一個電子經過細縫偏轉了角度，這是否違反了動量守恆？

單獨一個電子而言動量方向發生了變化，假如一個個的發射電子，應該也會發生這個現象。

電子和狹縫之間有相互作用啊，動量傳遞給了狹縫╮(╯_╰)╭

謝侯哥 @侯文輝 邀。這個問題狄拉克在《量子力學原理》中做了闡述，只不過他的闡述對象是光子，但原理是相通的。回答恐怕要讓題主失望了：這個「問題」不是一個「有意義的問題」，題主產生這樣的疑問還是在於沒能擺脫經典觀念，認為微觀粒子一定要有一個確定的偏振或動量方向，這是不準確的，準確的理解是疊加態原理。我將大意轉述如下。

讓光線通過方解石晶體（crystal of tourmahne ），後者有這樣的性質，只允許偏振方向垂直光軸的光通過，當二者夾角 時，通過的光強度為 ；在光量子的角度下看待這個問題則是，有的時候一個光子可以通過有的時候不能，通過幾率為 ，如果通過，只能是一個完整的光子，其能量與原來的完全一致，但其偏振方向變成了與光軸垂直。關於「一個傾斜偏振的光子通過方解石晶體時到底發生了什麼」不是一個精確的問題（not really very precise），因為這不可觀測， 不涉及實驗的觀測結果，只有涉及可觀測結果的問題才是一個有意義的問題。在量子力學觀點下，一個電子/光子有一定的概率處在這樣的動量/偏振模式下，有一定的概率處在那樣的模式下，你所認為的運動模式只是兩種垂直分量的（概率！）疊加而已，只不過在進行觀測之後發生了坍縮。動量根本不是一個確定的唯一的值，也就不涉及違反動量守恆的問題了。

"The result of this experiment will be that sometimes one will find a whole Photon, of energy equal to the energy of the incident Photon, on the back side and other times one will find nothing. When one finds a whole Photon, it will be polarized perpendicular to the optic axis. One will never find only a part of a Photon on the back side. If one repeats the experiment a large number of times, one will find the Photon on the back side in a fraction of the total number of times.

"A question about what will happen to a particular Photon under certain conditions is not really very precise.

"Only questions about the results of experiments have a real significance and it is only such questions that theoretical physics has to consider.

"Questions about what decides whether the Photon is to go through or not and

how it changes its direction of polarization when it does go through cannot be investigated by experiment and should be regarded as outside the domain of science.

"It is supposed that a Photon polarized obliquely to the optic axis may be regarded as being partly in the state of polarization parallel to the axis and partly in the state of polarization perpendicular to the axis. The state of oblique polarization may be considered as the result of some kind of superposition process applied to the two states of parallel and perpendicular polarization.

「When we make the Photon meet a tourmalme crystal, we are subjecting it to an Observation. We are observing whether it is polarized parallel or perpendicular to the optic axis. The effect of making this Observation is to forte the Photon entirely into the state of parallel or entirely into the state of perpendicular polarization. It has to make a sudden jump from being partly in each of these two states to being entirely in one or other of them. Which of the two states it will jump into cannot be predicted, but is governed only by probability laws.

動量守恆是空間平移對稱性導致的，狹縫的存在破壞的這個對稱性，從而在這裡對於電子不存在動量守恆。