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，只有三个要素限制了我们的太空船能把我们带到宇宙的多远处：我们投入的资源，我们现有技术的限制，以及物理定律。如果我们愿意作为一个整体把更多的资源投入其中，我们现在就有能力，可以把人类带到太阳系内任何已知的行星或卫星上，但不能带到奥尔特云或更远的任何星体上。载人太空旅行到另一个恒星系，至少以我们今天的技术，仍然是子孙后代才能实现的梦想。

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，如果我们能够开发出更先进的技术——核动力火箭、核聚变技术、反物质湮灭，甚至暗物质燃料——唯一的限制就是物理定律。当然，如果物理学像我们今天所理解的那样起作用，那么可穿越的虫洞可能就不存在了。我们可能无法折叠空间或实现曲率引擎。爱因斯坦相对论的局限性，阻止我们以比光速更快的速度传送或旅行，可能永远也无法克服。但即使不借助任何新的物理学理论，我们也能在宇宙中出人意料地旅行，到达目前距离我们不到180亿光年的任何物体。这是关于我们如何到达那里的解释：
原创翻译：龙腾网 http://www.ltaaa.cn 转载请注明出处



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).

当我们看一看我们从地球发射的传统火箭时，大多数人惊讶地发现，它们的加速度几乎没有地球引力加速我们的速度快。如果我们从高空跳下，地球的引力会将我们以9.8米/s2 （32英尺/s2 ）的加速度向我们的星球中心移动。当我们自由落体时，每过一秒，只要我们忽略空气阻力等外力，我们向下的速度就会增加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，我们所经历的加速度被称为“1g”（发音为“伊寄”），它对所有物体施加的力等于我们的质量乘以该加速度：即牛顿著名的F=ma。我们的火箭之所以如此特殊，并不是因为它们的加速度接近这个速度，许多物体，如汽车、子弹、轨道炮，甚至过山车，都经常轻易地超过它。相反，火箭是特殊的，因为它们在同一个方向上长时间保持这种加速度，使我们能够打破重力的束缚，实现从地球逃逸的速度。


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.

希望进行长时间太空旅行的人类面临的最大挑战之一是没有地球引力的生物反应。地球引力是人体健康发育和维持所必需的，如果我们在太空中呆得太久，我们的身体机能实际上就会衰退。我们的骨骼密度下降；我们的肌肉组织明显萎缩；我们会经历“空间盲症”。即使是国际空间站的宇航员，他们几个月来每天都要勤奋地锻炼几个小时，但回到地球后也无法支撑自己多走几步。

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.

克服这一挑战的一个方法是，如果我们能够持续1g的加速度，不是几分钟的时间，这只够推动我们进入太空。而是持续不断地保持这个速度。爱因斯坦的相对论有一个显著的预测——实验验证了多次——宇宙中的所有物体都无法检测到恒定加速度和重力加速度之间的差异。如果我们能使航天器保持1g的加速，那么在航天器上的宇航员与在地球上静止的房间里的人在生理上不会有什么不同。


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.

假设我们有朝一日能够无间断地实现持续加速，这需要一种质的飞跃，因为这就代表着我们拥有无限的燃料供应。即使我们掌握了反物质湮灭 - 一种100%有效的反应（湮灭一旦发生，正反物质的质量将全部转化为能量）- 我们也会受到我们能携带到飞船上的燃料数量的限制，我们很快就会达到一个收益递减的点：你携带的燃料越多，你需要维持这个体量的燃料就越多，燃料不仅加速你的飞船的质量，还加速飞船上所有剩余的燃料的质量。


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.

尽管如此，我们仍有很大希望在旅途中收集燃料资源。这些想法包括利用磁场将带电粒子“舀”到火箭的轨道上，提供粒子和反粒子，然后这些粒子和反粒子可以被湮灭用于推进。如果暗物质被证明是一种特殊类型的粒子，恰巧是它自己的反粒子——很像普通的光子——那么简单地收集并湮灭它，如果我们能够掌握这种操纵方式，就可以成功地为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.

如果没有爱因斯坦的相对论，你可能会想，每过一秒，你只需再增加9.8米/秒的速度。如果你在休息的时候出发，只需要不到一年的时间——大约354天——就可以达到光速：299792458米/秒。当然，这在物理上讲是不可能的，因为没有一个大型的物体能够达到，更不用说超过光速了。

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.

实际上，这样会导致的结果是，你的速度每过一秒就会增加9.8米/秒，至少在最初是这样。当你开始接近光速，达到物理学家所谓的“相对速度”（爱因斯坦的相对论效应变得重要）时，你会开始体验相对论最著名的两个效应：长度收缩和时间膨胀。


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.

长度收缩简而言之就是说，在对象移动的方向上，它所看到的所有距离都将被压缩。收缩的程度与它运动的速度有多接近光速有关。对于相对于快速移动的对象处于静止状态的人来说，对象本身看起来是压缩的。但是对于那些在快速移动的物体上的人来说，无论是粒子、火车还是宇宙飞船，他们试图穿越的宇宙距离都是缩短的。

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.

