Imagine a dense, solid projectile that can travel at lightspeed and is at least 1000 m^3 in volume. Aim the projectile so that it intercepts a planet. Launch the projectile at lightspeed. What happens to the planet? If it destroys the planet, the Death Star was overkill to the 100th power and quite a lame idea.

I think it would pass straight through the planet, without doing anything too tremendous.

I wouldn't really know, so I'm not going to vote.

I said nothing, but i thought you were talking about a laser projectile. 1000m cubed would certainly cause serious damage though.

I voted for cake.

Pure ballistics. Sure, only 1000m^3 leaves room for improvement, but think of it as a massive-scaled railgun.

At that velocity, it would go right through. Realize that the only reason light reflects is because it has no significant mass. That massive projectile at lightspeed would shear apart anything. Realize again that projectiles cause the most damage when they tear things. That is, the material must supply some resistance.

There would, of course, suddenly be a 1000m^2 hole through the planet. That material has to be either displaced or pushed right out the other side. Either way, that's a serious problem. The planet is toast.

But, it would take infinite energy to accelerate that mass to lightspeed. (Light is exempt. It doesn't accelerate to lightspeed. It already is at lightspeed.)

When it happens, saying I have 10 minutes left, I'll run to the nearest computer and post about it in this thread, even if it means necromancy.

um...who cares? sure its interesting to think about, but its never going to happen, so talking about it is pointless.

THE MAN'S IMAGINATION CARES


and so does mine

Really big hole in the planet. Bad. Lava. Death. Not good.

Yeah. And all the gravity'll escape if you punch a hole through the planet.

Remember that 1000 m^3 is actually pretty small, just 10 meters across. I think it would most likely just go right through the planet and not really do much. Well, anyone near it would be like "HOLY CRAP WHAT THE HELL WAS THAT"

Just a clarification: how fast exactly is it going? Near light speed, it makes a huge difference if it's going 99.9% or 99.99%. I want to calculate the energy but I can't unless I know the speed.

An object with any mass cannot go exactly c. It would take infinite energy for an object to go c.

For the sake of argument, assume a mass of.. ohhh, 1000 kg, just to be nice and pretty numbers. Speed of light we round to 3*10^8 m/s. Assume the Earth is stationary relative to the object. in collisions, mv (object) = mv (target), and let's assume, for the sake of this example, an elastic collision in which all energy of the object is transferred to the Earth. The Earth has a mass of 5.97*10^24 kg. This means that Earth would start moving all of... 5*10^-14 m/s. That's less than the radius of an atom per second.

Probably little to nothing, the planet wouldn't likely be there when the projectile got there. This is especially true the futher away the target is. The further away an object is, the more out of sync it is with you.

For instance, if you fired a projectile such as this at the nearest planet outside our solar system which is ~15 light years away, you'll miss it by a long shot. For one, when you line it up in your 'sight', 15 years would have already passed since it was in that exact location. When you fire your projectile, it'll take another 15 years to get there. In total, 30 years will have passed since the target was in the position you thought it was in and the projectile gets to where you thought the target was.

depends on the shape of the object... a super long thin object with a sharp point might make someone go OMGWTFBBQ while doing minimal damage while a wider object especially with a fairly flat side making the initial impact would do more damage

while you said 1000m^3 you didn't give any indication of the shape of the object so most would assume it's a cube

Yeah except things get wonkey approaching the speed of light. Which is why I wanted to know the exact speed; something traveling 1 m/s faster than something else can have a hundred times the energy if it's sufficiently close to C.



Well I think for the sake of argument, we're assuming that whoever fired it was smart enough to aim properly.

The planet as a whole would not be shifted. However, it can cause serious surface damage, like dropping a rock in a pond: The pond doesn't move; but the surface sure does.

