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

In 1960, physicist Freeman Dyson visionary envisioned that developed alien civilizations would one day no longer attach importance to kindergarten-level things such as wind turbines and nuclear reactors. Instead, starting from a larger pattern, design a device that completely wraps their stars to absorb all the energy they might get. They can use this large amount of energy to "mine", shoot short videos on social media, explore the deepest secrets of the universe, and enjoy the fruits of an inexhaustible civilization.

But what if one day we rise up and find that such an alien civilization is ourselves? What if we build a Dyson ball around our sun? Is it possible for us to do that? How much energy will it take us to rearrange our solar system, and how long will it take for our investment to be profitable? Before we consider whether human civilization has the ability to build such a crazy project, even if it is only a theoretical consideration, we should first determine whether it is worth building. Can we get a net benefit from energy by building a Dyson ball?

The Dyson Ball Project states in advance that the author is a theoretical cosmologist, not an engineer. I have no idea how to build a bridge, let alone the project to reshape the solar system. But I bet no one knows how to deal with such a big project challenge. We cannot say exactly in which areas and what progress is necessary to build a structure that only partially surrounds the sun. Speculating about this is a matter of science fiction, interesting, but not very fruitful.

Kevin Gill Art rendering of Dyson Ball (CC BY 2.0) but what I know is the laws of physics, and some of the physical principles behind Dyson Ball can be discussed. We can use the construction of a Dyson ball as a thought experiment to explore the basic principles of energy, orbit and motion. This is important, because no matter how advanced and magical technology our descendants are, they still have to face the cold laws of physics. They can't get something for nothing. If you want to reshape a planet, it takes energy. If you want to move solar panels the size of a mountain to different tracks, you also need energy.

For these and many other reasons, it takes energy to build a Dyson ball. Therefore, below we will discuss how long it will take to recoup the energy investment in building a building, and what is the optimal design that minimizes the initial investment.

In order for the calculation to have some initial data, we have to make a lot of assumptions. People like to laugh at physicists for simplifying complex problems, sometimes even beyond recognition. There is a classic joke that the dairy farmer went out to a nearby university for help to explain why his cow's milk production was so low, and the physicist's reply first assumed that the cow was spherical.

But such a simplified method has great power, which is why physicists have been trained in this method since the first day of introduction. First of all, it allows us to answer questions when we are not interested in accurate data at the initial stage. Here, we only need a general idea of feasibility-building a Dyson ball (relatively speaking) requires less, medium, or a lot of energy? Second, simplification helps us cover up mistakes (whether it's calculations or our initial assumptions). If what we are after is only a rough result, then twice as many errors (or even 10 or 100) will not really change the overall intuition of our calculations.

Finally, we don't really know how to build a Dyson ball, so trying something more complex will only lead us to introduce more assumptions to deal with all the small details. Each hypothesis increases the uncertainty of any number we get, and this uncertainty may eventually be hidden in the analysis rather than simply stated in advance.

To make assumptions, as for assumptions, this is what we will do later in this article. Please feel free to make your own amendments. I sincerely hope that this article does not provide a formula for building a Dyson ball, but provides a train of thought for more interesting discussions.

Our goal is to turn the entire planet into a solar energy collector. We don't know or care what our future generations use to obtain and store energy, so this article assumes that our energy collectors (such as the components of a Dyson ball) are made of substances that are still in stone, so the average density of these substances is similar to that of Earth, like solar panels. This article will maintain this assumption when we take apart other planets (focus only on the rocky parts we need).

This article will also assume that no matter what elements we need to build Dyson balls, they will exist in the raw material in the amount we need. I think this is a quite reasonable assumption, after all, we are talking about hollowing out the whole planet and turning it into something else, so we have a lot of material to process.

Finally, we assume that the thickness and density of the whole Dyson sphere are uniform, so that any part of the sphere is a good approximation of the overall structure. It doesn't matter whether you use the original Dyson ball idea or just a cluster of giant solar panels. Either way, all we care about is the part of the ball that is covered by the structure of energy collection when we put it on a particular orbit.

