Right now, there are only three things limiting how far our spacecrafts can take us in the Universe: the resources we devote to it, the constraints of our existing technology, and the laws of physics. If we were willing to devote more resources to it as a society, we have the technological know-how right now to take human beings to any of the known planets or moons within the Solar System, but not to any obxts in the Oort cloud or beyond. Crewed space travel to another star system, at least with the technology we have today, is still a dream for future generations.

目前，只有三個要素限制了我們的太空船能把我們帶到宇宙的多遠處：我們投入的資源，我們現(xiàn)有技術(shù)的限制，以及物理定律。如果我們愿意作為一個整體把更多的資源投入其中，我們現(xiàn)在就有能力，可以把人類帶到太陽系內(nèi)任何已知的行星或衛(wèi)星上，但不能帶到奧爾特云或更遠的任何星體上。載人太空旅行到另一個恒星系，至少以我們今天的技術(shù)，仍然是子孫后代才能實現(xiàn)的夢想。

But if we could develop superior technology — nuclear-powered rockets, fusion technology, matter-antimatter annihilation, or even dark matter-based fuel — the only limits would be the laws of physics. Sure, if physics works as we understand it today, traversable wormholes might not be in the cards. We might not be able to fold space or achieve warp drive. And the limitations of Einstein’s relativity, preventing us from teleporting or traveling faster than light, might not ever be overcome. Even without invoking any new physics, we’d be able to travel surprisingly far in the Universe, reaching any obxt presently less than 18 billion light-years away. Here’s how we’d get there.

但是，如果我們能夠開發(fā)出更先進的技術(shù)——核動力火箭、核聚變技術(shù)、反物質(zhì)湮滅，甚至暗物質(zhì)燃料——唯一的限制就是物理定律。當然，如果物理學像我們今天所理解的那樣起作用，那么可穿越的蟲洞可能就不存在了。我們可能無法折疊空間或?qū)崿F(xiàn)曲率引擎。愛因斯坦相對論的局限性，阻止我們以比光速更快的速度傳送或旅行，可能永遠也無法克服。但即使不借助任何新的物理學理論，我們也能在宇宙中出人意料地旅行，到達目前距離我們不到180億光年的任何物體。這是關(guān)于我們?nèi)绾蔚竭_那里的解釋：
原創(chuàng)翻譯：龍騰網(wǎng) http://www.top-shui.cn 轉(zhuǎn)載請注明出處



When we take a look at conventional rockets that we launch from Earth, it surprises most people to learn that they barely accelerate more rapidly than gravity accelerates us here on Earth. If we were to jump or drop from a high altitude, Earth’s gravity would accelerate us towards our planet’s center at 9.8 m/s2 (32 ft/s2). For every second that passes by while we’re in free-fall, so long as we neglect outside forces like air resistance, our speed increases in the downward direction by an additional 9.8 m/s (32 ft/s).

當我們看一看我們從地球發(fā)射的傳統(tǒng)火箭時，大多數(shù)人驚訝地發(fā)現(xiàn)，它們的加速度幾乎沒有地球引力加速我們的速度快。如果我們從高空跳下，地球的引力會將我們以9.8米/s2 （32英尺/s2 ）的加速度向我們的星球中心移動。當我們自由落體時，每過一秒，只要我們忽略空氣阻力等外力，我們向下的速度就會增加9.8米/秒（32英尺/秒）。

The acceleration that we experience due to Earth’s gravity is known as “1g” (pronounced “one gee”), which exerts a force on all obxts equal to our mass times that acceleration: Newton’s famous F = ma. What makes our rockets so special is not that they accelerate at approximately this rate, as many obxts like cars, bullets, railguns, and even roller coasters frequently and easily surpass it. Rather, rockets are special because they sustain this acceleration for long periods of time in the same direction, enabling us to break the bonds of gravity and achieve escape velocity from Earth.

