(spaceships) rocket (super)science

[Johnny1A.2]

Originally Posted by jacobmuller

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

It depends on your assumptions.

As far as we know now (emboldened because humility is important), the iron limit on travel time is the velocity of light. You just can't get there any faster than that (assuming causality is maintained and that relativity is the last word).

For interstellar travel, that means years, decades and centuries or more. No way around it (as far as we know).*

For interplanetary travel, the speed of light is not that big an issue. It only limits you to interplanetary trips in hours (or months if you want to go out into the cometary halo).

There is no law of physics that says you can't travel from, say, Earth to Saturn, in well under one day. It only requires energy sources and propulsion systems in excess of anything we know how to build.

The issue with postulating one-day trips across the Solar System is that if you have the energy resources to do that, then you can do a whole bunch of other things, too, that you might not want to deal with in a setting.

* I keep emphasizing the 'as far as know' because you can't know what you don't know. Imagine a group of Bronze Age thinkers trying to imagine travelling around the world in a few hours. It would seem as self-evidently impossible to them as FTL travel does to us. Many of them would not even be conceptualizing the world properly as they considered the matter (flat vs. round). Our own understanding of the universe might be equally erroneous, we can't know.

[malloyd]

Originally Posted by Johnny1A.2

* I keep emphasizing the 'as far as know' because you can't know what you don't know. Imagine a group of Bronze Age thinkers trying to imagine travelling around the world in a few hours. It would seem as self-evidently impossible to them as FTL travel does to us.

This is a popular take on this that is actually sort of false.

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

We on the other hand have a pretty good reason for concluding that things can't go faster than light (essentially the same one the Bronze Age guys have for concluding things you do now can't change the past) and few even apparent counterexamples - none that hold up to close scrutiny - despite different strong theoretical reasons for thinking we should have [lots] from all the quantum mechanical processes that should explore all possible states.

A lot of the difference is just difference in how interconnected the things we know are. Bronze Age facts largely aren't very connected - being wrong about the speed of the sun doesn't imply you are wrong about anything else. But being wrong about relativity basically breaks all of modern science and engineering in very central ways, and there is a [lot] more evidence it is at least broadly right (among other things it implies everything we know about electromagnetism is wrong, and yet generators and radios do work....)

There's a reason FTL proposals are always way out on the edges of known energies or require material properties we can't prove exist - if it could happen in the range of materials or energies we can regularly observe, we should have already seen evidence. One important corollary to that which I think deserves more attention is that if FTL were somehow possible, it is likely to be useless. We've can already see something of an example - hydrogen fusion - that's blindingly common in the universe but almost useless to us because it's so far from achievable conditions, FTL would presumably be much worse.

[Varyon]

Originally Posted by malloyd

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

I rather suspect Bronze Age thinkers recognized clouds were between them and the sun, given there would have been no known instances of the sun covering a cloud but plenty of clouds covering the sun, but certainly may have thought the difference in elevation was small enough that some clouds did indeed outpace the sun. There were plenty of myths involving gods, monsters, etc outrunning the sun at the same elevation as it, however, and many myths involved the sun being transported by (or itself being) a chariot, so the idea that it was possible to move that fast or faster was accepted (albeit considered rather exceptional). A man who had a machine that could do the same wouldn't be considered an impossibility - rather, it would likely be thought he must have received/stolen it - and/or the knowledge to create it - from the gods or similar entities.

[Johnny1A.2]

Originally Posted by malloyd

This is a popular take on this that is actually sort of false.

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

But in their ways of conceptualizing the world, the movements of the sun and the clouds are irrelevant to what is possible for mortals.

We on the other hand have a pretty good reason for concluding that things can't go faster than light (essentially the same one the Bronze Age guys have for concluding things you do now can't change the past)

Which is loaded with overt and hidden assumptions that we take on faith, because that's all we can do. The most science can ever say, as far as it can legitimately go, is 'as far as know now'.

A lot of the difference is just difference in how interconnected the things we know are. Bronze Age facts largely aren't very connected - being wrong about the speed of the sun doesn't imply you are wrong about anything else. But being wrong about relativity basically breaks all of modern science and engineering in very central ways, and there is a [lot] more evidence it is at least broadly right (among other things it implies everything we know about electromagnetism is wrong, and yet generators and radios do work....)

Which proves nothing about what is possible or not possible. The impossible always seems clearly impossible until it isn't, and there's no way to know what will hold up and what won't. It really doesn't matter how interconnected what we think we know is, as far as assessing it's reliability. We cannot go beyond 'as far as we know'.

There's a reason FTL proposals are always way out on the edges of known energies or require material properties we can't prove exist - if it could happen in the range of materials or energies we can regularly observe, we should have already seen evidence. One important corollary to that which I think deserves more attention is that if FTL were somehow possible, it is likely to be useless. We've can already see something of an example - hydrogen fusion - that's blindingly common in the universe but almost useless to us because it's so far from achievable conditions, FTL would presumably be much worse.

non sequitur. If it's possible at all, it means there's something basically wrong with out view of How Things Work, which means we can't make any meaningful predictions about how useful it will be or won't be. We're in the same position there that predictions about the utility of atomic energy once were. Or someone in 1930 pondering the precision of engineering necessary to create something like a modern processor chip.

For that matter, we can't say that fusion will remain useless to us with any confidence. Only that we can't do much with it (other than weapons) right now. It was known that flying machines and horseless carriages were possible long before they were practical, there was a long, long legacy of failed attempts before it all finally came together and worked. Ditto practical submarines.

[whswhs]

Originally Posted by johndallman

Smith did not go into details of just how this allowed travelling faster than light, but it certainly did in the Lensman series of space-operas.

As I understood it, the argument was that what stopped you from reaching the speed of light was that as you accelerated, your effective mass increased without limit, so that no amount of thrust could give you sufficient acceleration. Smith seems to have thought that if you didn't have mass (which was the effective result of inertialessness), you could just go on accelerating right past c. He doesn't seem to have thought about the fact that particles with rest mass zero such as photons travel at exactly c and cannot be further accelerated . . .

Technically, what was limiting your superluminal speed was that as you pushed forward, whenever you hit a particle, no matter how tiny, it stopped you dead. But your continuing thrust would then push you forward, accelerating the particle sideways. So this went on until your thrust exactly equalled the force you needed to shove aside all the particles in the local medium. Effectively you were experiencing the interstellar or intergalactic vacuum as a very tenuous fluid and overcoming its resistance. (It's not as simple as that, either, but that's a different story.)

[Fred Brackin]

Originally Posted by whswhs

Ac. He doesn't seem to have thought about the fact that particles with rest mass zero such as photons travel at exactly c and cannot be further accelerated . . .

T.)

Inertialess flight does not appear until the discovery of sub-etheric particles (and or radiations). These do travel faster than light and detection ssytems based on them are necessary for safe navigation (i.e. not blind) when traveling at FTL speeds.

I won't claim that the system is blasterproof but it covers more thna might be thought at first glance.