This mistake is about the gravitational lens, which is a small mistake. This kind of mistake exists. After all, Einstein is a man standing at the top of human wisdom. If he makes some mistakes, it is reasonable. Two important points, many people don't think about the problem, do it, and experience it, but they don't know the mystery. Some topics are very good, and the ideas and theorems contained in them can be transformed into many different topics with a little change.

The topic is endless. As long as the author wants to write a topic, you will never finish it in your life. However, there are only a few concepts and types. Once mastered, all changes are inseparable from its principle. Only by staying the same and coping with all changes can we win by surprise and achieve success. Therefore, we must think about this problem, not only what it is, but also why it is.

Moreover, people who study the universe think that if the mass of a celestial body only increases, the gravitational force it produces is unimaginable, and it can distort the light passing through it, just as a lens can change the path of light, so it is called a gravitational lens. The gravitational lens is Einstein's prediction, but he still thinks that human beings can't observe it at all. This gravitational lens is not important at all. But now, gravity lens has become one of the most useful technologies for scientists to draw the map of the universe.

The second Doppler effect, galaxies fly over the sky. Doppler published this theory in 1842, and a group of musicians played horns on the train in 1845 to prove this effect. A similar "Doppler effect" applies to light waves, but the term we use here is frequency rather than tone. The only difference between blue light and red light is frequency. Blue light has a higher frequency than red light.