> Well, yes. Smaller, lighter objects take less energy to get to high speeds. A one
> ton payload is going to take far less energy to accelerate than a ten thousand
> ton payload. And you eliminate all the practical problems of sustaining flesh and
> blood humans for so long, and the psychological problems of sending just a few
> people on a decades long journey.
The amount of energy necessary to accelerate the spaceship isn’t the point. The question I asked was whether it was possible to hold enough fuel for the spaceship, given that a smaller spaceship would have to hold a smaller amount of fuel. Can I please have an answer from someone who knows the physics? The posts replying to the OP seemed to say that the amount of fuel required would be more than one could possibly put in the spaceship. The posts seemed to say that this would be a matter of the weight of the fuel being a large multiple of the weight of the spaceship, regardless of how large or small the spaceship would be. Is that true? Again, I’d like a reply from someone who’s going to do the math to explain whether it would help to make the spaceship small enough. Please, if you can’t do the physics to calculate the answer for various sizes of spaceship, you’re not helping me.
How can you use “relative” in that sentence without realizing that the point of relativity theory is that C is a constant and that everything else depends on it being so?
O.K., then, you’re claiming that Xema’s calculations and JWT Kottekoe’s calculations are relevant for any size of ship, in which case Der Trihs’s post was utterly irrelevant to the discussion, right? So the solution proposed in the OP is no help, right? It wouldn’t matter if the spaceship was made very small and only carried information to reconstruct the bodies and download the minds when they reached their destination, because even small spaceships would be have the same limitation, right? No spaceship would work, regardless of the size of the ship, right? Any spaceship would take far too much energy to make it to another star system, right?
It depends on how fast you want to get there. The Voyager probes will probably *eventually *reach another star system, although that will be just from dumb luck since they weren’t aimed at anything in particular. It takes too much energy to get humans to another star within an acceptable fraction of a human lifetime, but generation ships or travel by virtualizing consciousness is still theoretically possible.
The point is that, for any given set of requirements (time of flight, speed, whatever) and any given set of capabilities (drive technology, etc.), what you ultimately calculate is a ratio of payload to fuel that you need. If, for instance, that ratio is 10,000 to 1, then that means that if you want a million-ton payload (as might be needed for embodied humans), then you’d need ten billion tons of fuel, which is impractical. But if your payload is a mere kilogram, as might be plausible for a robotic probe (with or without digitized human minds on board), then your 10,000 to 1 fuel ratio means you only need ten tons of fuel, which is much more practical. Basically, you can’t say whether a given fuel ratio is practical, without also saying how big your payload is.
Thank you, Chronos, for being the first poster to clearly answer my question. So it is possible for the situation in the OP to work in some sense. If it were possible to build a small enough spaceship, it would be theoretically possible to send that spaceship to another star system in less than a generation on Earth. O.K., next question then. If I recall correctly, there’s no way to code the information that would be required to reconstruct a human body or a human mind in a sufficiently small amount of space. Even if you could send this human reconstruction machine to another star, it couldn’t carry all the information necessary. Furthermore, even if you send the information to the ship after it reached the star, it would take far too long to send it because it was so much information. Is this true?
It is definitely possible to encode the information needed to construct a human body in a small enough space. You can fit all of the information into the nucleus of a single cell. For a mind, well, that’s a more difficult question, since nobody’s really sure just what constitutes a human mind, but you can certainly fit one into a few kilograms of brain tissue.
Even with current technology, the information in the human genome can fit on a single flash memory chip. Without all the packaging it might weigh a gram. Since we are talking matters of principle, what with antimatter fueled gamma ray lasers and all, we can assume that we could achieve atomic resolution storage, let’s say one bit per unit cell of the Silicon lattice. The human genome is less than a gigabyte, so that’s 8*10^9 Silicon atoms or about 10^-13 grams. Of course, it takes more than just the genome, we need to specify the cell that can bootstrap the code, so let’s throw in another factor of ten for good measure, bringing us up to a round picogram.
Yeah, it makes a big difference if you are trying to send a picogram that needs no life support, vs 5000 metric tons of spacecraft plus payload.
