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Double-Slit Experiment in Time, Not Space
Posted by
timothy
on Wed Mar 02, 2005 06:04 PM
from the that-young-guy-did-a-bunch-of-stuff dept.
from the that-young-guy-did-a-bunch-of-stuff dept.
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."
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Great minds think alike. (Score:5, Funny)
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.
Re:Great minds think alike. (Score:3, Insightful)
Ever read a biography of Feynman?
Re:Great minds think alike. (Score:5, Funny)
Gas? Check.
Femtosecond pulses? Not that kind of club, but Check.
Maxima with a minimum between them? Yup.
Dude, it was all there. What else did you need?!?
Parent
Re:Great minds think alike. (Score:5, Funny)
No, you have it wrong. See, It is possible to synthesize excited bromide in an argon matrix! Yes, it's an excimer, frozen in its excited state...As soon as we apply a field, we couple to a state that is radiatively coupled to the ground state.
Parent
Re:Great minds think alike. (Score:5, Funny)
Parent
Re:Great minds think alike. (Score:5, Insightful)
And yes, I am a physicist.
Parent
Re:Great minds think alike. (Score:4, Informative)
Yeah, I think you're right. No idea of your credentials, but the physicist often is right in such discussions.
Parent
Re:Great minds think alike. (Score:5, Informative)
The interesting thing about topological spaces is that they often have symmetries that are physically meaningful. In the case of a straight line the symmetry is called translational symmetry: this simply means that the line will look the same as you travel along it. To a physicist this means that there exists a "gauge group" for a line (a gauge group is essentially a group of transformations that generates a symmetry). The important thing is that the gauge group for a line corresponds to translational invariance, not to rotational invariance as the parent claimed.
Parent
Re:Great minds think alike. (Score:5, Informative)
Actually, Mathematicians don't say that. Mathematicians say that a closed curve is homeomorphic to S^1, and a line to R^1, ie, there exists a bijective, bicontinuous mapping between the sets.
The "topology" of a space is actually the set of all open sets in that space. (Which trivially could not be a set like S^1.) In essense, the thesis of general topology is that all continuity related problems can be redefined in terms of open sets. If you'll recall, in classic analysis an open set is defined as an open ball with respect to the metric of the space in question. This produced spaces that while perhaps not equivalent to R^n were very similar in many ways, in particular because there existed a way to meaningfully define the distance between any two points.
In topology, we do away with the metric definition of an open set entirely, and leave the concept of an open set essentially undefined (well, subject to a few sanity restrictions involving unions and intersections of open sets). This allows mathematicans to study spaces that really are nothing like the ones we experience regularly, and the vast majority of them are really, really unfriendly, which is one of the reasons that topology is the course that scares many math majors away.
However, it gives way to Algebraic Topology, which is without a doubt one of the most beautiful branches of pure math.
Physics is cool and all, if you're not quite bright enough to make it in Math. Ha ha. *jab*
Parent
Oblig. Feynman (Score:5, Funny)
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Re:Great minds think alike. (Score:4, Funny)
SB
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Re:Great minds think alike. (Score:5, Funny)
Well, you know, there's an old saying: physicists grow up to be engineers, and mathematicians grow up to be accountants. I don't think it's true (nothing bores me more than number crunching, I'd never go into accounting) but you have to admit, neither of our fields are particularly applicable, at least in their purest forms.
All my physics friends took jabs at engineers non-stop, too, back in school. Now they either work as engineers, or are paid much less. But then, look at Business majors.
Not that I blame them for making fun of engineers or anything. Engineers are... well, engineers, which reminds me of an old joke, as I veer dangerously off-topic. A mathematician, a physicist, and an engineer are all asked to define Pi.
The mathematician says, "Pi is equal to the ratio of the circumference of a circle to its diameter."
The physicist says, "Pi is equal to 3.141592653589, plus or minus 3 in the last digit."
The engineer says, "It's about 3."
Yuck yuck.
Parent
Re:Great minds think alike. (Score:5, Funny)
Parent
Re:Great minds think alike. : Moving Dimensions (Score:5, Insightful)
It's not a new way of thinking at all; crackpots have been coming up with stuff like this for decades. I take it as granted that you lack the mathematics to spot the logical inconsistencies in what you're saying, so you'll just have to trust me when I say there is *nothing* of value in your idea. As a simple example:
What about it doesn't make sense to you? It's a new way of thinking. It makes sense to me:
That's so ridiculous it's not even wrong. If you're truly interested in understanding this sort of thing you could do a lot worse than to buy a book on special relativity.
Parent
Re:Great minds think alike. : Moving Dimensions (Score:5, Funny)
Here ya go:
www.timecube.com [timecube.com]
There's gotta be a second or fourth corollary to Goodwin's Law here somewhere... mentioning the time cube guy....
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Re:Great minds think alike. : Moving Dimensions (Score:4, Funny)
Obviously only a Nazi would post a time cube link.
...