由于地球引力，我們所經(jīng)歷的加速度被稱為“1g”（發(fā)音為“伊寄”），它對所有物體施加的力等于我們的質(zhì)量乘以該加速度：即牛頓著名的F=ma。我們的火箭之所以如此特殊，并不是因為它們的加速度接近這個速度，許多物體，如汽車、子彈、軌道炮，甚至過山車，都經(jīng)常輕易地超過它。相反，火箭是特殊的，因為它們在同一個方向上長時間保持這種加速度，使我們能夠打破重力的束縛，實現(xiàn)從地球逃逸的速度。


One of the greatest challenges facing human beings who wish to take long-term journeys in space is the biological effects of not having Earth’s gravity. Earth’s gravity is required for healthy development and maintenance of a human body, with our bodily functions literally failing us if we spend too long in space. Our bone densities drop; our musculature atrophies in significant ways; we experience “space blindness;” and even the International Space Station astronauts who are most diligent about doing hours of exercise a day for months are unable to support themselves for more than a few steps upon returning to Earth.

希望進行長時間太空旅行的人類面臨的最大挑戰(zhàn)之一是沒有地球引力的生物反應。地球引力是人體健康發(fā)育和維持所必需的，如果我們在太空中呆得太久，我們的身體機能實際上就會衰退。我們的骨骼密度下降；我們的肌肉組織明顯萎縮；我們會經(jīng)歷“空間盲癥”。即使是國際空間站的宇航員，他們幾個月來每天都要勤奮地鍛煉幾個小時，但回到地球后也無法支撐自己多走幾步。

One way that challenge could be overcome is if we could sustain an acceleration of 1g not for a few minutes, propelling us into space, but continuously. A remarkable prediction of Einstein’s relativity — verified experimentally many times over — is that all obxts in the Universe can detect no difference between a constant acceleration and an acceleration due to gravity. If we could keep a spacecraft accelerating at 1g, there would be no physiological difference experienced by an astronaut on board that spacecraft as compared with a human in a stationary room on Earth.

克服這一挑戰(zhàn)的一個方法是，如果我們能夠持續(xù)1g的加速度，不是幾分鐘的時間，這只夠推動我們進入太空。而是持續(xù)不斷地保持這個速度。愛因斯坦的相對論有一個顯著的預測——實驗驗證了多次——宇宙中的所有物體都無法檢測到恒定加速度和重力加速度之間的差異。如果我們能使航天器保持1g的加速，那么在航天器上的宇航員與在地球上靜止的房間里的人在生理上不會有什么不同。


It takes a leap of faith to presume that we might someday be able to achieve constant accelerations indefinitely, as that would necessitate having a limitless supply of fuel at our disposal. Even if we mastered matter-antimatter annihilation — a 100% efficient reaction — we are limited by the fuel we can bring on board, and we’d quickly hit a point of diminishing returns: the more fuel you bring, the more fuel you need to accelerate not only your spacecraft, but all the remaining fuel that’s on board as well.

假設我們有朝一日能夠無間斷地實現(xiàn)持續(xù)加速，這需要一種質(zhì)的飛躍，因為這就代表著我們擁有無限的燃料供應。即使我們掌握了反物質(zhì)湮滅 - 一種100%有效的反應（湮滅一旦發(fā)生，正反物質(zhì)的質(zhì)量將全部轉(zhuǎn)化為能量）- 我們也會受到我們能攜帶到飛船上的燃料數(shù)量的限制，我們很快就會達到一個收益遞減的點：你攜帶的燃料越多，你需要維持這個體量的燃料就越多，燃料不僅加速你的飛船的質(zhì)量，還加速飛船上所有剩余的燃料的質(zhì)量。


Still, there are many hopes that we could gather material for fuel on our journey. Ideas have included using a magnetic field to “scoop” charged particles into a rocket’s path, providing particles and antiparticles that could then be annihilated for propulsion. If dark matter turns out to be a specific type of particle that happens to be its own antiparticle — much like the common photon — then simply collecting it and annihilating it, if we could master that type of manipulation, could successfully supply a traveling spacecraft with all the fuel it needs for constant acceleration.

