How We Knew Space Was a Vacuum (2021)
sky-lights.org
sky-lights.org
I don't know when this site was first made, but its design looks like the 90s web and I love it.
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.
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?
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.
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.
If you use a strong pump to depressurize and seal a glass bottle you still get a bottle with air inside; you can easily see that there is air inside because if you submerge it in water or oil and break the seal you will see air bubbles coming out.
If you were to try the same with a glass bottle full of water, you will find that is is much much harder to pump the water out than is it to pump the air out.
This is perfectly compatible with the impossibility of an actual vacuum.
The experiment that subverted this view had to use mercury to be practical at the time[1]
In this experiment you essentially use the weight of mercury to pump mercury out of a glass bottle and since air cannot traverse mercury nor glass [2] then it must truly be a full vacuum. [3]
[1] https://en.wikipedia.org/wiki/Torricelli%27s_experiment
[2] This is false, mercury evaporates, quantum tunneling exists, gasses likely can be dissolved in mercury and material are porous.
[3] In some sense you could say that the photons in the bottle make it not a vacuum, as they have mass or also that particles spontaneously appear from a base energy noise or a lot of other oppositions to that being a real vacuum. Still it is definitely a different thing from just low pressure air.
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.
The same with hot air balloons, there is no vacuum at play there, unless I'm miss-understanding what you're saying.
Just like future readers might shake their head “how did they not know warp drives work like this?”
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...
> 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".
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.
I think you got this upside-down. The Kármán line is named after an engineer called Theodore von Kármán 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 Kármán 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 Kármán 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.
or you could increase the size of the wings?
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.
... 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.
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.
If the earth were revolving in an atmosphere and air resistance was a thing, we’d have spiraled into the sun by the Cretaceous.
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.
:-)
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.
> (laws of physics would be have to be different obviously)
"Once the rockets are up, who cares where they come down?
That's not my department!" says Wernher von Braun.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.
The use of 'we' is fine in many cases - when talking about a holiday, I can say 'we went to the beach' and that is understandable because of the context.
Its not fine to talk about 'we' in the context of something that is subjective, eg understanding or knowing. It is an assumption to say 'we know' unless you have checked all the people you are referring to really do know.
If you think this usage is acceptable - and apparently most people do - I'd love to hear an explanation. To me, it is sloppy and confused thinking, and actually it's use is ultimately a type of casual lie, because it overstates whatever-it-is.