;)
(The real joke here will be clueless mods marking this as flamebait
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Re:Great minds think alike. : Moving Dimensions (Score:4, Funny)
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Hrm (Score:3, Interesting)
Re:Hrm (Score:4, Informative)
In this experiment we have an atom which has a 50% chance of being ionized at time t0 and a 50% chance of being ionized at time t1 (OK, the probablities cannot literally be those values but this is an example) so we have a superposition of two states - one corresponding to an atom ionized at one time and one ionized at another time. As the wavefunction for the atoms is essentially oscillatory it means that as the wavefunctions for these two separate states evolve they are out of phase with each other (or are sums of terms that are out of phase with each other). This means we can expect constructive or destructive interference depending on the exact value of t1-t0. This is what was observed.
Parent
Re:Hrm (Score:5, Informative)
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 [wikipedia.org] 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 [wikipedia.org] and quantum [wikipedia.org] 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
Parent
Ah yes... (Score:5, Funny)
Re:Ah yes... (Score:5, Funny)
Parent
Re:Ah yes... (Score:4, Insightful)
"I've been trying for years to do the double-slit experiment. Alas, the wife still won't go for it."
"That's a pity because your nanoscale penis is probably about the right size for quantum effects to be significant. "
That burnination was worthy of Trogdor himself.
Parent
huh?! (Score:5, Insightful)
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.
Re:huh?! (Score:3, Informative)
Re:huh?! (Score:5, Informative)
What the double slit experiment did was allow us to show that light is both. In the experiment, one shines a pinpoint of light onto two very thin slits. The physics of waves dictate that waves will interfere in a characteristic pattern. This was later used with any matter of particles to show that the wave/particle duality, that is, all suitable small things act like waves or particles depending on the circumstances.
The experiment depends on the fact that we have no idea which slit any particular particle passes through. This uncertainty, in a certain sense, allows particles to go through both slits, which is why a single electron will interfere with itself. If we do know which slit an particle goes through, then then interference disappears. In this way we can show that particles are a wave until, in Schrödinger terms, we collapse it into a wave. So the experiment can show the duality.
So, to summarize, when the state of any particular particle is left uncertain, and certain other conditions are met, it will interfere as a wave. What they are doing here is introducing the uncertainty through a ultra-short pulse of light. There are two ways that the pulse could interact with the surrounding particles, but the universe does not know exactly which interaction occurred. There, the strange and headache producing phenomenon of the sub atomic world are allowed to manifest. I am not sure how this is time instead of space, but it is neat.
Parent
Re:huh?! (Score:4, Informative)
What the double slit experiment did was allow us to show that light is both.
The double slit experiment showed us that light was a wave. This understanding allowed a Grand Unified Theory of Optics (not that they called it that) which explained reflection, refraction and diffraction in terms of waves.
We didn't know light was both until Einstein's 1905 paper on the photoelectric effect (for which he won his Nobel.)
For electrons, it was the other way around. First we knew they were particles, then the electron double split experiment proved that they also behaved as waves.
Parent
Question for /. subscribers (Score:4, Funny)
Re:Question for /. subscribers (Score:5, Funny)
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Re:Question for /. subscribers (Score:5, Funny)
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The Double-Slit Experiment (Score:5, Funny)
Not to mention flowers [angryflower.com], too...
FYI (Score:3, Informative)
"Maximum" is singular. "Maxima" is plural. Minima are similar.
So it's "two maxima and one minimum."
Elegant (Score:5, Informative)
For those of you who are unfamiliar with the double-slit experiment, there is a very clear, non-technical explanation here [utoronto.ca].
Speaking of time... (Score:4, Interesting)
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.
"Whoa." --Neo (Score:3, Insightful)
The very thought of making 5-femtosecond laser pulses (0.000 000 000 000 005 sec, right?) leaves me feeling dumb and slow.
That aside, someone please clue me in here:
So if the electrons hit the laser when the pulse was at maximum strength they would hit the detector, like the two "beams" of light passed through the slits in Young's experiment? and the ones that pass "between" the maxima and minima get distorted like the blurry edges of the light? thus making "slits" of electrons but at instants in time instead of separate points? (I'm no physics expert but I'm sure you guessed that by now...)
hmmm... (Score:4, Funny)
So what does this mean? (Score:5, Informative)
But in this case we're dealing with two different electrons fired at different times, so it's not quite the same.
Even so, if the electrons create the interference pattern, that means they must have collided... in time? So the second electron reached the point of collision before it was actually fired.
Does that mean that every electron travels every possible path in space AND in time? So whenever it is possible for an electron to be fired, it does, and interferes with all other electrons fired at all other times?
My head hurts. Damn you, Science.
Re:So what does this mean? (Score:5, Informative)
Basically, yup. Read Feynman's QED. He claims (and the math and experiments bear him out thus far) that all photons are particles, all electrons are particles, etc., and that this "all possible paths" concept is what accounts for their "wavelike" manifestations.
