201–210 of 500 posts
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> Mass matters. Exactly. You try accelerating 200kg up to anywhere close to the speed of light (say 80%). That is a lot of force. Technically even photons can exert force on objects, but they have such a small mass that it's a difficult effect to observe.
From memory, photons are massless, otherwise they could not move at light speed. They do have momentum though.
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Right, because space is known to be full of atmospheres to use to slow down. ???
> space is known to be full of atmospheres to use to slow down The places people talk about travelling to tend to have atmospheres and gravity, yes.
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an ion thruster that shoots ions out at the speed of light probably won't affect Earth
Yes because ions are... ions. You don't see photons knocking the planet out of orbit, do you? They can exert force but they don't exert that much force. But if you're trying to avoid self-propulsion and want to launch from Earth, say, even a 200kg craft, anywhere "close to the speed of light", then that will most probably require a significant enough amount of force to knock the planet out of orbit.
If you can apply small force over a long time, that will get you up to speed, too.
Someone did the math in the thread, and suggested that a constant 1g of acceleration would get you to the centre of the galaxy in 20 years (as measured by the clocks traveling on your spaceships). 1g of acceleration for 200kg is about 1962 Newton.
(This back of the envelope calculation assumes you have eg someone fire a laser at your ship to give you the energy you need. If you need to bring your own fuel, the rocket equation increases the total mass needed. But the same principle still applies: something like an ion drive has very little force, even if the top speed it can reach can be enormous.)
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Imagine a generation ship, consuming the energy of an entire star for the trip.
We have one of those (stars)! Just need to build a parabolic reflector on one side of it and point it towards the opposite direction where we want to go. When the reflector shoots to far from the same, tilt the reflectors, drop down closer to the Sun by gravity, tilt them back, do it again. We could be going places in a few billion years!
Btw, the sun is also an extremely inefficient engine. With a bit of extra engineering we could probably scoop up hydrogen from the sun, and 'burn' it much more efficiently.
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Right, because space is known to be full of atmospheres to use to slow down. ???
> space is known to be full of atmospheres to use to slow down The places people talk about travelling to tend to have atmospheres and gravity, yes.
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That is doable for "normal" speeds, not one where you accelerate 1G for twenty years, reach relativistic speeds that make you travel for hundreds of thousands of light-years in merely 20 years ship time
> doable for "normal" speeds, not one where you accelerate 1G for twenty years It's still a lot of energy you can bleed off, particularly if you're aiming for a system with gas giants. I'm not suggesting one only rely on passive deceleration. But especially given it's fuel saved at the very end of the journey, fuel you no longer need to accelerate and decelerate for the entire duration of the trip, the savings could…
Great visualization of the unintuitive nature of special relativity. It's kind of mind blowing to realize that if we could accelerate our spaceship at a mere 1G continuously, we could visit the center of the Milky Way in under 20 years (spaceship time), totally do-able in a human lifespan. Of course 27900 years would have passed on Earth, so you wouldn't be able to tell anyone about your vacation.
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Is the ship gaining mass (and shortening, and having its time slowed) in its own frame of reference? No, right? So wouldn't it be that the 1G acceleration from the viewpoint of the ship remains from the internal perspective, even though near the speed of light, more and more of that energy goes into higher mass and slower time?
I'm reading up again to refresh my relativity knowledge. Though: > even though near the speed of light, more and more of that energy goes into higher mass and slower time? This to me does not sound very different from saying the (relativistic) mass to be accelerated is larger, and is what's stopping a 1G (or any constant) acceleration to be maintained to bring the ship to the speed of light. The concept may have been…
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That is doable for "normal" speeds, not one where you accelerate 1G for twenty years, reach relativistic speeds that make you travel for hundreds of thousands of light-years in merely 20 years ship time
> doable for "normal" speeds, not one where you accelerate 1G for twenty years It's still a lot of energy you can bleed off, particularly if you're aiming for a system with gas giants. I'm not suggesting one only rely on passive deceleration. But especially given it's fuel saved at the very end of the journey, fuel you no longer need to accelerate and decelerate for the entire duration of the trip, the savings could…
trying to decelerate by "braking" anywhere close to a gravitionally significant mass sounds like a guarantee to total destruction from the impact of "stuff" (even individual photons).
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…
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…
https://en.wikipedia.org/wiki/Tau_Zero
Classic hard sci-fi. A colony ship’s Bussard ramjet has a glitch, and it cannot stop accelerating—and basically forever, given time dilation.