18 ms·

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler [1]. This neglects the expansion of the universe, the health and safety effects of traveling near to the speed of light, and the fact that we do not know how to do this (you would not be able to bring your own fuel). Nevertheless we do not know why it should be physically impossible. [1] https://en.m.wikipedia.org/wiki/Space_travel_using_constant_acceleration?wprov=sfti1 https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...

olooney 7y ago
> you would not be able to bring your own fuel I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem. For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations: https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation https://en.wikipedia.org/wiki/Relativistic_rocket#Relativist... Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics. https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities. https://en.wikipedia.org/wiki/Breakthrough_Starshot https://en.wikipedia.org/wiki/Breakthrough_Starshot I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have. So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!
- Koshkin 7y ago
  > (b) no deceleration at the end That kills it for me.
  - Filligree 7y ago
    Lithobraking is always an option.
    - jacobush 7y ago
      And is also a way to send a signal back...
      - olooney 7y ago
        Good point!
      - pugworthy 7y ago
        In this case (Breakthrough Starshot), the core of the probe weighs grams. It would never make it to the ground.
        jacobush 7y ago
        I was more thinking about the flash when something of relativistic speed hits something else. An atmosphere for instance.
    - saagarjha 7y ago
      When you’re going nearly the speed of light?
      - Filligree 7y ago
        I didn't claim it's a good option...
- cousin_it 7y ago
lachlan-sneff 7y ago
In science-fiction, engines that can accelerate at a constant 1g (more or less) are called torchships. Interestingly enough, there are theoretical designs that achieve that and give us huge amounts of ΔV and still large amounts of thrust. An example is the "Plasma Jet Magneto Inertial" [1] The main issue that may not be solved by eventual iterative progress is that these kinds of engines create way too much waste heat. [1]: http://www.projectrho.com/public_html/rocket/enginelist2.php#pjmif http://www.projectrho.com/public_html/rocket/enginelist2.php...
deleted 7y ago
[deleted]
macawfish 7y ago
This is fascinating! I had no idea! Thanks for sharing this, I'm inspired. From the point of view of conventional, reductionist models of human biology, the only basic physical health effects, given a reasonably humane spaceship life, would be the risks of collision. Otherwise, I'd think 1 g would actually make a space journey more livable! (Velocity is entirely relative, so traveling near the speed of light really doesn't mean much of anything in terms of basic physics/chemistry in the traveling frame. Physics undergrads are told that: light/physics under 1g of acceleration, by the theory of general relativity, is going to behave in exactly the same way as light/physics under 1g of gravity from mass.) Now, as far as psychological/spiritual/emotional health are concerned, that's another matter. Relativistic effects would extract you from the context of time that gives meaning to all of your relationships with other Earth life, and with Earth itself. It'd be very deep. Okay but now I'm remembering the article recently shared about Posner molecules and the possibility of meaningful quantum biological organizations in the body. If our bodies physically depend on, in some yet to be articulated way, quantum information being exchanged/entangled ecologically, and if we depend on the synchronicity of these exchanges (say for example to inform some deeper biological sense of meaningful time)... Well then moving close to the speed of light would take those processes to a different tempo. To what degree are our physical bodies in temporally dependent relationships with their environment or ecologies? How critical are these relationships to basic functioning? Now I have to agree that we don't know what effects on physical health might be!
- ajconway 7y ago
  According to the relativity theory, it’s physically not possible to determine if you’re on Earth, or in a spaceship accelerating at 1 g, just by observing physical processes.
  - idlewords 7y ago
    I think you're confusing special and general relativity here. The equivalence principle in accelerated systems only holds true for infinitesimal regions of spacetime.
    - 7y ago
stcredzero 7y ago
Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler But what kind of specific impulse are we talking about? It has to be quite high, or you're talking about unphysical amounts of reaction mass. Even with photon rockets, we're talking about godlike amounts of energy. Or, we could find clever ways to circumvent the rocket equation.
- cheerlessbog 7y ago
  Right we don't know how to do it but it's not unphysical either. You almost surely couldn't push your own fuel but a scoop is at least not physically impossible. The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft.
  - stcredzero 7y ago
    You almost surely couldn't push your own fuel but a scoop is at least not physically impossible. As far as we know, you always wind up with more drag than you can overcome by using what you scoop as fusion fuel. The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft. Exploiting time dilation would probably be rendered impossible by this.
  - deleted 7y ago
    [deleted]
jbotz 7y ago
This a key fact that a lot of people seem to misunderstand, even a lot of popular science journalists trying to explain relativity. But to me it really illuminates the connection between space and time and the inviolability of c. From your own perspective you can travel arbitrarily fast in terms of how much you age inside your spaceship... but as you move faster in space, time in the the Universe outside your ship appears to be moving faster as well such that always more than a year will have passed outside for every lightyear you've traveled, even if it was only days, hours, or minutes for you.
- disconcision 7y ago
  the way i put it is that relativity imposes no limit on how soon you can get from point A to B; it only limits how soon anyone else can do it.
- ridiculous_fish 7y ago
  Isn't it the other way around? From the perspective of the spaceship rider, time outside the ship appears to be moving more slowly. That is, as the spaceship rider flies by earth, she will observe earth-bound watches ticking more slowly than her wristwatch.
  - smallnamespace 7y ago
    It's both actually. But that gives an apparent contradiction - how can both observers see that the other party has slower clocks? Who is 'right'? The answer is that the 'moving' traveler needs to flip the ship around at some point and thrust back towards the starting point. Whoever turns back around will find that it's their clocks that passed more slowly.
    - ctdonath 7y ago
      And if the universe is spherical?
      - somab 7y ago
wwarner 7y ago
If you could harness the wave-particle interactions mentioned in the article, you wouldn't need to carry your own fuel.
jp555 7y ago
The halo drive concept is interesting. It uses a beam of light to extract gravity energy from a black hole and use to to maintain acceleration of your ship. Almost no fuel needed because its really just a kind of extreme gravity assist that we’ve been doing since what, the 1970s? https://youtu.be/rFqL9CkNxXw https://youtu.be/rFqL9CkNxXw
mr_toad 7y ago
If we could accelerate at 1g for just a few days, and reach just a fraction of a percent of the speed of light, the solar system would be our playground. We could go to the moon for lunch, visit Mars for the weekend, and take a trip to Neptune every year.
solipsism 7y ago
take a round trip to the edge of the universe The observable universe. That one word makes a huge difference.
deleted 7y ago
[deleted]
baron816 7y ago
The round trip is going to take 30 billion years (assuming no expansion). By the time you got back, Earth would be long gone.
- cheerlessbog 7y ago
  No need to return makes it even easier.

