## Galaxy simulation - Philosophical considerations.

The idea behind the programs "VB gal 2D" and "VB gal MOND" is to simulate a spiral galaxy. A spiral galaxy consists of a bulge and a disc.
The physical concept behind the simulation is to calculate the position of all the stars at different equally spaced moments. When you do that you will see that the stars rotate around a common center.
The two pictures below show the result such a simulations. Each simulation in both pictures involves 600 stars.
 Picture 1 ``` ^ | | | ``` Picture 2 ``` ^ | | | ```
Each picture shows the actual position of the 600 stars and the historical (past) positions.
• The left picture shows the initial position of the stars and almost no historical positions.
• The right picture shows a much longer simulation and more historical positions. As such you can follow the positions of each star. Each path follows a circle and the galaxy is stable. (*)
The simulation in this case consists of 20 rings (equally spaced) and 30 stars (equally spaced). That means 600 stars in total. Each star in each ring has the same mass. These stars are massif.

In the simulation all the positions of the stars are calculated as a function of the previous positions of all the stars and when that is done the display is updated. That means when you observe the display at each update you get an instantaneous diplay of the present position of all the stars.
In reality this is not possible. Suppose you are hanging in a helicopter straight above the center (Black Hole) of the galaxy. The most current view at that position is the center of the galaxy. All the other stars you observe in the past. This delay is a function of distance and the speed of light.

The left picture shows (almost) the initial position of all the stars. The structure is like the spokes of a wheel in straight lines. In each simulation that means there are 30 spokes.
In reality when you are hovering above the center when you start the simulation what you see is first the star of the most inner ring. This is "star 1". Next the star in ring two. This is "star 2" etc etc and finally star 30. All these 30 stars become visible in a straight line. However when "star 30" becomes visible "star 1" is not any more at its initial position but has moved forward, counter clockwise.

The reason why this exercise is important is because when you consider to perform a simulation based on real situation i.e. actual stars. Suppose you are capable to measure instantaneous the position of all the stars. In that case, even if you can measure all the positions simultaneous, what you have measured are not the positions of all the stars at the same moment but at different moments in the past. This time difference in the past is a function of the distance and the speed of light. To calculate the positions at the same moment you need at least the speed of all the stars.

The same problem arises when you want to test at the end of the simulation the predicted results in the future with actual observations: again you have to take the time delays into account.

The important lesson is here that only when actual observations are involved, that means at the beginning to calculate the initial positions and at the end to test the results, the speed of light is important, not during the actual simulation i.e. calculation. The simulation is a mathematical model that describe the processes involved, in this case the movements of the stars in a galaxy. These processes are almost completely independent of the speed of light, photons neutrinos and single atomic particles.

(*) When you observe the simulations sooner or later chaos starts and all structures disappear. The reason does not lie in the physical realm, but is purely caused by computer limitations i.e. accuracy.

### Spiral arms

In this simulation the total number of stars is 600. That is very small. In reality the Milky Way consists of 10^12 Solar masses and the radius is between 50000 and 90000 lightyears. This information comes from Wikipedia. Select this: Milky Way This same document also shows the GRC. Select this: Milky Way - GRC To observe the GRC from M33 select this:M33 GRC
The most important lesson to learn is that the two GRC's are rather different. This requires an explanation.

As mentions above the simulation only involves 600 stars. In fact each of these stars is a collection of thousands of stars. By combining the stars you make the simulation possible. In Picture B you see an image of a spiral arm. In reality when you perform a simulation with more stars you will not get spiral arms, but a structure which is only rotational symmetric like an elliptical galaxy. To observe spiral arms you have to inject huge "gas clouds" which will move radial towards the center of the galaxy.

### Dark Matter

The issue of spiral arms also touches a different issue and that is the concept of dark matter. Dark matter is supposedly not the same as ordinary matter or baryonic matter but something different i.e. non-baryonic matter. Baryonic matter we also call visible matter. This naming convention is rather strange because all matter is inprinciple invisible. When you place yourself in a room and you turn off all the light you become not visible, still the matter human beings are made from (= baryonic matter) is called visible matter.
This baryonic matter is only becoming visible when the internal processes involved are heated and the temperature increases. When this happens the radiation they emit change into the visible frequency range and become visible by the human eye. That means that baryonic matter depending about certain circumstances can be either observed or is not observed. As a consequence it is very difficult to measure directly (based on visibility) all the baryonic matter in a galaxy, because a huge amount can de invisible for the human eye.

