3 ms·

People often don't realize just how much energy is required to reach even the nearest star in reasonable time. Of course it depends on how long you're willing t

by jmyeet 3y ago

People often don't realize just how much energy is required to reach even the nearest star in reasonable time. Of course it depends on how long you're willing to take to get there. I forget the exact numbers but if you start making assumptions like 1G of acceleration up to somewhere between 0.01c and 0.1c you have a travel time between 40 and 400 years to reach Alpha Centauri and an energy cost per kg of payload in the exajoule range. That's as much as 1% of the mass as energy assuming you have a perfect conversion of mass to energy. This is getting into the realm of all the energy we produce and use for the entire planet in a year. Of course we have nothing like that. We don't even have a theoretical framework for that other than possible matter-antimatter annihilation but even then you have containment issues and have to perfectly convert that to motion in the desired direction. That is nontrivial. Our best hope for doing this and carrying our own fuel is nuclear fusion (or possibly by detonating nuclear bombs behind you; yes, seriously). Even here the tyranny of the rocket equation [1] just kills you from the weight of the fuel you have to carry. Also, exhaust velocity limits how fast you can get no matter how efficient your energy production. The only viable way may in fact be to capture energy from the sun and focus it at your ship to accelerate it. Photos have no rest mass but they do have (and can impart) momentum, which is what this article is talking about. I don't know why they'd be using ground-based arrays. You'll end up cooking the atmosphere this way and losing energy if you don't. A laser array like this really has to be space-based, practically speaking. Of course you have the issue of how you decelerate at the other end. This is a not-insignificant problem that probably rquires sending automated probes ahead to construct the necessary infrastructure to decelerate you. So this article uses gigawatt level power to accelerate a "spaceship" of a few grams. I'm not sure what you do with that exactly but this goes to just how high the energy budget requirements are. Even all the hopium over various FTL methods (eg Alcubierre drive, wormholes, space-folding) have no theoretical basis (eg requiring negative mass and/or energy) but even if they did the energy requirements are completely ignored. If you look into it and arrive at any numbers at all it's things like "convert 1 Solar mass into energy". [1]: https://www.kallmorris.com/columns/tyranny-of-the-rocket-equation https://www.kallmorris.com/columns/tyranny-of-the-rocket-equ...

semiquaver 3y ago
> Of course you have the issue of how you decelerate at the other end. This is a not-insignificant problem that probably rquires sending automated probes ahead to construct the necessary infrastructure to decelerate you. But how do you get the probes where you’re going in a reasonable time frame? You can’t use the light sail method because of the same deceleration problem. So you’re stuck with the whole operation taking tens of thousands of years, which means you might as well have sent the original payload with whatever sublightspeed mechanism you would use to send the probes.
- pmontra 3y ago
  I can't remember the book (maybe one by Egan?), they transported the endpoint of a wormhole to its final destination with a normal slow spacecraft and then started using the tunnel for FTL. I guess the crew used the wormhole all the time to refuel the ship and to go sleeping at home :-)
- jmyeet 3y ago
  People require life support and don't want to spend 80,000 years travelling 4 light years. Automated probes don't have that problem, although they still need to last that long in the hostile conditions of space, which is another not-insignificant issue. At the other end, an automated probe could both accelerate and decelerate at significantly higher than 1G if desired. So automated probes have a vastly wider set of possible mission parameters available to them. You can deploy a solar sail to decelerate as you approach another star. This limits how fast you can go as you need sufficient time to decelerate. Additionally, you have drag in interstellar space anyway just from dust and gas particles slowing you down. Fun fact: to accelerate beyond about 0.86-0.9c your ship would need to be aerodynamic because of the decceleration effect of the interstellar medium. What you're really building here is what's often called an "interstellar highway". If fusion ever becomes viable you can extend this to having waypoints to accelerate or decelerate (as required) a spaceship along the path but that's not strictly required. So building such a system will likely take hundreds, possibly thousands, of years but ultimately result in something where you can travel between the endpoints in a few decades.

by jmyeet 3y ago

semiquaver 3y ago

> Of course you have the issue of how you decelerate at the other end. This is a not-insignificant problem that probably rquires sending automated probes ahead to construct the necessary infrastructure to decelerate you. But how do you get the probes where you’re going in a reasonable time frame? You can’t use the light sail method because of the same deceleration problem. So you’re stuck with the whole operation taking tens of thousands of years, which means you might as well have sent the original payload with whatever sublightspeed mechanism you would use to send the probes.

pmontra 3y ago
I can't remember the book (maybe one by Egan?), they transported the endpoint of a wormhole to its final destination with a normal slow spacecraft and then started using the tunnel for FTL. I guess the crew used the wormhole all the time to refuel the ship and to go sleeping at home :-)
jmyeet 3y ago
People require life support and don't want to spend 80,000 years travelling 4 light years. Automated probes don't have that problem, although they still need to last that long in the hostile conditions of space, which is another not-insignificant issue. At the other end, an automated probe could both accelerate and decelerate at significantly higher than 1G if desired. So automated probes have a vastly wider set of possible mission parameters available to them. You can deploy a solar sail to decelerate as you approach another star. This limits how fast you can go as you need sufficient time to decelerate. Additionally, you have drag in interstellar space anyway just from dust and gas particles slowing you down. Fun fact: to accelerate beyond about 0.86-0.9c your ship would need to be aerodynamic because of the decceleration effect of the interstellar medium. What you're really building here is what's often called an "interstellar highway". If fusion ever becomes viable you can extend this to having waypoints to accelerate or decelerate (as required) a spaceship along the path but that's not strictly required. So building such a system will likely take hundreds, possibly thousands, of years but ultimately result in something where you can travel between the endpoints in a few decades.