As for the thickness and efficiency of solar panels, we will look at these data when considering our options.

Even if we cover the entire surface of the earth with solar panels, we can still capture less than 1/10000000000 of the total energy produced by the sun. Most of the energy radiates into the empty universe and cannot be used. If we want to build a "great galactic civilization" state, we need to stop the radiation of energy. So we need some minor modifications. We hope that not only the surface of the earth can be used to receive solar energy, we let the earth disperse and receive more energy.

Image source Pixabay so we're going to tear the earth apart and make huge plates that move it in the orbit of the planets, each of which captures sunlight and converts it into energy. In order to have an intuitive sense of the difficulty of this, we can introduce a quantity called binding energy. All the particles that make up the earth are bound together by the mutual gravitation between them. If you want to disintegrate the earth, you can imagine taking one particle at a time and throwing it out at an escape speed.

This process becomes easier as you work, and as each particle leaves, the earth's gravity decreases gradually, causing the escape velocity of the next particle to decrease slightly. Eventually, you will remove every particle on the planet and our world will officially disintegrate. In fact, humans have already started to do this, and we have sent 10000-20000 tons of material into orbit or beyond (and a considerable part of it has been left there). We only have 597199999999999999990000 tons of material to be sent away, and we are already very good.

While future generations may come up with some super-clever ways to minimize the effort required to process our planet into a series of tablets, the combination of energy allows us to estimate the energy needed to achieve this goal. For the earth, our binding energy is about joules. By contrast, the energy consumed by all mankind each year is only 1 trillion times less than joules.

Assuming that we have completed the task of dismantling our planet, it is time to rearrange it so that it can cover as many areas of the sphere as possible, and then use it to harvest more solar energy than it does now. We are ready to answer this key question: how long will it take to recover the energy we spent tearing the planet apart in the first place?

If we assume that our solar panel is 1 km thick, its surface area is equivalent to nearly 2000 of the earth's surface. However, it cannot completely cover our sun because it can only capture about 0.0004% of the sunlight in our orbit. Still, this is a huge step forward compared to the energy we can get on an undivided planet. Our sun releases about joules of energy every second. If we assume that our energy conversion efficiency is 10%, then even if we capture this tiny portion of solar energy, we can recover the energy consumption we used to tear down the earth in as little as 60,000 years. Considering the scale of the big project we are running, this is not bad.

If we can reduce the thickness of solar panels to one meter and increase efficiency to 90%, we can recoup these energy investments in a few years. From then on, it was all a natural gift.

What about the other planets? If we love the earth too much to tear it to pieces, that's not a problem-if we can tear it apart, we can tear down any planet. The advantage of Mercury is that it is already very close to the sun, so breaking it up will enable us to cover a larger part of the solar radiation. But it is also a smaller planet with less material to use. If Mercury is made into solar panels one kilometer thick, we can capture 0.0001% of the solar energy. If the efficiency is 10%, we will recover the energy consumption that broke up Mercury in about a thousand years. If it is a meter thick solar panel and 90% efficiency, our energy collector has an area equivalent to the surface area of more than 100000 Earths, and the investment will be recovered in less than a year.

On the other hand, Jupiter is by far the most massive planet in the solar system, so it should help build a giant Dyson sphere. But Jupiter is mainly made up of gas, and only about five Earths of solid matter (in theory-we can't know for sure) is buried under thousands of kilometers of mostly useless gas. We have to take the whole planet apart, but we can't even use most of its mass. All in all, we will get the surface area of about 10000 Earths, but in Jupiter's distant orbit, its coverage is no better than Mercury. Given the huge cost of dismantling the shackles of this gas giant, it will take hundreds of millions of years to recoup our energy investment.

