This chapter of the video:
• describes how detectors placed next to each slit reveal that half of the electrons went through each slit.
• describes how the act of measuring the electrons at the slits causes the interference pattern to disappear.
• outlines how researchers in Tübingen, Germany have verified these measurement disturbance results.

In the double-slit experiment with tennis balls, each ball passes through just one slit and no interference pattern is observed. With water, the wave passes through both slits and an interference pattern is observed. An interference pattern is also observed with electrons. This surprising result raises a question about how each electron passes through the slits. The answer is not obvious. The fact that we always observe electrons as localized particles suggests that each electron goes through just one slit, like a tennis ball. However, if that was true, electrons would form the same distribution that the tennis balls make. Instead, they form an interference pattern. Does this mean that each electron somehow goes through both slits, like a wave?

To find out exactly how electrons pass through the slits, we can place detectors next to each slit. Physicists in Tübingen, Germany did just this in 2002. Their detector consisted of a slab of silicon placed near both slits. When a (negative) electron went through one of the slits it attracted positive charges in the silicon creating an electrical current, which caused the silicon to heat up. From the heating data, the Tübingen scientists were able to determine that an equal number of electrons went through each slit.

The electrons significantly above the silicon slab did not interact and were not measured, shown by the intact interference pattern at the top of Figure 4.1. However, electrons passing near the bottom of the slits were measured, and the interference pattern was destroyed and replaced by a completely random distribution of hits, shown at the bottom of Figure 4.1. The act of measuring the electrons had disturbed them and the pattern they produced on the screen. This phenomenon is called measurement disturbance. It is one of the defining features of quantum physics. In classical physics, we can measure an object without affecting it. For example, we can measure the speed of a car with a radar gun without altering the car’s speed in any significant way. However, if we measure an electron, or any other quantum object, we change its behaviour in a significant way.

WHY DOES MEASUREMENT DISTURB?
To measure which slit an electron goes through, we have to physically interact with it. In the Tübingen experiment, electrical charges in the silicon “detector” exerted electromagnetic forces on the electrons. The interaction with each electron was largest when the electron passed close to the silicon, and it weakened as the distance increased. For example, if we measure which slit electrons go through by shining light on them, photons hit the electrons and bounce off them. Interactions like these have an effect on electrons. When a photon hits an electron, the electron rebounds and changes its direction of motion. As photons collide with each electron in a slightly different way, different electrons travel off in different directions. As a result, they hit the screen all over the place, destroying the interference pattern.

HEISENBERG'S UNCERTAINTY PRINCIPLE
The concept of measurement disturbance is closely related to Heisenberg’s uncertainty principle. This principle says there is a fundamental limit on how accurately we can make simultaneous measurements of the position and momentum of a quantum object. So, if we know the position of a quantum object with great accuracy, then we know very little about its momentum. When a detector measures which slit an electron passes through, we know its position with great accuracy. So Heisenberg’s uncertainty principle says that its momentum is highly uncertain. This means that the electron could be moving in one of a wide range of directions, as shown in Figure 4.2. This leads to electrons hitting the screen all over the place and to the pattern at the bottom of Figure 4.1.