How many watts of energy would it take to vaporize an entire planet?

[steve_v]

Just now, daniel l. said:

Now, my idea assumes that planet's have lost relevance as centers of population. And that they are only useful as massive ships. The majority of humanity lives in Dyson Sphere's/Swarms which can hold quintillions of people and be extremely difficult to destroy (Unless of course you somehow forced the explosion of the central star, which would be difficult if it was too small to go supernova naturally.)

Then you're so far into the realms of future fiction that a planet-killing laser is easy, you probably don't need to explain it at all. In fact you'll have more problems explaining why they're not available at the corner store.

[daniel l.]

Just now, steve_v said:

Then you're so far into the realms of future fiction that a planet-killing laser is easy, you probably don't need to explain it at all. In fact you'll have more problems explaining why they're not available at the corner store.

Perhaps, though as I've said, there is nothing more required here than simple labor. If we had a self replicating swarm of absolutely obedient drones (which we pretty much already have to ability to make.) then the creation of objects is merely a matter of time, not effort. You would only need to send one drone to a rich resource area to start the process, and soon you could have an entire dyson swarm waiting around a nearby star for human inhabitants to arrive.

[steve_v]

12 minutes ago, daniel l. said:

Perhaps, though as I've said, there is nothing more required here than simple labor.

Ehem:

23 minutes ago, Hannu2 said:

This would also need energy to break the gravitational binding of the planet.

Just "simple" labor, right?

Building things at the kind of scale you're suggesting will get pretty complicated, and gravity is going to be a real a pain in the ass.
If you hollow out a planet, what's going to keep it planet-shaped once you remove all that mass? Material strength gets progressively more problematic as you up the scale.
How do you keep all that matter you excavated from collapsing back into a planet-shaped ball again?

Edited April 12, 2017 by steve_v

[daniel l.]

Just now, steve_v said:

If you hollow out a planet, what's going to keep it planet-shaped once you remove all that mass? Material strength progressively more dicey as you up the scale.

If you were to take material from the core and use it to build support struts as you go. Then follow it up by using the leftover cutout core material to build the machines necessary for the function of the planet/ship.

1 minute ago, steve_v said:

Just "simple" labor, right?

If I like rubber duckies, then what scientifically stops me from building one the size of Burj Khalifa?

[tomf]

31 minutes ago, daniel l. said:

 

If I like rubber duckies, then what scientifically stops me from building one the size of Burj Khalifa?

Rubber.

Rubber/plastic isn't nearly strong enough to build something on that scale without it ending up a a massive pile of yellow rubble*. Even if you had a perfect arrangement of rubber supports on the inside it's just going to get crushed like a lego brick in a hydraulic press.

By the way, I once calculated that the energy released by proton-proton fusion of all the hydrogen in the earth's oceans was almost exactly the same as the binding energy from the earth, so if you are willing to sacrifice all the water of a planet that could provide your power source.

* I suspect that the exact failure mode is going to depend on how you try to build your duck, a giant flattened deflated duck is likely if you try a simple scaling. 

[HebaruSan]

10 minutes ago, daniel l. said:

If I like rubber duckies, then what scientifically stops me from building one the size of Burj Khalifa?

Scientifically, if you try to make it a literal scaled-up rubber ducky, it'll probably collapse under its own weight, because the mass will increase as the cube of the scaling, whereas the structural strength only increases as the square. You would have to use more and more non-traditional rubber ducky materials like steel, until what you had bore more resemblance to a skyscraper than a rubber ducky.

• http://galileoandeinstein.physics.virginia.edu/lectures/scaling.html
• https://physics.stackexchange.com/questions/72641/why-is-there-a-size-limitation-on-animals

[daniel l.]

Just now, HebaruSan said:

Scientifically, if you try to make it a literal scaled-up rubber ducky, it'll probably collapse under its own weight, because the mass will increase as the cube of the scaling, whereas the structural strength only increases as the square. You would have to use more and more non-traditional rubber ducky materials like steel, until what you had bore more resemblance to a skyscraper than a rubber ducky.

