mike3 wrote:

What would it look like if you blew up the Earth? Suppose you had a
device that could provide enough power in the form of a big ***** laser
beam (dunno how you'd get that much energy... but just suppose you
could, and form it into a coherent EM beam somehow). I've seen these in
movies, like Star Wars and the Death Star, and am curious to know if
what they depict is how it would happen. Suppose you were at the Moon
distance when you fire off the big laser. What would you see happen?
What would be left of the Earth?

The nearest approximation to "blowing up the Earth" since its
formation is when the Moon collided with it - Shortly after
the Earth was formed, lumps of rock and miscellaneous clods
of dirt collected at one of the Earth-Sun Lagrange points,
and as the mass of the Earth and the body accumulating at
the Lagrange point increased the latter became unstable
and started moving in a so-called "horseshoe orbit".

Soon this orbit, and further accumulation of mass by the Earth
and the body, put the latter on a collision course with Earth,
and it collided with Earth with a fairly glancing although
still substantial blow.

(Until a couple of years ago people thought the Earth's capture
of the moon was an amazing one-in-a-zillion coincidence; but,
as the above account shows, far from that, it was practically
inevitable except for the specific amount of "overlap" in the
impact.)

At that point the two from a distance would have glowed like
a small star for several days. Within 24 hours the ejected
material (which would become the Moon) was a glowing red-hot
sphere very close to the Earth. From the Earth's surface,
the new moon would have covered half the sky (if it had
been visible from the surface, which it almost certainly
wouldn't) and raised literally mountainous tides if any
surface water existed (which it almost certainly didn't).

Since then the Moon has been spiralling away from the Earth,
at first steaming away at a rate of knots but now at a much
slower pace, about six inches per century or something of
that ilk.

By a billion years from now the moon will become too far
away to exert enough of a gyroscopic effect on the Earth
to stabilize the direction of its axis of rotation; so due
to internal motion of magma the Earth's orbit could suddenly
flip over to one where the poles point roughly towards the
Sun which, to put it mildly, would be somewhat challenging
to any life still remaining at that time.

(That won't be the only problem, because the Sun increases
its radiation by c. 1% every 100 million years and by about
the same time will become hot enough to evaporate all the
world's oceans!)

I probably haven't answered your question very satisfactorily;
but hopefully the above, based on my recollection of articles
from New Scientist over the last couple of years, will be of
some interest to you and/or others.

One final point - The speed of light in a vacuum corresponds
to circling the Earth about seven times a second. So whatever
the effects resemble they would look fairly sedate and slow
from the vantage point of the moon.


Cheers

John R Ramsden