盡管如此，我們?nèi)杂泻艽笙Ｍ诼猛局惺占剂腺Y源。這些想法包括利用磁場將帶電粒子“舀”到火箭的軌道上，提供粒子和反粒子，然后這些粒子和反粒子可以被湮滅用于推進。如果暗物質(zhì)被證明是一種特殊類型的粒子，恰巧是它自己的反粒子——很像普通的光子——那么簡單地收集并湮滅它，如果我們能夠掌握這種操縱方式，就可以成功地為旅行的航天器提供恒速加速所需的所有燃料。

If it weren’t for Einstein’s relativity, you might think that, with each second that passes by, you’d simply increase your speed by another 9.8 m/s. If you started off at rest, it would only take you a little less than a year — about 354 days — to reach the speed of light: 299,792,458 m/s. Of course, that’s a physical impossibility, as no massive obxt can ever reach, much less exceed, the speed of light.

如果沒有愛因斯坦的相對論，你可能會想，每過一秒，你只需再增加9.8米/秒的速度。如果你在休息的時候出發(fā)，只需要不到一年的時間——大約354天——就可以達到光速：299792458米/秒。當然，這在物理上講是不可能的，因為沒有一個大型的物體能夠達到，更不用說超過光速了。

The way this would play out, in practice, is that your speed would increase by 9.8 m/s with each second that goes by, at least, initially. As you began to get close to the speed of light, reaching what physicists call “relativistic speeds” (where the effects of Einstein’s relativity become important), you’d start to experience two of relativity’s most famous effects: length contraction and time dilation.

實際上，這樣會導致的結(jié)果是，你的速度每過一秒就會增加9.8米/秒，至少在最初是這樣。當你開始接近光速，達到物理學家所謂的“相對速度”（愛因斯坦的相對論效應變得重要）時，你會開始體驗相對論最著名的兩個效應：長度收縮和時間膨脹。


Length contraction simply means that, in the direction an obxt travels, all of the distances it views will appear to be compressed. The amount of that contraction is related to how close to the speed of light it’s moving. For someone at rest with respect to the fast-moving obxt, the obxt itself appears compressed. But for someone aboard the fast-moving obxt, whether a particle, train, or spacecraft, the cosmic distances they’re attempting to traverse will be what’s contracted.

長度收縮簡而言之就是說，在對象移動的方向上，它所看到的所有距離都將被壓縮。收縮的程度與它運動的速度有多接近光速有關(guān)。對于相對于快速移動的對象處于靜止狀態(tài)的人來說，對象本身看起來是壓縮的。但是對于那些在快速移動的物體上的人來說，無論是粒子、火車還是宇宙飛船，他們試圖穿越的宇宙距離都是縮短的。

Because the speed of light is a constant for all observers, someone moving through space (relative to the stars, galaxies, etc.) at close to the speed of light will experience time passing more slowly, as well. The best illustration is to imagine a special kind of clock: one that bounces a single photon between two mirrors. If a “second” corresponds to one round-trip journey between the mirrors, a moving obxt will require more time for that journey to happen. From the perspective of someone at rest, time will appear to slow down significantly for the spacecraft the closer to the speed of light they get.

因為光速對于所有觀察者來說都是恒定的，所以以接近光速在太空中移動的人（相對于恒星、星系等）也會經(jīng)歷更慢的時間流逝。最好的例子是想象一種特殊的時鐘：在兩個鏡子之間反彈一個光子的時鐘。如果“1秒”對應于兩個鏡面之間的一次往返行程，則移動的物體將需要更多的時間來完成該行程。從靜止的人的角度來看，航天器的時間似乎會隨著接近光速而明顯減慢。


With the same, constant force applied, your speed would begin to asymptote: approaching, but never quite reaching, the speed of light. But the closer to that unreachable limit you get, with every extra percentage point as you go from 99% to 99.9% to 99.999% and so on, lengths contract and time dilates even more severely.

在同樣的、恒定的力作用下，你的速度將開始逐漸接近光速：接近但從未完全達到光速。但當你越接近那無法達到的極限，從99%到99.9%再到99.999%再增加一個百分點，如此類推，長度就會縮短，時間會更嚴重地膨脹。

Of course, this is a bad plan. You don’t want to be moving at 99.9999+% the speed of light when you arrive at your destination; you want to have slowed back down. So the smart plan would be to accelerate at 1g for the first half of your journey, then fire your thrusters in the opposite direction, decelerating at 1g for the second half. This way, when you reach your destination, you won’t become the ultimate cosmic bug-on-a-windshield.

