8 ms·

I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel be

I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel becomes somewhat viable provided you can continuously accelerate for the entire trip. Googling it, it would take approximately three years of observed time for a traveller to get to alpha centauri at 1g. Could any somewhat-plausible technologies (fusion etc) get us to that point? I don't know how to do relatavistic maths, but it seems that directly converting mass to energy and ignoring relatavistic effects requires 50 000kg to accelerate 100 000 kg (with a constant relationship, as the C^2 cancels out) to the speed of light.

JumpCrisscross 3y ago
> Could any somewhat-plausible technologies (fusion etc) get us to that point? Nuclear thermal or nuclear pulse propulsion. No fusion needed.
- ianburrell 3y ago
  Nuclear thermal or pulse, fission or fusion, aren't enough for interstellar relativistic speeds. They have way too low specific impulse. They don't even have enough for constant acceleration inside the solar system unless the acceleration is really low or the fuel ratio is huge.
  - JumpCrisscross 3y ago
    > unless the acceleration is really low or the fuel ratio is huge You’d need kilotons of nuclear fuel and a 10^3 fuel ratio. But that’s plausible. The economically-plausible answer is antimatter, where the ratio stays in the single digits and starts permitting deceleration. But I wouldn’t call that technologically plausible at this time.
- ben_w 3y ago
  For practical values, nuclear pulse propulsion would only get you up to a few percent of the speed of light. The best nuclear thermal, gas core, would give you 7000 seconds[0]. A "dusty plasma" rocket (suspend the nuclear fuel in dust form in a magnetic field) might get 100,000 seconds, or just over a day of accelerating at 1g. [0] for the benefit of non-space nerds: the unit of measure is "lb-force seconds per lb-mass of fuel", which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket.
  - ninkendo 3y ago
    > which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket. I think you mean “a very large fraction of the total mass”… generally the best efficiency comes if your fuel mass fraction is high, as it means there is little overhead of things in your spacecraft of things that are not fuel (the mass of the engine, etc)
    - ben_w 3y ago
      No, I mean small, because I'm talking about the quality of the approximation not the way to maximise delta-v. If you have 1 gram of fuel and a 1 ton payload and that fuel has 1e6 seconds of Isp, you can accelerate the ship for 1 second at 9.8m/s/s. If you have 1 ton of fuel and a 1 gram payload and the same fuel and burn at 1 gram/second, the first second is mostly spent accelerating the fuel, which means you're no longer able to just approximate the Isp as "seconds at 1g" in a nice linear fashion — it starts off at 1 gee in this example, but ends up at 10^6 gee in the last moment, a million seconds later.
      - ninkendo 3y ago
        Makes sense, I misunderstood your initial post. Thanks for clarifying!
  - mattclarkdotnet 3y ago
    g-seconds then? That seems like a very handy unit
    - Dylan16807 3y ago
      https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse
ianburrell 3y ago
It isn't possible without a magic reactionless drive; it might even require a zero-energy drive since the kinetic energy is huge and needs to come from somewhere. The rocket equation kills you. A reasonable fusion rocket would require an enormous (like an entire planet worth) to continuously accelerate to relativistic speeds. Antimatter drives are the only option and still require large amounts of reaction mass since have to absorb the energy to throw it out the back, or use a laser.
- eek2121 3y ago
  Well the obvious solution would be to figure out a mechanism to capture the particles that bombard your ship and figure out a way to turn those particles into thrust so you don’t need to take a bunch of fuel with you, which I get it, is another hard problem to solve, but maybe not impossible. We have done some incredible things before.
  - qayxc 3y ago
    The good old Bussard Ramjet. Problem is, recent calculations showed that it's not viable - even for a Kardashev Type II civilisation [1] [1] https://www.sciencedirect.com/science/article/pii/S0094576521005804 https://www.sciencedirect.com/science/article/pii/S009457652...
    - throwaway11460 3y ago
      It's not viable to use it to visit the galactic center during a lifetime, but it still flies, if I understand that correctly?
      - qayxc 3y ago
        Theoretically it works, yes, though not nearly as well as initially thought. The engineering and the required materials, however, are still questionable and way beyond our current understanding of physics and material science. It's a similar story for solar sails, unfortunately, though progress is being made in that area. Personally, I think that's the only realistic way of interstellar travel given our current knowledge of physics and engineering.
MilStdJunkie 3y ago
