h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The planets that orbit the sun, are able to do so because there's no atmosphere or similar to slow them down. So they just keep going round and round, mostly without slowing down. Dark matter is meant to not interact with normal matter, except gravitationally. If a planet is moving through dark matter in it's orbit, would the gravitational pull from the dark matter be able to slow it down? The pull would of course not come from one place, but rather from all over. Perhaps the pull from behind it's direction of movement would be cancelled out by the pull from in front of it's direction of movement. What would you say, would dark matter cause moving objects to slow down?

mark


Way-out guess: The earth is, I'm told, slowing down...
Set the earth free from the solar system & it would slow down a bit more

well, seeing as we don't know what dark matter is, because we can't detect it...
(it is after all, really just a fudge to make some equations work)

...We don't know.

And clearly, it does not - in our own local experience.

I should add to that of course, one of the reasons it is used is to explain orbital velocities of galaxies around their axes.

So you can certainly say that is affecting the rotation of these things on a rather gigantic scale - whether you can call that 'drag' is another thing though.

Fair enough, Orcus, point taken.
>>(it is after all, really just a fudge to make some equations work)<<
Aye, but in the absence of knowledge, it suffices pro tem.

However, I don't get this bit (the 'clearly'):
If it can explain galactic phenomena then how can it be said
>>And clearly, it does not - in our own local experience<<

A recent study of the local region of space out to a distance of about 30 light years (or it might have been 300 light years) has shown no gravitational effects attributable to dark matter. The scientists are very surprised.

The fact that they are surprised suggests that the dark matter should affect the paths of the planets.

The fact that there is no observed effect strongly suggests that dark matter doesn't exist. But this would mean there must be some other reason for the galaxies being the shape they are.

I believe the reason the planets are slowing down is due to somehow losing gravitational energy to the Sun. So if the Earth was set free, it would actually not be slowing down anymore. Assuming no dark matter at least.
It could be the reason the scientists don't think there's dark matter in our region of the universe is due to something other than the orbit of the planets. Though personally I find dark matter seems increasingly unlikely.

Indeed, what Gnomon said is my view (at least I think we agree, apologies if I'm misconstruing) - it *is* just a fudge to make the equations work and it really shows that we just don't really have the full picture. Lots of theories are like that though - we use one theory until it is demonstrably false and then we tweak it or change it according to our deeper understanding.


If the earth was 'set free' then it would just become gravitationally tied to some other more massive object (probably jupiter initially) and then gravitational tidal effects from other objects would still slow it down gradually.

I don't get the impression that physicists just think of this as a mathematical trick. They treat dark matter as an actual thing.

The study I cited was only published about a month ago.

Interesting article in nature.

http://www.nature.com/news/dark-matter-s-tendrils-revealed-1.10951 published today

Many things in physics begin as a 'fudge' which turns into something real when experiment allows more understanding - what I said is only my opinion. I am no expert in dark matter - but then, noone is here.

It's gone quiet...

Something that occurred to me earlier and which Orcus's latest post/link seems to not actually oppose:

Small accretions of matter tend to gather into larger accretions, maybe with different rules according to type.

If, say, we are now in a situation where, in our Solar System's volume of space, there is only one particle/item of this dark matter (perhaps the 'average' distribution?) then:

It could account for no local findings - and for findings, at great distance, in places where there are large accretions.


Yes / No / Maybe ?

>I don't get the impression that physicists just think of this as a mathematical trick.<

Well of course they don't. The rules of motion and gravitation work extremely well for most things and so there is no reason to think there is anything wrong.
_Except_ when you look at really massive things like galaxies, galaxy clusters and stuff.
So a 'fudge' must be used to make the same equations work and that works (I think) by way of introducing an extra constant to make the equations that fit our current theories still work.

As stated, the 'dark matter' cannot be detected (except indirectly as in the Nature article I linked to) so we don't know what it is.

That doesn't and shouln't stop people speculating on what 'it' is though.

It could of course be that we have also got it all spectacularly wrong and our general theories of gravitation and motion need entirely reworking on the large scale.
This did happen on the infinitesimal scale with quantum theory in the 20th century.

When I said it was clear that it didn't have any effect locally - the reason it's clear, at least to me, is that you don't need to invoke 'dark matter' to explain the motions of the planets around the sun and the motions of the nearest stars that I am aware of. Hence its effect is essentially negligible locally.

Re: post 12.

The fact that we don't need to invoke dark matter on a local level may well point to the fact that there is none nearby yes.

It could also point to the fact that it doesn't exist and that our theories need improving on the large scale.



In the absence of someone who _actually knows_ what they're talking about on this (and it's certainly not me) then we're just speculating in ignorance frankly.
Which is fine, as long as we're aware of that.

I wasn't saying that the planets are kept in their orbit by dark matter. The gravity of the sun etc works to explain that. What I was asking was *if* dark matter exists, would it cause a drag on the planets? Presumably dark matter would be able to pass through normal matter or be some kind of superfluid. Friction is as far as I know caused by molecular bonds being created and destroyed as two objects are close enough together. Similarly I'd imagine that any kind of dark matter particle would briefly pull on the atoms in an object they would be passing through. However some would pull the atom forwards and others would pull it backwards. Would this end up cancelling out, or would it cause a general drag effect?

One of the statements in that article is

"A ‘finger’ of the Universe’s dark-matter skeleton, which ultimately dictates where galaxies form, has been observed for the first time."

umm - how do you detect a finger when it is "Dark: matter ie. undetectable????

It's not undetectable, it can be detected by looking at it's gravitational effects.

Gravity doesn't cause drag, but transvestism does.

To be Sirius, drag is caused by conversion of kinetic energy into heat energy. Gravity which "causes" the earth "to orbit" the sun also generates solar tides and it is internal friction of the solar tides within the earth that converts gravitational energy into heat energy, which is lost from the earth by radiation. This energy conversion and loss causes the earth's spin to slow and it's orbit to get a smidgen closer to the sun. Of course the moon also causes tides etc.

My new Angel Theory will shortly be published which describes how the angels push the planets around the solar system and universe according to strict mathematical rules. Using Angel theory there is no need to resort to speculations of dark matter.