The discussion centers on the influence of local gravitational effects on the rotation of stars at the edges of galaxies, challenging the traditional view of dark matter. Participants debated whether nearby stars could alter their rotational speeds due to mutual gravitational interactions, similar to the Earth-Moon system. The consensus is that while local gravity affects individual star orbits, it does not change the overall orbital velocity dictated by the galaxy's mass distribution. The conversation highlights the complexity of gravitational interactions in galactic dynamics and the stability of mass distribution in galaxies.
PREREQUISITESAstronomers, astrophysicists, and students of celestial mechanics interested in understanding the dynamics of galaxies and the implications of gravitational interactions on star rotation.
Hi. Could the local effects of gravity between nearby stars at the edge of a galaxy cause these stars to rotate around the centre of the galaxy more quickly than predicted if the prediction is based upon individual stars only. This effect would be similar to the way the Earth "drags" the Moon around the Sun in a different orbit than if the Moon was alone.DaveC426913 said:Well, the net effect of gravity can be zeroed out as well, say, by having mass equally distributed about the target mass.
Of course. It's like the three body problem only much worse. Two stars, if they are far enough away from others, will 'orbit' around each other approximately but they will be affected by the next nearest ones in the details of their mutual orbit and also by the whole galactic mass. Sometimes the galactic orbit of the two will be sped up a bit and sometimes it will slow down. This is what produces the density variations which are seen as spiral arms.Viopia said:Hi. Could the local effects of gravity between nearby stars at the edge of a galaxy cause these stars to rotate around the centre of the galaxy more quickly than predicted if the prediction is based upon individual stars only. This effect would be similar to the way the Earth "drags" the Moon around the Sun in a different orbit than if the Moon was alone.
No. Just because two local stars happen to be gravitationally influencing each other doesn't mean that their combined centre of mass is affected. Whether singly, or as a pair, they will still orbit the galaxy at the same rate.Viopia said:It must be extremely difficult to calculate how the outermost stars of a galaxy are affected by "local" gravity in a complex galaxy. Is it possible that the calculations, which led to the conclusion that there is a mysterious "dark matter" were incorrect?
No.Viopia said:Would it still take one year for the Earth to orbit the Sun if the Earth was the same mass as the Sun and would the orbital speed of the Earth still be the same?
Er.. what?Viopia said:Thank you for the info. Taking the "same combined mass" scenario the common centre of gravity with the Earth and the Sun with the same mass as they have in reality, the rotation point would be near the Sun's centre and so the Earth would complete a circle with a radius of 93 million miles in one year. In the "each .5 of the sun's mass - orbiting each other" combined mass scenario the rotation point would be half way between the Sun and the Earth and their period will be still be 1 year. Does this mean that the Earth would be orbiting around the centre of gravity at a lower speed than it does at the moment with the mass of the Earth and the Sun the same as it is in reality?
Your suggested three body system would not be stable, though, whereas all the existing galaxies demonstrate that the distributed arrangement is, in fact, dynamically stable (i.e. there is a lot of shuffling around inside, over time but the overall shape is maintained. I think the whole analysis should b e very different from what you propose. I'm sure Google would throw up something to help you.Viopia said:The best representation if a galaxy that I can think of is a 3 body system.
I think as long as you idealise it to two identical bodies diametrically opposite one another and in identical circular orbits it's possible. There's no net force on the central body and both forces on the orbiting bodies are colinear. So it ought to be straightforward. As you say, it's unlikely to be stable and the instant it's not symmetric you need to go for numerical simulation. And the whole thing is irrelevant to galaxies, where (unless you're going for a star-by-star simulation) I expect you'd just model it as more or less continuous and apply Gauss' theorem.sophiecentaur said:I think the whole analysis should b e very different from what you propose.
Exactly.Ibix said:I think as long as you idealise it to two identical bodies diametrically opposite one another and in identical circular orbits it's possible. There's no net force on the central body and both forces on the orbiting bodies are colinear. So it ought to be straightforward. As you say, it's unlikely to be stable and the instant it's not symmetric you need to go for numerical simulation. And the whole thing is irrelevant to galaxies, where (unless you're going for a star-by-star simulation) I expect you'd just model it as more or less continuous and apply Gauss' theorem.
