[HN Gopher] How We Knew Space Was a Vacuum (2021)
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How We Knew Space Was a Vacuum (2021)
Author : susam
Score : 114 points
Date : 2023-03-11 10:08 UTC (1 days ago)
(HTM) web link (sky-lights.org)
(TXT) w3m dump (sky-lights.org)
| [deleted]
| [deleted]
| celeritascelery wrote:
| How weird would it be if we lived in some alternate reality where
| space was filled with air (laws of physics would be have to be
| different obviously). You would be able to fly a plane to mars.
| HPsquared wrote:
| It's not exactly air, but there is still some "stuff" filling
| the solar system.
|
| https://en.m.wikipedia.org/wiki/Interplanetary_medium
| Karellen wrote:
| I think you air-quoted the wrong word in that sentence. I
| think it would be more appropriate to say: there is still
| some stuff "filling" the solar system.
|
| :-)
| KineticLensman wrote:
| Bob Shaw's SF novel "The Ragged Astronauts" plays with this
| idea. A major plot point is a journey to a neighbouring planet
| - by balloon. The novel indicates that the laws of physics are
| different, e.g. a scientist character indicates that p = 3
| (IIRC).
|
| [0] https://en.wikipedia.org/wiki/The_Ragged_Astronauts
| zabzonk wrote:
| unfortunately, i think the earth would burn up, due to
| compression and friction
| epolanski wrote:
| Hence he wrote:
|
| > (laws of physics would be have to be different obviously)
| kibwen wrote:
| On the other hand, it might be a cold, lifeless rock, since
| the sun can already _barely_ penetrate a tiny sliver of
| atmosphere, as sunsets demonstrate.
| nottathrowaway3 wrote:
| We would live suspended in a great universal "ocean" and would
| navigate the heavens via submarine. Of course, it would be
| easier for other creatures to swim between planets as well...
|
| I wonder if advanced conscious life would be possible in this
| universe given that the number of isolated "buckets" is so much
| smaller. A lot of anthropic arguments would break down in such
| a universe.
| justsomehnguy wrote:
| It's quite amusing what this is the second time this week when
| I recommend 'Exhalation' by Ted Chiang.
|
| [0] https://archive.org/details/ExhalationByTedChiang
|
| [0] https://www.lightspeedmagazine.com/fiction/exhalation/
| zxexz wrote:
| In that case, we would perhaps still be a millennia from the
| technology required to get there. Imagine the fuel required to
| get to Mars with air resistance! The materials that would be
| required to keep the plane from being destroyed by the heat and
| vibrations from the speed for such a duration!
|
| The heat dissipation required would presumably be immense with
| that constant acceleration due to air resistance.
|
| I think we can be glad that space is a hostile vacuum.
| andrewflnr wrote:
| OTOH, sailing might be possible. Odds are against the
| atmosphere being perfectly still, certainly.
| snek_case wrote:
| It would just take too long. Modern space capsules travel
| at kilometers per second, and it would still take months to
| reach Mars. If were to travel there a 30km/h it would take
| centuries to get there.
|
| ... This is all ignoring the fact that orbital mechanics
| just wouldn't work if there was air resistance. The planets
| would quickly slow down and crash into the sun.
| andrewflnr wrote:
| Right, of course the real answer is that none of this is
| possible, at least not just replacing the interplanetary
| atmosphere without adjusting anything else for
| consistency.
|
| But imagine this: a breathable nitrox atmosphere co-
| orbiting with the planets. Each planet is roughly at rest
| with respect to the local atmosphere, so no drag. Just
| lots and lots of wind shear. Probably planetary scale
| vortices. That'll sap orbital energy into heat, so
| probably not stable, but pilots can probably exploit it
| to acheive velocities comparable to orbital velocities
| over time. I think you'd probably have to have two sails
| connected by a tether, sufficiently far that their air
| streams are at significantly different speed, with one of
| them acting like a keel.
| hinkley wrote:
| That Mark Twain story about the flying boat comes to mind.
| numpad0 wrote:
| Can't we just fly a U-2 all the way up and out of Earth SOI,
| then drift towards Mars in that situation? Do we still need
| to establish an orbit?
| Jeff_Brown wrote:
| If the air had no currents, the steady state would be
| motionlessness except gravity effects _. If it had
| currents, the steady state would be to follow them.
|
| _ Of course those gravity effects would also apply to the
| air, which makes the scenario inconsistent. I'd be
| interested to see a formulation of the laws of physics that
| actually makes a substantial interplanetary atmosphere
| possible.
| hinkley wrote:
| You'd have to have different physics.
|
| If the earth were revolving in an atmosphere and air
| resistance was a thing, we'd have spiraled into the sun by
| the Cretaceous.
| mannykannot wrote:
| Obligatory Tom Lehrer: "Once the rockets are up,
| who cares where they come down? That's not my department!"
| says Wernher von Braun.