• http://galileoandeinstein.physics.virginia.edu/lectures/scaling.html
• https://physics.stackexchange.com/questions/72641/why-is-there-a-size-limitation-on-animals

What about simply filling it with a high enough air pressure to counter gravity?

[Hannu2]

30 minutes ago, daniel l. said:

If you were to take material from the core and use it to build support struts as you go. Then follow it up by using the leftover cutout core material to build the machines necessary for the function of the planet/ship.

If I like rubber duckies, then what scientifically stops me from building one the size of Burj Khalifa?

Simple scaling do not work in engineering. There are very fundamental problems. For example, if you increase size of something by a factor of 10, all volumes and masses increase by factor of 1000 and areas by factor of 100. If you have a strut, it must carry 10 times more mass per area. You have to change material. It affects also to energy production, flows, hydrodynamics and practically everything. If you scale your rubber duck to a kilometer size, it can not carry its own weight and is flat layer of rubber sheet on floor. That is the reason why there are arbitrary small or large machines. You can make them, but they do not work.

Material strength is another problem. There are limited number of different atoms in our universe and bonds between them are known and they depend on electromagnetic interactions. They have limited strength. Therefore materials from which you can make hollow planets (or strut them) do not simply exist. You have to postulate whole new type of particles and physical interactions between them to get things which can strut hollow planets or store and handle energies of planet vaporizing lasers or act as rocket engine which moves planets. On that level you can say anything. In my opinion it would not be interesting scifi.

Real planets are practically liquid spheres. They may have solid layers, but all solid materials act as liquids at pressures inside planets. If two planets collide, they merge as viscous liquid drops instead of bouncing from each other or shattering like glass. You can drill just few of kilometers before solid rock becomes fluid and fills the hole.

6 minutes ago, daniel l. said:

What about simply filling it with a high enough air pressure to counter gravity?

Strength of rubber sheet (its ability to handle pressure) depends on its thickness which is linearly proportional to scale factor. Mass of the sheet depends on cube of the scale factor. At some point larger rubber duck can not handle enough pressure to prevent collapse. Also, such a pressure would form duck to a sphere, which is best shape to handle pressure stress, before collapsing.

[HebaruSan]

6 minutes ago, daniel l. said:

What about simply filling it with a high enough air pressure to counter gravity?

I'm guessing you'd end up exploding/melting the poor thing, since the force we're trying to counteract is already strong enough to override the structural integrity of the object. Dirigibles were a thing, but they were not rubber or duck-shaped.

[p1t1o]

Eek! Power is not a quantity of energy!

Pedantry aside, much relevant data here:

https://qntm.org/destroy

 

[Scotius]

Your idea has been done. Kinda.

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

And here. Warning: TVTropes ahead. Proceed at your own peril

http://tvtropes.org/pmwiki/pmwiki.php/Main/LensmanArmsRace

[WinkAllKerb'']

3 hours ago, HebaruSan said:

I'm guessing you'd end up exploding/melting the poor thing, since the force we're trying to counteract is already strong enough to override the structural integrity of the object. Dirigibles were a thing, but they were not rubber or duck-shaped.

this is an interesting metaphor ^^ sorry to be that weird ^^

[sevenperforce]

There is no material strong enough to "strut" the interior of a planet. A terrestrial planet like our own is a gigantic droplet of lava with big hunks of cooled-off rock floating on the surface. It would be like filling a swimming pool with pudding and then trying to "carve" out a cave inside.

I mean, if you just specify unobtainum with a compressive strength millions of times greater than chemical bonds can provide, then sure, you can basically do anything. But you'd be dealing with pressures at the core sufficient to sustain nuclear fusion.

Edited April 12, 2017 by sevenperforce

[p1t1o]

On the subject of membrane thickness, and scaling it up to contain large forces.

