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Double-Slit Experiment in Time, Not Space
TheMatt writes "Thomas Young's double-slit experiment is a classic experiment that helped establish the wave-like nature of light. Since then, it has been done with atoms, buckyballs, and biomolecules. It has even been seen in a single molecule, and the single electron version was voted the most beautiful experiment by Physics World readers (covered previously on Slashdot). Now, PhysicsWeb is reporting that Gerhard Paulus and coworkers have conducted the double-slit experiment using a double-slit in time, not space. The "slit" was a crafted femtosecond pulse consisting of one-and-a-half cycles--say, two maxima and one minima--passed through an argon gas. Each maxima has a probability of ionizing an argon atom and producing an electron. The electrons were accelerated to a detector which observed an interference pattern since the detector had no idea which maximum produced the electron."
I know I'm probably going to be rated down for not being all-knowing, but could someone try to explain this in a bit more simplific terms? I know what the dual-slit experiment was, but I don't understand the purpose of this particular one.
I just glanced at the calendar. Nope, not April Fools. So why exactly is a large paragraph of nearly incomprehensible text on the front page of Slashdot?
Relativistic time dilation has been demonstrated by synchronizing atomic clocks and sending one of them into space for a while at high speed. The one sent into space slows down a tiny bit. As I interpret this, one of the clocks is slightly in the past relative to the other one.
Suppose you did the same thing with two entangled particles. The particle sent into orbit be slightly in the past relative to the other one. So would they then be entangled across the dimension of time? Seems like this has big implications, though what they are is beyond me.
Looks to be that they have redone the classic double-slit experiment in a new variation.
Instead of having the two slits existing at the same time but in different 3d space, they made the slits in different time, but in same 3d space.
Probably we have the same quantum effect as in the traditional double-slit experiment: When trying to determine which slit the particle passes through the interference pattern goes away, as the waves change change to particles.
It doesn't look to me like they have seen that experimentally yet. Their setup that did not produce the interference pattern looks more like a single-slit to me.
But I think that an attempt to find out at which of the two maxima are ionizing an argon atom should make the interference pattern go away.
I'd love to see a geometric illustration of how this demonstration is identical to Young's, rotated in spacetime.
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make install -not war
I have seen pictures of the double slit experiment in action, but have never had an opportunity to play with it myself.
So you have two slits, and slight goes through both slits and creates an interference pattern on the far plane.
So the experiment looks like this:
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But what happens if you set up the box so that there is a divider between the slits, like so?
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Pretend that line extends to both planes to form a solid barrier.
So what would happen in this case? If you fire one photon at the two slits, would it choose one side only? And why would the fact that there is a barrier on the other side of the plane with the slits affect how light passes through the slits which it arrives at before it knows that the barrier exists?
And what happens if you change that divider in the middle to make it so it doesn't extend all the way to the side with the slits? If you keep making it shorter and shorter towards the side the light is being projected onto, at what point does the experiment return to the way it is expected to behave, assuming it has not been behaving as expected up until this point?
Isn't there something meaningful in this observation?
Why would energy change with time? Or is it just that the frequency of electron hits adding an negating are causing the variances in energy?
I'd like to stare at the experiment and the graph... Maybe after burning it into my retinas for a while, then sleeping restlessly, then waking and going to work tomorrow, then forgetting about it for a while, maybe then the understanding will come...
Best.... reference.... ever
All three answers are actually correct, in that they're accurate, however differently precise descriptions of Pi.
This reminds me of my earlier years in high school when my physics teacher would get really annoyed at the students who would put in answers like "3.52302881055" when clearly, the margin of error was at the first decimal.
My point: when we were kids, there was a stigma associated with the number of digits after the decimal you could get out of your pocket calculator. A sort of "More is better" mentality.
Without digressing, my point is that the engineer needs no more than 3. Knowing more, or wanting to cram more would be like driving an SUV inner city... it would be overkill.
Aside from the elitism of how precise our representations of numbers are, I think the real debate comes as to how much creativity is involved in the three disciplines. I personally believe that all three have the potential to be extremely boring and also extremely creative disciplines.
Fyi. I grew up in pure physics, switched to pure math, and eventually ended up being a software 'engineer'.