當然，這是一個糟糕的計劃。當你快到達目的地時，你不想還在以99.9999+%的光速移動；你想放慢速度。因此，明智的計劃是在你的旅程的前半段以1g的速度加速，然后朝相反的方向發(fā)射推進器，在下半段以1g的速度減速。這樣，當你到達目的地時，你就不會成為擋風玻璃上的終極宇宙小飛蟲。

Adhering to this plan, over the first part of your journey, time passes almost at the same rate as it does for someone on Earth. If you traveled to the inner Oort cloud, it would take you about a year. If you then reversed course to return home, you’d be back on Earth after about two years total. Someone on Earth would have seen more time elapse, but only by a few weeks.

如果堅持這個計劃，那么在你旅途的上半程，時間的流逝速度幾乎和在地球上的任意某個人一樣快。如果你想去奧爾特云內(nèi)旅行，大約需要一年的時間。若你們倒轉(zhuǎn)方向回家，你們將在大約兩年后回到地球上。地球上的時間會過去更久，但只多出幾個星期。

But the farther you went, the more severe those differences would be. A journey to Proxima Centauri, the nearest star system to the Sun, would take about 4 years to reach, which is remarkable considering it’s 4.3 light-years away. The fact that lengths contract and time dilates means that you experience less time than the distance you’re actually traversing would indicate. Someone back home on Earth, meanwhile, would age about an extra full year over that same journey.

但你走得越遠，這些差異就越嚴重。距離太陽最近的恒星系統(tǒng)比鄰星大約需要4年才能到達，考慮到它距離太陽4.3光年，這會是一次非凡的旅行。長度縮短而時間膨脹的事實意味著你經(jīng)歷的時間比你實際穿越的距離要少。同時，回到地球的人在同一次旅行中會多衰老一整年。


The brightest star in Earth’s sky today, Sirius, is located about 8.6 light-years away. If you launched yourself on a trajectory to Sirius and accelerated at that continuous 1g for the entire journey, you’d reach it in just about 5 years. Remarkably, it only takes about an extra year for you, the traveler, to reach a star that’s twice as distant as Proxima Centauri, illustrating the power of Einstein’s relativity to make the impractical accessible if you can keep on accelerating.

今天地球天空中最亮的恒星，天狼星，位于大約8.6光年之外。如果你將自己發(fā)射到天狼星的軌道上，并在整個旅程中以持續(xù)1g的速度加速，你將在大約5年內(nèi)到達它。值得注意的是，作為旅行者，你只需再花大約一年的時間就能到達一顆距離是比鄰星兩倍的恒星，這說明了愛因斯坦相對論的力量，如果你能持續(xù)加速，就可以實現(xiàn)不切實際的目標。

And if we look to larger and larger scales, it takes proportionately less additional time to traverse these great distances. The enormous Orion Nebula, located more than 1,000 light-years away, would be reached in just about 15 years from the perspective of a traveler aboard that spacecraft.

如果我們往更大的尺度上來看，穿越這些遙遠的距離所需的額外時間就會相應減少。巨大的獵戶座星云位于1000光年之外，從飛船上的旅行者的視角來看，他們將在大約15年內(nèi)到達。

Looking even farther afield, you could reach the closest supermassive black hole — Sagittarius A* at the Milky Way’s center — in about 20 years, despite the fact that it’s ~27,000 light-years away.

放眼更遠的地方，你可以在大約20年內(nèi)到達最近的超大質(zhì)量黑洞——銀河系中心的人馬座A*，盡管它距離我們約27000光年。

And the Andromeda Galaxy, located a whopping 2.5 million light-years from Earth, could be reachable in only 30 years, assuming you continued to accelerate throughout the entire journey. Of course, someone back on Earth would experience the full 2.5 million years passing during that interval, so don’t expect to come back home.

而距離地球250萬光年的仙女座星系，如果你在整個旅程中繼續(xù)加速，只需30年就可以到達。當然，地球上的某些人會在這段時間內(nèi)經(jīng)歷整整250萬年的時間，所以不要指望你還能回到家里。


In fact, so long as you kept adhering to this plan, you could choose any destination at all that’s presently within 18 billion light-years of us, and reach it after merely 45 years, max, had passed. (At least, from your frx of reference aboard the spacecraft!) That ~18 billion light-year figure is the limit of the reachable Universe, set by the expansion of the Universe and the effects of dark energy. Everything beyond that point is currently unreachable with our present understanding of physics, meaning that ~94% of all the galaxies in the Universe are forever beyond our cosmic horizon.

