disappointing that we don't know more about the system than the probabilities for the outcome of a measurement, but that's how Nature seems to be, according to quantum theory. Physics doesn't provide explanations for why Nature is as it is but only describes, how Nature is (or at least that part of Nature that is objectively and reproducibly established by observations).
 
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  • #44
DrChinese said:
You are free to visualize it any way you find convenient. It would be reasonable to picture it as if 2 waves are interfering, as that matches the detection pattern. However, that is just a convenient and helpful picture and should not be taken too literally.

Wiki says that it is caused by two probability wave

http://en.wikipedia.org/wiki/Double-slit_experiment
The appearance of interference built up from individual photons could seemingly be explained by assuming that a single photon has its own associated wavefront that passes through both slits, and that the single photon will show up on the detector screen according to the net probability values resulting from the co-incidence of the two probability waves coming by way of the two slits

Professor Benjamin Schumacher also says that the interference is the constructive and destructive pattern of two waves.
http://www.youtube.com/watch?v=7y5RJHiDOK8

and in many other places.

are all wrong? isn't the interference is caused by to wave?
 
  • #45
Can we say that an event is a measure when it gives a "robust mark" which can be read?
The mark is robust when it contains many duplicate copies of the same information.
 
  • #46
vijayantv, your question is asked and answered.
 
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