Double-slit experiment revisited....again....

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Double-slit experiment revisited....again....

Post by Learner »

http://www.upscale.utoronto.ca/PVB/Harr ... eSlit.html

Found this flash anime on the net, which reminds me in the dual slit experiment for light of a single photon. Why would it pass the slit if the photon is being projected in the middle of 2 slits?
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

That animation, like the one in "What the bleep to we know?" is misleading.

Animations are always dangerous: they tell you what the designer of the animation thinks would happen. Film clips are better: they tell you what the universe did (at least from the point of view of the camera).

In the case of quantum physics, animations are particularly worrying, because so many people have misguided ideas.

A photon doesn't have a trajectory. It is not a little ball with a well defined position at any point in time. Any animation that shows it thus is misguided.

(Further, the same is true for electrons, unless they have very high energy. Heisenberg's principle does allow localisation if the uncertainty in the momentum is small.)

You can concentrate photons on a limited area (limited by the Heisenberg condition). e.g., if you shine a torch on the ceiling, little of the beam will be radiated (directly) out the window. However, some will.

Your understanding of physics will be improved if you disregard any animation in which the photons are shown as balls.

Joe
Last edited by joe on Tue Mar 11, 2008 3:33 pm, edited 1 time in total.
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

joe wrote:
A photon doesn't have a trajectory........

(Further, the same is true for electrons, unless they have very high energy. Heisenberg's principle does allow localisation if the uncertainty in the momentum is small.)

Joe
I don't understand what you mean about photon doesn't have a trajectory, it is trajectory which allows us to make use of light? eg

Image
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

I don't understand what you mean about photon doesn't have a trajectory
A ball has a trajectory. At time t1, it is at (x1,y1); at time t2, it is at (x2,y2) etc. t1 and t2 can be arbitrarily close together, and we can make as many recordings as we like: for instance, with a high speed camera. That's how we could determine a trajectory for a ball.

A photon does not have a trajectory. If a photon is absorbed by a phosphor molecule at (x1,y1), we know that it interacted with something at (x1,y1). But that is all. We can't say where it was before. And after, it was nowhere.

If we have a large number of photons, we can put phosphors (or other detectors) all over the place and say "one interacted here, another interacted here... that traces out a line on the screen" but we cannot determine a trajectory for a photon.

The diagram that you draw is this time is perhaps less misleading than the animation, but misleading still.

Joe
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

If we have a large number of photons, we can put phosphors (or other detectors) all over the place and say "one interacted here, another interacted here... that traces out a line on the screen" but we cannot determine a trajectory for a photon.
Point taken.

Instead of tracing out the trajectory of a photon, this method can be used to confirm the the believe of electron intefering with itself in the dual slits experiment; by placing phosphor screen on both slits and we should expect to see 2 dots of illumination at a lower intensity since the mass of the electron has been halved?

Likewise, since we already know that projection of particles travels forward(direction x). We should be able to approximate the oscillation pattern of electrons and photons by observing & accumulating locations of interaction on the phosphors screen(direction y and z), in reference to the center point of projection (y=0, z=0) and super impose that motion onto movement of a particle moving in direction x.

Thoughts?

PS. the image

Image

was taken from the following location, if anyone wants more info...

http://hyperphysics.phy-astr.gsu.edu/hb ... ig.html#c2
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

we should expect to see 2 dots of illumination at a lower intensity since the mass of the electron has been halved?
No. Electrons and photons are similar in that they are all-or-nothing. You don't subdivide them. When you put up a screen for photons (film, CCD, eye etc) or electrons (eg phosphor on CRO screen), you get a spot when you interact with one photon or one electron respectively and nothing otherwise. Usually, there are statistically important numbers of photons or electrons in such experiments, and so one builds up a histogram that shows the interference pattern.

The mass of the electron does not divide in two. It is misleading to say that 'half goes through each slit' -- the idea of 'going' somewhere implies a trajectory, and that is an idea that must be abandoned at the quantum level.

Beware of pictures. Be even more careful with animations. I cannot remember who it was who (wisely) said something like "If you have a mental picture of a photon, then you have probably not understood what a photon is."

In the last decade or so, some physicists and science writers have discovered that there is money to be made from books about quantum weirdness written for the general public. Some of these books are misleading. Science journalists also write articles about such things. Often they think that a picture will help the reader understand. Very often, such pictures help the reader misunderstand. Often, the journalists don't mind -- I've spoken to such journalists and some of them would really prefer that the reader understood something, even if it were wrong, rather than risk them not understand. And I strongly suspect that some of the book writers are concerned mainly with selling books. (Of these, one the worst I've encountered is called "The Tao of Physics".)

For example, the picture you show of the laser is misleading, not only in that reinforces the idea of a photon trajectory, but also in that it suggests that a laser has a coherence length of a few microns (instead of kilometres).

Joe
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

The mass of the electron does not divide in two. It is misleading to say that 'half goes through each slit' -- the idea of 'going' somewhere implies a trajectory, and that is an idea that must be abandoned at the quantum level.
The perception in regards to trajectory is based on the interaction occurs between 2 points of potential difference, such as the interaction of phosphor on CRO screen example. If nothing has moved from points of potential difference, there would be no interception/interaction and yet the illumination intensity is based on the kinetic energy of the electron(potential of 2 points).