One gee over the course of months is insane. Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission nuclear gas rockets might max out at 12% but those numbers are dodgy as we don't have sharp numbers on gas core reactors. As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I imagine a sort of neo-Polynesian culture, with travellers never quite knowing how the place will look if they ever come back that way.
- 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.
Keyframe 3y ago
as far as we understand, you have to eject mass in order to move there. pick your choice in that regard.
floxy 3y ago
"Roundtrip Interstellar Travel Using Laser Pushed Lightsails" https://ia600704.us.archive.org/view_archive.php?archive=/24/items/wikipedia-scholarly-sources-corpus/10.2514.zip&file=10.2514%252F3.8632.pdf https://ia600704.us.archive.org/view_archive.php?archive=/24... "A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. " "...The third mission uses a three-stage sail for a roundtrip manned exploration of e Eridani at 10.8 light years distance."
- qayxc 3y ago
  Holy cow! I just read the energy requirements of that third mission. Total electricity generation capacity of the entire world is about 12TW today [1], whereas that 3rd mission would require a "formidable" - as the author admits - 45,000TW at least. A mere three orders of magnitude more :) [1] https://www.statista.com/statistics/267358/world-installed-power-capacity/ https://www.statista.com/statistics/267358/world-installed-p...
  - fifticon 3y ago
    I have a confused question here. I asked chatGPT what the approx terawatt output of the sun is. It claims it is 384.6 terawatts. I find that hard to reconcile with the earth having 12TW production. Given that your number is probably OK, that would suggest the sun output calc is way off. I wonder if any non-AI contributors have better ballpark estimates for the sun's energy output.. Hmm, I found a non-chatGPT source, which claims the same.. https://www.rmg.co.uk/file/2277/download?token=KyEQPN9O https://www.rmg.co.uk/file/2277/download?token=KyEQPN9O If this relation is really true, I am somewhat shocked - us burning 1/30th of the suns energy output, can that really be true..?? If so, scary. And also 'illuminating' concerning how unsustainable what we are doing is..? I do hope my numbers are off.
    - rootusrootus 3y ago
      This says we use about 1/10000 of the solar energy that hits the Earth at any given moment. https://phys.org/news/2011-10-vast-amounts-solar-energy-earth.html https://phys.org/news/2011-10-vast-amounts-solar-energy-eart...
Workaccount2 3y ago
Even if you can figure out the problem of thrust, you still have to deal with particles slowly swiss-cheesing your ship. And even beyond that, you either witness the ships leaving and never hear back or you embark on one and comeback to whatever Earth is in thousands of years from now.
darreninthenet 3y ago
Even the GPS satellites have to take into special relativity - they have to correct for a tiny amount of time per day due to time dilation causing the moving clocks on the satellites to tick slightly slower than the stationary ones on Earth.
- Koshkin 3y ago
  Curiously, general-relativistic effects needed to be accounted for as well. Quoting Wikipedia, GPS “must account for the gravitational redshift in its timing system, and physicists have analyzed timing data from the GPS to confirm other tests. When the first satellite was launched, some engineers resisted the prediction that a noticeable gravitational time dilation would occur, so the first satellite was launched without the clock adjustment that was later built into subsequent satellites. It showed the predicted shift of 38 microseconds per day. This rate of discrepancy is sufficient to substantially impair function of GPS within hours if not accounted for.”
phire 3y ago
Things get more insane the further you go. After an observed 12 years of constant 1g acceleration, you will be 113,000 lightyears from your point of departure, which is enough distance to cross from earth to the opposite side of our galaxy. Of course, you are now traveling at 99.999999996% of the speed of light. If you actually wanted to stop at the other side of the galaxy to take a look around, you would need to turn around at the halfway point to decelerate, which roughly doubles the travel time.
- galangalalgol 3y ago
  Is there any relativistic bonus that makes deceleration easier? Does it take less energy to go from 0.999C to 0.99C than to go from 0.009C to 0C?
  - phire 3y ago
    Acceleration and Deceleration are symmetrical. It takes the same amount of delta-v to accelerate between up to 0.999c as it takes to decelerate all the way back down to 0. Though as you burn fuel your ship gets lighter and the actual energy usage to maintain that constant delta-v of 1G will go down. As for the energy used to accelerate from 0C to 0.009C compared to 0.99C to 0.999C, I'm not sure. I know the time taken (from an external reference frame) changes, but part of me suspects the total energy stays the same and the difference in time taken is caused entirely by time dilation. However, I suspect I might be messing up reference frames, I don't actually know the equations.

