

The Moon’s Giant-Collision Theory Proven Incorrect - jjp9999
http://techzwn.com/finding-shows-the-moon-comes-only-from-earth-not-giant-collision/

======
inconditus
I'm being picky here, but the word "proven" doesn't really make sense in the
context. A more accurate way of expressing the title would be "Evidence
supports a different theory" or "Less support for this theory", as theories
can never be proven.

~~~
arethuza
If theories could be "proven" in the mathematical sense then they wouldn't be
falsifiable and therefore it wouldn't be a "theory" in any scientific meaning
of the word. So yes, I completely agree with your being picky about this!

~~~
srl
> If theories could be "proven" in the mathematical sense then they wouldn't
> be falsifiable

Ack! This is _not_ what "falsifiable" means.

Falsifiability is the property that states, there exists some hypothetical
evidence which would be accepted as conclusive disproof of the theory. For
instance, newton's third law ("equal and opposite") can be falsified by the
discovery of a method of applying force which avoids any opposite application.

Extrapolating some, we can see that for all falsifiable theories, there is an
extensive class of evidence which doesn't quite prove the theory wrong, but
does weigh against the theory. Non-falsifiable theories don't have this
property - for any method of assigning a probability of correctness to a non-
falsifiable theory, there is a positive number below which that probability
cannot go.

The reason the giant-collision theory should not be said to be "proven"
incorrect is that it has not been. Its probability of correctness is still
non-zero (and significant). However, it's not true that scientific theories
cannot be "proven" incorrect - classical mechanics has been, for instance.

~~~
arethuza
In what sense can a scientific theory be "proven" correct of incorrect in the
same way that a purely mathematical statement can? Aren't they two completely
different things - with the mathematical argument being binary (it's either
true or false) and the scientific argument, as you say, involving degrees of
confidence based upon supporting or contradictory evidence?

[Of course, a scientific theory could be disproven by being based on incorrect
mathematical reasoning - but that's presumably not very interesting]

------
kristianc
My understanding of the theory is that Theia hit the earth, that most of it
was absorbed into the earth's surface/ molten core, and that the rest, along
with some of the earth was ejected out into space.

If that's the case I don't see how there is any way of 'proving' that the
isotopic signatures we are matching with those of the moon are 'native' and
not already part of some Earth/Theia mix - in which case they'd match with a
Moon that was already part of an Earth/Theia mix. Without knowing the
composition of the earth 4.5 billion years ago, I'm not sure you can say
otherwise?

~~~
jbri
If the giant impact theory is correct, Theia must have been about the same
size as Mars - roughly a tenth of the mass of the earth. Hence it would have a
much larger impact on the makeup of the moon than it would on the makeup of
the earth - a difference we would expect to be able to detect if the theory is
correct.

~~~
Sharlin
It would have a negligible impact on the makeup of the Earth _as a whole_ ,
but should have a large impact on the composition of Earth's _crust_ (and the
mantle, to a lesser extent.) It seems to me that the Theia theory should also
predict that Earth's crust and Moon's crust (which are the only parts we can
get samples of!) have quite similar composition.

------
abc_lisper
I don't understand. If "theia" hit earth and formed moon, why would the
istopic signatures be any different. Now both earth and moon come from same
pair of different objects.

~~~
hartror
We would expect different proportions of the isotopes on the Earth and the
Moon. This is because the Moon according to the models would have a greater
proportion of its mass made up of the theorised collider "Theia" than Earth.

In this particular experiment they showed the isotopic proportions matched
between here and the Moon.

------
nsns
I think we should remember the vast differences between reporting about
scientific papers and the peer-reviewed papers themselves; I have a good
feeling, based on previous experience, that most of the caveats raised here
will be tackled by the paper, whose tone will probably be less decisive.

~~~
schiffern
Indeed. The actual title of the paper is "The proto-Earth as a significant
source of lunar material."

------
sp332
One thing I've never understood about this model: according to orbital
mechanics, you can't "boost" yourself into an orbit that doesn't include the
point you're already in. So if the mass that became the moon started at the
surface of the earth, its orbit would just smash it back into the earth again.
You can see what I mean here: <http://isthis4real.com/orbit.xml>

~~~
mokus
In a 2-body system this is true, but adding many more bodies to the system
makes things very chaotic. An object passing by or colliding with the "proto-
moon" could easily deflect it from its crash course. Whether that's probable
given the evidence is a different question, but mathematically it absolutely
is possible.

And that's even assuming that the "mass" is more or less solid - if it were
ejected from a violent impact it's more likely that it would be more of a
cloud of debris, in which case things get even more complex. So many "bodies"
are involved that it becomes more of a fluid dynamics problem than an orbital
mechanics one. The individual particles in the cloud interact with each other
gravitationally, deflecting each other from their "return-to-sender" orbit.
Parts of the cloud would fall to earth, parts would escape to space, and parts
would stay in orbit to become a ring system or (as is hypothesized) one or
more moons.

The theorized situation isn't so much like skipping a stone across the earth
as totally vaporizing a big chunk of both the earth and the planet that hit
it, and a fairly small part of the resulting cloud (about half according to
some models) of debris managing to end up in orbit and eventually collapsing
into a single body (the moon).

------
frogly
Strictly identical? Maybe the samples they're using are not really from the
moon in the first place! Hehehe...

~~~
transphenomenal
Or that the mix of Earth and Theia in the moon is very non homogenous and they
took most of their samples from the Earths part.

Actually, now that I think about it, wouldn't the result of the collision
cause most of Earths contribution to be on the surface of the moon? If it was
otherwise, then Earth would have had an ring much like Saturn at one point
which then collected into whole to become the moon. That couldn't be true
since we would have easily seen evidence on the moon. This theory of mine
would mean that almost any samples taken from the moon would would be
originally from Earth.

~~~
jbri
The giant impact theory predicts that Earth _did_ , at one point, have a
proto-ring of debris from the collision. That proto-ring slowly accreted into
the moon.

I'm not sure what evidence we would expect to observe on the moon to indicate
whether it accreted from a proto-ring or was captured intact - could you
elaborate a little?

------
mekwall
The theory was merely proven to be theoretically falsifiable, and not
incorrect. They clearly state in the article that they couldn't find any proof
of two parents, which for the moment points to the moon having only one
parent. This contradicts the previous theory, but doesn't make it false.

------
tocomment
So what are some alternative theories that now look more likely in light of
this evidence?

