If the earth responds to civilizations 6 billion light-years away, how long will it take them to receive the signal from the earth?

If the earth responds to civilizations 6 billion light-years away, how long will it take them to receive the signal from the earth?

There is a funny metaphor. We responded to the electromagnetic pulse signal 6 billion light-years away, which is equivalent to somebody else's alien bombing a big Ivan (58 million tons equivalent hydrogen bomb), and we humans responded with cannons! Maybe with the ability of us earthlings, we can't even hit the gun! Can the other party hear you? I hope so!

If the attenuation of the signal is ignored, how long will it take for the "aliens" 6 billion light-years away to receive this signal? When the universe reaches this scale, it is necessary to consider the distance increase effect brought by expansion!

According to various measurements, the expansion speed of the universe is 70KM/S/MPC, that is, the expansion speed increases by 70KM/S per million parsec (3.26 million light years). It is very simple to calculate that celestial bodies 6 billion light-years away are moving away at a speed of more than 1.29 million kilometers per second! So, six billion years after the signal was sent, how far did they reach?

About 8.5 billion light years. Simply put, even if the other party is no longer far away from this position, it will take 8.5 billion years for the signal from the earth to reach their present position, and in 8.5 billion years, their expansion distance will exceed 3 billion light years, which means that it will take more than 20 billion years for the other party to receive this signal!

So we always thought that aliens also used this kind of radio communication. This way of communication in the era of space travel is equivalent to a bonfire thousands of years ago. We think about building the beacon tower higher and bigger every day, but who knows that people don't rely on this way at all!

Just FRB (Fast Radio Burst). What about repeating? It's just celestial activity that we haven't discovered yet. Not every time it thunders, you can shout "We are here, where are you"! It's strange not to be treated as a psycho!

First of all, human beings are not able to communicate with distant alien civilizations at present. Six billion light-years away from the Milky Way, the signal propagation distance will be greatly attenuated (inverse square law), and the result is no different from the background noise. With the current technology of mankind, let alone the alien civilization of galaxies outside the river, even if the alien civilization of galaxies sends signals to mankind, it is difficult for us to respond. So the following discussion is purely theoretical.

If the universe is static, then we will respond to alien civilizations 6 billion light years away, and they will receive human signals after 6 billion years. Because the carrier of signals is electromagnetic waves, and their propagation speed in vacuum is the speed of light, it takes 6 billion years to travel 6 billion light years. This means that it will take 654.38+02 billion years at the earliest for alien civilizations 6 billion light-years away to receive a reply from human beings.

But in reality, the universe is expanding, making distant galaxies farther and farther away from the Milky Way, so this problem will become more complicated. At large scale, the expansion of the universe is very uniform, and Hubble constant can measure the expansion speed of the universe.

Here, the Hubble constant is 70 (km/s)/Mpc, which means that every 3.26 million light-years (1 million parsec), the speed of galaxies leaving the Milky Way will increase by 70 km/s, which means that galaxies 6 billion light-years away are rapidly leaving the Milky Way at the speed of129,000 km/s (43% of the speed of light).

In addition, Hubble constant is not a real constant, it will change with time, because the expansion rate of the universe will change with time. At first, under the action of gravity, the universe was slowing down and expanding. By 5 billion years ago, when dark energy was dominant, the universe began to expand at an accelerated rate.

If the signal we receive travels through 6 billion light-years in the universe, with the expansion of the universe, the current distance of this signal source from the Milky Way will reach 7.6 billion light-years, and 6 billion light-years is only an optical distance. After human beings send signals to alien civilizations, with the passage of time, the galaxy where the alien civilizations are located will move away from the Milky Way faster and faster, which makes the signal spread to alien civilizations far more than 7.6 billion light years. According to friedmann equations's calculation results, the required propagation distance of signals will reach 654.38+006 billion light years, so the required time will also reach 654.38+006 billion years, instead of 6 billion years or 7.6 billion years. Therefore, it will take166 billion years at the earliest to wait for human reply after the alien civilization sends a signal.

