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One day, the earth suddenly gets covered by a spherical barrier that allows waves (light from the sun, radio, etc.) to pass both ways, but not matter. Let's ignore the feasibility of that kind of barrier existing and say that it's just there now. Anything with mass bounces right off as if it were an impenetrable wall. The barrier hovers at around 200km from the surface.

Will there be a noticeable change to atmospheric conditions on a timescale of around 1000 years?

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    $\begingroup$ One very notable effect would be our inability to to send satellites beyond that relatively low boundary $\endgroup$
    – M S
    Commented May 21, 2023 at 11:30
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    $\begingroup$ Rocket launches would become a lot less successful but a lot more...spectacular. Very bad news for the ISS crew. We'd also probably see bright flashes at night where space debris or meteorites hit the barrier. $\endgroup$
    – biziclop
    Commented May 21, 2023 at 14:35
  • $\begingroup$ Op, I hope you exclude relativistic mass like Photons posses from the shield $\endgroup$
    – Trish
    Commented May 22, 2023 at 6:54
  • $\begingroup$ Possible duplicate: worldbuilding.stackexchange.com/questions/138380/… $\endgroup$
    – Ryan_L
    Commented May 22, 2023 at 22:01

5 Answers 5

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Absolutely nothing.

From Wikipedia:

Atmospheric escape of hydrogen on Earth is due to charge exchange escape (~60–90%), Jeans escape (~10–40%), and polar wind escape (~10–15%), currently losing about 3 kg/s of hydrogen. The Earth additionally loses approximately 50 g/s of helium primarily through polar wind escape. Escape of other atmospheric constituents is much smaller.

Some math:

  • 3 kg/s would be 9.461×1010 kg in the span of 1000 years.
  • The atmosphere is 5.15×1018 kg.
  • 9.461×1010 / 5.15×1018 = 0.00000001837

The atmosphere would gain 0.000001837% mass.

Completely negligible.

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    $\begingroup$ well there is one thing, all aroura would cease. $\endgroup$
    – John
    Commented May 21, 2023 at 11:38
  • $\begingroup$ @John not technically: OP seems to exclude quasi-particles like Photons and also not cut magnetic or gravitational pull, and some of the Aurora is generated by EM alone $\endgroup$
    – Trish
    Commented May 22, 2023 at 6:53
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    $\begingroup$ Hello! For the uninitiated, it's hard to draw conclusions from the figures in your answer. So, we lose 3 kg/s of hydrogen. The OP's scenario would prevent us from losing these 3 kg/s of hydrogen. Would that have a huge effect or be completely negligible? You conclude "completely negligible" without any explanation and I think there is nothing obvious about this conclusion. I mean, maybe it's obvious to you based on your knowledge of geology and biology and chemistry, but it's not obvious at all for me, at least. $\endgroup$
    – Stef
    Commented May 22, 2023 at 12:22
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    $\begingroup$ @Stef, feel that this is completely clear. If the barrier started 1000 years ago and ended just before we sent out our first satellite, the difference between that Earth and our Earth would be undetectable. $\endgroup$ Commented May 22, 2023 at 16:23
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As for loss of Atmosphere...

The atmosphere weighs $\pu{5.148 \times 10^{18} kg}$.

Earth loses $\pu{3050 \frac{g}{s}}$ or about $\pu{96.24\times 10^{6}\frac{kg}{year}}$. Or about a fraction in the ballpark of ${2}\times10^{-11}$ of its mass is lost per year. In other words: nigh nothing compared to the mass of the atmosphere is lost.

Stopping that neglectible loss will have neglectible effect. To even be noticeable as a 1% difference from the predictions, earth will need to run on about $10^9$ years. Remember that earth is only about $4.5\times10^9$ years old.

Non atmosphere side effects

200 km is well below the common Low Earth Orbit, and thus almost no new satellites can be put up, especially nothing geostationary or with sensible retention times.

The ISS is at a 413-422 km orbit, and thus will be cut off.

Meteorite impacts and debris will stop at the 200 km line, changing the pleiads and others from straight shooting-stars into ones with a bend.

If it does not ignore relativistic mass...

Now, there is one thing that we need to ignore for the above to be true: Relativistic mass. As long as we ignore that light can behave like a particle that has a mass, everything is hunky-dory and earth is fine.

The moment that the shield does not ignore relativistic mass, things get bonkers: the shield turns the Albedo of the planet from something like 0.3 to 1. That means not 30% of the light falling in is reflected to the stars, 100% are. That means multiple things:

