Single vs. Double slit coherence clarification please

[PhysicsIsReallyFun]

TL;DR

Single slit electron detection - Is wave nature still observed?

The woman in the video below is claiming everyone has the double slit experiment wrong. She is claiming that when electrons are detected as they pass through the slits decoherence does not occur, what is happening is that the electron wave only passes through one of the slits.

Her "proof" is that if you detect the electrons in the double slit experiment the result is not two bands of particles on the screen behind the slits but rather two diffraction produced interference patterns.

We can simplify this experiment to a single slit. If electrons are directed through a single slit a slight interference pattern is produced due to the electron wave diffracting through the opening. I had thought that if a detector was used the electron wave would disappear and the result would be electron particles and no interference pattern observed due to diffraction?

She seems to know what she is talking about and now has me confused?

 

[PhysicsIsReallyFun]

Specifically at 9:00 in the video.
She says that when you have a detector before both of the slits, this "forces the electron to collapse to just one of these (waves) and you see a single slit interference pattern."

Is this the biggest lie in physics as her video title claims or is she greatly mistaken, to put it nicely?

 

[Demystifier]

She is claiming that when electrons are detected as they pass through the slits decoherence does not occur

That is certainly wrong. Any detection is associated with decoherence.

 

[PeroK]

The woman in the video below is claiming everyone has the double slit experiment wrong.

You have to ask yourself whether a single Physics PhD graduate really understands QM better than everyone else? It's not a credible claim.

PS that said, I watched Jim Al-Khalili's lecture on QM at the Royal Institution and he strongly implied that there was no wave-like behaviour for a single-slit (i.e. no diffraction); but only wave-like behaviour when there were two slits. I emailed him about this, but didn't get a reply.

She is correct that this misinformation is widespread. And, bizarrely, is promoted even by physicists who should know better.

 

[gentzen]

The woman in the video below is claiming everyone has the double slit experiment wrong.

She is unhappy with the way popularizers use the double slit experiment to illustrate wave-particle duality. She seems to try to better understand the meaning of wave-particle duality for herself, and those oversimplified depictions of the double slit experiment are not helpful for her.

However, her own attempts to make sense of wave-particle duality are on the wrong track, in my opinion. As long as she doesn't understand in which situations (classical) light acts like a particle, and in which sense, she will have an extremely hard time "investigating" whether electrons act more particle-like than light or not.

One such situation is depicted on the (last) slide Coherence Time and Coherence Length. For "sufficiently" short coherence length, light starts to act particle-like. One can build lasers with extremely long coherence length, so this particle-like behavior can be suppressed (for light). But lasers exploit the fact that photons are bosons, so this won't work for electrons. Of course, electron wavepackets still have a finite extent, but the point particle picture has always been an idealization, even in classical mechanics.

She is claiming that when electrons are detected as they pass through the slits decoherence does not occur

No, that is not really what she tries to convey. Her point is rather that the wave-like behavior is still there.

 

[sophiecentaur]

You have to ask yourself whether a single Physics PhD graduate really understands QM better than everyone else? It's not a credible claim.

This is the filter that we should always apply. Her video is a bit 'superficial' / arguable, I think. I am skeptical about the idea of using Youtube videos to publish theories. It's a very cheap and cheerfull medium and relies on equally cheap and cheerful comments to justify or debunk the ideas. Otoh, if you post on PF, you can rapidly be put back in your place by 'peer' reaction.

She doesn't do a comprehensive comparison between electrons and photons. For a start, she uses the term "interaction" to link the patterns from her ripple tank simulator with the light wave situation. EM waves do not 'interact'. There is only apparent 'interaction' between the pairs of waves over the whole distance when there is 'measurement / detection everywhere but that's not interaction, it's just what the detector array would see. If you were to do the (long winded) two slit experiment with electrons over a range of distances, you would get the same pattern (2D this time) as the ripple tank.

She implies a dichotomy with regard to coherence although it seems to me that the only reason for any apparent difference is method of detection of the presence of either particle as it goes past. Any change of coherence could be due to the measurement method.

I was a bit disappointed to learn that Jim Al-Kalili denies diffraction of single slit electron. I wonder if this has been reported accurately? Electrons are far less frequently measured than photons but, afaiaa patterns are uncannily similar but not in scale. An electron microscope uses much shorter wavelengths but the fine structures produce diffraction patterns. Why ever would a single hole be different?

 

[PeroK]

I was a bit disappointed to learn that Jim Al-Kalili denies diffraction of single slit electron. I wonder if this has been reported accurately?

You can check out his video here. The single versus double-slit experiment starts at 2:40, approximately:



I would say it's at best highly misleading regarding the importance of the width of the slits (which determines the amount of diffraction). This strongly implies that it's the number of slits that deternmines the wave-like behaviour; rather than the width of the slit - with interference being a function of the two diffraction patterns.

 

[sophiecentaur]

I would say it's at best highly misleading regarding the importance of the width of the slits (which determines the amount of diffraction). This strongly implies that it's the number of slits that deternmines the wave-like behaviour; rather than the width of the slit - with interference being a function of the two diffraction patterns.

Yes; very highly misleading. The arm waving description of what goes on as the slit gets narrower omits the fact that the pattern will be sinx/x (measured). Mr. Al-Kalili; the numbers count!!!

 

[QuarkyMeson]

imho, neither electron nor photon can act like a particle and a wave at the same time. however, when propelled, they can both create a wave. neither act as a wave until they are propelled.

This is a misconception. When people say it's like a particle or wave it doesn't mean it's actually either. It's just in general, sometimes it better to model quantum objects as one or the other, but it remains a quantum object described by a quantum state.

In fact, when you actually learn quantum mechanics standing waves in potential wells are classic problems. "Propelling" simply changes p.

[Mentor Note: the quoted text above has been deleted from this thread because it is personal speculation]

 

[hutchphd]

In fact, when you actually learn quantum mechanics standing waves in potential wells are classic problems. "Propelling" simply changes p.

And in fact the solution for a "generic" two-slitted plane wave is easy to write down (at Fraunhoffer limit) and so why is this well-intentioned human wasting so much energy retpatedly (and obsessively) just waving her hands at it?? The actual answer, although of large historical significance, is neither particularly subtle nor surprising.

 

[bhobba]

At the undergrad level, see the following:
https://arxiv.org/abs/quant-ph/0703126

It's not the last word on the subject, but at the level of this thread, it's fine.

There have been more advanced discussions on this forum, but you have to start somewhere.

The great Feynman, a noted educator, lamented this, but never found a way around it despite trying. It just seems to be how nature is.

Thanks
Bill