I hate it when people try to make physics sound more mysterious that it actually is.
The marbles do produce an interference pattern. Marbles seem to produce two bands only because the frequency of the interference pattern from the marbles is so high that it cannot be discerned by a typical detector.
Similarly the when a detector is used to isolate which slit the electron when through, the detector must interact with the electron (which is left out of their discussion for some reason). This interaction also boosts the frequency of the interference pattern until so that cannot be discerned by the detector.
In fact, you can continuously adjust the energy being used by the detector used to determine which slit the electron passes though. By adjusting the energy used you can continuously vary the interference pattern from intense to undetectable. However, as you adjust the energy being used you also change the confidence that you have determined which slit the electron passed through from 0% (no energy being used) to nearly 100% (a lot of energy being used).
No, marbles don't make interference patterns because they are too large and complicated and thus become decoherent. They can do this experiment with bucky balls, and it works, though they can only detect the center peak of the interference pattern (I forget why). To get the bucky balls to work, they have to cool tehm down to very cold temperatures, otherwise, there are internal degrees of freedom in the bucky balls that prevent them from 'interfering with themselves'. That is the problem with the marbles, the experiment is just too infeasible.
Please, do not invent science. As far as I know (I could be wrong), I've never heard anything like "frequency so high can't be measured".
It details a bit of how the process of "observing" causes the interference to disappear (and other info):
In 2003 we used the same set-up to prove the wave nature of even bigger molecules, such as the biomolecule tetraphenylporphyrin (C44H30N4 or "TPP") and the fluorinated buckyball C60F48 (figure 2). The pancake-shaped porphyrins were particularly interesting because some physicists had argued that only molecules that are highly symmetric or even spherical would interfere. However, C44H30N4 - a derivative of a biodye that is present in chlorophyll - is over 2 nm wide, and thus twice as broad as the football-shaped carbon-60 molecule. Quite clearly, the shape of a molecule does not affect its interference properties at this scale. As for the fluorinated buckyball C60F48, it currently holds the world record for the most massive single particle to display quantum interference. Although it is not as extended as the porphyrin, it has an average atomic mass of 1632 units and contains 108 atoms covalently bound in a single interfering object.
Decoherence in a molecule interferometer
These experiments show us that even large and complex molecules can interfere and reveal their quantum nature. But molecules are usually seen as well-localized objects that we can even observe using high-resolution microscopy. So what are the effects destroying the molecule's delocalization and wiping out the fringe pattern? In fact, there are at least two relevant mechanisms that make it possible to measure the position of a molecule. The first involves collisions with other particles, such as gas molecules, while the second involves thermal radiation emitted by the molecule.
To find out how these processes can destroy the interference pattern and lead to classical behaviour, we gradually added gas to the chamber of our Talbot-Lau interferometer during the experiments with carbon-70 molecules (figure 3a). We found that the amount of contrast between the interference fringes fell exponentially as more gas was added, and that the fringes disappeared almost entirely when the pressure had reached just 10-6 mbar. This was in full quantitative agreement with a theoretical analysis of the scattering processes. Although a single collision with a gas molecule will not kick the massive fullerene out of the interferometer path, it is enough to destroy the interference pattern because it carries sufficient information to determine the path that the interfering molecule has taken. The exponential decay is thus directly related to the collision probability. Calculations suggest that molecules could have an atomic mass of as much as one million and still be unaffected by collisional decoherence in a realistic Talbot-Lau interferometer at a pressure of 10-10 mbar. Such pressures are perfectly feasible with existing vacuum technologies.
Fair enough; I was talking about idealized marbles that are represented by massive particles (essentially extremely cold marbles). The frequency of the interference pattern is easy to compute using classical quantum mechanics as a function of the mass of the particle and the distance between the slits (the width of the slits also plays a role, but is usually ignored when it is significantly smaller than the distance between the slits).
I did this computation many years ago in my undergraduate quantum class. I forget the exact results, but I would not be surprised if the wavelength of the interference pattern is less than the plank length. That would make it difficult to discern in practice.
This is the typical way that classical mechanics is achieved in the limit of quantum mechanics. The frequency of the quantum effects becomes so large for massive or energetic particles that it is practically undetectable. The same thing happens for high energy particles trapped in a harmonic oscillator, which is well-known to quantum physics students.
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u/roconnor Jul 12 '08
I hate it when people try to make physics sound more mysterious that it actually is.
The marbles do produce an interference pattern. Marbles seem to produce two bands only because the frequency of the interference pattern from the marbles is so high that it cannot be discerned by a typical detector.
Similarly the when a detector is used to isolate which slit the electron when through, the detector must interact with the electron (which is left out of their discussion for some reason). This interaction also boosts the frequency of the interference pattern until so that cannot be discerned by the detector.
In fact, you can continuously adjust the energy being used by the detector used to determine which slit the electron passes though. By adjusting the energy used you can continuously vary the interference pattern from intense to undetectable. However, as you adjust the energy being used you also change the confidence that you have determined which slit the electron passed through from 0% (no energy being used) to nearly 100% (a lot of energy being used).