  • Earth's sky now is dark. Welcome to a sunless, starless world where nothing grows and everything will equalize its temperature to about the average of the planet's temperature over astronomical time scales.
  • Because Gravity is not traveling but existing as a potential field, we'll still feel the pull of the sun and Moon and can measure the tides.
  • We stop being able to communicate with our satellites already out there. We couldn't send any out there anyway due to the shield, but we can't call them anymore either if it does not ignore light. Or see them, see above.
  • We suddenly can see beyond the curvature of the planet! Light and radio waves will be bounced from the shell back to Earth. In the right conditions, you can even see yourself looking away from you reflected around the planet once... provided that you stand under a streetlight.
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  • $\begingroup$ This brings up an interesting consequence of such a shield though: no new satellites (even if it ignores relativistic mass). That may mean little in the short term, but it would certainly restrict technological advantages in that area as satellites eventually start failing one by one without replacement. And what happens if all that scrap crashes into the shield and sits there? $\endgroup$
    – Katai
    Commented May 22, 2023 at 7:53
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    $\begingroup$ @Katai: As for crashing into the shield and sitting there... it's not much more different than being in orbit and sitting there, so I would say "nothing unusual". Given the surface of the shield, I'd also expect that even if comets were to "stick" to it, "nothing unusual" would happen. $\endgroup$ Commented May 22, 2023 at 15:09
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    $\begingroup$ @thegreatemu which is irrelevant for the point I was trying to make. $\endgroup$
    – Trish
    Commented May 22, 2023 at 22:36
  • $\begingroup$ The amount of incoming material is not as negligible as the amount of atmospheric outgassing. In a few millenia, the shield and the dust on top of it would become the new planet surface, probably inhabited by bacteria, fungi, arthropodes... some hitchhickers go really far. $\endgroup$
    – Oxy
    Commented May 23, 2023 at 7:24
  • $\begingroup$ @Oxy not within the specified timeframe of just 10000 years. $\endgroup$
    – Trish
    Commented May 23, 2023 at 8:10
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Major volcanic eruptions could be noticably worse

On the day-to-day of things, you'd not notice much difference, but anytime a major volcano erupts somewhere in the world, the climate impact it has would be increased. For example, the recent Tonga volcano eruption is reported to have sent about 8% of its mass ejection off into space. This % only goes up as a volcano becomes more powerful; so, the bigger the volcano, the bigger difference this shield will make. For example, the Mount Tambora eruption in 1815 was about 15-20 times as powerful as the Tonga eruption. As it was, this eruption caused the "year without a summer", but had there been a shield like this, it could have possibly doubled the intensity of the volcanic winter.

In the past 1000 years, there have been 8 eruptions on Earth big enough to cause noticeable climate impacts that would have been made at least 10% worse by this shield. So, most of your next 1000 years, would be unaffected but within that time period, you'd have a few bad years where it makes a significant difference, and probably at least 1-3 where things would get really really bad for anyone unfortunate enough to be alive at the time.

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There would be very little effect upon Earth itself as others have pointed out. All existing satellites would eventually cease to function and would not be able to be replaced so no more space exploration and GPS, Satellite telecommunications, Earth observation and more would be ended or very seriously disrupted.

On a more positive note there would (presumably) be no danger of a civilization destroying asteroid impact.

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  • $\begingroup$ We can replace the lost sattelite capacity with weather balloons that go all the way up to the shield and stay there, or maybe we can bolt them into the shield on the inside? $\endgroup$
    – Jacques
    Commented May 22, 2023 at 12:49
  • $\begingroup$ Can we? The current balloon record is just 52km. $\endgroup$
    – Slarty
    Commented May 24, 2023 at 16:36
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The escape layer of the atmosphere is several thousand kilometers up, which is many times as high as the shield at about 200 km.

So the atmosphere above the shield would continue to slowly escape into space, but the shield would stop gas from lower down from moving up to the upper atmosphere. Thus the atmosphere above the shield would diminish faster than it does now. However, the slow rate of atmospheric loss should mean that will not be a problem in the long range, and certainly not during the time scale of most stories.

Micrometeoroids burn up lower than the shield, so people would no longer see meteor trails in the night sky. Instead space dust would hit the shield and be shattered or even vaporized by the impact, or possibly merely sit on the shield. Larger space rocks and asteroids would also be stopped by the shield, perhaps exploding spectacularly.

Since dust particles are opaque, the higher the percentage of the shield's surface which was covered by dust particles, the lower the percentage of the sunlight hitting the shield that would reach the surface of the Earth. I expect that dust buildup on the shield would probably be far too slow to be an important factor in the story.

The ionosphere (/aɪˈɒnəˌsfɪər/)[1][2] is the ionized part of the upper atmosphere of Earth, from about 48 km (30 mi) to 965 km (600 mi) above sea level,[3] a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth.[4] It also affects GPS signals that travel through this layer.

https://en.wikipedia.org/wiki/Ionosphere

So about a fifth of the Ionosphere would be inside the shell and about 4 fifths would outside the shell. If being outside the shell caused major changes in the Ionosphere it could affect long distance radio on Earth. But I don't think changes in the ionosphere would happen fast enough to affect the story, unless you are planning to write a multi billion year future history like Stapledon's Last and First Men.

So far I have not been able to think of any changes to the atmosphere that the shell at 200 kilometers would make happen fast enough for the purposes of a story. But if an expert in atmospheric physics can think of a change which would happen fast enough to be important for a story, they should mention it in their answer.

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  • $\begingroup$ Only by making the shield reflect anything (including light) you get any changes that make it worth writing about... I mean, besides stopping any space exploration. $\endgroup$
    – Trish
    Commented May 22, 2023 at 18:43

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