Better yet, avoid rockets alltogether and send the bits electromagnetically. The bits won’t get bored or need sustenance and they travel at the speed of light.
P.S. In my calculation of the fuel requirements, I was rash to assume that I could do it all in one boost of staggeringly high acceleration at the beginning and end of each one-way trip. While there is nothing wrong with that model, I think we would all agree that it is even more impractical than our antimatter gamma ray drive. In reality, you will have to bring the fuel with you and then things get very much worse, since most of the energy is spent pushing fuel in the direction of travel, only later to throw it out the back to boost you along. In the non-relativistic case, some of you know that the result is that the final velocity is equal to the velocity of the rocket exhaust times the logarithm of the ratio of the fully fueled spacecraft mass to the payload mass. Stated a different way, the required fuel goes up in direct proportion to the payload mass, and exponentially with the desired final velocity. Something similar will happen in the relativistic case, and I haven’t calculated that. Thus, a million years of global energy consumption will be a mere fraction of the required energy.
It’s necessary to have a receiver at the other end to do the reconstruction of the human. Of course, you could assume that aliens at the other end would have a receiver and a reconstruction machine, but do you want to assume that? If you don’t assume that, something has to be sent, so you have to worry about rockets.
JWT Kottekoe, wouldn’t a burst of staggeringly high acceleration smush the spaceship flat? If you don’t assume that you use such a burst, wouldn’t even a small spaceship use too much energy?
As I understand it, if you are in deep space and not slingshoting past a massive body, how quickly you burn the fuel and therefore how quickly you accelerate are irrelevant to the bigger picture of total trip time.
IFAIK, if you are going on a 1000 year voyage, it doesnt make much difference if you burn all your fuel in 1 second or 1 month. I’d go so far as to say a slower burn within reason makes most things much easier engineering wise and results in overall better performance.
There are a few prior threads on this and associated topics:
[THREAD=398234]Stephen Hawking says humanity must go to the stars. Is he right?[/THREAD]
[THREAD=398767]Getting to the stars[/THREAD]
[THREAD=414857]Human-supporting ecosystem on a permanently dark planet?[/THREAD]
[THREAD=439659]Would space aliens be good or bad? Why?[/THREAD]
[THREAD=441422]How do you slow down your colony ship?[/THREAD]
[POST=9254281]Why wouldn’t an “atomic rocket” (as in “Rocket Ship Galileo”) work?[/POST]
[THREAD=476087]How will humanity react to a habitable world close by?[/THREAD]
[THREAD=477444]Do we have the technology to build a probe to visit another star?[/THREAD]
[THREAD=503991]Orphans of the sky: Does Human nature prevent multi-generational interstellar travel?[/THREAD]
[THREAD=512426]Speck of space dust + traveling at speed of light = Death?[/THREAD]
Readers are invited to examine these threads, but the summary of them comes to the same conclusions stated in this thread: that the energy requirements and mass ratios make interstellar transit prohibitive, even for “generation ships” and the like.
The thrust of this thread [sorry, couldn’t help myself] has been focused on the difficulties with propulsion, but there are other significant difficulties, too. Aside from the energy required to move the vessel, the complex food-air-water cycles necessary to sustain a living crew demands enormous energies. We don’t notice this on Earth because except for generating electricity and the labor necessary to move commodities from one place to another, all of our energy comes for free from the Sun. But if you actually trace how much energy it takes to get, say, a glass of water from a waste stream through the hydrological cycle (evaporation, precipitation, filtration through soil) you’d realize that you are actually holding the equivalent of a stick of dynamite, albeit one in which most of the energy is not accessible, having been “wasted” to get the water to your faucet. And along with that wastage comes entropy and waste heat; again, we take this for granted because the Earth can moderate large heat impulses and ultimately radiate away all of the energy it takes in, but for a space vessel whose only means of eliminating waste heat is by radiation to the microwave background, it means having massive radiators. Then there is the difficulty of getting anything like closed-system recycling; for anything like a generation ship even a couple percent short of complete recycling of all comsumables is going to require carrying a prohibitive amount of provisions.