It is highly likely that the amount of electron passing through each slit is unequal, therefore explains the random electron points that makes up the interference pattern?
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

A ball travels along a trajectory. At time t1 it is at x1,y1. At time t2 it is at x2,y2. The locus xi,yi gives the trajectory. When you say a ball passes through a hole, then it means that the line xi,yi passes through the hole. A ball either goes through a hole or it doesn't.

For Young's experiment, whether with photons or electrons, the above does not apply. There is no trajectory. It is meaningless to say it went through a hole. Rather, photons or electrons were produced from a source. They produced a certain distribution on a screen.

At some point, you have to give up the idea that electrons and photons are like little balls. They simply are not.

Many people say that the quantum world can't be like that, that it *can't* be so different from the macroscopic world. Well, it can be different and it is. I think what people really mean when they refuse to accept the differences of the quantum world is that they don't *like* it to be thus. If they had designed the universe, it would have been one in which photons behaved liked balls.

We don't have a choice about the universe. It is like that. Richard Feynman said something approximately like this "If you don't like quantum mechanics, then go and live in a universe where it isn't true."

Or else learn to live with it.
Joe
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

For Young's experiment, whether with photons or electrons, the above does not apply. There is no trajectory. It is meaningless to say it went through a hole. Rather, photons or electrons were produced from a source. They produced a certain distribution on a screen.

At some point, you have to give up the idea that electrons and photons are like little balls. They simply are not.
The problem is that in the case of single slit, in order for the distribution to appear indicates projection has taken place and in order for the projection to take place the slit/hole are essential. Blocking the slit/hole is not unlike occupying a fixed point of trajectory at all time since it would prevent distribution to appear.

I do agree that that electrons and photons are not like little balls but I don't see how this part would influnce the idea of trajectory? Is it because trajectory is not a property of wave?
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

Single slit diffraction is also something that is inconsistent with the trajectory picture (which would give a single bright bar, and no diffraction pattern). It is also inconsistent with the photons-as-balls picture.

The "photons-as-balls picture" is difficult to give up. It is so consistent with common sense -- a common sense, however, that is developed in the macroscopic world in which quantum events are not observed. So it is no surprise that the common sense picture doesn't work outside of the regime in which it was observed.

I'm sure that there is not a physicist who has not said 'this is crazy' or 'it can't be true' when first encountering quantum effects. It has even been said (perhaps by Bohr?) that, if you don't find quantum mechanics crazy, you haven't understood.

The fact that you keep finding all of these illustrations and animations show how easy it is to be wrong and/or misleading about quantum events.

However, if you've embarked on a career in physics or engineering, at some stage you have to abandon the photons-as-balls picture, because it is quite simply wrong. The universe is not like that, and we have to make calculations and predictions about the universe.

Many people find Newton's first law counter-intuitive. Surely, they say, Aristotle is correct that the 'natural' state of matter is at rest. It often takes quite a bit of convincing to overcome common sense in this instance. Okay, you are far beyond that, and I hope that you now find Newton's laws beautiful and even intuitive. With quantum mechanics there is another difficult step to take.

Joe
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

Was meant to mention this at the very start of the thread, in this anime

http://www.upscale.utoronto.ca/PVB/Harr ... eSlit.html

showed deflection off the interior of the slit as part of the diffraction pattern in particle mode, I am aware of the particle illustration is inaccurate and is refering to only the possibility of deflection which seems plausible? or are there any reasons why this would be impossible?
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

joe wrote:Single slit diffraction is also something that is inconsistent with the trajectory picture (which would give a single bright bar, and no diffraction pattern)
Single slit diffraction is caused by the size of the slit which causes the interferece with the light source, it would be consistent with the trajectory picture if the slit is large enough not to cause any interference. (done in Laser beam would show agrrement?single bright dot, and no diffraction pattern)

Likewise the macroscopic world, if a slit is being placed on the path of trajectory its size will determine if the interference will occur to the object being projected.
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

Yes. A macroscopic object like a ball can bounce off the walls of a corridor and, if you aim well enough, it can bounce of the inside of a door frame, or the edge of a hole, as shown in that animation.

If you shine a laser beam on a mirror, yes, ray optics will work, too. You have a macroscopic system and very many photons.

However, when you talk of the double slit experiment, you are talking of objects whose wavelength (whether light or particle) are not hugely smaller than the distance between the centres of the slits. And of course the slits have to be narrower than the distance between their centres. It is under this condition that the particle & trajectory picture don't work.

Joe
Learner
Posts: 40
Joined: Wed Mar 22, 2006 11:12 pm
Location: Area 621.xxx

Post by Learner »

http://www.hqrd.hitachi.co.jp/em/movie/ ... lite-n.wmv

Found this video of the double-slit experiment on the hitachi site, it seems that the electron do not appear in consistantly with time increment in the first (40sec) part of the video(same zoom shot).

May be its the video editing thing or it actually takes different amount fo time for each electron to get to the screen?
User avatar
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Post by joe »

I can't open that version, but I have seen a similar experiment and I have seen the stills on that page.
takes different amount fo time for each electron to get to the screen?
The electrons have a well defined momentum and so take a very similar time to pass through the apparatus. However, their arrival time at the apparatus is probabilistic.

In the beginning, when there are only few arrivals recorded, they probably show the film at a slow rate, and at the end, when dealing with lots of electrons, they probably show it at a fast rate.

Joe
Post Reply