事實上，只要你堅持這個計劃，你就可以選擇目前距離我們180億光年以內(nèi)的任何一個目的地，并在僅僅40多年后到達它，最多45年 ( 從你在宇宙飛船上的參照系來看?。┻@180億光年的數(shù)字是可視宇宙的極限，由宇宙的膨脹和暗能量的影響決定。在我們目前對物理學的理解中，超出這一點的一切都是不可能實現(xiàn)的，這意味著宇宙中約94%的星系永遠超出了我們的宇宙視界。

The only reason we can even see them is because light that left those galaxies long ago is just arriving today; the light that leaves them now, 13.8 billion years after the Big Bang, will never reach us. Similarly, the only light they can see from us was emitted before human beings ever evolved; the light leaving us right now will never reach them.

我們能看到它們的唯一原因是因為很久以前離開這些星系的光今天才剛剛到達；而現(xiàn)在才離開它們的光，在宇宙大爆炸138億年后，永遠不會到達我們這里。同樣地，他們能從我們身上看到的唯一的光是在人類誕生之前發(fā)出的；現(xiàn)在離開我們的光永遠無法到達他們。

Still, the galaxies that are within 18 billion light-years of us today, estimated to number around 100 billion or so, are not only reachable, but reachable after just 45 years. Unfortunately, even if you brought enough fuel, a return trip would be impossible, as dark energy would drive your original location so far away that you could never return to it.

盡管如此，今天距離我們180億光年以內(nèi)的星系，估計有1000億左右，不僅可以到達，而且只需45年就可以到達。不幸的是，即使你帶了足夠的燃料，回程也是不可能的，因為暗能量會把你的出發(fā)點推得很遠，以至于你永遠無法回到那里。


Even though we think of interstellar or intergalactic journeys as being unfeasible for human beings due to the enormous timescales involved — after all, it will take the Voyager spacecrafts nearly 100,000 years to traverse the equivalent distance to Proxima Centauri — that’s only because of our present technological limitations. If we were able to create a spacecraft capable of a constant, sustained acceleration of 1g for about 45 years, we could have our pick of where we’d choose to go from 100 billion galaxies within 18 billion light-years of us.

盡管我們認為星際或星系間的旅行對于人類來說是不可行的，因為涉及到巨大的時間尺度——畢竟，“旅行者”號宇宙飛船需要將近10萬年的時間才能到達比鄰星——這僅僅是因為我們目前的技術(shù)限制。如果我們能夠制造出一個能夠在45年內(nèi)保持1g恒定加速度的航天器，我們可以從180億光年內(nèi)的1000億個星系中選擇我們要去的任意地方。

The only downside is that you’ll never be able to go home again. The fact that time dilates and lengths contract are the physical phenomena that enable us to travel those great distances, but only for those of us who get aboard that spacecraft. Here on Earth, time will continue to pass as normal; it will take millions or even billions of years from our perspective before that spacecraft arrives at its destination. If we never ran out of thrust, we could hypothetically reach anywhere in the Universe that a photon emitted today could reach. Just beware that if you were to go far enough, by the time you came home, humanity, life on Earth, and even the Sun will all have died out. In the end, though, the journey truly is the most important part of the story.

唯一的缺點是你再也不能回家了。事實上，時間的膨脹和長度的收縮是物理現(xiàn)象，使我們能夠旅行這些遙遠的距離，但只對我們這些登上宇宙飛船的人來說。在地球上，時間將一如既往地流逝；從我們的角度來看，宇宙飛船到達目的地需要數(shù)百萬年甚至數(shù)十億年的時間。如果我們有無限的推力，我們可以到達宇宙中今天發(fā)射的光子可以到達的任何地方。只是小心，如果你走得夠遠，到你回家的時候，人類，地球上的生命，甚至太陽都將湮滅了。但無論如何，過程才是一個故事最重要的部分，而不是結(jié)果。