One of the great challenges of modern physics is to try to wrap our minds around the idea that electrons behave differently depending on how we look at them. Electrons going though a double slit scatter in such a way that they demonstrate their fundamental wave nature. If we watch carefully to see which electron goes through which slit, the wave nature apparently disappears and a particle behavior predominates.

The problem is we don’t know how the electron knows that we’re watching! Why should its behavior change depending on whether we looking or not. It doesn’t have eyes to see us does it?

Well maybe. Not eyes exactly, but it appears that the classic way of performing this experiment is unavoidably preferring one sort of scattering mechanism over the other. A team of experimental physicists report from Italy, having looked very closely at the phenomenon:

“Although the electrons (which were shot one by one) could still pass through the filtered slit, the filter caused more of the electrons to undergo inelastic scattering rather than elastic scattering. As the physicists explained, an electron undergoing inelastic scattering is localized at the covered slit, and acts like a spherical wave after passing through the slit. In contrast, an electron passing through the unfiltered slit is more likely to undergo elastic scattering, and act like a cylindrical wave after passing through that slit. The spherical wave and cylindrical wave do not have any phase correlation, and so even if an electron passed through both slits, the two different waves that come out cannot create an interference pattern on the wall behind them.

The physicists also found that the thickness of the filter determined the interference effects: the thicker the filter, the greater the probability for inelastic scattering rather than elastic scattering, and so the fewer the interference effects. They could make the filter thick enough so that the interference effects canceled out almost completely.

‘When the electron suffers inelastic scattering, it is localized; this means that its wavefunction collapses and after the measurement act, it propagates roughly as a spherical wave from the region of interaction, with no phase relation at all with other elastically or inelastically scattered electrons,’ Frabboni said. ‘The experimental results show electrons through two slits (so two bright lines in the image when elastic and inelastic scattered electrons are collected) with negligible interference effects in the one-slit Fraunhofer diffraction pattern formed with elastic electrons.’”

Read the full article here.

So, the problem isn’t solved exactly, but the situation isn’t as simple as theorist imagined when they set up the paradox. And, isn’t it interesting that the electron’s classical or quantum behavior is dependent on which scattering mechanism is being probed? That probably means something…