13
$\begingroup$

So I went and saw Rogue One recently. Wonderful movie...

If you want absolutely no spoilers STOP READING NOW. The Death Star shoots up a couple planets in the movie. (Not much of a spoiler but still.) They don't poof like Alderaan in episode 4. (They aren't completely destroyed.) In Rogue One the laser hits the planet causing a massive explosion (Looks very similar to an atomic blast.) followed by an eruption of dirt flowing like a tsunami outward destroying everything is sight. Now I think that this is not a very accurate depiction of a planet destroying laser This article seems to confirm my suspicion. And I was curious how an actual planet destroying laser would destroy a planet.

My question is: Assuming we have a laser that has enough energy to destroy a planet (Greater than or equal to 2.24 × 10^32 Joules.) what effects would it have on the planet? What exactly would happen to the planet? Would the laser just super heat the planet liquefy the surface or would the planet actually explode and turn into an small asteroid field?

Here are some things that might narrow down the situation just in case this question is too broad.

  • The planet we are shooting is Earth.
  • The laser is just an extremely, extremely powerful laser there is nothing special or unique about it. (No Kyber crystal fuel.) It doesn't matter how it works it just does.
  • The definition of destroyed is very vague. However if you have seen the death star in action you should have an understanding of what destroyed means. The planet should no longer be considered a planet after the laser has done it's job.
$\endgroup$
  • $\begingroup$ You may get some inspiration from reading Gamma Ray Burst, some claims suggest it contributes to past extinction event(s). $\endgroup$ – user6760 Dec 27 '16 at 6:43
  • $\begingroup$ "So I'm just gonna say the planet is "destroyed" when the planet is completely uninhabitable for the foreseeable future." - Luke, fix that, we need more destruction, because to make a planet surface inhabitable it an easy thing to do, a less powerful laser will do it fast and well. $\endgroup$ – MolbOrg Dec 27 '16 at 7:02
  • 1
    $\begingroup$ Just a note about plot of the movie - the beams fired were meant to cause local destruction, not planetary. The first full-powered test was when Alderaan gets blown up in A New Hope. $\endgroup$ – JNW Dec 27 '16 at 7:40
  • 4
    $\begingroup$ Was kinda answered on xkcd - it is not open content so I can't use it to answer. But xkcd's vision seems strangely consistent with Star Wars. Just like the droid design SW used years after it was posted on xkcd, by the way... $\endgroup$ – Mołot Dec 27 '16 at 10:48
  • 1
    $\begingroup$ @XandarTheZenon It sort of hints at the how, but there really isn't any "what" to give away if you are familiar with the SW universe. $\endgroup$ – James Dec 27 '16 at 20:06
11
$\begingroup$

The sheer physical energy of the laser will rapidly turn matter into plasma (certain efficiency and coupling effects will be enhanced or degraded depending on the wavelength of the beam, but I suspect that at these energy levels the amount of difference it will make will be overshadowed by the scale and scope of the beam itself).

In the first microsecond, the atmosphere under the beam will immediately be turned into a superheated ionized plasma, expanding rapidly outwards from the strike. The idea anyone within a few hundred kilometres will survive being lashed by supersonic waves of plasma is questionable, at best.

Striking the surface will result in substantially the same thing, rock, soil, plants etc will be converted into a star hot plasma. Interestingly, the plasma will try to "run up the beam". but the extreme heat and electrostatic repulsion of the ionized plasma will tend to drive it apart, resulting in a massive inverted cone of plasma leaping out of the ground and into the vacuum the beam has created. For areas outside the immediate strike zone, a supersonic shockwave will blasting away from the area, continuing to excavate a larger and shallower crater entered on the strike zone. Something very similar will happen if the beam lands in the ocean or body of water, the main difference being the plasma will be largely hydrogen and oxygen, rather than an unholy mess of silicates, metals and organics.

Burrowing through the crust will take a bit of time, although the massive pressures developed by the plasma will clear out the laser channel and prevent the hole from collapsing due to static pressure. The oceans might actually allow the planet to survive for a time, since water will continually try to flow into the plasma "hole" and the boiling sea water will carry a great deal of energy into the atmosphere, meaning the beam might have to dwell for a while to evaporate enough ocean to strike the seabed.