This reminds me of a time when I was taking some engineering courses subsequent to completing my undergrad in mathematics.
... Try it out with a DVD case or some such ;-)
The professor had given a "challenge problem" in dynamics. I've long since forgotten the specifics of the problem, but this, I do remember:
I spent several days pondering the problem, trying to figure out how to decouple the equations or do a gradient walk or some such - in order to obtain a closed form global solution.
Having had no luck, I asked an engineering major student in the same class how he was coming on the problem. He said he had solved it a couple of nights ago. As I excitedly began to quiz him on what math wizardry he had employed, he began to look at me as if I was from some strange and alien planet. He informed me that he had "plugged it into TK-solver" and out came the answer.
Talk about an "AHA!" moment - it would have never occurred to me that numerical analysis was "good enough" for the job, which of course was to obtain a numerical answer that could be "engineered" with. The problem probably didn't even have an analytical solution proper.
Sounds funny, but we were coming at the problem from two completely different perspectives. (BTW, it was then that I decided that I was not cut out to be an engineer!)
Oh, and one more thing...
I did gain one other valuable insight from that dynamics class that has stuck with me to this day. Namely, that a rotating body is stable only when rotating about its major or minor axis - rotation about any other axis will induce a "flip"
You actually see the image on the phosphor screen yourself through a window at the base of the column. The image is a bit dim, you you have to have the lights out, but what you see is being imaged directly.
The electrons all have roughly the same energy - a million eV or so - so they are the equivalent of nearly monochromatic light. If your target film varies in thickness, then you get electron Newton's rings because of reflections from the top and bottom surfaces. You can get lots of fringes - out to the 50th or 100th order because the electrons are pretty monochromatic.
Suppose you have a 1 MeV electron beam travelling about 50 cms from your target to the screen. You cannot put more than a few hundred picoamps through your target without frying it. Now you do not get many electrons per second in a picoamp, and they are moving very fast at 1 MeV. I remember doing the sums, and finding out that the whole TEM column for my beam current spent 97% of its time completely empty. The film is only a few nm of this 50 cms, so the odds of it having two transmitting electrons in it at once is really tiny.
You actually see the image on the phosphor screen yourself through a window at the base of the column. The image is a bit dim, so you you have to have all the lights out, but what you see is being imaged directly by the electrons. Or electron, rather, because what you are looking it is the image formed by a single electron interfering fifty or a hundred times with itself after having passed through every point of the target film, and reflecting (or not reflecting) multiple times off each surface.
This as much as anything got me to believe in the wave equations. Trust in the sums and leave your common sense by the door, and it all seems to work.
Great minds think alike and all; actually Feynman and co-workers was seriously thinking about this possibility once. If there is only one single electron it would explain why all electrons are exactly similar, with exactly the same charge , mass etc, because all the electrons we observe are just the same one (Bob if you like).
Now why did Feynman consider this wild hypothesis; well, because one valid mathematical representation of a positron (the anti-particle of the electron) is as an electron traveling backwards in time. It is still unresolved if there exist any fundamental particles that actually travel backward in time instead of in the same time directions as we experience. The attitude in theoretical physics is always if the fundamental equations don't disallow it, one has to consider it a possibility to check for. One argument against such particles would be if they could be used to communicate with the past with all the possible paradoxes such a time communications would create (just like time travel).
Anyway, Feynam was considering if the electron Bob would sometimes become the positron anti-Bob, travel back in time and then after a while return back to normal Bob. To us, these events would look like anti-matter matter anihilation, with the creation of a gamma-ray to preserve momentum.
The reason Feynman dropped the idea is not because it was too wild, but because the hypothesis had a serious deficit since it could not explain why there were so little anti-matter around.
--- guns don't kill people, people with guns kill people ---
The argument against space having a physical existence is that it would provide a fixed frame of reference that doesn't really sit well with relativity.
Space-time, however, can be considered to have a "physical" existence, but it can't really be compared to normal matter. Space-time probably is a manifestation of something (perhaps a discrete, causal network, as suggested by Stephen Wolfram), but any descriptions of it are purely speculative.
The engineer says, "It's about 3." A much better approximation for pi that's easy to remember is 355/113 (113355 is the mnemonic). As an engineer, I say that's engineering at its best.