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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."
Just today at lunch I was saying "Wouldn't it be cool to craft a femtosecond pulse consisting of 1.5 cycles, say 2 maxima and 1 minima, passed through argon gas? We could get electrons which could be accelerated then observe the resulting interference patterns!"
Well, that didn't fly. The guys got pissed off and yelled "Shut up and watch the stripper!" so I sheepishly went back to my titties and beer.
Trolling is a art,
I've been trying for years to do the double-slit experiment. Alas, the wife still won't go for it.
"Adressen på den hjemmeside, du ønsker at finde, er enten forkert, eller også eksisterer hjemmesiden ikke længere. Du kan prøve følgende:
Tjekke om adressen er stavet rigtigt. Bemærk at det har betydning, om du bruger store eller små bogstaver!"
that may as well have been the writeup, because i don't understand a word of it.
Not to mention flowers, too...
Obliteracy: Words with explosions
For those of you who are unfamiliar with the double-slit experiment, there is a very clear, non-technical explanation here.
This space for rent.
"In the classic version of the experiment, electrons pass through a mask containing two parallel slits and produce a pattern of bright and dark interference fringes on a screen." Wasn't this called Pong?
Don't worry, the second time this story is posted you'll have figured it out.
:-)
Basically, you can look at light, or electrons, or whatever, as either a particle or a wave. Sometimes one interpretation will work better (light as a particle explains the photoelectric effect, light as a wave explains interference patterns, diffraction, etc). Current state of play is that the wave interpretation is always the best way to look at things, except when you observe the system everything collapses to particles, and when something mathematically inconvenient happens (you can explain the photoelectric effect in terms of waves, but the maths is horrible).
Classic two slit experiment with light consists of shining laser light on a barrier with two slits; each slit produces a diffraction pattern (http://en.wikipedia.org/wiki/Diffraction), the diffraction patterns interfere to produce the classic two slit pattern, see same link. This basically works because the laser light is coherent, you can (sort of) treat all the photons coming from the laser like one photon.
If you do this with electrons, because electrons are waves, you get the same patterns. Ditto any other particle.
Even if you do this experiment firing only one electron at a time you will get the same two-slit interference pattern, although 'common sense' tells you the electron can only pass through one of the two slits what actually happens is it passes through both at once. If on the other hand you fit a detector over one slit to register the passage of electrons, so you can tell which slit the electron passes through, you lose the interference pattern, you get two overlapping single slit diffraction patterns, which is not the same thing.
Roughly, if you have two slits and whenever an electron is fired at the slits you do not know which slit it went through, but the classical probability (what you'd expect if you didn't know quantum mechanics) of either slit is 0.5, then you will get a two-slit pattern.
This is basically the same experiment, except instead of two slits in space a little distance apart there are two possible source times for the electron, separated by a small time gap. There is no way to know whether a detected electron was produced at the first or second time, so the maths works out (roughly) the same as for the two slits in space case and you would expect to see the classic two-slits pattern. But it is kind of neat that someone's actually found a way to test that idea.
**Skip the first part if you know the basics.
If you pass a water wave through a wall with two slits in it, you will get interference. If you put another solid wall (no slits) beyond and parallel to the first wall, you will see that the water line on the 2nd wall looks like a sinewave with magnitude tapering off as you get further from the slits.
If you pass particles (electrons, photons, etc) at a wall with two slits, and place a "detecting wall" beyond the first wall, then the distribution of electrons hitting the detecting wall would be similar to the wave observed against the 2nd wall in the water example.
--New Experiment--
In the new example, two pulses of light can trigger an electron to be released. Think of these two pulses as pulling a trigger on a gun while playing russian roulette. The electron is the bullet and the detector is your head. If you pulled the trigger at 0 secs and 2 secs, you'd expect to see a person die at 0.01 seconds and/or/neither 2.01 seconds, assuming it took 0.01 seconds for the bullet to reach the person and kill him.
The detector, however sees an interference pattern. This is like seeing deaths at 1 second or 1.5 seconds. The interference pattern is measured as a function of time, and instead of seeing two blips in time, they saw a range.
The double-slit experiment classically involved sending light through two small slits closely separated, onto a dark screen. If light was particulate, you'd expect to see only two bright spots on the screen. But you see a whole interference pattern, with the brightest spot located between the two slits.
This is because of diffraction, and that light acts like a wave, so you get constructive and destructive interference on the screen.
What we didn't know until the 20th century is that light consists of photons, which are individual quanta of electromagnetic radiation. These photons interfere with each other in space as they go through the slits, to give the characteristic interference pattern on the far screen. Or, that the photons don't go through a single slit, but the photons actually go through both slits, and you don't know where the photon is until you measure it (ie, let it hit the screen).
The current experiment effectively used a laser to create two 'slits' in time. They made two quick laser pulses (really two maxima and one minimum). The pulses have some probability of creating an electron, and by making two discrete pulses in time, there is a similar 'interference pattern' associated with observing the electron at various points in time. This means that the electron wasn't created from one laser pulse or the other, but was effectively created through both slits, the time separation of which created an interference effect.
There's no new quantum mechanics here, but here's an attempt at a layman's explanation of what's called the propagator. In classical mechanics you have a well-defined trajectory from a set of well-defined initial conditions (ie, a ball on a spring has a well-defined position and momentum at some time, and you can exactly predict where the ball will be at future times). See this article for example.
Quantum mechanics extends this because there is a classical path the ball would take, but also infinitely many other 'quantum' paths that can also bring the ball from position X at time 0 to position Y at time T. Many of these are classically impossible. But Quantum Mechanics deals with a wavefunction (which describes the state of the system) which is complex. So you need to consider all these other paths too, but each path has an associated phase with it. When you maintain this phase coherence between all paths, you are basically building a similar interference pattern. So when you take the modulus squared of the wavefunction to find the probability of finding the electron, you have interference from the wavefunction going through either of the two slits in time.
The difficulty is that you have to repeat the experiment many times to see when you measure the electron, just like w/ the classical double-slit experiment you need enough photons to give a relative intensity that can be measured.
Here's a little math for anyone curious. The time progression of a wavefunction looks like
|Psi(t)>=exp(-i*H*t/hbar)|Psi(0)>
where |Psi(t)> is the wavefunction at time t, i is the square root of negative one, H is the Hamiltonian Operator, hbar is the Planck constant. See here for more information on the Hamiltonian for classical and quantum mechanics. In many cases it's the energy operator (expressed in terms of position and momentum), and acts on discrete energy eigenstates.
But you can see that time translation evolves the 'phase' of the wavefunction. And if the wavefunction isn't in a single energy eigenstate but a combination of them, each individual component will have have the phase evolve at a different
Will the second story in time interfere with the first one?
Mmmm.. Donuts
"Physics is cool and all, if you're not quite bright enough to make it in Math" "Physics is to math as sex is to masturbation." -R. Feynman