Blocking microwave emissions from earth to outer space and vice versa

[kos]

We all know what would be the effect if we block emission of infrared radiation from earth. Actually we call this global warming . But let imagine a hypothetical situation that because of some strange reason someday someone decide to build a shield and wrap up entire earth in some more distant orbit even than that of the moon. The cage around the earth would be from some exotic material allowing the passage of all other electromagnetic spectrum except for microwave region. So in this scenario what would be the consequence for our planet and life eventually ?

[pzkpfw]

You need to explain what the shield does with the energy received, that it's blocking from further transmission.

[kos]

Change it into electricity

[pzkpfw]

And then hide the electricity in a sock?

[Enthalpy]

The material wouldn't be so exotic. A bunch of nonconnected conducting wires of proper set of lengths should do it on Earth. It's more a matter of amount...

 

Change to electricity: sure, make DC current with rectifiers at the antennas, since they work at microwave frequency. The absorption can't be perfect, but only for technological reasons. The field strength is expectedly too small for usual rectifiers, but again I'd say not for physical reasons - as long as the received power clearly exceeds kT. And with the DC power, emit light to some direction that is neither Earth nor the shield.

 

An easier way would convert the microwaves to heat radiated away as far infrared.

 

And then?

 

Our microwave emissions are insignificant to the thermal balance of the planet. Do whatever you want with them.

 

For the thermal emission of microwaves, the theoretical feasibility of the shield gets difficult. It depends on if you want to reflect the waves (works) or absorb them (the absorber emits microwaves too, necessarily). But in every case, the effect on Earth's heat balance is negligible since bodies around 288K radiate mostly around 10µm, and if one band is blocked, the heat passes over the others.

[Airbrush]

Blocking electromagnetic radiation from a planet may be a strategy of an advanced ET civilization that has realized that they have nothing to gain my announcing their location in space to all listeners. The reason is they don't want their nice, habitable planet to be taken over by a more advanced civilization that is predatory. I mean "predatory" not in the sense the more advanced civilization would want to EAT inhabitants on a certain planet, just wipe them out and take custody of their planet. Like what Stephen Hawking said that when Europeans discovered America, that was a disaster for the native Americans. For a predatory, more advanced ET civilization, looking for a planet to inhabit, and such habitable planets are very rare, they would probably not hesitate to destroy the inhabitants on the planet they desire.

Edited April 12, 2016 by Airbrush

[kos]

Finally can we be unclosed in some box around the earth with suspisious of noone . Is there a way to make this kind of cage around the globe without anyone ever notice it? What are constrains here. Gravity,turbulence,tidel forces due to earth movement . .. ?

[Enthalpy]

So we'd block the man-made radiowaves? Such a shield is made from a metallic fabric, for instance at Faraday cages, and lets much (enough?) light through.

 

I feel difficult to let the shield orbit the Earth if it must cover all directions. Several layers would be needed, and it gets quite complicated. But one single part would be badly difficult too, almost as difficult as a space elevator, because it must resist its own weight. The cage would also need active steering with thrusters to stay centered on Earth instead of falling.

 

Then you have to check the amount of matter. A huge enterprise for Mankind was to put wires to many homes, but they don't cover the whole area between the homes, and they run densely in towns and cities, not in countryside, deserts and oceans. Covering all Earth with metal wires isn't within our reach.

 

The construction high there is a problem. Shield elements won't stay where we put them because of their weight, but we can't give them the orbital speed neither.

 

Plus more objections. In short: impossible for Mankind presently.

 

If really we wanted to stop transmitting radiowaves, we could. Connecting more homes to fiber optics is under way. Other communications, say with aeroplanes, could use light beams. Then we shut off all transmitters. All unintented radiations can be reduced as much as needed. This is within graps if badly necessary.

[kos]

Why metalic carbon nanotubes or greaphen is said to block perfeclty microwaves

[swansont]

Finally can we be unclosed in some box around the earth with suspisious of noone . Is there a way to make this kind of cage around the globe without anyone ever notice it? What are constrains here. Gravity,turbulence,tidel forces due to earth movement . .. ?

To what end? Having the emission from the earth being a conspicuous deviation from a blackbody would be a blatant announcement that it's inhabited by beings with some technological ability.

[kos]

Thought experiment. Let's suppose This is for example China no matter. So where is the problem to pack itself some of theese three ways. In the last one shield is managed by some veichules in the space. Shield is from carbon nanutubes. It's light

[Enthalpy]

Who said graphene or nantubes block microwaves perfectly? At least I didn't.

 

We can make masts to carry a shield, or at least their strength can carry their own weight. The best choice today is graphite composite, with optimistic 1550MPa compression strength and 1550kg/m³ density, so at 10m/s² its section multiples by e every 100km. To reach 300km (or is there a better choice?) the basis must have a 20x bigger cross-area than the tip. The shield can then weigh as much as the truncated part of the masts.

 

Then, there are some worries and limitations, for instance with the wind, the cost, and so on.

 

Pulling the shield under rockets would not be possible. The best chemical rockets offer isp=470s, which means that the propellants can lift their own weight for 8 minutes only. Ion engines do it for 200 hours.

[kos]

Great answer I just couldnt catch the last part about the energy. Why not with iin engines so if they are better

[Enthalpy]

Reaction engines must eject mass, here downwards, to achieve a pull, here lifting the shield. Though, the ejected mass must first be carried by the rocket engine. Ion engines are about the least bad and they're not up to the task.

 

Let's say the engine expels ions at 720km/s. If it uses 1kg propellant, it achieves 720,000 N*s, and if adjusting the thrust to 10N to lift this 1kg, it can hover for 72,000s or 200h. If it uses 1t propellant, it achieves more N*s, but since it needs more force to lift the heavier propellant, it still hovers for 200h at most. (OK, the mass diminishes over the time, it would be an exponential, blah blah who cares). That's why we introduce this "specific impulse", which characterizes the propellant and engine independently of the propellant amount: here it's 200h.

 

After 200h, the shield would fall back to Earth, impractical. Since this is for the best available rocket engine, I consider this option is out.

 

Not that masts would be practical... I only checked that for 300km height they can carry their own weight.

[kos]

And what ifmengines are constantly updated!!?

Till last ones are exhausted new ones are coming

[kos]

Constantly changing vheicules that grasp thr shield ?

[aliceinwonderland]

I'd like to add that I think another constraint would be constant damages to the shield. Smaller meteoroids fall down on earth every day. A shield all around the earth would sooner or later be damaged by those meteoroids. So, who would be able to repair and maintain the shield so it keeps working? The ISS for example has radar and can avoid larger ones, but a global shield can't go anywhere and in the battle shield vs. meteoroid the shield won't win.

[kos]

What about the static shield gonna produce. Maybe around it there would be cloud from dudt particles. Or the particles will stick on the walls of the shield !

[kos]

I did the math and I calculated that for a case our shield being stationary it should be pushed by engines in an opposite direction of this of the wind by the averege net force 1x10^8 N . if we assume that our shield is verry light and its area is 100 x 20 sq km of one of its walls and the average wind speed is 3 m/s !

If it is from somemetamaterial absoulutly transperant could it be absoloutly invisible and undetectable for the casual citizen ? What are the issues?

[Enthalpy]

I wouldn't put such a shield in the atmosphere but above it, because of the wind.

 

The "casual citizen" doesn't exist. Humans communicate, are fantastically gifted, people are 40dB more clever than politicians, so anything will be noticed. Even if someone thought to have checked all possibilities, other people will find something he hadn't thought at.