# Gravity and the bullet cluster.

1. Apr 1, 2012

In the bullet cluster, a large quantity of matter in the form of gas has been removed from two galaxies as a result of them passing through each other.

What i'm wondering is, how do we know that such gas constitutes a significant portion of the gravity a galaxy would exert?

I know that gravity has been found to be proportional to mass, but it seems to me that directly studied massive systems are also always involve a high level of order in the sytem.

So, how is it determined that a large quantity of disordered material in the form of gas exerts a gravitational force equal to the same quanity of material, but highly ordered, in suns, planetary bodies, etc?

Is the relationship between mass and gravity such that gravity must be considered a consequence of an amount of material?

I suppose if it is, the lensing effect of light would be observed as a result of intergalactic clouds of gas, so has this been observed?

In the bullet cluster, the lensing effect is essentially consistent with what you would expect to see in two ordinary galaxies that hadn't had lot's of gas removed from them, so it's concluded that there must be additional unseen material in the galaxies to account for the loss of the gas. But couldn't it be concluded that the bullet cluster is evidence that gravity is related to the level of order in material?

(EDIT: i'm not sure whether the forum rules indicate i should remove the last speculative last paragraph...)

Last edited: Apr 1, 2012
2. Apr 1, 2012

### Drakkith

Staff Emeritus
What does "level of order" mean?

3. Apr 1, 2012

Hmmm.... i'm not sure how to define that convincingly, or even what the options might be. It just seems intuitively plausible to me that for e.g. a sun would be in some sense more ordered than an equivalent amount of material in a more dispersed form.

I suppose i'm really trying to find out what we know about gravity as opposed to what assumptions we make about it, and how that might affect the interpretation of observations infered from gravitational affects, such as bullet cluster.

(EDIT: Maybe order could be defined as interaction of parts of a system that result in additional properties not carried by those parts? So, e.g., in a star, the formation of heavier elements with time is a more ordered system than an equivalent body of dispersed gas in an equivalent time. )

Last edited: Apr 1, 2012
4. Apr 1, 2012

### Drakkith

Staff Emeritus
Well, I'm not sure how to answer you really. However, I did type in "experiments testing gravity" in google and got plenty of results, so you can see some of the experiments that we have performed there.

5. Apr 1, 2012

Fair enough. I'll go blunder around somewhere else :)

6. Apr 1, 2012

### Drakkith

Staff Emeritus
I'm not an expert, so I can't really help you much in this situation. There may be some other here who can go into detail on what you are asking. In the meantime I would suggest learning about General Relativity, as it is our primary theory at describing gravity. There is a wealth of information here and elsewhere online, and you can pick up many books on the subject at any bookstore.

7. Apr 2, 2012

I just don't understand why dark matter is preferable to a coupling of gravity with a correlate of mass rather than directly with mass.

8. Apr 2, 2012

### Drakkith

Staff Emeritus
What?

9. Apr 2, 2012

Why is the gravitational mass of an object directly coupled with the amount of stuff in it? Couldn't gravitational mass be coupled with something correlated with amount of stuff?

10. Apr 2, 2012

### Chronos

Gravity is mass exerting an influence on mass. It is that simple. It does not 'matter' if it is clouds of primordial gas or stars. Your conjecture fails on many levels.