across the universe in a lifetime. with a catch.

September 29, 2009 — 17 Comments

New Horizons is the fastest spacecraft launched so far by humans. After a gravity assist from Jupiter, it’s on a course to rocket past Pluto at 47,000 miles per hour. At that speed it would circle our world in a little under 32 minutes, make it to the moon in just five hours, reach Mars at its closest point in a bit over one month and get to our nearest stellar neighbor in… 87,633 years? Oh. Ok, maybe it’s not really all that fast in the grand scale of things. In fact, when it comes to space travel, it’s about as fast as a limping tortoise on sodium pentothal. But what if New Horizons could keep accelerating as close as was physically possible for it to get to the speed of light? A 2005 paper on the subject argues that it could cross half the visible universe in half a century. And of course, since I’m writing about this paper, you know there’s some sort of major catch to the whole thing…

light speed

When people talk about exploring space, their greatest frustration is that the vast distances involved and how slow our spacecraft travel and could travel according to the laws of physics, make exploring any notable part of the universe in a single human lifetime impossible. In a straight-line, arithmetical projection, things look very bleak for future interstellar explorers. Even at relativistic speeds, it would take hundreds, if not thousands and thousands of years to get to their destinations. However, things don’t work in straight-line projections when an object is traveling at relativistic speeds according to Einstein’s theory of special relativity. The faster an object moves, the slower time flows for it relative to stationary or sluggish observers. The phenomena is known as a time dilation and we know it really happens after experiments involving atomic clocks and airplanes.

If you’ve ever flown in an airplane across a good stretch of land, you’re a tiny fraction of a second younger than you should be if you traveled by car at sea level. Astronauts experience even greater dilations. Now, all that is taking place at the speed of a napping snail as far as the universe is concerned. So to make things a little bit more interesting, physicist Jeremy Hoyl decided to ratchet up both the speed and the timeline to see what will happen if we launch a manned spacecraft able to accelerate by 9 meters every second well into the territory of relativistic speeds and keep on going. His results show that when the astronauts age by half a century, they would cover almost 15 billion light years. For us, their trip would take eons and the Sun would consume our world before shriveling into a white dwarf. On our hypothetical spacecraft however, the astronauts would be a good deal older, but still very much alive and unaware of how fast time passed for the slowpokes on Earth.

By accelerating at a constant 9 m/s/s, not only would their craft be incredibly fast, it would also be a very good simulation of our planet’s gravity. The explorers could have showers, relax with a cup of hot coffee without the liquid floating into their faces as a boiling hot bubble, and the other little comforts of home. In just over a year, they would hit the speed of light. Computationally. In reality, even if we ignore how big the power source would need to be to keep firing for over a year, the spacecraft would be stopped by the interstellar medium. Usually, it’s just gas and dust scattered in extremely low densities over thousand of light years. However, when a ship tries to plow through this matter at a significant percentage of the speed of light, the sheer force with which an ordinarily harmless speck of dust hits the hull is multiplied exponentially. And as the ship barrels on through, it’s basically sandblasted away. Not a good start for a trip across the cosmos.

The numbers get even worse if the spacecraft has to cross a solar system. Bands of dust around stars’ inner solar systems would vaporize a relativistic rocket going just 10% the speed of light. In the craft’s control room, the crew would never even know what hit them before meeting their high friction doom. Using time dilation to our advantage isn’t a new idea by any stretch of the imagination, but Hoyl does something innovative with it by showing us how over a long period of time, the phenomenon can give us an amazing return on investment. Still, what’s the practical use of taming special relativity to our advantage if the spacecraft wouldn’t survive the trip in the first place?

See: Heyl, J. (2005). The long-term future of space travel Physical Review D, 72 (10) DOI: 10.1103/PhysRev…

[ photo illustration by Paul Louis Villani ]

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  • KGB

    We are the penguins who want to fly. I think it’s going to take a few more decades before we realize that humans and this planet are bonded. Our fates are intertwined. We are not meant to travel to the stars. That’s probably happening somewhere else in our galaxy, and likely in thousands of others, where beings came into existence without the fetters to tie them to a specific atmospheric pressure and extremely narrow bands of tolerance to heat and cold. Beings who can go without sustenance of any kind for long periods, who can hibernate for years. We’re stuck here though. We were born here. We live here. And we’ll die here in the arms of the star that gave birth to us. It’s not so bad. We’ve come a long way, but we must accept that this is our tiny little home in a backwater system of an enormous galaxy. Our abilities will not meet our ambitions before we run out of time.

  • intercoastal

    Wow… Maybe by the time we develop power sources good enough for relativistic travel, we will be able to shield spacecraft against collisions. (Some type of magnetic thing, ionizing the hydrogen atoms and flinging them away with a powerful magnetic field?)

    A relativistic ship would almost certainly do most of its acceleration and deceleration in interstellar space — and inner solar systems are tiny, compared to the vastness of space. So it would be easy to avoid solar systems on the way to your goal, and you’d be mostly slowed down by thousands of AU away.

    @KGB: If that turns out to be true — that we are naturally bound to the Earth by our biological requirements — perhaps we will be able to change that by genetic engineering?