Switching to thinner solar panels and assuming higher conversion efficiency have improved the situation to some extent, enabling us to achieve a positive return on investment in just hundreds of thousands of years. But we are not a particularly patient civilization, so it will be difficult to adopt.

All of the above calculations of the Mountain removal Project assume that we leave the matter of each planet in its current orbit. But if we want to reconstruct our solar system, let's do everything we can. The amount of radiation we can capture at a given surface area is inversely proportional to the square of the distance between the energy collector and the sun. If we shorten the distance between the solar collector and the sun, the energy harvested will be increased. If we can move the planetary debris to a closer orbit, we can cover a larger portion of the sun's output.

Relative position of planets in the solar system indicates | Source pixabay but there is no such thing as a free lunch. Yes, the sun is at the center of the gravitational trap of the solar system, so in a way, the sun is "lower" than other planets. You might think it wouldn't take much energy to move anything closer to the sun. But the planets are already in higher orbits, and in order for them to change their orbits, you must first change their speed.

There are many ways to move an object from one orbit to another. In our calculation, we will use perhaps the most direct one: Horman transfer. In our case, to move the planet, we first have to slow down the planet and it will fall to the sun. But when it is close to the sun, it will get a greater speed. If we don't take any follow-up action, the planet will revolve around the sun and fly back to where it started, that is, along a long ellipse. It's of no use to us, so we have to push it again and put it on the track we want.

The author likes to think of Horman transfer as an orbital version of passing the ball from the mountain to his friends. First of all, you have to play football to make it move. It takes energy. The ball will roll all the time and accelerate as it moves. If your friends do nothing, the ball will roll past them. Instead, they have to play again and release energy again in order to stop the ball at their feet.

We can use the vis-viva equation (conservation of mechanical energy) to estimate the relationship between the orbit and velocity of a planet, as well as the energy required to move from one orbit to another. Vis-viva, which means "vitality" (kinetic energy) in Latin, is a relic of medieval concepts of energy and motion. But I guarantee that future generations will still use it to calculate the energy consumption of moving planets.

Looking back to Earth, we can never hope to capture all the solar output with a kilometer-thick Dyson panel. But if we get a little closer, maybe we can. If we move the earth to 1/10 of the current orbital radius (or 0.1 astronomical units), we can cover 0.04% of the sun-a hundredfold increase in energy output. But moving the earth consumes about 10 times more energy than we need to disassemble it.

Fortunately, with the increase in the proportion of energy capture, our return on investment time has been shortened to only 10,000 years, even if the efficiency of solar panels is only 10%. Then we can enjoy the extra captured energy for hundreds of millions of years to come.

Moving orbits is not good for Mercury. Moving it to 0.1 astronomical units will increase energy costs, thus extending our payback period to thousands of years.

Moving Jupiter to the same orbit, or at least the rocky part of its center to the same orbit (we can let hydrogen and helium drift in the original orbit)-will consume a lot of energy, about joules. Through our efforts, we can cover nearly 20% of the sunshine. It will still take us more than 1 million years to see a positive return on investment, but after that, our efforts will be well worth it.

For thinner, one-meter-thick solar panels that run at 90% efficiency, this has completely changed. In the orbit of 0.1 astronomical units, the earth will cover 1/3 of the sun, and we will get a return on our energy investment in about a year. As for Jupiter, we don't even have to reach 0.1 astronomical units. About 30% farther away than this, we can achieve the unimaginable goal of completely encircling the sun. It only takes us a few hundred years to recover the energy cost, and from then on we can have all the output energy of the sun.

So now we know: based on our technical level and construction capabilities, we can follow Dyson's advice and reconstruct our solar system, capture a large portion of the sun's output energy, and use that energy for any purpose we want. But as I said at the beginning, I don't know how to actually build a Dyson ball-it can be used as homework for engineer friends.

Author: PAUL SUTTER

Translation: there is a good future.

Revision: * 0

Original link: What is a Dyson sphere? | Space

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Paul Sutter

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