I’m looking for dope help on a fairly obscure and geeky argument. A friend of mine has argued with me that interstellar travel by humans is not possible even in theory because the energy requirements for accelerating the mass required to keep a human sustainable ecosystem going for the length of the trip is just not possible.
Instead she argues only uploaded minds will be sent to other star systems in spaceships that may only weigh a few kg but contain many hundreds of “human equivalent intelligences” in a computer. Think Greg Egan scifi to get the full picture.
So someone smarter than me do the math. Let’s imagine that you have a spaceship capable of supporting 10 people for an extended voyage. I’m going to go out on a limb and say it needs to weigh 10 times the mass of the ISS to have a complete recycling biosphere to sustain 10 people. That means it weighs 4500 metric tons according to this page: http://www.boeing.com/defense-space/space/spacestation/overview/facts.html
Now lets imagine you have a VASIMR type drive with an antimatter energy system fueling it. How much fuel mass is required to accelerate that 4500 metric ton ship to 90 percent lightspeed and decelerate it at the other end?
How much relativistic effect do you get at 90 percent light speed? eg how long will a 4 light year journey take for the inhabitants of the craft? What about for fusion powered fuel instead of antimatter, how much fuel mass is required then?
found at least part of my own question, from this graph here:
It looks to me like time dilation is approx 0.4 at 0.9 c, which means that the 4 light year journey to proxima cenatauri would take approx 1.6 observed years for the spaceship inhabitants…
The total energy of a body traveling at relativistic velocities is given by:
E[sub]total[/sub] = mc[sup]2[/sup]/sqrt(1 - (v/c)[sup]2[/sup])
At 0.9c, this works out as:
E[sub]total[/sub] = 2.29 * mc[sup]2[/sup]
To get the energy required for acceleration (i.e. the kinetic energy) we subtract out the rest energy, which is of course mc[sup]2[/sup] - so:
E[sub]kinetic[/sub] = 1.29 * mc[sup]2[/sup]
which is indeed a formidable amount, the carrying and delivery of which would be decidedly challenging.
Note that this is the amount needed for each acceleration or deceleration; a round trip involves four such events.
I don’t think you can sustain people for many years in something with 10X the mass per person allocated by the ISS. Besides, this might amount to being able to deliver several living humans at an unliveable destination before they die, which is setting the bar pretty low from the point of view of a travel agency. Think about how difficult it sounds to get somebody to Mars. It is pretty unlikely that you would find something extrasolar that is as hospitable as Mars if you went a million times further to a few nearby stars, and when you increase the distance, you increase both the energy it takes to get every kilogram delivered, and the number of kilograms needed to keep the cargo alive.
I don’t think it is imaginable with our current understanding of technology. The question is how many decades of changing understanding it would take. How reasonable were mid nineteenth century ideas about visiting the Moon? You might consider that visiting the Moon and another star are practically incomparable. For example, most humans would survive the travel time to the Moon without any food at all, whereas travel to another star might practically require multiple generations of reproduction.
If we assume a magical propulsion system that converts fuel to kinetic energy with 100% efficiency - and if my calculations are right - then the trip with four accelerations to and decelerations from 0.9c requires that you start with fuel whose mass is 27.5 times that of the ship.
Napier, maybe we can’t sustain life or 10 people in something 10x the mass of the ISS but it provides a useful measure for a discussion.
We do know know the theoretical limits of mass/energy conversion so this is a question that has a factual answer, namely how much fuel mass would it take to accelerate 4500 metric tons to a 0.9c velocity?
edit, thanks for the answer xema, but presumably, they can refuel at the far end by using some device that converts normal matter to antimatter so we need 13.75 times the mass of the ship as 100 percent efficient fuel to accelerate and decelerate for the journey.
You don’t need to move at .9c, any speed will do. Running at .10c is more realistic.
Moving at .10c it will take roughly 43 years to get to Alpha Centauri. The realistic answer to this question is that biological advancement is most certainly going to lead to much more “sci-fi” type scenarios than advances in physics and engineering. The simple reason is there is no science whatsoever to suggest we won’t someday be able to produce humans with “no” or “very long” expiration dates. You put someone whose life expectancy is 1,000 years in a ship and then blast them off towards our nearest stellar neighbor and the 43 years it takes them to get there is only a relatively small portion of their life.
It is highly unlikely we’ll use any sort of antimatter, it’ll probably be nuclear fusion (a much more likely technology for powering interstellar travel.)
It isn’t linear - the fuel for a one-way trip need be only 5.2 times the mass of the ship.
But by our current understanding of physics, that matter-to-antimatter device would need to consume vast amounts of energy (the energy that the antimatter fuel will later deliver for propulsion). So either it will itself need some magical source of energy or the star travellers will need to pause for a few thousand years while the necessary energy is slowly accumulated (perhaps from a nearby star).
Let’s convert the Moon, firstly bio-engineering an underground breathable atmosphere, then using it as our spaceship*.
Now we’ve got room for lots of brave explorers.
(The effect on the Earth of the Moon departing is left as an exercise for the reader…)
We still cannot get a sealed ecosystem to work. So its all science fiction anyway.
On top of it, its a little naive to think that unmodified plants and humans would be sent anyway. Without some high level of genetic engineering its most certainly not possible or not practical (cost more than the GDP of several countries).