I was reading about battleship guns the other day, 16 inchers, that sort of lark. Turns out the barrels of these things are constructed in a more complex way than one would imagine. Referred to as "built up guns" a rough description is that they have a 1-piece "liner" that is the innermost metal layer that would actually be in contact with the shell. Around this they forge "sleeves" or "hoops" which compress the liner under great pressure, I forget the exact mechanism they use to achieve this but its around on the internet somewhere. It involves hydraulics of immense power IIRC.

The strength of the barrel dictates the maximum power of the propellant charge and thus the capabilities of the weapon, so stronger is better.

Anyway, you'd think that you could just keep increasing the thickness of the layers (oh yeah, there can be multiple layers, the barrel of the 16 inchers on Iowa-class ships consisted of 12 components) to get greater resistance to the pressures of firing and thus allow greater propellant charges. But you cant.

When you fire a gun like this, the barrel expands from its under-pressure state, through the neutral state, towards the point of elastic failure. This is the limit that sets the max pressure.

Making the barrel stronger/thicker helps, to a certain point.

With a very, very thick barrel of large excess, as you increase the pressure what happens is the barrel expands, and is constrained by the outer layers, those layers expand slightly too, but the lateral expansion is not as much due to the decreasing radius as you travel inwards from the surface, so there is actually no thickness that can prevent the inner surface expanding to the point of cracking at a certain pressure, and once cracks are formed, even a very very thick breech will shatter after multiple firings.

Hence, there will be a limit to the pressures that you can contain even with very, very thick rubber, and I doubt those pressures are going to be that high, in this context.

 

[Mad Rocket Scientist]

9 hours ago, p1t1o said:

Eek! Power is not a quantity of energy!

Pedantry aside, much relevant data here:

https://qntm.org/destroy

 

Aww, you beat me to it.  

[sevenperforce]

36 minutes ago, Mad Rocket Scientist said:

10 hours ago, p1t1o said:

Eek! Power is not a quantity of energy!

Pedantry aside, much relevant data here:

https://qntm.org/destroy

Aww, you beat me to it.  

And me.

Asking "precisely how many watts of energy would it take to vaporize Earth" is a bit like asking "precisely how many inches of force would it take to lift a tank?"

[wumpus]

On 4/12/2017 at 5:15 PM, sevenperforce said:

And me.

Asking "precisely how many watts of energy would it take to vaporize Earth" is a bit like asking "precisely how many inches of force would it take to lift a tank?"

I would have thought that was wrong myself, but apparently there (should be) roughly a hard Watt limit as well.  Since the power needed to disperse the mass >> the power needed to "vaporize" the planet, the minimum power needed is the power needed to accelerate all mass slightly more than 1g.

- note that the units might still not drop out.  I tried to use the engine of the "Wright Flyer" as proof that 19th century helicopters were impossible, but couldn't get hp/weight to convert to thrust/mass.

[sevenperforce]

10 minutes ago, wumpus said:

I would have thought that was wrong myself, but apparently there (should be) roughly a hard Watt limit as well.  Since the power needed to disperse the mass >> the power needed to "vaporize" the planet, the minimum power needed is the power needed to accelerate all mass slightly more than 1g.

- note that the units might still not drop out.  I tried to use the engine of the "Wright Flyer" as proof that 19th century helicopters were impossible, but couldn't get hp/weight to convert to thrust/mass.

Well, since any power source could simply be pulsed in order to increase its effective power, there's no minimum. If your chosen "dispersal" method requires 100 watts, but your power source is only 1 watt, then you can run your 1-watt power source to a capacitor bank and fire it for one second every 100 seconds.

Weight and mass convert easily enough, but your problem is trying to convert between horsepower (power) and thrust (force). Power needs to operate on working mass in order to generate force. In your case, the working mass is the air being pulled through your main helicopter rotor. The conversion is simple enough if you know your rotor efficiency, your rotor length, and the density of air. Keep in mind that a longer rotor will mean a more massive system, which is going to drive up your weight.