Relativity 1 1: Poetics on the Edge of Black Hole

Relativity: Poetics on the Edge of Black Holes

Every four years, there will be a World Cup champion and a lot of Olympic gold medals, and more than a dozen people will win the Nobel Prize every year. And some heroic feats can only happen once in the history of human civilization.

16 10, that is, in the thirty-eighth year of Wanli in the Ming Dynasty, Galileo wrote a book called Star Messenger in six weeks. This book introduces what Galileo saw with the world's first telescope. He told people that there are countless stars in the sky far away from the earth, there are mountains on the moon, Jupiter has its own satellites, Venus has phases, and the sun has sunspots-these evidences show that the sun may be just an ordinary star, and all the planets we see revolve around it.

At that time, people thought that all celestial bodies were smooth spheres revolving around the earth. When it comes to books that change people's world view, no one can surpass Star Messenger.

General relativity soon became a routine tool for astronomers after 19 19. It is much more complicated than a telescope, and the astronomical discoveries it brings come out one by one.

However, the general theory of relativity tells us that the news of this universe is as shocking as the interstellar messenger. Like a black hole. What's so amazing about black holes? Starting from the most basic hypothesis of general relativity, an unimaginable but poetic situation is deduced step by step.

General relativity also has time dilation effect. Where the space is seriously curved, that is, where the gravitational field is strong, the time will be slower than where the gravitational field is weak. In ordinary people's words, it is faster to be at high altitude than on the ground.

To understand this, you should first know that there is a phenomenon in physics called "Doppler effect". The following reasoning process is very interesting, don't miss Einstein's exquisite thought!

Doppler effect means that for a wave, if it comes to you, its frequency will increase as each period becomes shorter; If it leaves you, the frequency will decrease.

For example, when a train comes to you, you will hear its sharper whistle; When the train leaves you, the whistle becomes lower.

The same is true of light waves. We now know that the speed of light is constant, but the frequency of light can be changed. If the luminous point is coming towards you, the frequency of light will appear higher, which means that when you see it, the color of light will become bluer. This is the so-called "blue shift" And if the luminous point is leaving you, the frequency of light will become lower and the color will turn red, which is called "red shift".

It is through the red shift and blue shift that astronomers can judge which stars in the universe are leaving the earth and which are flying to the earth.

Speaking of which, there is a cold joke that physicists like. American physical society once got a red bumper sticker that read

I don't know if you can feel the beauty of it. In short, you can judge the relationship between the light source and your relative speed by the color change of light.

Now let's go back to the thought experiment in the free-fall elevator. We imagine the elevator shooting a beam of light from the floor to the ceiling. Let's consider two situations.

Scene 1, the elevator is in a space without any gravity, moving freely in a straight line at a uniform speed. Then we can imagine that this beam of light should be neither red-shifted nor blue-shifted, as it is now.

In the second scene, the elevator will fall freely in the gravitational field of the earth, and there will be an acceleration from top to bottom. The ceiling will accelerate towards the light wave that just left the floor. Of course, the ceiling still sees the same speed of light-however, the ceiling will notice that this beam of light has a blue shift.

Okay, that's a problem. According to the equivalence principle, the physics in the elevator in scene 1 and scene 2 should be exactly the same, which means that no matter what experiment you do, you won't find any difference between them. What's the blue shift in the second scene?

When people think about this, they may say, Ah, this shows that the principle of equivalence is wrong-so people are not Einstein. Einstein believed in the principle of equivalence.

So Einstein said that the second scene should not see the blue shift of light. In order to do this, the gravitational field in scene 2 must provide a red shift to offset the blue shift of light caused by accelerated motion!

To this end, Einstein demanded that the gravitational field-or curved space-time-must have a property: it must have a red shift! This is the "gravitational redshift".

That is to say, in the gravitational field, the light emitted by a star from a height will have a natural red shift.

In other words, the same beam of light, when I look at it from high altitude, will feel its frequency slow down.

In other words, if you do something on the ground and I look at you from high altitude, I will think that you are doing slow motion. And if you look at me lying on the ground, you'll think I'm doing something fast.

That means you are older than me.

In other words, gravity can cause time to expand.

The time expansion caused by the near-earth gravitational redshift is proportional to the height, and the higher the ground is, the faster the time passes.