If the signal sent by human civilization can reach 6 billion light-years away, it has long been background noise, and no one can distinguish it. In three-dimensional space, the intensity of the signal is inversely proportional to the square of the distance, that is to say, the intensity of the signal decays quickly. If it is to be received 6 billion light years away, the intensity of the signal must be particularly high. For example, the fast radio burst received before can release the energy equivalent to that released by the sun all day in a few milliseconds. Can humans achieve it? I can't.

Let's go back to this question. How long does it take for the signal from the earth to reach 6 billion light years away? Some people may think that the radio signal is the speed of light, so it only takes 6 billion years. Actually, this is not right. At a distance of 6 billion light years, the effect of the expansion of the universe is already very obvious. If the Hubble constant is 70 (km/s)/Mpc, it means that the distance of galaxies increases by 70 km/s for every 3.26 million light years. That means that the galaxy where the civilization is located 6 billion light-years away is moving away from us at the speed of129,000 km/s.

Simply put, the signals we receive are 6 billion light years away, and at this time, with the expansion of the universe, they are 8 billion light years away. If the earth immediately restores the signal at this time and the universe continues to expand, then the time for them to receive the signal is about 1 1 billion years.

If the civilizations in the universe are too far apart, it is almost impossible to communicate with each other. Human civilization is on an isolated island, and it may be difficult to meet other cosmic civilizations at the end of civilization.

Welcome to pay attention to us: scientific black hole! Image source network intrusion.

A: It takes more than 654.38+0 billion light years, and it may never reach the earth.

In astronomy, light years are often used to indicate the distance of distant stars. A light year is about 9.46 trillion kilometers. If humans receive a signal from a civilization 6 billion light years away, and then we reply, we must use a transmitter with tens of thousands of times the power of a star.

If the universe is static, it will take 6 billion years for the other side to receive information from human beings. By that time, the earth had been swallowed up by the red giant sun.

The universe is constantly expanding, and the current expansion speed is 67.8(km/s)/Mpc, which means that the retrogression speed caused by the expansion of the universe is 67.8km/s every 3.26 million light years.

Then, 6 billion light years away, because of the cosmic expansion effect, the retrogression speed relative to the earth is 65.438+0.25 million kilometers per second, which is 0.42 times the speed of light.

If the expansion speed of the universe remains the same, and the distance between the two continues to widen after we send messages to each other, then the time for signals to spread to each other will also be extended. It is estimated that it will be 654.38 billion years before the other party can receive the signal.

According to astronomical observation, the universe is currently in the process of accelerating expansion, so the actual propagation time will be longer.

It should be reminded that generally speaking, astronomy says that a star is hundreds of millions of years away from the earth, which is not the real distance at present.

In fact, this distance is not the real distance now, because this distance does not consider the expansion effect of the universe. The actual distance is far greater than 65.438+0.5 billion light years, and it is estimated to be around 65.438+0 billion light years. If you reply to the other party at this time, you have to use the real distance and the subsequent cosmic expansion effect to calculate the propagation time.

Let's start with a conclusion. Today's human beings can't communicate with such a distant civilization. Even if human beings gather the energy of the whole earth to send signals, they can't reach such a distant place. Before they got there, the signal had decayed into cosmic noise.

Let's talk about if, if humans have the ability to send such a signal, how many years will it take them to receive it?

The universe has been expanding. The farther the galaxy is from us, the farther it is from us. Hubble constant can give the answer, that is, 67.80 0.77 (km/s)/Mpc, and MPC is 1 million parsec(326 million light years), which means that every 3.26 million light years increase in distance, the speed of galaxies away from us will increase by about 70 km/s, and it is foreseeable that it will be 6 billion light years.

When we receive the signal, we can know how long the signal has traveled in the universe and the distance from the earth by measuring the red shift value. When we launched, they were still keeping 43% of the speed of light away from us, and the speed changed with time, getting faster and faster. In this case, it is more complicated to consider. Of course, the time is much longer than 6 billion years, at least 10 years.

