The discussion centers on the double slit experiment and its implications for understanding wave-particle duality, particularly in the context of single photons. Participants explore the paradox of single photons producing interference patterns while appearing as localized particles upon detection. Key points include the distinction between classical electromagnetic waves and quantum states of photons, as well as the role of measurement in determining the behavior of photons. The conversation also touches on holography and its relation to interference patterns, questioning whether diffraction can fully explain the phenomena observed in these experiments.
PREREQUISITESPhysicists, quantum mechanics students, optical engineers, and anyone interested in the foundational principles of light behavior and quantum theory.
Demystifier said:The double slit "mystery" is a mystery when you consider a single photon, because the detection of a single photon does not show interference, and yet the statistics of many single photons does. To the extent that it is a "mystery" at all, it is a mystery even for a single slit interference.
To the extent that it is a "mystery" at all, it is a mystery even for a single slit interference.
The problem is that a single photon, when detected, appears as a particle with a relatively well defined position, and not as a wave widely expanded in space. And yet the associated wave is widely expanded in space. It looks like a contradiction or at least a puzzle.Zelebg said:What equation makes us think "single photon" equals only one wave? Why wouldn't a single wave interfere with itself like waves usually do?
Demystifier said:The problem is that a single photon, when detected, appears as a particle with a relatively well defined position, and not as a wave widely expanded in space. And yet the associated wave is widely expanded in space. It looks like a contradiction or at least a puzzle.
Zelebg said:The problem with the single photon double slit experiment is that single photon should not interfere with itself, but it’s not supposed to be a photon until it gets fully absorbed, so I ask where is the problem then if not photon, but a wave is interfering with itself?
EPR said:It's not just a wave. It lands as if it was a localised particle.
Why do you think so?Zelebg said:but it’s not supposed to be a photon until it gets fully absorbed
Demystifier said:Why do you think so?
EPR said:Those are single photons and single photons probably have very very small em wave width(due to very low energy).
In the double slit it's the probability wave that goes through both slits. Or that's the predominant mode of thinking. It's nonsense but still...Zelebg said:Looks like it's wide enough to go through, otherwise it would get absorbed between the slits and there wouldn't be a pattern. Isn't it the wavelength that needs to be in some special relation to the slits separation?
I do not.Zelebg said:You do not?
In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space...Zelebg said:In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space...
Demystifier said:By my bolding, you see that photons can be propagating too.
Well, nearly no-one.EPR said:In the double slit it's the probability wave that goes through both slits. Or that's the predominant mode of thinking. It's nonsense but still...
No one understands this. I mean no one.
EPR said:'photons' travel through space as oscillating EM waves
Optics?Mentz114 said:
Stuff like this is why Wikipedia is not in general an acceptable source under the Physics Forums rules. The struck-through part above is just plain wrong.Zelebg said:https://en.m.wikipedia.org/wiki/Electromagnetic_radiationIn physics, electromagnetic radiation (EM radiation or EMR) refers to the waves(or their quanta, photons)of the electromagnetic field, propagating (radiating) through space...
Who said that photons do not exhibit their wave nature while propagating through space?Zelebg said:Never heard of it before. If photons do not exhibit their wave nature while propagating through space, why would you expect them to interfere like waves?
Nugatory said:Stuff like this is why Wikipedia is not in general an acceptable source under the Physics Forums rules. The struck-through part above is just plain wrong.
You will also find that it has been removed from the most recent version of the article.
Just to make this explicit, you get the same type of "mystery" for a single slit. The only difference is in the shape of the probability distribution for many photons, which corresponds to the difference in the intensity patterns for classical light waves:Demystifier said:The double slit "mystery" is a mystery when you consider a single photon, because the detection of a single photon does not show interference, and yet the statistics of many single photons does.
Read the paper I cited. QFT gives the correct prediction for 'matter-waves'.EPR said:Optics?
The double slit experiment has been performed with matter as large as C60.
Explain the interference.
Zelebg said:Double slit - single photon experiment
Mystery: why is the pattern still there when emitting single photons.
Q1. Why is this a mystery, why would not a single EM wave interfere with itself like other kinds of waves do?
Cthugha said:I think you are asking the wrong question. The interesting question is why single photons show interference in a double slit and antibunching in a Hanbury Brown-Twiss experiment at the same time. In case you are not familiar with it: The HBT experiment is simply realized by directing a light field to a beamsplitter and placing single-photon-sensitive diodes at both output ports. If you send single photons towards the beamsplitter, you will find that both detectors will never click simultaneously and for each photon only one of the detectors will receive a detection event. This cannot be realized by considering only EM waves. In fact, the detectors can be placed arbitrarily far apart. Even so far apart that they are not within each others light cone.
Zelebg said:I don't see a problem with HBT. I would be surprised if a single photon could split in two. It seems there is an assumption that for a photon to interfere with itself it would first need to split, but in holography interference pattern is not the result of wave interference, but wave diffraction, which is simply direction bending of EM wave.
With respect, surely using analogies like of wavefronts and updates is old-fashioned, as is looking for mechanisms. The problem is handled in QFT by creating whatever is required in the inputs of the BS and evolving this state. The calculation agrees with experiment but it tells us nothing about a 'mechanism'. Perhaps assuming these things will lead to apparent paradoxes.Cthugha said:I think you still avoid the relevant problem. A photon certainly carries energy. If the photon is detected, all of the energy arrives exactly at the position of the detector. One may naively assume that the energy travels to the detector in a localized manner. Then the appearance of the double slit pattern is puzzling. One may also naively assume that the energy is somehow delocalized within the wave. In that case at the time of the detection, all the other positions of the wavefront must be "updated" instantaneously in order to ensure that the photon gets detected only once at the position of the detector. In this scenario the double slit pattern is trivial, but you run into problems with relativity and have non-locality as this "update" would happen faster than the speed of light. Of course you may also assume that this property is simply undefined between emission and detection. However, not everybody considers this as a satisfying answer.
Mentz114 said:With respect, surely using analogies like of wavefronts and updates is old-fashioned, as is looking for mechanisms. The problem is handled in QFT by creating whatever is required in the inputs of the BS and evolving this state. The calculation agrees with experiment but it tells us nothing about a 'mechanism'. Perhaps assuming these things will lead to apparent paradoxes.