因为光速对于所有观察者来说都是恒定的，所以以接近光速在太空中移动的人（相对于恒星、星系等）也会经历更慢的时间流逝。最好的例子是想象一种特殊的时钟：在两个镜子之间反弹一个光子的时钟。如果“1秒”对应于两个镜面之间的一次往返行程，则移动的物体将需要更多的时间来完成该行程。从静止的人的角度来看，航天器的时间似乎会随着接近光速而明显减慢。


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.

在同样的、恒定的力作用下，你的速度将开始逐渐接近光速：接近但从未完全达到光速。但当你越接近那无法达到的极限，从99%到99.9%再到99.999%再增加一个百分点，如此类推，长度就会缩短，时间会更严重地膨胀。

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.

当然，这是一个糟糕的计划。当你快到达目的地时，你不想还在以99.9999+%的光速移动；你想放慢速度。因此，明智的计划是在你的旅程的前半段以1g的速度加速，然后朝相反的方向发射推进器，在下半段以1g的速度减速。这样，当你到达目的地时，你就不会成为挡风玻璃上的终极宇宙小飞虫。

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.

如果坚持这个计划，那么在你旅途的上半程，时间的流逝速度几乎和在地球上的任意某个人一样快。如果你想去奥尔特云内旅行，大约需要一年的时间。若你们倒转方向回家，你们将在大约两年后回到地球上。地球上的时间会过去更久，但只多出几个星期。

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.

但你走得越远，这些差异就越严重。距离太阳最近的恒星系统比邻星大约需要4年才能到达，考虑到它距离太阳4.3光年，这会是一次非凡的旅行。长度缩短而时间膨胀的事实意味着你经历的时间比你实际穿越的距离要少。同时，回到地球的人在同一次旅行中会多衰老一整年。
原创翻译：龙腾网 http://www.ltaaa.cn 转载请注明出处



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，位于大约8.6光年之外。如果你将自己发射到天狼星的轨道上，并在整个旅程中以持续1g的速度加速，你将在大约5年内到达它。值得注意的是，作为旅行者，你只需再花大约一年的时间就能到达一颗距离是比邻星两倍的恒星，这说明了爱因斯坦相对论的力量，如果你能持续加速，就可以实现不切实际的目标。

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.

如果我们往更大的尺度上来看，穿越这些遥远的距离所需的额外时间就会相应减少。巨大的猎户座星云位于1000光年之外，从飞船上的旅行者的视角来看，他们将在大约15年内到达。

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.

放眼更远的地方，你可以在大约20年内到达最近的超大质量黑洞——银河系中心的人马座A*，尽管它距离我们约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.

而距离地球250万光年的仙女座星系，如果你在整个旅程中继续加速，只需30年就可以到达。当然，地球上的某些人会在这段时间内经历整整250万年的时间，所以不要指望你还能回到家里。


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.

事实上，只要你坚持这个计划，你就可以选择目前距离我们180亿光年以内的任何一个目的地，并在仅仅40多年后到达它，最多45年 ( 从你在宇宙飞船上的参照系来看！）这180亿光年的数字是可视宇宙的极限，由宇宙的膨胀和暗能量的影响决定。在我们目前对物理学的理解中，超出这一点的一切都是不可能实现的，这意味着宇宙中约94%的星系永远超出了我们的宇宙视界。

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.

我们能看到它们的唯一原因是因为很久以前离开这些星系的光今天才刚刚到达；而现在才离开它们的光，在宇宙大爆炸138亿年后，永远不会到达我们这里。同样地，他们能从我们身上看到的唯一的光是在人类诞生之前发出的；现在离开我们的光永远无法到达他们。

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.

尽管如此，今天距离我们180亿光年以内的星系，估计有1000亿左右，不仅可以到达，而且只需45年就可以到达。不幸的是，即使你带了足够的燃料，回程也是不可能的，因为暗能量会把你的出发点推得很远，以至于你永远无法回到那里。
原创翻译：龙腾网 http://www.ltaaa.cn 转载请注明出处



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.

尽管我们认为星际或星系间的旅行对于人类来说是不可行的，因为涉及到巨大的时间尺度——毕竟，“旅行者”号宇宙飞船需要将近10万年的时间才能到达比邻星——这仅仅是因为我们目前的技术限制。如果我们能够制造出一个能够在45年内保持1g恒定加速度的航天器，我们可以从180亿光年内的1000亿个星系中选择我们要去的任意地方。

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.

唯一的缺点是你再也不能回家了。事实上，时间的膨胀和长度的收缩是物理现象，使我们能够旅行这些遥远的距离，但只对我们这些登上宇宙飞船的人来说。在地球上，时间将一如既往地流逝；从我们的角度来看，宇宙飞船到达目的地需要数百万年甚至数十亿年的时间。如果我们有无限的推力，我们可以到达宇宙中今天发射的光子可以到达的任何地方。只是小心，如果你走得够远，到你回家的时候，人类，地球上的生命，甚至太阳都将湮灭了。但无论如何，过程才是一个故事最重要的部分，而不是结果。