As technology progresses we will start seeing better ways of solving this issue. Robotics, artificial intelligence, etc. Not to mention there needs to be a complete change as how we see space projects. Can the space agencies of the world even handle a 200 year mission? Can human governments have that kind of patience and vision?
Uploaded minds are science fiction also, so I dont see how thats a good argument. We have no theoretical framework to do this and if you think about it, if we can do some kind of high level human brain simulation then we are pretty much doing AI, not “uploading.” So Io still think that of all the scifi concepts thrown around that AI or complex robotics is the only way to go.
If closed-ecology habitats are feasible (a starting premise for virtually any interstellar exploration other than a straight shot to a planet known to be habitable), then one scenerio is that habitats gradually spread first to the Kuiper belt and then out in the Ort cloud, until eventually the “frontier” is only a light-year or two from the outer fringes of the next solar system.
An interesting related question: for how long has anything ever consistently been believed by experts to be impossible?
When my grandfather was little, a fair number of technical people still believed heavier-than-air vehicles would forever be impossible, at least as I understand the history. I still remember being taught in school that Mars probably had cities on it connected by the canals that astronomers watched turning more or less green with the passing seasons. The Einsteinian relativistic limit c was unknown when some people still alive today were old enough to talk. How long will it remain as an apparently certain limit?
This is a great question. Since we are talking about matters of principle, I will assume that we can bring as much antimatter as we like and convert it with 100% efficiency into gamma rays directed out of the front or back of the spaceship as desired. For mathematical convenience, I’ll assume that our travelers are in suspended animation in tanks that enable them to withstand enormous accelerations that I will assume to be infinite. The problem reduces to a simple relativistic kinematics problem in which we have to balance conservation of energy and momentum. If we limit the acceleration to several g’s, it will take longer, but no more than a few years (since one g times one year is approximately the speed of light).
If M is the original rest mass of the loaded spacecraft and eta is the fraction of the original rest energy (Mc^2) that we convert to gamma rays, then the rest mass (m) of the remaining spacecraft plus fuel and payload is given by:
m = Msqrt(1-2eta).
Note that the total spacecraft rest mass after acceleration goes to zero as eta goes to one half, since this would be the case if we consumed all the matter with antimatter and our spacecraft and payload is, unfortunately, now all gamma rays with zero rest mass.
The speed of the spacecraft is beta*c where beta is given by:
beta = eta^2/(1+eta^2-2*eta)
If we want beta = 0.9, as the original poster wants, we can solve for eta to get 0.4868. We deduce that the ratio of the rest mass of the rocket after acceleration to the original rest mass is 0.1622. Thus, the original rocket, plus payload, plus fuel, must weigh 6.165 times the rest mass of the spacecraft, that is now traveling at 0.9c. To decelerate at our destination, we need the same ratio, so a one way trip requires 6.165^2 - 1 = 37 times as much fuel as spacecraft plus payload. A two way trip would require an amount of antimatter equal to 6.165^4 - 1 = 1445 times the mass of the loaded spaceship. That’s a lot of antimatter.
If you use thermonuclear fusion instead of antimatter annihilation, things get much, much worse.
My math gives a different answer from Xema’s, but I could easily have made a mistake.
The problem with antimatter annihilation is there is no evidence we’ll ever be able to do that to nearly enough scale to power an interstellar ship. While the possibility that we can throw something together in say, the next 1,000 years fueled by fusion is quite realistic.
You could restock your antimatter using a fusion reactor at the other side. Hydrogen is plentiful on planets. If you think in terms of self-replicating nanotech to build the reactors, you’ll get an exponential curve of energy production. It may be a few years or even decades in the other star system, but it’s not like we’re in any real hurry when the trip out already took 40+ years.
Just to put my calculation in perspective, if I assume that the spacecraft has a mass that is a mere ten times higher than the ISS, the round trip journey would require an energy of 4*10^23 joules, which is more energy than the entire world energy consumption for a million years at today’s rate of energy consumption.
This can’t be emphasized enough: The statement “it’s impossible to travel faster than c” is fundamentally different from “it’s impossible to build a heavier-than-air flying machine”, or “it’s impossible to break the speed of sound”, or “it’s impossible to build a closed ecosystem that can last long enough to get humans to another star”. Those latter statements are all things that are extremely difficult, and require a great deal of money and engineering, but there’s no fundamental, inherent property of the Universe that forbids them. When people said that heavier-than-air flying machines were impossible (if indeed they ever did say that), what they really meant was that they were difficult enough to be impractical. But when physicists say that travel at faster than c is impossible, they mean that, so far as we can tell, it really is inherently impossible, no matter how much time, money, and effort you throw at the problem, and no matter how little you care about practicality. Now, we may be wrong, of course, and it may be that there’s some hitherto-unanticipated quirk in the laws of physics that makes it possible. But it’s also possible that we’re right.
JWT, I think that the source of the discrepancy between your answer and Xema’s is that Xema is just conserving energy, not momentum, whereas you’re taking into account the momentum of the exhaust. That said, though, there are more efficient ways to handle the exhaust than having it just be all gamma rays, since photons require a huge amount of energy for any given amount of momentum. You can still use antimatter as your energy source if you want, but best to keep a separate supply of mass that you heat up using the antimatter and shovel out the back.