olooney 7y ago

> you would not be able to bring your own fuel I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem. For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations: https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation https://en.wikipedia.org/wiki/Relativistic_rocket#Relativist... Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics. https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities. https://en.wikipedia.org/wiki/Breakthrough_Starshot https://en.wikipedia.org/wiki/Breakthrough_Starshot I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have. So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!

Koshkin 7y ago
> (b) no deceleration at the end That kills it for me.
- Filligree 7y ago
  Lithobraking is always an option.
  - jacobush 7y ago
    And is also a way to send a signal back...
    - olooney 7y ago
      Good point!
    - pugworthy 7y ago
      In this case (Breakthrough Starshot), the core of the probe weighs grams. It would never make it to the ground.
      - jacobush 7y ago
        I was more thinking about the flash when something of relativistic speed hits something else. An atmosphere for instance.
  - saagarjha 7y ago
    When you’re going nearly the speed of light?
    - Filligree 7y ago
      I didn't claim it's a good option...
cousin_it 7y ago

lachlan-sneff 7y ago

In science-fiction, engines that can accelerate at a constant 1g (more or less) are called torchships. Interestingly enough, there are theoretical designs that achieve that and give us huge amounts of ΔV and still large amounts of thrust. An example is the "Plasma Jet Magneto Inertial" [1] The main issue that may not be solved by eventual iterative progress is that these kinds of engines create way too much waste heat. [1]: http://www.projectrho.com/public_html/rocket/enginelist2.php#pjmif http://www.projectrho.com/public_html/rocket/enginelist2.php...

