7 ms·

I don't have references in front of me, (EDIT: I do!) but IIRC it takes about a kilogram of mass-energy to accelerate a kilogram mass to about 0.85c. But that

by jp57 2y ago

I don't have references in front of me, (EDIT: I do!) but IIRC it takes about a kilogram of mass-energy to accelerate a kilogram mass to about 0.85c. But that kilogram would have to be carrying another kilogram of matter/anti-matter fuel to decelerate again. So there is a kind of relativistic Tsiolkovsky equation for mass-energy propulsion vehicles that carry their own fuel. Zipping around the galaxy at 0.95c, stopping at destinations and then zipping off again will require carrying a lot of antimatter with you. EDIT: Thanks to Wolfram Alpha I was able to see that it the kinetic energy of 1 kg at 0.87c is very close to the mass energy of 1kg of matter.

m463 2y ago
If you could avoid accelerating so much and go a little slower, you could get better lypkg.
- jp57 2y ago
  Sure, and if you want to go at non-relativistic speeds you hardly need to carry much fuel at all.
api 2y ago
This is why there’s ideas like the Bussard ramjet, which may not work but try to work around this problem by using in situ mass-energy. Now I have read that it may be possible to use a powerful magnetic field to assist with slowing down by braking against the interstellar medium, which helps. The Avatar films have (in spite of very derivative plots) fairly realistic (at least physics wise) interstellar ships. They accelerate using beamed laser propulsion from the Sun and use antimatter rockets to decelerate, then repeat this in reverse to come home. One assumes they somehow recharge at their destination but this is not shown. Too bad all that cool tech is in service to humans who decided to be the bad guys from War of the Worlds. Still traveling that close to c brings up tons of other problems. Collision with a micrometeorite would be like an atomic explosion, and blue shifting of incident and cosmic background radiation would blast you in the head with x-rays and gamma rays. Those problems would demand more mass for active or passive shielding, and you’re already mass constrained. All things considered it’s way more practical to go slower — which could still be insanely fast e.g. 0.25c — and figure out how to cryosleep or become an AI that can just turn yourself off for the trip. Cryosleep for humans is a brutally hard biomedical problem but way easier than trying to approach the speed of light. There are other multicellular animals that can do it, albeit much simpler ones, so it’s probably possible. 0.25c allowing for acceleration and deceleration gets you to Centauri in around 25 years and to further star systems with promising exoplanets in hundreds of years. Then there’s generation ships, but that’s the kind of thing Mormons would do. :)
- PaulHoule 2y ago
  I'd imagine interstellar travelers who have mastered D+D fusion could break the journey down into hops of maybe 10,000 or 100,000 AU looking for large comets or plutoid objects which they could use to replenish their supplies. I'd imagine it would take them 10,000 years or so to make it to the next star system but they might not care if they can live a comfortable lifestyle in the great dark.
  - api 2y ago
    Something I didn’t think about: there is debate over how empty the space between stars is or whether there are a lot of rogue planets, comets, asteroids, maybe even exotic objects like asteroid or planet mass primordial black holes (these are a dark matter candidate). If the space between stars is not as empty as we imagine it opens other possibilities like flybys and refueling.
    - giantrobot 2y ago
      Interstellar space can be filled with junk yet still be effectively empty because it's so mind bogglingly large. Comets and rogue planets also aren't helpful unless 1) you can see/chart them ahead of time and 2) they're on the way to your destination. The first problem is a big challenge as they're cold, dark, and small. Just finding them to begin with is a giant problem. Accurately charting them is another order of magnitude increase in difficulty. Even tiny error bars in the measure of their proper motion means your spaceship can miss them by millions of miles. Even missing them by a dozen miles is the difference between life and death. Even with a huge catalog of extremely accurate interstellar fuel-capable objects they don't do you any good if they're not on the way to where you want to go. A meandering route to a destination in order to visit refueling stops adds tons of extra complexity and points of failure.
  - RajT88 2y ago
    Might I humbly recommend the disaster of a TV show "The Starlost".
    - 2y ago
thechao 2y ago
Even at .99c, everything interesting is years, decades, centuries, and millennia apart. All interstellar solutions include maintenance over millennia. Once you're doing that, relativistic velocity is just a hazard, not a boon. If we do conquer interstellar travel it'll probably be at 1% c. The factor of 10-20x scale won't matter to such long lived civilizations.
- weberer 2y ago
  From the perspective of someone on the ground, yes. But due to time dilation, it could be just a few days for the people on the ship.
  - Sanzig 2y ago