How accurate is this effect? You take two atomic clocks, one on the ground and the other on a building several tens of meters high-you can find the difference when they leave. Because the altitudes of Paris and London are different, their time difference is 1 nanosecond every day! Physicists have also launched satellites near the sun to verify the time expansion effect of general relativity, and the results are consistent with the theory.

The GPS satellite is far from the ground, and the time expansion effect is very strong, so the correction of general relativity must be considered in the calculation time. Without this correction, the positioning accuracy will be ten kilometers worse! This is also an example that ordinary people can directly use general relativity.

We can also fully explain the aforementioned twin effect. Why did my brother turn around and find that my sister on earth suddenly aged a lot? Because my brother's U-turn is a violent acceleration movement, and the acceleration movement is equivalent to a powerful gravitational field. My brother is on the surface of a massive celestial body, and my sister stands on a high place and looks at my brother-my sister feels the gravitational redshift.

In this way, people who live at the top of the mountain age faster than those who live at the foot of the mountain. Speaking of it, it is not necessarily true that pilots and flight attendants who often fly in the sky are younger than us because of high-speed sports-it depends on which of the two effects is stronger: high-speed youth and high-speed aging. Scientists from the National Bureau of Standards have done research, saying that even the speed of 40 kilometers per hour or the height of 30 centimeters is enough to have a measurable impact on the atomic clock-and for ordinary flight, the impact of height is slightly greater than the speed. People who fly 10 million miles are 0.059 seconds older than people on the ground?

Of course, all these effects are not obvious on the earth, including the whole solar system. Living at high altitude, you don't have to feel sorry at all. Even the gravity of the sun is not strong.

The greater the mass and the smaller the volume of a star, the greater its curvature to the surrounding space. The so-called "black hole" is that it bends the surrounding space to the point where even light can't come out.

Black hole and the aperture at the edge of the planet, the picture comes from the movie Interstellar.

From the outside, the black hole itself is a … black hole. But if there are other substances near the black hole, such as interstellar gas or charged particles, you will see an aperture around it. Those lights are radiation from the accelerated motion of charged particles.

The picture below shows the bending of ordinary stars, massive and small neutron stars and black holes to time and space-

Hawking has talked a lot about black holes, so I won't go into details here. But you need to know a concept: "horizon". The so-called event horizon is a boundary that separates the inside and outside of a black hole. Outside the event horizon, at least light can leave the black hole; No matter what enters the event horizon, it will never escape from the black hole.

Ok, now let's think about a particularly poetic thing-other places won't give you such a feeling: what kind of experience it is to fall into a black hole.

For example, if you go to a black hole, I will sit in a spaceship far away and watch you.

Because of the strong time expansion effect, when you approach the black hole, I will see your movements become slower and slower. You will be older than me!

Being close to a black hole does not necessarily mean falling into a black hole. In fact, because the size of black holes is often relatively small, it is not easy to fall in. You can think of a black hole as an ordinary planet, and you turn around it several times. You are in a state of free fall and won't feel any discomfort. But because the gravitational field of the black hole is too strong, your turn is too long in my eyes. If you come back to me twice, I may have died of old age, and you will be a teenager when you come back.

But if you don't think it's enough to go around the periphery twice, and you want to enter the horizon to see what's going on inside the black hole, it's in trouble.

On the event horizon, your time dilation will reach infinity.

In other words, when you fall into a black hole, what I see is that you walk slower and slower, and finally your figure will stay in the event horizon forever. I don't think you will ever move there ... your image will remain in my world forever. Your moment is my eternity.

But time expansion is relative to me, you won't feel it yourself, you will only fall into the black hole naturally. After the horizon, you won't feel anything strange. The black hole didn't hold a welcoming ceremony for you at the border. You can also see the light inside the black hole, and there is no difference between inside and outside the horizon in your eyes.

However, this is a road of no return. You will be killed by a black hole. But you didn't hit the ground and die. The bending space of a black hole is so large that the gravity in the lower part of your body is much stronger than that in the upper part of your body, and this difference in gravity will tear you apart. ...

We can't directly observe a black hole, but we can judge its existence from the way the stars near it move. Astronomers have enough evidence to find many black holes in the universe.

All the knowledge about black holes has been studied by other physicists, and Einstein didn't look back at the explosion caused by relativity. He just wants to do the most important research. Maybe next time.