Some, like our Sun, are pretty consistent, keeping a distance of around

夜空に浮かぶ天の川銀河では数十億もの星々が周回している

30,000 light years from the galactic center, completing an orbit every 230 million years.

太陽を含むいくつかの星の軌道は

This dance is not an orderly ballet, more like a skating rink filled with drunk toddlers.

銀河の中心から3万光年を保ち、 2億3千万年で一周している

This chaos makes the galaxy dangerous.

銀河系の星々の軌道は、息の整ったバレエというより 泥酔者だらけのスケートリンクのようだ

Our solar neighborhood is constantly changing, with stars moving hundreds of kilometers every second.

この混沌が銀河を危険な場所にしている

Only the vast distances between objects protect us from the dangers out there.

太陽系のご近所も刻々と変化しており

But we might get unlucky in the future. At some point,

星々は秒間数千kmも移動している

we could encounter a star going supernova or a massive object passing by and showering Earth with asteroids.

衝突を避けられているのは 単に広大な距離があるからだ

If something like this were to happen, we would likely know thousands if not millions of years in advance.

だが、いつか不運が訪れるかもしれない

But we still couldn't do much about it.

超新星爆発に巻き込まれたり

Unless,

通りかかった大質量の天体に引き寄せられて

we move our whole solar system out of the way.

小惑星が降り注いだりすることもありえる

[Snazzy Kurzgesagt intro music]

このような出来事が起こる場合 数百万～数千年前には予測できるはずだ

To move the solar system,

だが、予測しても打つ手がないだろう、

we need a stellar engine, a megastructure used to steer a star through the galaxy. It's the kind of thing

ただひとつの…

that might be built by an advanced civilization with Dyson sphere-level technology

太陽系をまるごと動かしてしまうという手法以外には

that's thinking about their future millions of years ahead of time.

太陽系を動かすにステラエンジンが必要だ

But how do we possibly move the hundreds of thousands of objects in the solar system?

ステラエンジンは銀河の中で星を動かす巨大な構造物で

The good news is we can ignore all of that.

ダイソン球レベルの高度な技術を持つ未来の文明が

We only need to move the Sun; all the other stuff is glued to it by gravity and will follow it wherever it decides to go.

数百万年後を見据えて建造するかもしれないものだ

There are lots of ideas about what a stellar engine might look like and how it would work.

しかし、太陽系にある数十万もの物体を どうやって動かすというのだろう

We've picked two, grounded in our current understanding of physics, that could be built in theory.

実はいいニュースがある。他の星を動かす事に関しては無視していい

The simplest kind of stellar engine is the Shkadov thruster, a giant mirror.

動かす必要があるのは太陽だけだ

It works on the same principle as a rocket.

他の星々は重力によってひっついており、 どこに行こうがついてくるのである

Like rocket fuel, the photons released as solar radiation carry momentum, not a lot but a bit.

ステラエンジンのデザインや仕組みについては 多数のアイディアがある

For example, if an astronaut turned on a flashlight in space, it would push them backwards very very slowly.

ここでは、現代の物理学に基づき、理論上は可能な 2つのタイプを取り上げる

A stellar engine will work a little better than a flashlight because the Sun produces a lot of photons.

最もシンプルなステラエンジンは シュカドフスラスター

The basic idea of the Shkadov thruster is to reflect up to half of the solar radiation

巨大な鏡だ

to create thrust, and slowly push the Sun where we want it to go.

ロケットと同様の原理で動作し

In order for the Shkadov thruster to work, it must be kept in the same place, not orbiting the Sun.

ロケットの燃料のように、太陽から放たれる光子は、運動量により

Although the Sun's gravity will try to pull it in, it would be supported by radiation pressure which props the mirror up.

僅かな推進力を持っている

This means the mirror would have to be very light, made of micron thin reflecting foil from materials like aluminium alloys.

例えば、宇宙飛行士が懐中電灯を点けると

The mirror's shape is important too,

極めてゆっくりと、体が後方に押し出されるだろう

enveloping the Sun in a giant spherical shell wouldn't work because that would refocus light back to the Sun,

太陽は懐中電灯よりも活躍が期待できる

heating it up and creating all sorts of unpleasant problems.

作り出す光子が大量だからだ

Instead we use a parabola, which sends most of the photons around the Sun and in the same direction, which maximizes thrust.

シュカドフスラスターの基本は 太陽放射の半分を反射して推進力を作ることにある

To prevent accidentally burning or freezing Earth with too much or too little sunlight,

そしてゆっくりと、太陽を望む方向に押していく

the only safe place to build a Shkadov thruster is over the Sun's poles.

シュカドフスラスターを機能させるには 太陽を周回しないような方法でその位置を固定する必要がある

This means we can only move the Sun vertically in the plane of the solar system and one direction in the Milky Way,

太陽が引力で引き込もうとするが

which limits our travel options abate.

太陽放射の押し上げる力で支えられる

But that is basically it.

鏡はごく軽量となるはずで、

For a civilization capable of building a Dyson Sphere, this is a relatively simple endeavor.

厚さ数ミクロンで反射性のある アルミ合金のような素材になるだろう

Not complicated, just very hard to build.

鏡の形状も重要だ

At full throttle, the solar system could probably be moved by about a hundred light years over 230 million years.

巨大な球で太陽を包んではならない

Over a few billion years, it gives us near complete control over the sun's orbit in the galaxy.

そうすると光が太陽に集まり、加熱させ

But in the short term, this might not be fast enough to dodge a deadly supernova. That's why we thought we could do better.

他に大量の問題が発生する

So we asked our astrophysicist friend if he could design a faster stellar engine for this video.

パラボラアンテナの形状がいいだろう 多くの光子を同一の方向に返し、推進力を最大化する

He did and wrote a paper about it that's been published in a peer-reviewed journal.

