I have these following two experiments that seemed not so clear for me

  1. Michelson Morley experiment - The first one is the experiment from Michelson and Morley who proved (if I'm not wrong) that the speed of light is constant no matter what the speed of the observer is by using an interferometer. I'm not sure if the following set up is correct, they set the mirror to split the light beam into two identical waves traveling perpendicular to one another and set one direction in the same direction of the Earth moving around itself. Suppose the Earth moves along the right in the figure, and the light travels in the direction of green line would have the speed $c+v$ and the speed $c-v$ after it bounces the mirror, where $v$ be the speed of the Earth around itself. Hence, the average speed of the light along the green path is $c$ and culminates in no interference shift detected when they mingle at the center mirror? I don't know what's the exact experiment is, so please describe if I got things wrong (I hope not the whole)enter image description here

  2. Double slit experiment for electron - Where exactly the electron gun shooting an electron at a time? Is it in the middle of the two slits? The electron was shot in the middle and as depicted in the figure, it can travel in any direction as a superposition? So, putting the gun in the middle can yield the result that the electrons act like a wave? enter image description here

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    $\begingroup$ I recommend you to post these as two separate questions $\endgroup$ – Superfast Jellyfish Oct 23 '20 at 17:25

With regard to your first question, I think there is a misconception. Photons move at a constant speed (c) and cannot be accelerated in a vacuum no matter how much momentum is applied to its emitter (see Einstein's Train). As counterintuitive as it may seem, light is always emitted at the same speed in a vacuum no matter how much acceleration the emitter is undergoing. That's the problem with your first diagram. Your setup is mostly correct, but the photons do not move at a speed of c+v, and then a speed of c-v. Rather, they move constantly at c both ways.

A good way to visualize this is to imagine the following experiment: 1 photon emitter is at rest relative to an observer. A second photon emitter is moving at an arbitrarily fast speed v (v<c) relative to the observer. When both emitters are lined up relative to the observer, they are observed to simultaneously emit light beams in the same direction. At first blush, it might seem that the light emitted from the stationary emitter moves at c, while the light emitted from the mobile emitter is emitted at c+v. In fact, in a vacuum, both beams of light will move at the same speed, c.

With regard to your second question, there is an important caveat to the double slit experiment. From Encyclopedia Brittanica:

An important parameter in the double-slit geometry is the ratio of the wavelength of the light λ to the spacing of the slits d. If λ/d is much smaller than 1, the spacing between consecutive interference fringes will be small, and the interference effects may not be observable. Using narrowly separated slits, Young was able to separate the interference fringes.

The scale of the double-slit experiment is quite small, so either the slits must be very thin and very close to each other, or the light source must be quite far away. For more information, I suggest this Britannica article.


The experiment of Michelson shows that the speed of light can not be c+v or c- v. First your misconception : traveling with c+v and c-v the same distance gives not the mean v of c. Imagin you start at your house and drive in your car with 100km/h. after 10 km you stop, and return very slow with 1km/h, so you need 1/10 h with the car and 10 h on foot, you average speed ist not 50.5km/h but 20/10.1 km/h but still to proof that the light travels independent of the direction Michelson put his experiment on a heavy stone swimming in mercury, so he could turn it in different directions. one hat to be orthogonal to earth movement, one in direction of earth movement one perpendicular to it. but the interference pattern did never change! So it is not with one experiment he could proof, that light travels independent of the movement but this series of movement.


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