A similar problem exists when you study the spiral arms in a spiral galaxy. Spiral arms are parts of a galaxy disc which contains more visible mass (stars) than in the space in between. That means it does not make sense to proclaim that there is more baryonic matter in this "in between" space than in the arms. The issue is how do you explain this apparent discrepancy, the fact that the supposed involved dark matter is not every where available in the same amounts. With explain meaning the processes involved i.e. galaxy evolution. A much more logical assumption is that throughout the disc there is baryonic dust in order to explain the missing matter issue.

### Black Hole

In the center of the Milky Way there exists a Black Hole. A Black Hole is nothing more than a huge massif object consisting of baryonic matter. The name is in fact a misnomer, because it makes a link to human quality i.e. our eyes. The fact that a Black Hole does not emit photons could be related to its mass as a function of escape velocity. How ever that is not for sure. A better explanation whould be linked to the internal processes inside a very large mass which should not produce photons. However that is a pure guess.
Common understanding is that a Black Hole is invisible and does not emit anything. Also this is wrong. In fact each Black Hole is "visible" by all other masses in the Universe. The clearest prove are the stars and "gas clouds" which rotate around the BH in the center of the Milky Way". This "visibility" requires a different way of thinking. It is the same for a fish in the water which does not "understand" the concept of water. For a fish an ocean empty. Also for us the so called empty space is not empty but filled with photons. When you look towards the sun the only photons you can see (observe, measure, detect) are the one's who travel in a straight line towards your eye. In reality the sun emits each second "trillion" of photons in all directions.
The concept of light travelling in a straight line is also requires some thoughts. The problem is that light, the way we humans observe it, reaches us almost always in a straight line except in the case near matter. In reality light is emitted in al directions, in a sphere, with a speed independent of the source. Near sun-like masses a single light ray is bended.

### Simultaneous - Absolute versus Relative

The starting point of any simulation are "at least" 3 collections of the observed positions of the masses of the objects involved at equally spaced moments. (This last is not a must). Using these positions and the speed of light you can calculate the initial positions for the same objects but now for the (equally spaced) moments or simultaneous events. When you perform such an exercise "in principle" starting from a different observation point you should get the same initial positions and velocities.

When you are in a train at rest at a platform you can calculate the speed of an approaching train by performing different observations. For an observer at rest not only the platform, but also the horizon and the earth are at rest.
From the point of view of an observer on the moving train, the moving train is at rest. For this observer the train at the platform is moving, so is the platform, so is the horizon and so is the earth.
Not so long ago the Earth was considered the center of the universe. Than along came Nicolaus Copernicus who unveiled that this is not correct, but that the Sun is at the center of the solar system. The result is that the Sun is more or less at rest and the planets are moving. For the Earth Moon system a very similar solution exists: The Earth is at rest and the Moon is moving.
The same for any subject on the surface of the Earth: The Earth is at rest and all the objects (including humans) are considered moving.
For a galaxy the same logic applies: The center of the Milky Way galaxy is at rest and all the stars are moving.
But when more galaxies are involved also that is true.

This type of reasoning should be considered when you want to simulate a galaxy

### Super nova

A super nova is an exploding star which emits radiation at "all" frequencies including light, in a sphere. The question is: how do you draw this sphere inside the rotating galaxy. In picture 2 above the supernova starts as a point at a certain position P. This point grows, becomes a small circle, which grows and grows. The question does the point P stay at the center of this circle.

A rather similar question is: Suppose you have two galaxies (at large distance) which move around/towards each other. In each galaxy there is a supernova. The tricky part is you can only have one answer which should equally apply to both. The points where the supernovae happen are P and Q. For example: it can not be that point P is at the center of his circle but point Q not. They should be either stay both at the center or both not.

Created: 24 August 2015

Back to my home page Contents of This Document