Einstein once predicted the phenomenon of "gravitational time expansion", arguing that the greater the gravitational pull of an object, the slower the time it experiences. Subsequent studies have shown that his predictions were correct, as early as 1960, the "Pound-Rebka experiment" for the first time confirmed the existence of "gravitational time expansion", and to this day, the global positioning system we use must also be time-corrected according to "gravitational time expansion".
So the question is, how did Einstein know that gravity slows time down?
I have to say that "gravitational time expansion" is a bit confusing, because in our impression, gravity should only act on matter with mass, and time is not matter, there is no mass at all, so why can gravity slow down time? To answer this question, we need to start with the "principle of invariance of the speed of light."
Simply put, the "principle of invariance of the speed of light" means that the speed of travel of light in a vacuum is a constant constant (c) regardless of the frame of reference in which you are observing, and does not change due to the relative motion of the light source and the frame of reference in which the observer is located.
This means that no matter what state of motion a beam of light in a relative vacuum is, the speed of the light you observe is always c, for example, if you move against a beam of light in the vacuum at 10% of the speed of light, then the speed of the light you observe is still c, not 1.1c, and if you move in reverse with a beam of light in the vacuum at the same speed, then you observe the speed of the light is also c, not 0.9c.
The "invariant principle of the speed of light" is obtained by scientists through theory and experiment, which is also one of the public settings of special relativity, according to which we can deduce from a thought experiment that speed can slow down time.
The picture above is a simplified mode of "photon clock", in which the photon clock is a vacuum state, and the photons can be continuously reflected vertically at the speed of light (c) in a vacuum between two parallel mirrors, so the time for each reflection of the photon is "h/c" (Note: h is the vertical distance between the two mirrors).
Now we take two such photon clocks, one placed on the ground, called "photon clock a", and the other placed on a spaceship moving at high speed relative to the ground, called "photon clock b". In this case, if the ground is used as a stationary frame of reference, the photons placed in the "photon clock b" will have an additional motion in addition to the vertical motion with the direction of the spacecraft's motion (as shown in the following figure).
That is to say, if we observe on the ground, we will find that the distance of each reflection of the photon in the "photon clock b" increases, and according to the Pythagorean law we can conclude that this distance is "√ (h^2 + x^2)" (Note: "√" refers to "under the root number", x is the translation distance of the photon).
According to the usual thinking, the photons in "photon clock b" superimpose the speed of the spacecraft, and its speed also increases, so that the photon in "photon clock b" will complete the reflection time equal to that of "photon clock a".
However, due to the "principle of invariance of the speed of light", the speed of photons does not increase because of the speed of the spacecraft, that is, if we observe on the ground, we will find that the time of each reflection of the photon in "photon clock b" is "[√ (h^2 + x^2)]/c", while the time of each reflection of the "photon clock a" is "h/c".
However, for the people on the spaceship, because they are moving with the spacecraft, they will not observe an additional movement of the photons in the "photon clock b", so they observe that the photons in the "photon clock b" are still "h/c" every time they complete a reflection.
This means that for every time "h/c" spent on the spaceship, "[√(h^2 + x^2)]/c" time is spent on the ground, compared to the former, which passes more slowly than the latter, in other words, the time on the spaceship slows down.
It can be seen that the cause of this phenomenon is precisely the speed of the spacecraft, according to which we can also speculate that the faster the speed of an object, the slower its time. So what does this have to do with Einstein's idea that gravity slows down time?
In the experience of taking the elevator, we often feel that when the elevator is just started, there will be a force opposite to the direction of the elevator movement (such as the elevator running upwards, the direction of this force is downward), and this force is actually the "inertia force" caused by the acceleration of the elevator, which is essentially the embodiment of the inertia of the object.
Suppose there are two scenarios, one of which is to have a person in a spaceship and then place the spaceship on the surface of the Earth.
Another scenario is to place the spacecraft in a weightless environment and let it continue to fly upwards at an acceleration exactly equal to the gravitational acceleration on the Earth's surface.
It is conceivable that in both scenarios, the person in the spaceship will feel the downward force, and it is exactly the same force, if the person can not observe the outside world, then he will not be able to distinguish whether he is subject to the force of the earth's gravity or the "inertial force" caused by the acceleration of the spaceship.
It follows that a frame of reference with acceleration is indistinguishable from the frame of reference in the gravitational field, and they are equivalent, and this is the important theory proposed by Einstein in General Relativity, the "equivalence principle".
Einstein argued that since the two are equivalent, each gravitational field can be replaced by a frame of reference with a specific degree of acceleration, so that time in the gravitational field can be described using special relativity, and since speed can slow time, the gravitational field can of course also slow time, and the greater the gravitational force of the gravitational field, the faster the acceleration of its equivalent frame of reference, and the slower time will become.
Well, today we will talk about this first, welcome to pay attention to us, we will see you next time.
(Some of the pictures in this article are from the Internet, if there is infringement, please contact the author to delete)