But I don't think it is irrelevant to the way galaxies behave because it could model how the velocity curve can be affected by significant mass on the opposite side of the rotation point.Viopia said:Exactly.
If you want an approximate way to the behaviour then assume a uniform disc of mass and calculate how the Potential varies with position. Forget about using just two / three masses and treat the galaxy as a distributed mass. For a spherical galaxy, there is no contribution from the shells outside the orbit.Viopia said:But I don't think it is irrelevant to the way galaxies behave because it could model how the velocity curve can be affected by significant mass on the opposite side of the rotation point.
Yes, I can see it would be a better representation but more difficult to calculate. By using the 3 bodies I was merely trying to ascertain if the velocity curve can be changed by a significant mass on the other side of the rotation point. I am not interested in the values but only if my assumption is correct. Using the 3 bodies the velocities can be plotted against the distance from the "sun" which can be increased incrementally to give specific values. These can then be plotted on a graph to show the shape of the velocity curve. If my assumption is correct, and the shape of the curve can be altered by different mass distributions, it could provide evidence for an alternative explanation to "dark matter" being present. I am not a physicist and so my assumption may be incorrect, but if there is no obvious flaw in my reasoning it would be very easy to produce such a graph.sophiecentaur said:If you want an approximate way to the behaviour then assume a uniform disc of mass and calculate how the Potential varies with position. Forget about using just two / three masses and treat the galaxy as a distributed mass. For a spherical galaxy, there is no contribution from the shells outside the orbit.
Depends what you mean. Modelling any three bodies is pretty pointless because you are neglecting the other hundred billion bodies that make up the galaxy. The obvious approach to it is just to model the thing as a rotating body of fluid, and work out the gravitational effect of all the fluid. Gauss' Theorem cuts out the hard work of that if you assume the galaxy is rotationally symmetric, in which case you can say that the velocity at some radius ##r## depends on the radius ##r## and the total mass inside ##r## and everything else drops out. If you want to handle asymmetric situations you'll typically have to go to numerical simulation, and you'll need a good integrator.Viopia said:But I don't think it is irrelevant to the way galaxies behave because it could model how the velocity curve can be affected by significant mass on the opposite side of the rotation point.
Viopia said:although I can think of instances where physicists have been wrong. It was only around 100 years ago that most physicists believed that an ether permeated the whole of space.
Viopia said:Also, physics needs a healthy degree of scepticism to function properly.
Are methods "way more scientific and empirically justified". Where is the empirical evidence four string theory?weirdoguy said:The bases for claims "ather permeats space" and "we know how gravity works" were and are different and the base for latter is way more scientific and empirically justified than for the former. So sorry, but you should learn how scientific method works now, not what some people thought long time ago, when scientific methods were not so strict.
Yes, but from people who know the subject in all its details (namely - other physicists), not from layman.
Viopia said:Where is the empirical evidence four string theory?
But, if you want to emerge with a justification of your idea, your calculation has to be so accurate that your idea is needed to explain the discrepancy. The error, using your approximation would be vast, compared with any effect that you could attribute to dark matter.Viopia said:Yes, I can see it would be a better representation but more difficult to calculate.
That's not the string story at all. String theory is a possible explanation for the way things work at a very low level. Last time I read about it, it was a fairly coherent story and that makes it attractive. The claims for string theory are only based on that but the ideas of string theory are not just in isolation; they are not disproved by observation, which is different from saying they are proved.Viopia said:Where is the empirical evidence four string theory?
If our maths was incorrect we wouldn't be able to predict the motions of planets and spaceships correctly. Yet we can do that precisely enough that we need to take into account the non-spherical shapes of the planets. Non-trivial mass distributions are well understood.Viopia said:The gravitation formula is simple but it's amazing how complicated things can become when there are hundreds of billions of gravitational bodies in a system.