deleted 7y ago

[deleted]

macawfish 7y ago

This is fascinating! I had no idea! Thanks for sharing this, I'm inspired. From the point of view of conventional, reductionist models of human biology, the only basic physical health effects, given a reasonably humane spaceship life, would be the risks of collision. Otherwise, I'd think 1 g would actually make a space journey more livable! (Velocity is entirely relative, so traveling near the speed of light really doesn't mean much of anything in terms of basic physics/chemistry in the traveling frame. Physics undergrads are told that: light/physics under 1g of acceleration, by the theory of general relativity, is going to behave in exactly the same way as light/physics under 1g of gravity from mass.) Now, as far as psychological/spiritual/emotional health are concerned, that's another matter. Relativistic effects would extract you from the context of time that gives meaning to all of your relationships with other Earth life, and with Earth itself. It'd be very deep. Okay but now I'm remembering the article recently shared about Posner molecules and the possibility of meaningful quantum biological organizations in the body. If our bodies physically depend on, in some yet to be articulated way, quantum information being exchanged/entangled ecologically, and if we depend on the synchronicity of these exchanges (say for example to inform some deeper biological sense of meaningful time)... Well then moving close to the speed of light would take those processes to a different tempo. To what degree are our physical bodies in temporally dependent relationships with their environment or ecologies? How critical are these relationships to basic functioning? Now I have to agree that we don't know what effects on physical health might be!

ajconway 7y ago
According to the relativity theory, it’s physically not possible to determine if you’re on Earth, or in a spaceship accelerating at 1 g, just by observing physical processes.
- idlewords 7y ago
  I think you're confusing special and general relativity here. The equivalence principle in accelerated systems only holds true for infinitesimal regions of spacetime.
  - 7y ago

stcredzero 7y ago

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler But what kind of specific impulse are we talking about? It has to be quite high, or you're talking about unphysical amounts of reaction mass. Even with photon rockets, we're talking about godlike amounts of energy. Or, we could find clever ways to circumvent the rocket equation.

cheerlessbog 7y ago
Right we don't know how to do it but it's not unphysical either. You almost surely couldn't push your own fuel but a scoop is at least not physically impossible. The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft.
- stcredzero 7y ago
  You almost surely couldn't push your own fuel but a scoop is at least not physically impossible. As far as we know, you always wind up with more drag than you can overcome by using what you scoop as fusion fuel. The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft. Exploiting time dilation would probably be rendered impossible by this.
- deleted 7y ago
  [deleted]

jbotz 7y ago

This a key fact that a lot of people seem to misunderstand, even a lot of popular science journalists trying to explain relativity. But to me it really illuminates the connection between space and time and the inviolability of c. From your own perspective you can travel arbitrarily fast in terms of how much you age inside your spaceship... but as you move faster in space, time in the the Universe outside your ship appears to be moving faster as well such that always more than a year will have passed outside for every lightyear you've traveled, even if it was only days, hours, or minutes for you.

disconcision 7y ago
the way i put it is that relativity imposes no limit on how soon you can get from point A to B; it only limits how soon anyone else can do it.
ridiculous_fish 7y ago
Isn't it the other way around? From the perspective of the spaceship rider, time outside the ship appears to be moving more slowly. That is, as the spaceship rider flies by earth, she will observe earth-bound watches ticking more slowly than her wristwatch.
- smallnamespace 7y ago
  It's both actually. But that gives an apparent contradiction - how can both observers see that the other party has slower clocks? Who is 'right'? The answer is that the 'moving' traveler needs to flip the ship around at some point and thrust back towards the starting point. Whoever turns back around will find that it's their clocks that passed more slowly.
  - ctdonath 7y ago
    And if the universe is spherical?
    - somab 7y ago

wwarner 7y ago

If you could harness the wave-particle interactions mentioned in the article, you wouldn't need to carry your own fuel.

jp555 7y ago

The halo drive concept is interesting. It uses a beam of light to extract gravity energy from a black hole and use to to maintain acceleration of your ship. Almost no fuel needed because its really just a kind of extreme gravity assist that we’ve been doing since what, the 1970s? https://youtu.be/rFqL9CkNxXw https://youtu.be/rFqL9CkNxXw

mr_toad 7y ago

If we could accelerate at 1g for just a few days, and reach just a fraction of a percent of the speed of light, the solar system would be our playground. We could go to the moon for lunch, visit Mars for the weekend, and take a trip to Neptune every year.

solipsism 7y ago

take a round trip to the edge of the universe The observable universe. That one word makes a huge difference.

deleted 7y ago

[deleted]

baron816 7y ago

The round trip is going to take 30 billion years (assuming no expansion). By the time you got back, Earth would be long gone.

cheerlessbog 7y ago
No need to return makes it even easier.