Most of these problems go away or are at least significantly diminished if we don’t bring the familiar but inefficient bodies that require food, sleep, and sex to function. To that end, it seems not only likely but inexorable that if and when we send spacecraft to other star systems it will be with some proxy for humanity rather than actual people. And I suspect it won’t be with one monolithic ark as envisioned by generations of science fiction writers and fans, but with small, modular, autonomous, semi-sentient, self-modifying and perhaps self-replicating machines, albeit likely organic construction or at least in a form similar to organic life i.e. built up from a cellular level rather than bolted together like a children’s Lego set.
Yes, in an idealized conventional rocket, the final velocity is independent of how fast you burn the fuel, it only affects the acceleration and the total duration of your journey.
This is exaclty how I went awry in my calculation. I figured that the acceleration didn’t matter, so I simplfied by assuming it was infinite. The way I set it up, all of the fuel was used to push the remaining payload, none of it pushed a portion of the fuel. If it were truly instantaneous (or at least if the acceleration was over before the rocket moved any significant fraction of its length) you might actually be able to engineer a spacecraft that worked this way. It would be more like the Jules Verne craft to the Moon that was shot out of a gun. For example you could blow up your entire fuel tank filled with equal mixtures of matter and antimatter inside an ellipsoidal gamma ray reflector. The gamma rays would only push on the spacecraft empty of fuel.
For any reasonable acceleration, you’ll have to bring the fuel with you, and then you are stuck with an exponential increase in the amount of fuel needed.
Sorry, didn’t read the intervening posts, but this is a common way of thinking about interstellar travel:
For Distance X, it takes Time Y.
Given our current technology, Time Y will be longer than the average person’s lifetime to travel even the smallest distance between stars.
Therefore, if we can increase a person’s lifetime, interstellar travel is possible with less than the speed of light. Alien and Planet of the Apes use the suspended animation theory to increase Y without having X>speed of light.
Regarding the intelligences part, I don’t think it is possible to send a human intelligence without the human body. Without the body, the intelligence will either just be a collection of human knowledge, or an advanced AI.
So what about the Bussard ramjet or light sail options? Both of these don’t need to carry their fuel mass. If we allow these two options and decrease the journey speed to 0.12 c is it then “just” an engineering problem or are there still other theoretical issues that would stop (non hibernating) humans make a journey?
0.12c is the theoretical upper limit (within the interstellar medium density of the local area); the actual top speed would be much less. In addition, there is a minimum speed at which the momentum of the craft will collect enough matter and compress it sufficiently to achieve fusion. This limit depends on the details and efficiencies of the fusion system, but it may be as high as 2-3% of c. Getting a vessel up to even these speeds may be prohibitive, and there may not be much of a band in which the fusion ramscoop concept is even practicable even if the minimum speed can be obtained.
Interstellar propulsion via light pressure is a non-starter. Photon flux drops off as a square of distance, so inside the orbit of Mars there is a credible amount of pressure, but by the time you’ve gotten to the orbit of Jupiter light pressure is 4% of what it is at Earth’s orbit. At Saturn it is less than 1%. By the time you’ve gotten to the Oort cloud light pressure from the Sun is only immeasurably more than it is from other stars in the neighborhood; as weak as gravity is, it is still more of an influence. Robert Forward fronted a proposal called “Starwisp” that involved propelling a lightweight craft using a solar sail propelled by an orbital laser, but setting aside the impracticalities of operating such a facility over the span of a couple hundred years or more, the actual payload mass was on the order of a few dozen grams, and this for gigawatts of energy. Using solar sails for interstellar propulsion is nothing more than uninformed science fantasy.
The non-propulsive technical issues are addressed in a previous post.
Some previous threads on this specific topic:
[post=9580272]How could a Bajorian Light Ship get all the way to Cardiassia[/post]
[post=6520779]Using A Laser To Alter An Objects Path In Space[/post]
[post=6334046]Questions about how Solar Sails work[/post]