Parent
Interesting (Score:5, Interesting)
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.
pi in the sky (Score:4, Interesting)
Of course the most important question is.. (Score:5, Funny)
A Brief Explanation (Score:5, Informative)
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.
What this means (Score:5, Funny)
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.
Anyone who has a femtosecond pulse generator should feel comfortable with this. If not, get access to a two-photon UV femtosecond pulse generator which uses nanosecond-time-scale infrared laser to deplete the terminal state of an F2 laser, based on F2 transitions.
Next, you'll want a healthy dose of argon gas. Argon is used to reduce heat loss in sealed units by slowing down convection inside the air space. You can get argon gas cartridges to prevent wine oxidation, which is a neat little side benefit. A 50L cylinder filled with argon gas to a pressure of 10130 kPa at 30C has approximately 201 moles of argon. Just remember that if you're going to lase with argon, its most efficient transitions are at 488 nm and 514.5 nm.
So now you'll need to create an ion chamber using the argon gas. You'll need a metal conducting can, and a wire electrode in the center which is well insulated from the chamber walls. The chamber, of course, will be filled with argon.
Next, you'll need to use your femtosecond pulse generator to apply a DC voltage between the outer can and center electrode. This will create an electric field, of only a few volts, that sweeps the ions to the oppositely charged electrodes. For some additional fun, if you apply a few hundred volts, the electron emissions will produce "secondary emissions", which amplify the results. I wouldn't recommend creating one of these by hand if you haven't already done so, but remember to use a 4.7uF capacitor with non-polar film, a 100,000 megohm resistor and a 2N4117A electrometer-grade JFET.
Anyways, generating a local maxima shouldn't be too difficult if you keep the phase dynamics of your pulse generator within one half delta of the wavelength propogation delay of your argon gas cylinder. This, as always, varies according to room temperature, so be sure to calibrate your scales before attempting the experiment.
The trickiest part of the experiment is to build a ray tube to display your intereference pattern. I suggest using a Tektronix Type 453 Oscilloscope, which may be hard to find but has the best bang per buck.
In no time at all, you'll be generating double slits in time!
Re:Full Text (Score:5, Funny)
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Re:Sooo... (Score:5, Informative)
It tells us nothing new about waves and particles, but it does confirm that there is no difference between a pair of slits separated by space and a pair of slits separated in time.
IOW it confirms that time is just another dimension.
Parent
Bored... (Score:5, Informative)
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.
Parent
Re:So is this saying ... (Score:5, Insightful)
The experiment is the same as a known one, with a single difference: In the traditional experiment the slits are separated by a difference in the normal 3d space, But in this experiement the slices are at the same place in the normal 3d space but separated by a difference in time.
Parent
Re:So is this saying ... (Score:4, Informative)
No, time isn't a wave. As another poster mentioned, time is another dimension.
But it's much more tricky than that, time is very different from space. If you rotate a vector in 3-D space, it's length (x^2+y^2+z^2) will remain the same, even though the x,y, and z components are different and kind of mixed together. What Einstein showed is that in 4-dimension space-time, the quantity (-t^2+x^2+y^2+z^2) is what is conserved if you 'rotate' in 4-D spacetime (in other words, if you change reference frames, like going from standing on the ground to standing on a freigh train). So spatial dimensions look spherical while the time dimension looks hyperbolic.
There are obvious parallels between Space and Time in non-relativistic quantum mechanics, namely a time translation evolves the wavefunction by a factor exp(-i*H*t/hbar) and a spatial translation evolves the wavefunction by a factor exp(-i*p*x/hbar). What this means is that momentum is the 'generator' of space translations, and the 'Hamiltonian' is the generator of time translations.
But making relativity works in quantum mechanics isn't as straightforward as physicists hoped, and involved alot of extra work, which finally culminated as quantum field theory. You can read more detail here [wikipedia.org]. But here's a quick summary :
In quantum mechanics, position and momentum aren't just parameters but are operators. They don't commute, which is why you cannot simultaneously know a position and momentum. But time is NOT an operator, it is a parameter, it's the corresponding Hamiltonian that is the operator. So you have 4-dimensional space, 3 dimensions act like operators, 1 dimension acts as a parameter.
So anyway, back to this experiment, what the physicists did was to show that an electron, with a probability of being created during two discrete times (each of the laser pulses) turns out to have an interference pattern just like photons traveling through two slits in space.
The resulting electrons weren't produced from laser pulse 1 or laser pulse 2, but were produced from a superposition of both pulses, and the complex phase that I showed previously with time evolution causes an interference pattern between the two pulses.
Parent
Re:Physics Explained! (Score:4, Informative)
We defeated the Nazis, the next evil: Libertarians
Who is this "We" you refer to?
And since when do people who work hard to support civil liberties get lumped in with people who work even harder to take then away?
I think you need to spend a bit more time at Cato's website and learn what Libertarianism really represents. (Hint: diminished state control of our lives)
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Re:Time is an illusion? (Score:5, Funny)
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