JumpCrisscross 3y ago

> Could any somewhat-plausible technologies (fusion etc) get us to that point? Nuclear thermal or nuclear pulse propulsion. No fusion needed.

ianburrell 3y ago
Nuclear thermal or pulse, fission or fusion, aren't enough for interstellar relativistic speeds. They have way too low specific impulse. They don't even have enough for constant acceleration inside the solar system unless the acceleration is really low or the fuel ratio is huge.
- JumpCrisscross 3y ago
  > unless the acceleration is really low or the fuel ratio is huge You’d need kilotons of nuclear fuel and a 10^3 fuel ratio. But that’s plausible. The economically-plausible answer is antimatter, where the ratio stays in the single digits and starts permitting deceleration. But I wouldn’t call that technologically plausible at this time.
ben_w 3y ago
For practical values, nuclear pulse propulsion would only get you up to a few percent of the speed of light. The best nuclear thermal, gas core, would give you 7000 seconds[0]. A "dusty plasma" rocket (suspend the nuclear fuel in dust form in a magnetic field) might get 100,000 seconds, or just over a day of accelerating at 1g. [0] for the benefit of non-space nerds: the unit of measure is "lb-force seconds per lb-mass of fuel", which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket.
- ninkendo 3y ago
  > which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket. I think you mean “a very large fraction of the total mass”… generally the best efficiency comes if your fuel mass fraction is high, as it means there is little overhead of things in your spacecraft of things that are not fuel (the mass of the engine, etc)
  - ben_w 3y ago
    No, I mean small, because I'm talking about the quality of the approximation not the way to maximise delta-v. If you have 1 gram of fuel and a 1 ton payload and that fuel has 1e6 seconds of Isp, you can accelerate the ship for 1 second at 9.8m/s/s. If you have 1 ton of fuel and a 1 gram payload and the same fuel and burn at 1 gram/second, the first second is mostly spent accelerating the fuel, which means you're no longer able to just approximate the Isp as "seconds at 1g" in a nice linear fashion — it starts off at 1 gee in this example, but ends up at 10^6 gee in the last moment, a million seconds later.
    - ninkendo 3y ago
      Makes sense, I misunderstood your initial post. Thanks for clarifying!
- mattclarkdotnet 3y ago
  g-seconds then? That seems like a very handy unit
  - Dylan16807 3y ago
    https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse

ianburrell 3y ago

It isn't possible without a magic reactionless drive; it might even require a zero-energy drive since the kinetic energy is huge and needs to come from somewhere. The rocket equation kills you. A reasonable fusion rocket would require an enormous (like an entire planet worth) to continuously accelerate to relativistic speeds. Antimatter drives are the only option and still require large amounts of reaction mass since have to absorb the energy to throw it out the back, or use a laser.

eek2121 3y ago
Well the obvious solution would be to figure out a mechanism to capture the particles that bombard your ship and figure out a way to turn those particles into thrust so you don’t need to take a bunch of fuel with you, which I get it, is another hard problem to solve, but maybe not impossible. We have done some incredible things before.
- qayxc 3y ago
  The good old Bussard Ramjet. Problem is, recent calculations showed that it's not viable - even for a Kardashev Type II civilisation [1] [1] https://www.sciencedirect.com/science/article/pii/S0094576521005804 https://www.sciencedirect.com/science/article/pii/S009457652...
  - throwaway11460 3y ago
    It's not viable to use it to visit the galactic center during a lifetime, but it still flies, if I understand that correctly?
    - qayxc 3y ago
      Theoretically it works, yes, though not nearly as well as initially thought. The engineering and the required materials, however, are still questionable and way beyond our current understanding of physics and material science. It's a similar story for solar sails, unfortunately, though progress is being made in that area. Personally, I think that's the only realistic way of interstellar travel given our current knowledge of physics and engineering.

MilStdJunkie 3y ago

One gee over the course of months is insane. Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission nuclear gas rockets might max out at 12% but those numbers are dodgy as we don't have sharp numbers on gas core reactors. As an SF writer, it's pretty interesting, though, to think of the weird shapes a society would take when your astronauts are outliving entire civilizations. I imagine a sort of neo-Polynesian culture, with travellers never quite knowing how the place will look if they ever come back that way.