| twawaaay wrote:
| > At 100 km most of the atmosphere (99.9999%) is below you. 100
| km also marks the Karman Line (shown in the main graphic),
| generally considered the "official" start of outer space. Above
| this altitude aerodynamic flight is no longer possible. Wings are
| useless. Only rocket thrusters and gravity can affect your
| motion.
|
| I would like to nitpick here a bit.
|
| 100km is just "administrative" end of atmosphere. The atmosphere
| does not really have an end. If there are any hard, measurable
| boundaries it would be where solar winds meet our atmosphere
| (kinda like discussion about when did Voyagers leave Solar
| system). But this boundary does not have defined shape, moves a
| lot due to solar wind or Earth magnetic field situation.
|
| And we do actually use our atmosphere above 100km to "affect
| motion". We place satellites in LEO well above 100km to assure
| they will eventually fall to Earth.
|
| And anybody who played KSP knows how useful it is to skim the
| atmosphere for aerocapture...
| whywhywouldyou wrote:
| Nothing you said contradicts anything the article said. What
| are you nitpicking, exactly?
|
| > The atmosphere does not really have an end.
|
| Article clearly doesn't state it has an end. They said 99.9999%
| is below you. They then say the Karman Line is considered the
| "official" (IN QUOTES) start of outer space, implying that this
| is a somewhat arbitrary definition.
|
| > And we do actually use our atmosphere above 100km to "affect
| motion". We place satellites in LEO well above 100km to assure
| they will eventually fall to Earth.
|
| The article clearly stated "affecting motion" in relation to
| aerodynamic flight ("aerodynamic flight is no longer
| possible"). Wings are useless to generate lift at those
| altitudes. I wouldn't classify falling to Earth as "aerodynamic
| flight".
| proto_lambda wrote:
| > Wings are useless to generate lift at those altitudes.
|
| They aren't, though. They still work, just much less well.
| There isn't one altitude where wings suddenly stop working
| entirely - there is however an altitude where generating
| enough lift to sustain flight would require going so fast
| that you're already in orbit (regardless of lift), according
| to wikipedia it's somewhere around 80km.
|
| The Karman line has nothing to do with lift, it's just an
| arbitrary, round number.
| 123pie123 wrote:
| >there is however an altitude where generating enough lift
| to sustain flight would require going so fast that you're
| already in orbit
|
| or you could increase the size of the wings?
| krisoft wrote:
| > The Karman line has nothing to do with lift, it's just an
| arbitrary, round number.
|
| I think you got this upside-down. The Karman line is named
| after an engineer called Theodore von Karman who studied
| the problem of how high aeronautical flight is possible
| theoretically. That has everything to do with lift, and
| speed an orbiting body needs to stay in orbit. That is the
| Karman line.
|
| Then yes of course organisations need clear and bright
| lines for things like "when do we give someone an astronaut
| badge?" For that different organisations come up with nice
| round threshold numbers. And those numbers have nothing to
| do with lift. But those numbers are not the Karman line.
| They are just convenient approximations of it for
| administrative purposes.
|
| It's like if a country's tax authority declares that for
| tax purposes you shall calculate the area of a circle using
| the first 5 digits of PI. That doesn't mean that suddenly
| the PI become not a transcendental number. It is just an
| approximation for practical reasons.
| abecedarius wrote:
| > 1643 when Evangelista Torricelli invented the mercury barometer
|
| iirc Tycho Brahe needed to make careful allowances for
| atmospheric refraction many decades before this. Also iirc a
| rougher notion of this effect goes back to Ptolemy.
|
| I guess in thinking the following I'm just showing my lack of
| historical imagination, but: watching a sunset, and knowing it's
| at a distance many times the size of the earth, you'd think there
| must be _something_ attenuating light which forms a layer much
| thinner than the size of the earth. This comports with a view of
| distant mountains vs. ones nearby, versus the sharp outline of
| the moon. You might still be very unsure whether this factor
| might include the air itself, and not just dust and vapor. If
| anyone wrote about this early on, I haven 't heard of it.
| teraflop wrote:
| You have to bear in mind that until the 17th century or so, the
| philosophical doctrine that "nature abhors a vacuum" seemed to
| be backed by very reasonable evidence: namely, the fact that it
| was basically impossible to actually create a vacuum through
| any technological means available. And until we had basic
| theories of gravity and the properties of gases, there was no
| theoretical justification for how a vacuum could form
| naturally.
|
| So it makes sense that to most thinkers of the time, it was
| more plausible for space to be completely filled with some kind
| of perfectly transparent but substantial aether.
|
| You raise a very interesting point, though. I would be very
| interested to learn what ancient philosophers thought about
| atmospheric haze. Did they recognize that the bluish appearance
| of distant mountains and the reddish appearance of
| sunrises/sunsets had a common cause? And if so, did they
| associate it with "air" as an elemental substance, or something
| else?