日射量の激変により 地球が燃えたり凍ったりしないよう

You can find it in our sources document.

シュカドフスラスターは 太陽の極点を覆うように作るのが安全だ

We're going to call at our new stellar engine the Caplan thruster.

これは太陽の移動が太陽系と垂直方向に 限定されることを意味する

It works a lot like a traditional rocket: shoot exhaust one way to push yourself the other.

銀河内で一方向のみの移動となると

It's a large space station platform powered by a Dyson sphere that gathers matter from the Sun to power nuclear fusion.

選択肢は限定的だ

It shoots out a very fast jet of particles at nearly 1 percent the speed of light out of the solar system.

だが、そういうものと思うしかない

A second jet pushes the Sun along like a tugboat.

ダイソン球を建造できるレベルの文明なら 努力すれば作れるだろう

The Caplan thruster requires a lot of fuel, millions of tons per second.

複雑な工程はなく、ただとても大変なだけだ

To gather this fuel, our thruster uses very large

全速力なら、太陽系は100光年を 2.3億年で移動できるかもしれない

electromagnetic fields to funnel hydrogen and helium from the solar wind into the engine.

数十億年あれば、銀河内での太陽の軌道を ほぼ制御できるかもしれない

The solar wind alone doesn't provide enough fuel though, and that's where the Dyson sphere comes in.

しかし短期的にみると 超新星爆発を回避できるほどには早くない

Using its power, sunlight can be refocused to the surface of the Sun.

だからもっといい方法はないか考えた

This heats small regions to extreme temperatures, lifting billions of tons of mass off the Sun.

この動画向けに、より高速なステラエンジンを デザインできないか友人の天体物理学者に頼んでみた

This mass can be collected and separated into hydrogen and helium.

彼は実行し、論文を書き 査読付き雑誌に掲載された

The helium is burned explosively in thermonuclear fusion reactors.

論文は詳細欄にあるソース一覧にも入っている

A jet of

この新たなステラエンジンをカプランスラスターと呼ぶ （訳者注：友人の天体物理学者の名前がCaplan）

Radioactive oxygen at a temperature of nearly a billion degrees is expelled and becomes our primary source of propulsion from our stellar engine.

これはロケットのような機構で動く 残り滓を一方に放出し、逆方向に進む

To prevent the engine from just crashing into the Sun, it needs to balance itself.

ダイソン球を動力源とする宇宙ステーションであり 太陽から核融合の素材を集め

To do this,

分子を超高速（光速の約1%）で 太陽系の外部にジェット噴射し

We accelerate the collected hydrogen with electromagnetic fields using particle accelerators and shoot a jet back at the Sun.

もう一つのジェット噴射で太陽をタグボートのように押す

This balances the thruster and transfers the thrust of our engine back to the Sun.

カプランスラスターは 秒間数百万トンという大量の燃料を必要とする

In as little as a million years, this engine can move the Sun by 50 light years, more than enough to dodge a supernova.

この燃料を集めるため、巨大な電磁場を作り

At full throttle, the solar system can be completely redirected in its galactic orbit in 10 million years.

太陽風に含まれる水素とヘリウムを エンジンに流し込む

But wait, will we use up the Sun this way?

太陽風だけでは燃料が不足するので

Fortunately the Sun is so massive that even billions of tons of material will barely scratch the surface.

ここでダイソン球の出番となる

In fact, this megastructure will actually extend our Sun's life, since lower mass stars

その力で太陽光を太陽表面に集中させ

burn slower, keeping the solar system inhabitable for many more billions of years.

一点が極度の高温となることで 太陽の質量を数十億トン持ち上げる

With a Caplan thruster, we could turn the entire solar system into our spaceship.

この質量を集め 水素とヘリウムに分ける

For example, by orbiting backwards in the galaxy and

ヘリウムは熱核融合炉で燃焼させる

colonizing hundreds or thousands of stars as we pass by them.

10億度近い放射性酸素がジェット放出され

It may even be possible to escape the galaxy entirely and expand beyond the Milky Way.

カプランスラスターの一次動力源となる

Stellar engines are the kind of machines built by civilizations thinking not in terms of years or decades but eons.

太陽への衝突を防ぐため 一定の距離を保つ必要がある

Since we know that our Sun will die one day, a stellar engine could allow the far future descendants of humans to travel to other stars

そこで、集めた水素を粒子加速器で加速し

without ever having to venture into the terrifying dark abyss of interstellar space.

太陽に向けてジェット放出し、バランスをとる

Until we build a stellar engine, we're adrift and subject to the whims of the Galactic sea.

これで太陽との距離を維持すると同時に 不要物を太陽にお返しできる

We may not like where it leads us.

理論上、100万年を待たずに 50光年を移動できると考えられ

Maybe our descendants will set sail and become an interstellar species for millions of years to come.

超新星爆発をかわすのに十分である

This was our last video for the year 12,019 of the human era, and what a year it was.

全速力なら、一千万年で 太陽系の軌道を逆向きにできる

So much stuff happened everywhere, to so many different people.

ちょっと待って この方法だと太陽を使い切ってしまわないか？

Calendars and holidays are just imaginary, but they help us to cut our lives into pieces that our brains can handle.

幸い、太陽の質量はとても大きく、数十億トン といっても表面を引っ掻いた程度のものだ

We're leaving 12,019 behind with a weird mixture of disillusion and hope. The world is screwed up,

実際、カプランスラスターは 太陽の寿命を伸ばすだろう

But we can fix it. In a few days, this year will be over, and we'll get to try again.

これは質量の低い恒星ほど寿命が長いためで

Thank you for watching our videos, and for sticking around for so many years. See you all in 12,020.

太陽系に住んでいられる年数は 数十億年伸びるだろう