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What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow

by Teever 3y ago

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

pc86 3y ago
Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.
- jandrese 3y ago
  There is also the question of how you get the lasers to your destination in the first place.
  - Teever 3y ago
    Use lasers for the first half of the journey to send a rocket that can decelerate and build the laser station at the destination.
  - MilStdJunkie 3y ago
    Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.
    - eru 3y ago
      How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?
      - MilStdJunkie 3y ago
        Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.
NotYourLawyer 3y ago
The farther you get, the dimmer the laser gets. The faster you get, the less energy each photon has.
- slow_typist 3y ago
  Note that the 2nd statement only applies because of the Doppler effect - energy of the photon is a function of its frequency.
cypherpunks01 3y ago
I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed? I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.
- PeterisP 3y ago
  Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.
MilStdJunkie 3y ago
That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down. I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.

by Teever 3y ago

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

pc86 3y ago

Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.

jandrese 3y ago
There is also the question of how you get the lasers to your destination in the first place.
- Teever 3y ago
  Use lasers for the first half of the journey to send a rocket that can decelerate and build the laser station at the destination.
- MilStdJunkie 3y ago
  Send a mirror ahead, bounce the beam back toward the ship to push it backwards. Loses a lot of efficiency, though. Laser propulsion, a lot of the thrust is coming from ablation of the surface.
  - eru 3y ago
    How do you 'send a mirror ahead'? Do you want it to stop, or just keep flying away indefinitely?
    - MilStdJunkie 3y ago
      Yep, throwaway mirror, it goes off to wherever once it's done bouncing light back at your ship. Again, much less efficient, so when you get where you want to go, you'd want to put in a laser station there too.

NotYourLawyer 3y ago

The farther you get, the dimmer the laser gets. The faster you get, the less energy each photon has.

slow_typist 3y ago
Note that the 2nd statement only applies because of the Doppler effect - energy of the photon is a function of its frequency.

cypherpunks01 3y ago

I think the perceived output from stationary lasers on Earth would appear to decrease as your speed increases, since you're accelerating away from the laser photon packets. Because of that behavior, I'm not sure it's a feasible way to get up to relativistic speed? I think the lasers would also need ~10,000 years of fuel, as opposed to ~10 years of fuel if you could somehow figure out how to carry it. But the energy to sustain in either case is probably measured in Dyson spheres, or some fraction of all energy in the universe pretty quickly too.

PeterisP 3y ago
Time dilation also has an effect, since you're receiving 10000 Earth-years of laser energy over 10 ship-years.

MilStdJunkie 3y ago

That's the drive of the eponymous generation ship in Kim Stanley Robinson's Aurora. It sends a mirror ahead of itself, then the Sol-Mercury lasers bounce off the mirror, back on the ship to slow it down. I could see the light spacecraft with their matter-antimatter engines laying down "lightways", like railroads in the old west. Once a lightway is established, then it's much less of a big deal to get from system to system. Apart from the tens of thousands of years in subjective time, of course.