Can someone with real knowledge of "relativity, physics and everything" help me out on this one?

Question

Help me, I'm an amateur! But Newton's 2nd Law, saying force equals mass times acceleration, seems unquestionably true. As a kid riding on a merry-go-round, I loved to get in the center and ride straddling the axis, to see the big outer edge make the circuit in the same time that I made it. But later I learned that a galaxy doesn't spin quite like a carousel: gravitational force toward the outside is weaker,so stars near the center should go around faster, with rotation speed varying precisely in proportion to distance. Wrong! Astronomers detect a "drag" starting about halfway toward the outside, easily explainable by dark matter. But MOND theory tweaks Newton's 2nd Law to explain away dark matter. Weird! WHY?

Answer 1

What you have here is two competing theories which both explain the velocity curve of galaxies pretty well. Conventional theory needs dark matter to explain the non Keplerian orbits of stars in a galaxy's disk; because given the inverse squared rule that gravity holds true by under nromal conditions of acceleration, as you move further away from the source of the gravitational pull, the actual force decreases. It decreases such that the velocity curve falls away to zero at an arbitrarily long distance.

But the non Keplerian curve of a galaxy's rotation and the subsequent "flat" velocity profile of the outer areas requires one of two things.....either that the mass outside of the outer regions of a galaxy matches and/or exceeds that within the galaxy itself.....or.....Newton's Laws of Gravitation need to be modified, without invoking any special conditions such as "extra mass" or "dark matter". All it needs is a new constant Ao and the function u(x).....which is not actually specified except where values of x are greater or less than 1 by a substantial margin. You could say that u(x) is an acceleration function. The constant Ao, applies to acclerative forces which are small compared to the normal situation, such as on a planet or in the solar system. It is premised that gravity under conditions of very low acceleration behaves somewhat differently than in normal situations. At present both the conventional theory and MOND hold promise however scientist don't like to part with dear "sacred cows" and will not consider MOND as a whole. What they do have against MOND is that it's an effective theory....one which just describes a condition without giving any physical reasons or observations to describe the condition. However MOND has been shown to be consistent with all the observed data to date, except where opponents of MOND have shown data claiming otherwise. What rankles the physicist is that they believe invoking dark matter is easier given the observations and current threory than having to modify Newtonian dynamics. However, the "explanations" for what the dark matter is are just as "left of field", even more so, than the explanation given by MOND.

Answer 2

I've researched MOND, and actually there's no reason "why". This is just a formula that lets the calculation fit the data we've collected. MOND theory has been solved without dark matter - the outstanding challenge in physics is either to identify this dark matter, or come up with a good, mathematical explanation why MOND should work (other than "it fits our numbers").

In other words, it hasn't been solved yet.

Answer 3

You should read the wikipedia article about MOND, it explains the "why" quite nicely:
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Although it accurately *describes* the motion of the stars in the galaxy, it does not *explain* their motion; it gives no physical basis for the mathematical formulation. It's merely a convenient description.

Which is nice, although it defeats the point of physics - to describe physical nature. Hence, i personally don't take it very seriously.

Answer 4

Isn't it obvious? There is another force active that we don't perceive because it works on a cosmic scale, throwing a monkey wrench into everything. This only makes sense, considering every other scale we work with has its own force.

The only question is this. Does it behave like it acts on a closed system or an open one.

Answer 5

dude what?
sorry i didnt answer it