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Old 07-31-2022, 03:27 PM   #11
Anthony
 
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Default Re: (spaceships) rocket (super)science

Quote:
Originally Posted by malloyd View Post
Hours. The speed of light is about 8500 G-hours.
Conveniently, very close to a G-year. For less cinematic spacedrives that's actually a sorta useful measure.
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Old 08-01-2022, 03:26 AM   #12
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Default Re: (spaceships) rocket (super)science

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Originally Posted by malloyd View Post
Hours. The speed of light is about 8500 G-hours, so 2.8 hours,
Yup, you're right. When i was doing my back of the envelope calculation I'm pretty sure I either dropped or added a multiplier or divisor by 60 (for 60 sec or 60 min) which turns hours into minutes.
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Old 08-02-2022, 01:46 PM   #13
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Default Re: (spaceships) rocket (super)science

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Originally Posted by jacobmuller View Post
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.
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Old 08-03-2022, 09:11 AM   #14
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Default Re: (spaceships) rocket (super)science

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Originally Posted by Johnny1A.2 View Post
* 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.
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Old 08-03-2022, 10:10 AM   #15
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Default Re: (spaceships) rocket (super)science

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Originally Posted by malloyd View Post
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.
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Old 08-04-2022, 12:07 AM   #16
Johnny1A.2
 
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Default Re: (spaceships) rocket (super)science

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Originally Posted by malloyd View Post
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.

Quote:

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'.

Quote:


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'.

Quote:

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.
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Old 08-04-2022, 05:05 AM   #17
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Default Re: (spaceships) rocket (super)science

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Originally Posted by Johnny1A.2 View Post
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'.
In a sense. But denying that there are levels of certainty misses much of the point of science. As far as we now know I cannot destroy the universe by sneezing while holding a green apple. That does not mean that sneezing should be taken seriously as an existential risk.

Edit: Actually better example: as far as we now know, the Earth is not flat. This is a much closer analog to the relativity case - not quite a strongly supported because the observations are necessarily only local, since the Earth is, but there is still a lot of evidence and strong connections to the rest of our physics in the theoretical reasons why it *should* be approximately spherical. Sure we could discover tomorrow that geometry doesn't work the way we thought it does and therefore the evidence is nonsense, but if we did, well "flat" is also a geometric concept, and would probably be rendered equally meaningless, and geometry would take most of mathematics, kinematics and astrophysics down with it.

I think this is quite closely analogous to the relativity case, which is fundamentally just the behavior of another coordinate system. If we found something FTL tomorrow the most likely reason would be time doesn't work the way we thought, and therefore velocity is a meaningless concept and putting events in chronological order is impossible. What kind of things would be possible in a physics where time ordering wasn't a real thing? I dunno, even that "would be" has to be called into question, but that's the order of magnitude of the issue - FTL is the least of it.
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Old 08-04-2022, 06:06 AM   #18
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Default Re: (spaceships) rocket (super)science

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Originally Posted by Johnny1A.2 View Post
But in their ways of conceptualizing the world, the movements of the sun and the clouds are irrelevant to what is possible for mortals.
I mean, if we ignore all the myths about mortals who do go that fast - such as Phaethon, who briefly drove Helios' chariot as it circled the world (he made a mess of things, because he wasn't able to properly control the horses, and wound up struck down by Zeus, but he did go as fast as the sun) - then sure. If we don't ignore said myths, of course, then we'll recognize that while they would have found a mortal traveling that fast to be amazing and exceptional, it wouldn't have been anything as world-shaking as someone going faster-than-light would be to us. For them, it would just mean this person had figured out how the gods managed such feats, or had learned/stolen from the gods how to do so (or been gifted such knowledge and/or artifacts that made it possible), making it just another notch in a long line of things like fire, architecture, medicine, booze, etc. But someone going faster than light - or creating a reactionless drive, or whatever - isn't something that would just fit in like that with our current knowledge, because it would violate what we thought were some of the fundamental laws of physics.
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Old 08-05-2022, 04:23 PM   #19
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Default Re: (spaceships) rocket (super)science

(Side question: what’s a “g-hour”? The amount of velocity you get from accelerating at one G for an hour?)
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Old 08-05-2022, 05:32 PM   #20
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Default Re: (spaceships) rocket (super)science

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Originally Posted by SydneyFreedberg View Post
(Side question: what’s a “g-hour”? The amount of velocity you get from accelerating at one G for an hour?)
Yes. 35,301.6 meters per second.
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