# Does accretion need gravity?

1. Nov 16, 2005

### -Job-

The phnenomenon of accretion doesn't really seem to need the effect of gravity, as long as particles have different speeds, sizes and intersecting trajectories, then they'll eventually form bigger and bigger clumps of matter. These clumps don't really need gravity to remain together either, if you think about it, when two objects collide, except when they're about the same size with about the same velocity, one of the objects will "win" and carry the other object in its path. Gravity seems to play only a small role in this process, what do you think?

2. Nov 16, 2005

### Integral

Staff Emeritus
Why would the material stick together? Wouldn't they rebound from the collision? Gravity provides a force which activily draws the particles towards each other. It is not clear to me how random motions of particles would create larger lumps of mass without some driving force.

3. Nov 16, 2005

### -Job-

I was thinking if i have a big object, say the size of a house, going north at 10 meters per second, and another, smaller object, the size of a car, going also north at 1 meter per second, then when the bigger object comes up from behind the smaller one and impacts head on, then the smaller object will not rebound away from the bigger object if the velocity obtained by the impact of the bigger object is smaller than the velocity of the bigger object (the house would sweep the car). At this point the two objects are touching eachother and going in the same direction with the same speed, so we don't need gravity to keep them together, they stay that way (unless acted upon).

4. Nov 17, 2005

### Integral

Staff Emeritus
Pretty special circumstances :surprised . How many collisions do you suppose will occur with just the right parameters? If you make more realistic assumptions. That is, a random mass at a random velocity collides with a different mass at a different velocity, what are the chances for your collision? How many will happen?

Next throw in the fact, that if the density of particles is low, there may be a low odds of any collision occurring at all.

Beyond that, this is another case of creating a useless imaginary universe. I, personally, do not find such pointless speculation of much interest. There is so much that I do not understand, and want to learn, about the universe that we live in, why should I waste a minute speculating about a universe that does not exist? There could be some value in it if I were writing a SciFi story. but I am not, so what is the point?

5. Nov 17, 2005

### -Job-

Not really that rare i would think. Suppose i have the object the size of a house going south and the object the size of a car going east, the trajectories intersecting at a 90 degree angle, if the house hits the car on the side, then it will sweep the car as well. I can think of a number of scenarios where the same would happen. If the house is going faster than the car, then in most of the scenarios the house will sweep the car, right?
I have to ask, what is so imaginary about this universe? I have to consider that on the small scale at which accretion begins, gravity is practically negligeable on a per-object basis, and somewhat evenly distributed in the whole cloud.
Scifi story? Pointless speculation? I don't know what to say to that. When i posted this thread i thought people were going to reply saying stuff like "well, obviously".

6. Nov 17, 2005

### samalkhaiat

So where did gravity come from? We use vravity to explain the phenomena that you mentioned as well as the motion of bodies in the sky.
So, can you, using your "theory", explain why the moon goes round the earth? If you can not, then don't waist your time?

sam

7. Nov 17, 2005

### -Job-

Are you guys dense or something? I'm not saying that there isn't gravity, i'm only saying that it only plays a minor role in accretion.

8. Nov 17, 2005

### cotarded

This is absurd that you all found it necessary to insult Job over a fanciful question. I think it's an interesting line of reasoning, and, while admitting I am far too ignorant to have an opinion on the manner, I agree with you, Joe, that some sort of accretion would probably take place even without gravity, but I don't think it would be at all like the accretion we have now (do you think roughly spherical planets would form for instance?). It probably isn't correct though to go as far as to say gravity plays a minor role in accretion in our universe just because a related phenomenon could occur without it.

I think this sort of speculation is far from idle, and that you can note it's presence in the minds of all sorts of productive and highly esteemed figures in scientific history. Albert Einstein often thought about what it would be like to race a beam of light in a train - I suppose you could call it idle but I'd bet most of us respect what it inspired.

lates,
cotarded.

edit:
This is totally flimsy reasoning here, but it seems to me that masses accreted(sp) that way would have some sort of upper limit (and it doesn't seem to me that it would be very large at all) where further additions would be exceedingly likely to cause the components of the mass to separate (as there's no real cohesion).

Take your house sized object and the car for instance. Once they are aggregated by their collision, imagine another house-sized object slammed into either one. If you neglect the insignificant friction one object will recieve more velocity and depart the group. If that doesn't seem immediately apparent, imagine the following two scienarios(sp). The objects are hit parallel to their border: unless they're of identical mass (and therefore inertia) and perfectly aligned, one will recieve more velocity by either being hit first or by mass difference. If otoh they're hit perpendicular to their border unless the mass is much larger more velocity should be transmitted to the far one. If somehow they've been intermeshed by the collision enough to create significant friction some rotation might be introduced which, it seems to me, would cause the objects to split.

Of course it is probably patent I know nothing about physics, so feel free to correct me. It seems intuitive though that even if the limit is larger than I suppose, that it has to be there.

Last edited: Nov 17, 2005
9. Nov 17, 2005

### Integral

Staff Emeritus
Surely accretion is a big player in the formation of solar systems, but gravity is the driving force. Imagine adjacent rocks following parallel paths, they will never collide so accretion will not occur.
Gravity provides the small force acting over long periods of time that draws the paths together causing low velocity accretion style collisions. The combined mass now exerts a larger gravitational force to work on other nearby objects. It is this mechanism that accretion lacks.

10. Nov 17, 2005

### -Job-

But how big do the largest objects have to be before gravity starts having a meaningful effect? I imagine in the earlier stages there were all kinds of small sized debris everywhere, with gravity being low and somewhat evenly distributed in this "cloud of debris and gas", so there must be a process that can create the larger objects needed to get things going. Consider that at this level, if two objects collide in such a way that they deflect, then how will the weak gravitational force of either body prevent it?
Also, it's not necessary that every collision results in a bigger body, there's also a number of deflections and shatterings. Even if the probability of the objects sticking together is relatively small, maybe <25%, things are still looking good, because it will require two big objects to have shattering of a big object. So the shatering of the bigger bodies is determined by how likely it is that these bodies form at all, hence it's beneficial that the probability of two bodies staying together on impact be less than 25%.
This way, over time, a number of collisions will form a few larger objects (without a particular meaningful effect from gravity). These larger objects, though still small, are big enough so that the probability of them being shattered (by colliding with another big sized object, of which there are few) is small, and the probability of them growing in size after a collision much bigger.
Gravity would come into play later, when the objects are considerably bigger, to clean up the mess, but it doesn't guarantee accretion by itself (consider the asteroid belt), the dynamics of the system are also very important.

Thanks cotarded for the support and Integral for giving this another thought.