# Screen with slits, on rollers, a la Feynman

1. Jan 5, 2004

### Swamp Thing

Plate with slits, on rollers, a la Feynman

In one of Feynan's lectures, he describes a thought experiment about trying to 'beat' the uncertainty principle. Without going into all the details, this version of the two-slit experiment has the diffracting plate mounted on rollers. The text and the figure seem to imply that when an electron passes through a slit and is deflected downwards, the plate will acquire an upward momentum. If the electron travels upwards, the plate will recoil downwards.
Now, is this literally true, or is it just a pedagogic device that Feynman has used to make a specific point? Is there really a collision and an exchange of momentum when an individual electron deflects while passing through a slit?

Another question (one that I found, unanswered, somewhere on the web) : When an electron is diffracted at a slit, and there is acceleration in the upward or downward direction ... should we not expect EM radiation due to the accelerating charge?

Last edited: Jan 5, 2004
2. Jan 7, 2004

### Chi Meson

I was actually waiting for someone else to respond to this, but...

I can answer your first question: yes and no. The law of conservation of momentum is even better than the law of conservation of energy, since the latter is allowed to have exceptions (within short periods of time). Photons do have momentum, so when their direction chages, there must be a equal and opposite change in momentum in the thing that interacted with the photon.

It is a though experiment, and not a practical experiment, because the momentum of a photon (let's pick 600 nm)is about 1 x 10^-27 kg m/s. If this photon was diffracted at an extreme angle (ie, stopped going "forward" and went essentiall "up," then the rolling slits would go through equal/opposite changes, but the magnitude would be so small that a 1 gram apparatus would change its magnitude of velocity on the order of 10^-24 m/s . This amount is not detectable.

I'm still waiting for someone else to take the second question. I think all the college guys are on vacation.

3. Jan 7, 2004

### Njorl

The problem is, the electron doesn't go up or down. It fills the entire diffration pattern. There is no change of momentum to the electron, and so, no momentum imparted to the plate. A single particle will interfere with itself.

Because the electron is "squeezed" into the slit, its location is restricted and hence less indeterminate. Therefore its momentum is made more indeterminate in the directions that the space is restricted, the vertical. This does not mean that the electron will veer up or down, just that it will be less well defined.

Njorl

4. Jan 7, 2004

### pallidin

Chi Meson, In your comment you said "The law of conservation of momentum is even better than the law of conservation of energy, since the latter is allowed to have exceptions (within short periods of time). "
Not being a trained physicist, I am sure your statement is very clear to those who are. But I am curious as a layman physicist... what are the exceptions you spoke of?

5. Jan 7, 2004

### Njorl

Most readers of this forum are familiar with the form of the uncertainty principle dealing with momentum and location (delta p delta x >=hbar), but there is an analog of it dealing with time and energy. For very short periods of time, a very small amount of enrgy can be gained or lost to the universe.

Njorl

6. Jan 8, 2004

### Nacho

Swamp Thing,

You might be right about taking Feynman's lecture point with a grain of salt. Here is what he wrote in "QED, The Strange Theory of Light and Matter", as a footnote on pages 55 & 56:

3 This is an example of the "uncertainty principle": there is a kind of "complementarity" between knowledge of where the light goes between the blocks and where it goes afterwards--precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as, light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when ..." If you get rid of all the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures--adding arrows for all the ways an even can happen--there is no need for an uncertainty principle!

Now I don't know how anybody else reads that .. but I read it as quite a snub to Heisenburg and Bohr and their ideas. He doesn't identify them -- appears to me not wanting to give them credit for something he doesn't believe in, and downplay their discoveries. Heisenburg's Uncertainty Principle and Bohr's Complemetarity are usually identified with proper names, and capitalized. Here he uses lower case and even quotes the words .. quotes not for quoting somebody, but to set aside the word. And in the end he says they are not needed at all. I was about floored when I read that.

So, why would he reference them in another lecture? Maybe because most other people believe in them? And not conceding their points to them (Heisenburg and Bohr), but not wanting to argue that specific thing .. just use it as a learning point on something else?

Last edited: Jan 8, 2004
7. Jan 9, 2004

### Chi Meson

I'm quite certain that each electron, like photons, arrives as a discreet bundle on the viewing screen. Am I mistaken?