    At .99c, the Lorentz factor is only about 7, so 1 year of ship time for 7 years of earth time.
  - brandall10 2y ago
    .99c only dilates time by a factor of 7, so one year of time passed on a vehicle would yield ~7 light years. Heck, you have to get better than 5 nines to even compress a year into a day, .9999963c, which would take a freakish amount of energy to accelerate a KG to (3.3 × 10^19 J).
    - Retric 2y ago
      On top of this at 1g it takes ~1 year accelerating to that speed and another year decelerating. So even the nearest star is going to be a multi year subjective journey or a really unpleasant trip.
      - llm_trw 2y ago
        This is assuming that you have to be conscious during the trip. Antimatter storage is a lot more scifi than human hibernation.
jmyeet 2y ago
The energy budget for any kind of interstellar travel is large, almost incomprehensibly large. Like many orders of magnitude greater than what our entire planet produces and consumes. The numbers you're quoting seem reasonably accurate but they also assume perfect mass-to-energy conversion, which we'd never get. It's why some kind of generation ship, that is basically a colony, is really the only conceivable method of traveling between stars. Also remember that whatever energy is produced by antimatter, you need more than that to produce the antimatter to begin with. Where are you getting that energy? I believe it's from solar power from a Dyson Swarm.
- mrybczyn 2y ago
  We just have to get creative, or discover some new physics :-) In addition to the ark-ship colony, or the cryosleep slow ship: 1. Assuming it's a stream of robotic probes doing flybys, without decelerating, we have the Breakthrough Starshot approach. Maybe there's a way to use the target system's sun for solar sail braking? Send smart enough robots that have agency, that can do the exploring for us. 2. For human travel - it could just be a bunch of frozen embryos with a robotic nursemaid, accelerated via external propulsion and decelerated via nukes / high-g aerobraking... (Raised by Wolves had a cool introduction like this in the first episode - then went quickly downhill) 3. ...
  - jmyeet 2y ago
    "we need new physics" basically means "I hope this isn't true". I get it. I think many of us would like to wander the stars in a reasonable timeframe but there's simply no evidence the Universe works this way. As for cryosleep, this curently seems unlikely but not impossible. For one thing, the decay of radioactive elements in your body (primarily Carbon-14) would give you about a lethal dose of radiation after about a century. Some organisms have natural antifreeze and other means of surviving low temperatures. We do not. Freezing water tears our cells to shreds. Cryobabies and artifical wombs are another vector. This is nontrivial too but also, woould you trust the automation? Some AI might have to raise humans hundreds or thousands of years in the future without any context of what's happened in that time. We might be able to communicate with such a ship and update it but should it trust such updates? You're also creating a whole bunch of people who haven't consented to never see Earth. Generation ships have this problem too to some degree. That has questionable ethics. As for using the target star to decelerate, that's entirely possible. It's just a solar sail. And that might be the only way we could do interstellar travel anyway because of the reaction mass problem. But solar sails can only accelerate so fast. Travel too fast and you might not have time to decelerate as well. So you're still looking at hundreds of years most likely. It really seems like we need radical life extension while maintaining quality of life to make these time frames reasonable (relatively). That actually does seem doable.
benlivengood 2y ago
Hopefully Von Neumann probes will be much lighter than a Kg and we can construct receiving stations on the other end and transport the information we care about back and forth directly at light speed.
TOMDM 2y ago
Which is to say nothing of the mass of whatever containment equipment would be necessary. For stop and go; we'd be looking at something that could deploy a dyson swarm, generate and contain its own antimatter and then go again right? I should play universal paperclips again.
davedx 2y ago
Yup the problem with antimatter propulsion is it’s still subject to the tyranny of the rocket equation. The only way to escape that is to leverage energy external to the vehicle, like laser pushed light sales or relativistic railguns. You just can’t be carrying your own fuel around for an interstellar trip.
- vivzkestrel 2y ago
  would be real nice to have a small device in your pocket that works at nano scale and converts input matter into output antimatter. Like feed it rock powder and you get antimatter rock powder. Would be real nice such a device could be made
pdonis 2y ago
> there is a kind of relativistic Tsiolkovsky equation for mass-energy propulsion vehicles that carry their own fuel Yes, it's called the relativistic rocket equation. https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/...
deleted 2y ago
[deleted]

by jp57 2y ago

m463 2y ago

If you could avoid accelerating so much and go a little slower, you could get better lypkg.

