We can see an object because it emits light or reflects light, bringing the information of the object to our eyes, and then the brain images the light signal, so that we can see the object. Although the speed of light is very fast, this speed is not infinite, about 300,000 km/s.
Therefore, everything we see is not the present moment, but the past scene. If there is an object outside 1 m, what we see is actually a scene 300 millionth of a second ago. This time interval is very short. If you are far away, the time difference will get bigger. For example, the moon is 380,000 kilometers away, and it takes 1.3 seconds for the moonlight to travel to the earth. The sun is located at1.500 million kilometers away, and it takes 8.3 minutes for sunlight to reach the earth.
Celestial bodies outside the solar system are more distant, and the light they emit takes thousands of years, even billions of years, to spread to the earth. For example, the light emitted by the GN-z 1 1 galaxy took13.4 billion years to reach the Earth. The longest-lasting light in the universe is cosmic microwave background radiation. These photons came from 65.438+0.38 billion years ago, that is, 380,000 years before the birth of the universe.
But no matter how long time has passed, it doesn't mean that what we see is false, and the information carried by these lights is true. The microwave background radiation we see in Sirius, Andromeda galaxy and the universe is real, and these things do exist in the universe.
Just because of the limitation of the speed of light, we can't see distant celestial bodies in real time. We don't know what those celestial bodies look like now, and we don't know whether they are still there. The celestial bodies that still exist in our field of vision may not exist now.
For example, Haishan II is located 7500 light years away, and what we are seeing now is the scene of this star 7500 years ago. According to observation, Haishan II has evolved to the final stage, and it may explode into a supernova at any time in the next 200,000 years. Haishan II may no longer exist. It may have exploded 7000 years ago, but the light is still on its way to the earth, so there is no explosion at present. It will be another 500 years before the explosion light can reach the earth and be seen by us. Or Haishan II exploded before 7499, and we will see it evolve into a supernova next year.
In a sense, the universe is like a grand documentary. Everything that happened in the universe in the past was carried by photons, and these photons have been moving forward in the universe. When photons reach the earth and are received, we can understand the past of the universe. If we observe the farther universe and receive older light, what we see is the earlier universe.
Astronomers have always been eager to understand the early universe. Although the energy of light emitted by distant celestial bodies has been greatly attenuated after long-distance propagation, we can capture it through astronomical telescopes. Hubble Space Telescope is a window for us to understand the universe. It photographed very distant galaxies, greatly expanding our horizons and giving us an unprecedented understanding of the universe.