| flavius29663 wrote:
| > the fact that it was basically impossible to actually
| create a vacuum through any technological means available
|
| Do you have more sources on this? I would like to read up. As
| far as I'm concerned, vacuum is trivial to produce: by
| putting your palms together and then pulling them apart, by
| using blacksmith's bellows, and a lot of devices that used
| leather and/or bladders. I think the problem is in
| recognizing that you created vacuum, rather than creating it,
| so I would like to know if you have more insight into why you
| said that.
| maweki wrote:
| > by using blacksmith's bellows
|
| Creating a pressure differential where air flows in
| immediately. As vacuum seemingly cannot be sustained
| without constant work, it seems that nature does indeed not
| like vacuums.
|
| > vacuum is trivial to produce
|
| But not trivial to sustain. And it would be a sustained
| vacuum if it were between the earth and the moon or sun.
| flavius29663 wrote:
| you can sustain it if it's properly sealed, but I see
| your point, people would have a hard time understanding
| why the outer space would apparently sustain such a large
| amount of vacuum. It seems unnatural to us, because we're
| so used to atmospheric pressure that we're taking it for
| granted.
| prox wrote:
| This is the key take away. As a modern human you
| obviously reason "how they hell could there NOT be
| vacuum?" , but in those times you had islands of
| knowledge (no email, mail if you are lucky) and you
| always reason from what you know at that point.
|
| Just like future readers might shake their head "how did
| they not know warp drives work like _this_?"
| seiferteric wrote:
| I also thought of several examples; breathing in/out of a
| paper bag, hot air balloon, or that experiment where you
| put heat up or metal container and put the opening on
| water and it gets sucked in (or crushes a thin can). I
| can see that it takes work to sustain these, but what
| about just putting a glass bottle in your mouth and
| pulling a vacuum and carefully putting your thumb on top.
| You can create a slight vacuum in a glass bottle this way
| and it will stay there until you move your thumb and hear
| the "whoosh". But I guess it still "wants" to fill the
| void as soon as you unplug it, so I guess I understand.
| abecedarius wrote:
| That kind of effect was studied and used in antiquity, as
| pneumatics. Aristotle's point of view did not hold sway
| everywhere or for all of that period; afaik pneumatics
| started with Empedocles and the clepsydra, and obviously
| the atomists favored the vacuum.
|
| I've kept out of this conversation because I'm too
| ignorant, but it seems worth saying that while "it
| doesn't fit with Aristotle's scheme of the world" may be
| a good enough explanation for most premodern western
| thinking, it's hardly all of it.
| flavius29663 wrote:
| How does breathing in and out of a paper bag create
| vacuum? As soon as you inhale, the atmospheric pressure
| collapses the bag. Bellows are different because they can
| hold their shape: if you seal all the ends and then
| expand the bellows, you will have vacuum and the 1atm of
| pressure trying hard to stop you from doing that.
|
| The same with hot air balloons, there is no vacuum at
| play there, unless I'm miss-understanding what you're
| saying.
| seiferteric wrote:
| Another interesting one is when we "knew" how the sun worked. Of
| course we didn't know about nuclear reactions until the early
| 20th century, so it makes sense we didn't figure it out for a
| while after that. But before that it was a complete mystery!
| Scientists had calculated how much energy would be required to
| produce the amount of light we see and it seemed impossible given
| the estimated mass of the sun, for any know physical method to
| produce it (chemical, gravity compression). Some thought it was
| made of coal (sign of the times I guess) but that would of course
| require oxygen to react with... And would have only lasted a few
| million years I think...
| sebzim4500 wrote:
| By my inexpert calculations, if we ignore the requirment for
| oxygen (because I don't know what ratio you need) the sun would
| only last 4000 years.
| seiferteric wrote:
| Actually ya your right, searching for it, they thought it
| would only last about 5k years. Actually they also thought it
| might be meteors falling in may add more energy that might
| allow to last 100s of millions of years...
| https://www.scientificamerican.com/article/experts-doubt-
| the...
| verisimi wrote:
| I hate the use of 'we', as it is being used in the title, the
| question and the answer.
|
| Put simply, 'we' cannot know anything. Knowledge only exists in
| the mind of an individual. No one is able to say what another
| knows. And unfortunately, most people don't even know the
| difference between knowing and believing!
|
| At best, use of the term 'we know' is a shortcut to saying 'some
| scientists claim that'.
|
| I can only know that claim to be true once I have verified it for
| myself. Until that time it can only be a belief or hypothesis.
| Jeff_Brown wrote:
| I don't think anyone is in danger of being confused by the
| imprecision.
| citrusynapse wrote:
| And within epistemology and your process of "verifying" there
| are hundreds of contemporary schools of thought from
| Wittgenstein to Chbosky.
| jedberg wrote:
| I miss web pages from the 1990s.
|
| I don't know when this site was first made, but its design looks
| like the 90s web and I love it.
| prox wrote:
| It's the frames type of design, left an outline, right content.
| jedberg wrote:
| Yeah, that's probably it. Late 90s, early 2000s, we all used
| these magical _frames_.
| [deleted]
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