jp57 2y ago
Sure, and if you want to go at non-relativistic speeds you hardly need to carry much fuel at all.

api 2y ago

This is why there’s ideas like the Bussard ramjet, which may not work but try to work around this problem by using in situ mass-energy. Now I have read that it may be possible to use a powerful magnetic field to assist with slowing down by braking against the interstellar medium, which helps. The Avatar films have (in spite of very derivative plots) fairly realistic (at least physics wise) interstellar ships. They accelerate using beamed laser propulsion from the Sun and use antimatter rockets to decelerate, then repeat this in reverse to come home. One assumes they somehow recharge at their destination but this is not shown. Too bad all that cool tech is in service to humans who decided to be the bad guys from War of the Worlds. Still traveling that close to c brings up tons of other problems. Collision with a micrometeorite would be like an atomic explosion, and blue shifting of incident and cosmic background radiation would blast you in the head with x-rays and gamma rays. Those problems would demand more mass for active or passive shielding, and you’re already mass constrained. All things considered it’s way more practical to go slower — which could still be insanely fast e.g. 0.25c — and figure out how to cryosleep or become an AI that can just turn yourself off for the trip. Cryosleep for humans is a brutally hard biomedical problem but way easier than trying to approach the speed of light. There are other multicellular animals that can do it, albeit much simpler ones, so it’s probably possible. 0.25c allowing for acceleration and deceleration gets you to Centauri in around 25 years and to further star systems with promising exoplanets in hundreds of years. Then there’s generation ships, but that’s the kind of thing Mormons would do. :)

PaulHoule 2y ago
I'd imagine interstellar travelers who have mastered D+D fusion could break the journey down into hops of maybe 10,000 or 100,000 AU looking for large comets or plutoid objects which they could use to replenish their supplies. I'd imagine it would take them 10,000 years or so to make it to the next star system but they might not care if they can live a comfortable lifestyle in the great dark.
- api 2y ago
  Something I didn’t think about: there is debate over how empty the space between stars is or whether there are a lot of rogue planets, comets, asteroids, maybe even exotic objects like asteroid or planet mass primordial black holes (these are a dark matter candidate). If the space between stars is not as empty as we imagine it opens other possibilities like flybys and refueling.
  - giantrobot 2y ago
    Interstellar space can be filled with junk yet still be effectively empty because it's so mind bogglingly large. Comets and rogue planets also aren't helpful unless 1) you can see/chart them ahead of time and 2) they're on the way to your destination. The first problem is a big challenge as they're cold, dark, and small. Just finding them to begin with is a giant problem. Accurately charting them is another order of magnitude increase in difficulty. Even tiny error bars in the measure of their proper motion means your spaceship can miss them by millions of miles. Even missing them by a dozen miles is the difference between life and death. Even with a huge catalog of extremely accurate interstellar fuel-capable objects they don't do you any good if they're not on the way to where you want to go. A meandering route to a destination in order to visit refueling stops adds tons of extra complexity and points of failure.
- RajT88 2y ago
  Might I humbly recommend the disaster of a TV show "The Starlost".
  - 2y ago

thechao 2y ago

Even at .99c, everything interesting is years, decades, centuries, and millennia apart. All interstellar solutions include maintenance over millennia. Once you're doing that, relativistic velocity is just a hazard, not a boon. If we do conquer interstellar travel it'll probably be at 1% c. The factor of 10-20x scale won't matter to such long lived civilizations.