Because of this, communication between distant galaxies becomes impossible, unless in the future, human beings can solve it in some way.

My humble opinion, welcome comments!

It can be said that no matter how much electromagnetic signal transmitting equipment we use, alien civilizations as far away as 6 billion light years can't receive the signals sent by the earth.

The attenuation rate of electromagnetic waves is directly proportional to the square of the distance, so when electromagnetic waves reach other planets through a distance of 6 billion light years, the attenuation is basically the same as the background noise of the universe, and alien civilizations can't tell it out at all. Of course, even if we use the sun as energy supply. Although Harper can now see celestial bodies 6 billion light-years away, it should be noted that the celestial bodies here are all galaxies, and the stars in the galaxies can't be distinguished at all! Therefore, only by using the energy of the whole galaxy can we send a signal to the alien civilization 6 billion light years away.

When we can use the energy of the Milky Way, I don't know how many billions of years have passed. Just before the destruction of the sun, mankind reached the level of using the energy of the Milky Way, which is 6 billion years. At this time, human beings will send out past signals, and with the expansion of the universe, it will take more than 654.38+0.5 billion times. Coupled with the time delayed by the progress of human civilization, it will take at least 2 1 100 million years for alien civilizations to receive our information. At this time, whether the alien civilization is still there or not is two kinds of statements!

Therefore, we only need to receive the signal from a long distance, so we don't have to consider the reply!

Mathematically, the answer is simple. At a distance of 6 billion light years, the signal travels at the speed of light, which of course takes 6 billion years.

But this is only the conclusion reached under ideal conditions. In fact, if humans really receive signals from alien civilizations 6 billion light-years away, it is of little practical significance to humans. At most, let us confirm the existence of alien civilization, which has no other practical significance.

Moreover, our universe has been expanding at a faster and faster speed. Theoretically, it takes 6 billion years to analyze the distance signal of 6 billion light years, but considering that the universe has been expanding, the actual propagation time is far more than 6 billion years, probably more than 65.438+0 billion years, and even the signal will never reach the alien civilization.

After in-depth analysis, we will find that the so-called "alien signal 6 billion light years away" hides a very horrible fact, that is, alien civilization sent a signal 6 billion years ago, and it is impossible to imagine what height aliens will reach after 6 billion years of development. Simply think about what height human science and technology will reach in 6 billion years!

Probably because of an accident, alien civilization has been extinct for a long time. Even if they still exist, electromagnetic waves are unlikely to still be used as a means of communication. Moreover, the long span of 6 billion years has made aliens forget the signals they once sent.

Even if humans reply to aliens, electromagnetic wave signals can't span billions of light years, and they have already weakened into cosmic radiation in the process of propagation!

Therefore, the speed of light should be quite fast in the eyes of human beings. Once it rises to the height of the universe, the speed of light is too slow. In the future, if human beings realize interstellar travel and emigrate to other planets, maybe we need to find a faster way to spread information, otherwise the speed of light will really limit the height of human civilization!

When the distance is measured in one year, the unit of quantity is 100 million, which is really too long for human beings. At present, the time recorded by our human civilization does not exceed 1 10,000 units, and 1 10,000 units is 1 100 million, which is 6 billion units. Perhaps the earth human civilization has been repeated many times. The other side estimates that civilization has been repeated many times.

Assuming that human civilization can last for 65.438+000 billion years, you receive a civilization signal of 6 billion light years, and you have to travel the same distance and time to respond to its signal (the wireless signal rate and the light rate are almost equal, which is assumed to be equal here), and the round-trip time is 65.438+02 billion years, which is really meaningless for short-lived human life.

It is expected that this boring news will be hyped for another month or so!

If they fly to the earth, it is probably the distance divided by the speed of light plus their speed. It may also be affected by the rotation of galaxies. Anyway, the time will not be short. My humble opinion, welcome to correct me.