The vacuum is not empty
A ball that rotates in a vacuum, if the external gravitational and electromagnetic forces are excluded, then it will spin to the earth and will not stop. Classical physics textbooks say so, because angular momentum and energy are to remain conserved in the absence of external forces.
But quantum mechanics, which is often opposed to classical physics, once again makes a startling prediction: the speed of the ball spinning in a vacuum will gradually slow down until it stops completely!
Is it possible that angular momentum and energy can not be conserved in quantum mechanics? Definitely! The reason is, vacuum also has friction!
How do you say that?
We usually think of a vacuum, which is a space where there is nothing and nothing. This is also the view of classical physics.
But quantum mechanics tells us that the vacuum is not really empty, but full of quantum fluctuations one after another: a large number of positive and negative particle pairs are constantly created and annihilated. Because these particles exist for a very short time, they are called "virtual particles".
In principle, a vacuum can produce any virtual particles. However, since in quantum fluctuations, the smaller the fluctuation amplitude, the greater the probability of the occurrence of quanta, and among all elementary particles, the rest mass of the photon is zero, and the quantum fluctuation amplitude that produces it is the smallest, so the virtual photon is the easiest to generate.
Vacuum friction of **
Normally, we don't detect these virtual photons, but they can have a real impact. For example, it can hit an object in a vacuum like a real particle.
Since the motion of the virtual photon is completely random, when the object is stationary, the impact from different directions cancels each other out, so it has no effect on the object. But once the object is in motion, it's a different story.
Suppose there are now two virtual photons A and B, which are moving at the same speed but in opposite directions, A is in the same direction as the object, and B is in the opposite direction of the object. aHit the object from behind, so that it gains energy, accelerates; And B hits the object from the front, causing it to lose energy and slow down.
Literally, one accelerates, one decelerates, and after counteracting, it seems that the object should maintain its original velocity. But in fact, this is not the case. Anyone who has studied physics in middle school knows that the energy gained by the impact of A is always less than the energy lost by the impact of B, and the end result is that the object slows down. In life, a face-to-face collision of a car is often more costly than a rear-end collision, which is exactly the reason.
It is not difficult to understand that for a ball spinning in a vacuum, although the impact of the virtual photon comes from all directions, the end result is to slow down its rotation until it stops.
Actually, it's the same as the ball spinning in still air and eventually stopping. It's just that one is hit by a virtual photon, and the other is hit by an air molecule.
Factors that determine vacuum friction
The strength of this effect of vacuum depends on the composition and size of the object. Objects made with materials that do not easily absorb electromagnetic waves, such as gold, may slow down little or no at all. Because an object collides with a virtual photon, it involves the absorption and re-emission of the photon (which is different from a collision between two macroscopic objects). The more massive the object, the less likely it is to slow down, because the friction of the vacuum is very small after all. Therefore, if a rotating object is a small particle that easily absorbs electromagnetic waves, the deceleration will be more obvious.
The speed of deceleration also depends on the temperature, because the higher the temperature, the more virtual photons are produced in the vacuum. At room temperature, a 100-nanometer diameter graphite ball (about the order of most interstellar dust) will take about 10 years to slow down to one-third of its initial rotational speed. At 700, it only takes 90 days to drop the same rate. At close to absolute zero (the temperature of most interstellar space), it would take 2.7 million years.
When an object slows down, the rotational energy it loses is emitted in the form of real, detectable photons. Unfortunately, detecting such photons requires a high degree of precision, beyond our current state of the art.