weberer 2y ago
From the perspective of someone on the ground, yes. But due to time dilation, it could be just a few days for the people on the ship.
- Sanzig 2y ago
  At .99c, the Lorentz factor is only about 7, so 1 year of ship time for 7 years of earth time.
- brandall10 2y ago
  .99c only dilates time by a factor of 7, so one year of time passed on a vehicle would yield ~7 light years. Heck, you have to get better than 5 nines to even compress a year into a day, .9999963c, which would take a freakish amount of energy to accelerate a KG to (3.3 × 10^19 J).
  - Retric 2y ago
    On top of this at 1g it takes ~1 year accelerating to that speed and another year decelerating. So even the nearest star is going to be a multi year subjective journey or a really unpleasant trip.
    - llm_trw 2y ago
      This is assuming that you have to be conscious during the trip. Antimatter storage is a lot more scifi than human hibernation.

jmyeet 2y ago

The energy budget for any kind of interstellar travel is large, almost incomprehensibly large. Like many orders of magnitude greater than what our entire planet produces and consumes. The numbers you're quoting seem reasonably accurate but they also assume perfect mass-to-energy conversion, which we'd never get. It's why some kind of generation ship, that is basically a colony, is really the only conceivable method of traveling between stars. Also remember that whatever energy is produced by antimatter, you need more than that to produce the antimatter to begin with. Where are you getting that energy? I believe it's from solar power from a Dyson Swarm.

mrybczyn 2y ago
We just have to get creative, or discover some new physics :-) In addition to the ark-ship colony, or the cryosleep slow ship: 1. Assuming it's a stream of robotic probes doing flybys, without decelerating, we have the Breakthrough Starshot approach. Maybe there's a way to use the target system's sun for solar sail braking? Send smart enough robots that have agency, that can do the exploring for us. 2. For human travel - it could just be a bunch of frozen embryos with a robotic nursemaid, accelerated via external propulsion and decelerated via nukes / high-g aerobraking... (Raised by Wolves had a cool introduction like this in the first episode - then went quickly downhill) 3. ...
- jmyeet 2y ago
  "we need new physics" basically means "I hope this isn't true". I get it. I think many of us would like to wander the stars in a reasonable timeframe but there's simply no evidence the Universe works this way. As for cryosleep, this curently seems unlikely but not impossible. For one thing, the decay of radioactive elements in your body (primarily Carbon-14) would give you about a lethal dose of radiation after about a century. Some organisms have natural antifreeze and other means of surviving low temperatures. We do not. Freezing water tears our cells to shreds. Cryobabies and artifical wombs are another vector. This is nontrivial too but also, woould you trust the automation? Some AI might have to raise humans hundreds or thousands of years in the future without any context of what's happened in that time. We might be able to communicate with such a ship and update it but should it trust such updates? You're also creating a whole bunch of people who haven't consented to never see Earth. Generation ships have this problem too to some degree. That has questionable ethics. As for using the target star to decelerate, that's entirely possible. It's just a solar sail. And that might be the only way we could do interstellar travel anyway because of the reaction mass problem. But solar sails can only accelerate so fast. Travel too fast and you might not have time to decelerate as well. So you're still looking at hundreds of years most likely. It really seems like we need radical life extension while maintaining quality of life to make these time frames reasonable (relatively). That actually does seem doable.

benlivengood 2y ago

Hopefully Von Neumann probes will be much lighter than a Kg and we can construct receiving stations on the other end and transport the information we care about back and forth directly at light speed.

TOMDM 2y ago

Which is to say nothing of the mass of whatever containment equipment would be necessary. For stop and go; we'd be looking at something that could deploy a dyson swarm, generate and contain its own antimatter and then go again right? I should play universal paperclips again.

davedx 2y ago

Yup the problem with antimatter propulsion is it’s still subject to the tyranny of the rocket equation. The only way to escape that is to leverage energy external to the vehicle, like laser pushed light sales or relativistic railguns. You just can’t be carrying your own fuel around for an interstellar trip.

vivzkestrel 2y ago
would be real nice to have a small device in your pocket that works at nano scale and converts input matter into output antimatter. Like feed it rock powder and you get antimatter rock powder. Would be real nice such a device could be made

pdonis 2y ago

> there is a kind of relativistic Tsiolkovsky equation for mass-energy propulsion vehicles that carry their own fuel Yes, it's called the relativistic rocket equation. https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/...

deleted 2y ago

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