But at that speed, it would overwhelm the local rock before it could transfer any force. Like I said before, to cause any damage, the rock has to resist the impact. A mass at nearly the speed of light has used nearly infinite energy to accelerate to that speed, and thus will strike with nearly infinite force. And the most common material in the earth's crust - granite - can be broken by a few hundreds pounds of force. And everything below it is molten. Maybe if the core is solid, and it hit it, there might be some shifts in volcanic patterns.

Ok. Let's learn about relativistic effects.

As some mass approaches c, the mass observes that everything is getting shorter. While observers outside will see the mass become more compressed. The mass will have a constant m but it will have a length of zero. Eventually it will be an infinitely stretched mass. So when this object crashes into the planet, it's going to leave a line the whole north-south of the planet.

This is of course neglecting that it takes infinite energy to get a mass to c. Assume that we have gained the infinite energy required to accelerate the mass to c. Energy is conserved always so that energy has to go somewhere. That energy is going to go into the planet at least some.

Moral of the story. It is with most certainty that the planet will be completely obliterated.

No, it won't. It's the same way a bullet can go through a person, and do no damage at all besides the actual hole.

A bullet doesn't have to dissipate infinite energy.

The faster the projectile is going the more energy it's going to dissipate, which leads to more of an effect on the thing you're shooting.

Also, humans don't have a hard crust around a molten core, so what happens after you get shot is going to be completely different.

In short, the human-being-shot-by-a-bullet scenario is a poor analogue for this discussion.

And yet when you start getting up there in caliber (say, .50), you pretty much explode when you're hit.

You must have missed the part where he said "Aim the projectile so that it intercepts a planet" and not "Aim the projectile right at a planet."

Short answer, I believe, is that it will pass right through the planet, but not without significant negative effects on the planet. It will not, however, destroy the planet outright.

Exactly. The bullets transfer energy on inpact. That energy results in flesh exploding around you. The only reason why the bullet goes through is because the KE is so great that it still maintains enough to go through you (unless it's a .22)

Like kyle90 said, things get wonkey as you approach c. You stretch out, time begins to slow...

Okay, I see that setting the velocity at lightspeed was a bit ambiguous. Let's say that at most it is going at half lightspeed. Also, the shape is probably something similar to a cylinder or a conventional missile, where the tip is pointing at the planet. I originally said lightspeed because I was comparing it to the Death Star laser, which I assumed would release energy traveling at light speed.

Good discussion though.

Some numbers:

γ = 1.15
0.5c = 149,896,229 m/s
m = 22.67 kg (a 50 lb warhead)
E = γmc[sup]2[/sup] = 2.35x10[sup]18[/sup] J
K = mc[sup]2[/sup](γ - 1) = 3.15x10[sup]17[/sup] J

I tried to use the impact calculator but it didn't like my 93,141 MPS speed.

My guess: It would put a dent in the Earth, nothing huge.

Hmm, that's only about 75 megatons worth of energy. Not much bigger than the Tsar Bomba. So it'd do some superficial damage but it would be nowhere near destroying the Earth.

Of course that's for a 50 pound projectile; something that weighs on the order of thousands of tonnes (which would be suggested by the initial 1000 m^3 volume) would do a bit more. Initial calculations show that it would be hundreds of times as powerful as the world's entire nuclear arsenal. So call it an extinction-level event, like the one that killed the dinosaurs.

[QUOTE=Michael MacFarlane]You must have missed the part where he said "Aim the projectile so that it intercepts a planet" and not "Aim the projectile right at a planet."[/QUOTE]

Aye, I did. But none the less, calculating the window for such a launch would be daunting in and of itself. 15 lightyears isn't the closest system (~4.22 is, however). So you'd have an astonding amount of varibles to consider in avoiding other bodies that might cross the path, compensating for all of the gravity shifts as it darts in and out of various gravitational fields from those crossing bodies, etc.

Though, judging from his last comment, that wasn't the intent of the question. If you target the core of the planet, it would certainly go boom boom (maybe even more so depending on the chemical make-up of the projectile and the approx. temperature at impact to the core).