Shulou(Shulou.com)11/24 Report--

One of the biggest mysteries of our existence is also one of the biggest mysteries of physics: time. When we experience the passage of time, there is a special moment that we call the present. Now you are reading this article. You may have been doing something else half an hour ago. No matter what you do, there is nothing you can do to change it. This is our view of time. But the time that physics tells us is very different from what we think, and it was Albert Einstein who figured it out. Know what Einstein taught us about the past, the present and the future, and that's what we're going to discuss today.

We think that time is something that is equally effective for everyone and every object, which makes time a universal parameter that marks how much time has elapsed and what we mean by "now". Minkowski, who was the first to notice that this may not be correct, believes that Maxwell's electrodynamic equation makes more sense if time is seen as a dimension rather than a parameter. If you rotate one direction of space to time, the Maxwell equations will not change.

So, Minkowski said, we just combine space and time into a four-dimensional space-time, and then we can rotate space to time, just as we can rotate two spatial directions to each other. This naturally explains the symmetry of the Maxwell equations, which has nothing to do with electric and magnetic fields, but comes from the characteristics of space and time itself.

We can draw the time dimension in the Cartesian coordinate system: one for time and the other for at least one-dimensional space, which is called a time-space graph. If it is only motionless, then the motion in this kind of graph is a vertical straight line; if it moves at a constant speed, it is a straight line tilted at a certain angle in the picture. The maximum speed is the speed of light, which is usually drawn at an angle of 45 degrees. We can move back and forth, left and right, or up and down in space. But with the passage of time, we can only go forward, not turn back. So time is still different from space in some ways.

So what does the time we're talking about mean? If we want to understand it further, we will enter Einstein's special theory of relativity. There are two important principles of special relativity: the principle of constant speed of light and the principle of relativity. These two hypotheses will have a far-reaching impact.

The speed of light is limited. Nothing is faster than the speed of light. You may be reading this article in front of your phone screen, so do you still have your phone "now"? Your answer is yes, but physicists may point out that you don't actually know, because the light from your "right now" phone hasn't reached your eyes yet. If someone grabs your phone and it takes a while for the lost information to reach your brain, how do you know where the phone is now?

Although this seems to be the case physically, this is not the way normal people use the word "now". We always talk about what is happening "now" and never have to worry about how long it will take for light to spread. Why can't we agree on some "now" and continue? To this end, we must come up with a way to determine what we mean by "now".

Einstein's thought experiment did this, assuming that we put a mirror on the left and a mirror on the right, and then emit a photon to the left and right mirrors at the same time. Because we have the principle of constant speed of light, we know that the left and right photons must return at the same time. In the final step, at half the time required for the whole process, we know that the photon must have bounced off the mirror. Now, we can say that this moment is the so-called "now", even if the light has not yet reached us. The representation in the space-time map is shown in the following figure.

Suppose a friend moves relative to you at a constant speed, and he moves along a sloping straight line in your space-time map. He does exactly the same experiment as you do, but from your point of view, half of the current time is not what you call the "present". The space-time map is shown below. Obviously, his "present" is no longer what you call "present", and you think his "present" is wrong. On the other hand, from his point of view, he is motionless and you move opposite to him, so he thinks his concept of "present" is correct, while your "present" is not correct.

This is the opportunity to use the principle of relativity, and the views of all observers are equally valid, so you are both right. In other words, the concept of "present" depends on the coordinate system, and your "present" is different from his "present". If you like technical terms, this is also called simultaneous relativity. Different observers have very little difference in the concept of "present" in daily life, and they become obvious only when the relative speed is close to the speed of light.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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