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.

Keyframe 3y ago

as far as we understand, you have to eject mass in order to move there. pick your choice in that regard.

floxy 3y ago

"Roundtrip Interstellar Travel Using Laser Pushed Lightsails" https://ia600704.us.archive.org/view_archive.php?archive=/24/items/wikipedia-scholarly-sources-corpus/10.2514.zip&file=10.2514%252F3.8632.pdf https://ia600704.us.archive.org/view_archive.php?archive=/24... "A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. " "...The third mission uses a three-stage sail for a roundtrip manned exploration of e Eridani at 10.8 light years distance."

qayxc 3y ago
Holy cow! I just read the energy requirements of that third mission. Total electricity generation capacity of the entire world is about 12TW today [1], whereas that 3rd mission would require a "formidable" - as the author admits - 45,000TW at least. A mere three orders of magnitude more :) [1] https://www.statista.com/statistics/267358/world-installed-power-capacity/ https://www.statista.com/statistics/267358/world-installed-p...
- fifticon 3y ago
  I have a confused question here. I asked chatGPT what the approx terawatt output of the sun is. It claims it is 384.6 terawatts. I find that hard to reconcile with the earth having 12TW production. Given that your number is probably OK, that would suggest the sun output calc is way off. I wonder if any non-AI contributors have better ballpark estimates for the sun's energy output.. Hmm, I found a non-chatGPT source, which claims the same.. https://www.rmg.co.uk/file/2277/download?token=KyEQPN9O https://www.rmg.co.uk/file/2277/download?token=KyEQPN9O If this relation is really true, I am somewhat shocked - us burning 1/30th of the suns energy output, can that really be true..?? If so, scary. And also 'illuminating' concerning how unsustainable what we are doing is..? I do hope my numbers are off.
  - rootusrootus 3y ago
    This says we use about 1/10000 of the solar energy that hits the Earth at any given moment. https://phys.org/news/2011-10-vast-amounts-solar-energy-earth.html https://phys.org/news/2011-10-vast-amounts-solar-energy-eart...

Workaccount2 3y ago

Even if you can figure out the problem of thrust, you still have to deal with particles slowly swiss-cheesing your ship. And even beyond that, you either witness the ships leaving and never hear back or you embark on one and comeback to whatever Earth is in thousands of years from now.

darreninthenet 3y ago

Even the GPS satellites have to take into special relativity - they have to correct for a tiny amount of time per day due to time dilation causing the moving clocks on the satellites to tick slightly slower than the stationary ones on Earth.

Koshkin 3y ago
Curiously, general-relativistic effects needed to be accounted for as well. Quoting Wikipedia, GPS “must account for the gravitational redshift in its timing system, and physicists have analyzed timing data from the GPS to confirm other tests. When the first satellite was launched, some engineers resisted the prediction that a noticeable gravitational time dilation would occur, so the first satellite was launched without the clock adjustment that was later built into subsequent satellites. It showed the predicted shift of 38 microseconds per day. This rate of discrepancy is sufficient to substantially impair function of GPS within hours if not accounted for.”

phire 3y ago

Things get more insane the further you go. After an observed 12 years of constant 1g acceleration, you will be 113,000 lightyears from your point of departure, which is enough distance to cross from earth to the opposite side of our galaxy. Of course, you are now traveling at 99.999999996% of the speed of light. If you actually wanted to stop at the other side of the galaxy to take a look around, you would need to turn around at the halfway point to decelerate, which roughly doubles the travel time.

galangalalgol 3y ago
Is there any relativistic bonus that makes deceleration easier? Does it take less energy to go from 0.999C to 0.99C than to go from 0.009C to 0C?
- phire 3y ago
  Acceleration and Deceleration are symmetrical. It takes the same amount of delta-v to accelerate between up to 0.999c as it takes to decelerate all the way back down to 0. Though as you burn fuel your ship gets lighter and the actual energy usage to maintain that constant delta-v of 1G will go down. As for the energy used to accelerate from 0C to 0.009C compared to 0.99C to 0.999C, I'm not sure. I know the time taken (from an external reference frame) changes, but part of me suspects the total energy stays the same and the difference in time taken is caused entirely by time dilation. However, I suspect I might be messing up reference frames, I don't actually know the equations.