  • Greg Fish

    “Our abilities will not meet our ambitions before we run out of time.”

    That’s a rather grim prognosis, isn’t it? Over the last few hundred years, there have been plenty of people quick to say that we’ll never accomplish this or do that and yet, we’ve been able to do it all and then some.

    History is generally not written by naysayers and there’s a good reason for that.

  • Believer

    I disagree. I think as long as humans have the dream, we will manage to make it. We are known to rise to the challenges given. We have plenty of time to get where we want to go (sort of). We have the will and I’m sure we will have the power, just maybe not as soon as we see it. Sure we will doubt ourselves and someone will calculate that there isn’t enough time, but we will blow that apart.Just wait and watch. Also scientist have calculated that there in our galaxies and for quite a few light years out there, there is no other life that seems to have made that sort of progress unless they picked up extremely quickly. We are destined to colonize the stars (well the planets by them at least).

  • TomJ

    There was one significant advantage to the fictional ‘inertialess drive’ of E.E. Smith. Due to a lack of inertia, if you hit a dust cloud the ship slowed instantly. Your ship moved at whatever speed friction allowed…and if you added more thrust you’d better add more shielding on the front to deal with the increased friction.

  • Hank Roberts
  • Greg Fish

    “There was one significant advantage to the fictional ‘inertialess drive’ of E.E. Smith…”

    Unfortunately, a spacecraft without inertia would be permanently stationary. It would violate Newton’s 3rd Law which is impossible as far as we know.

    And Hank, while the scoop concepts are great, they would encounter so much friction from the interstellar medium that the craft would come screeching to a halt after a certain velocity.

  • Thorne

    Poul Anderson dealt with this problem in his classic scifi novel, “Tau Zero” (http://en.wikipedia.org/wiki/Tau_Zero). He proposed using a “Bussard ram jet” which used the interstellar dust and gas to provide thrust for the ship. It’s become cosmologically outdated, but still a good read and interesting science.

  • Victor

    @website

    And I suppose your statement isn’t ignorance masquerading as “thoughtful contribution”.

    Yes, thought experiments are vague, and maybe viewed as unconventional, but it allows the mind to do what machines cannot.
    Considering what thought experiments have been shown to accomplish (Galileo, anyone?), I’d say they’re a bit more than just “personal bias”.

  • DaveM

    Interesting idea, but I don’t think people tend to think of all the implications in terms of physical laws.

    For example, you can calculate from the Lorentz Factor described (15 billion light years traveled in 50 years of ship’s time) that the ship speed would have to be 0.99999999999999999444 c, where c is the speed of light.

    At this speed, the relativistic kinetic energy would be 2.7 * 10^25 Joules per kilo. This is about 300 million times as much energy as is present in the mass, if you were to convert the entire mass directly into energy as per Einstein’s formula. We’re not even considering reaction mass here, obviously this would be some propulsion system that doesn’t rely on ejecting mass out of the rear, a reactionless drive. And we’re talking about converting the entire rest mass of the ship into energy, including the astronaut and the engines. And even then, the energy is short by a factor of 300 million.

    Obviously, the solution would be to pick up more mass along the way, picking up interstellar dust like a Bussard Ramjet. According to Wikipedia, the density of interstellar dust is about 1 gram per 10^18 cubic meters. So if you had a scoop that was a square kilometer in cross section, you’d have to move through 700 million light years of space to even collect the mass needed to drive our 1 kilo spaceship. Of course, if you want a bigger spaceship, you need even more mass.

    I do not think this will ever happen. We better find wormholes or some other means to travel large distances, because light speed is going to be a bummer.

  • Victor

    Wow. his/her comments were deleted right after I posted. what a coinkidink.
    .

  • Greg Fish

    Victor,

    These comments were just random spam that got past the filter but was flagged. I try to clean those up as fast as I can but sometimes they do hang around for a bit before they get deleted.

  • jab49

    You could use a high energy laser grid around the spacecraft to vapourise down to component atoms all interstellar dust, at atomic levels it would be possible to armour the ship accordingly

  • http://www.reddit.com Michael

    What is amusing to me is that anyone doubts that this is ever going to be possible to do. Half a century is easily doable without even engineering our own bodies. We’ve only just begun to try that.

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  • Jim Gagnon

    To KGB: We are on the verge of uncovering our latent abilities to hibernate. Turns out that breathing a dilute mixture of hydrogen sulfide triggers a hibernation mechanism in humans. There’s a lot of effort to learn how to control it going on now, but if they find technology alone isn’t enough then the bioengineers could grab a few bear genes and slice then into humans. That means sleeper ships are possible. Also on the table are realistic engines that will allow a craft to reach about 5% of the speed of light (fission fragment engines, look it up in Wikipedia). The part that’s missing is a way to handle interstellar dust; it’s a real problem and all the proposed solutions only work if the dust is charged. However, we are tricky apes; we might not find the answer this century, but I’m sure there’s a way. We’ll have more answers once we truly understand the nature of the matter field(s).

    We aren’t the penguins that want to fly. We are the penguins that do fly.

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