Assuming you haven't targeted the edge of a crustal plate or the magma chamber of a super volcano, the beam will continue to the point where static pressure is finally able to overcome the pressure of the expanding plasma. You will already be feeling earthquakes, but now the beam is "pushing" on inflowing rock (at the scale of planets, matter should be treated as a liquid). Depending on the depth of the hole, magma or even plastic rock from the mantle might be flowing into the hole to be immediately vapourized. An observer in orbit will see the huge inverted cone of star hot plasma exiting the atmosphere and accelerating into space. Some of this may "condense" around the edges of the cone, and shower down around the planet, igniting a global firestorm. Much of the remaining atmosphere may well already be poisoned by various oxides as the hot plasma reacts with atmospheric oxygen.

If at this point the laser is stopped, there will be a sudden inflow of rock, water and gasses into the huge vacuum that was created by the laser and expanding clouds of plasma, generating more violent storms, earthquakes and tsunamis. The surface of the planet will have been scoured by supersonic waves of plasma streaming outwards from the strike zone, and as the plasma cools, the surface may well be covered with a thin glassy layer of rock vapour that condenses on the cooler surfaces.

If the laser continues for a prolonged period of time (I don't have the math to figure this out, but intuition tells me this will take several hours), the pressure waves of the plasma will be moving through the mantle and around the core, creating violent earthquakes on the opposite side of the planet. As an aside, the planet will also be getting some severe earthquakes due to the plasma plume acting as a rocket engine and pushing against the Earth as well.

In terms of planetary destruction, the laser will fairly rapidly strip off the planetary atmosphere, by heating the gasses to well beyond the escape energy required to leave the gravity well. The water will be eliminated by a similar process, but lasting for a considerable period of time, given the massive amounts of water on the Earth. Carving a hole in the crust will trigger violent earthquakes, but shattering the crust or lifting massive chunks of the Earth bodily into space is likely not going to happen for a while yet. The increasing amounts of energy in the mantle will stir the plastic rocks there violently, triggering massive earthquakes and probably shattering continental plates along the edges, but until the mass of rock reaches the point where it is essentially turning into "steam", you won't see the crust blow off into space.

Once the crust is blasting into space because the mantle is turned into vapour, we should consider the core. The violent pressures in the mantle will actually increase the pressure on the core, but asymmetrically. As the amount of matter over the core is suddenly reduced, the core will violently be disrupted, and the huge amount of energy from the heat and pressure will be dumped into the remaining mantle, "splashing" it violently into space.

So I do hope someone can do the math to get some real times, but my sense of the matter is the laser will strip life off the surface in a short period of time, but will need more than a day to impart enough energy throughout the mass of the planet to actually blow it up.

$\endgroup$
3
$\begingroup$

It's highly likely your laser of doom will actually start generating antimatter. See, when you focus a powerful enough laser into a small enough spot, you start generating electron-positron pairs (this is what the Extreme Light Infrastructure laser is supposed to accomplish when finished - and it's only a petawatt class laser, focused so that the beam has a radiance of >10^23 W/cm2).

So the death star likely generates quite hazardous quantities of positrons and electrons which will almost certainly annihilate each other to produce lots of gamma rays, whether this has much of an effect on the overall level of destruction is open to debate. Needless to say, when the thing's running there'll be supersonic shockwaves full of molten rock bullets, plasma that makes the sun look like an ice cube, xrays, gamma rays and antimatter. Just something to be aware of (you know, so you can attach the correct safety labels to it)

$\endgroup$
  • $\begingroup$ hm, gamma will very handy to distribute energy to the core. Probably laser itself have to be a gamma laser from the start. $\endgroup$ – MolbOrg Dec 27 '16 at 22:04
  • $\begingroup$ @MolbOrg That would probably make the whole thing more plausible as a gamma ray "laser" could potentially be built either from a nuke or from a pile of positrons (you have to use the right kind of antimatter, antiprotons make all kinds of short-lived rubbish when annihilated) $\endgroup$ – Samwise Dec 28 '16 at 20:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.