# Thread: Is dark matter and energy needed?

1. Does a universe that begins and ends in a "big bounce" need dark matter/energy? In the big bounce theory gravity would eventually pull the universe back to a single point only to start expanding again. If dark matter would cause the universe to infinitely expand then wouldn't this interfere with the collapse?

2.

3. Originally Posted by IrishStu
If dark matter would cause the universe to infinitely expand then wouldn't this interfere with the collapse?
Don't you mean dark energy?
I understand that there is another theory to explain the expansion of the universe, unlikely that it may seem: Our U is surrounded by larger ones, causing ours to expand in accordance with their gravitational pull, and with gravity being an accelerating force this explains the accelerating rate of our U's expansion, so there is no such thing as dark energy. It might also explain why galaxies are variable distances apart.

4. Does a universe that begins and ends in a "big bounce" need dark matter/energy? In the big bounce theory gravity would eventually pull the universe back to a single point only to start expanding again. If dark matter would cause the universe to infinitely expand then wouldn't this interfere with the collapse?
First off, I think you mean the 'big crunch' theory, wherein gravity pulls back the matter off the universe back into a single point. Nobody can really hypothesise that the universe will start expanding afterwards - singularities arise by the time you get all of the matter in the universe together, making attempts at predicting what happens afterwards nigh on impossible.

Secondly, it is also only a possibility. There are three kinds of models for the way the universe might, for want of a better word, evolve, called Friedmann models. The first deals essentially with a 'big crunch' state of affairs, whereby the acceleration of the universe is slower than the force of gravity pulling it back together. The second deals with the case where the acceleration of the universe outwards is greater than that offered by the force of gravity - such a universe will end, instead, in a 'big freeze', as heat sources move progressively further and further away. A third - and unusual - case is when the two forces are roughly equal. While this universe will never actually come to as magnificent a conflagration as the 'big crunch' theory seems to promise, it will not be quite as rapid an expansion as the 'big freeze' version, either. The 'big crunch' model is therefore only one of three possible alternatives the universe might consider.

With that in mind, I'd ask you to reconsider your use of the word 'need' in your question. There is no given, after all, that the universe has to obey our laws and theories; there can never be any such thing as the universe 'needing' something or not - only human beings requiring something or the other in their theories to help understand the universe better. Dark matter is not a necessary condition for the universe to cease from collapsing (if that is so) - it could very well be that the force of gravity is much stronger than the expansive force dark matter seems to promise, resulting in collapse nevertheless.

Furthermore, it is important to remember that dark energy/matter (the two terms are practically equivalent in their usage) is only a theory. No experiment has offered evidence for it; it is simply a theory which scientists seem to like at the moment.

I hope that answers your question, and if it does not, please feel free to tell me; I'll try to do better next time.

5. Thanks guys. That grand! I was asking strictly from a "bouncing universe" point of view though but I believe you have answered my question.

6. Originally Posted by IrishStu
Thanks guys. That grand! I was asking strictly from a "bouncing universe" point of view though but I believe you have answered my question.
I'd pretty much ignore everything said by the last two posters.

A "bouncing" universe would require that the expansion of the Universe slows over time due to its own gravitational attraction, and that this slowing will be enough to eventually stop the expansion. The analogy is throwing a ball into the air and having it fall back.

Another possibility would be that gravity does slow the expansion, but that the expansion is never slowed to a complete stop. The analogy would be throwing a ball upwards at greater than escape velocity. The loses speed as it climbs, but since gravity gets weaker as it climbs, gravity never can stop it completely and pull it back down.

Until the 90's these were the two possibilities in consideration. In order to determine which was the case, a method was used to test which was right. Observations were made which allowed us to compare the distance of a galaxy to it its observed speed of recession. Since the further away a galaxy is the further from its past the light we get from it left, we can use this to plot the expansion of the universe over time, and get the trend its expansion was following. Depending on the rate at which the expansion has slowed over time, we could tell it s eventual fate.

The surprise came when the data showed that the universe's expansion was actually speeding up over time. Something appeared to be actively pushing the universe apart. Just exactly what, was unknown, so the term "dark energy" was coined for it. (the word "dark" is that only thing it has in common with "dark matter", a term that had already been coined.) It was a label for whatever caused the accelerating expansion, just like x-rays were a label for an unknown type of radiation when they were first detected("x" standing for "unknown".)

So the answer to as to "Why do we need dark energy?", is that it explains what we observe.

Dark matter, on the other hand refers to mass, that by it gravitational effect, appears to be there but isn't visible. It was first hypothesisied to explain rotation curves of galaxies. Dark matter, despite what Liongold says, is on much more firm footing than dark energy. The evidence that it is some type of particulate matter rather than a quirk in gravity has grown over time.

It works roughly like this: You determine what type of properties DM needs to have to explain the galaxy rotation curves. You then use these properties to predict what effect DM would have on other observations (Gravitational lensing, the large scale structure of the Universe, etc) You then compare the actual observations against what the DM model predicts. To date, no model that tries to explain things by modifying gravity has been able to fit all the observations, while DM does.

The most recent is the observation of the Bullet cluster, which is the result of two clusters of galaxies colliding. By looking at the different components of the collision, you can model what it would take to get the results seen. The DM model fits what we see, and it can be shown that no model of modified gravity can be made to do so with out including some form of DM.

7. I'll just add that the current best understanding of how the universe works suggests it will end in a Big Rip as dark energy pushes things apart faster than light until even basic particles fall apart. (Note that this does not contradict relativity, which only says that nothing massive can move at or above light speed through space. Space is free to expand as fast as it wants.)

There's a possibility that when things smooth out enough, there could be some string-theoretic event that would cause another big bang, but that's very speculative until we develop a much better understanding of string theory and can actually start testing it.

The Big Crunch and the Big Freeze are pretty much out of the running.

8. Originally Posted by IrishStu
Does a universe that begins and ends in a "big bounce" need dark matter/energy? In the big bounce theory gravity would eventually pull the universe back to a single point only to start expanding again. If dark matter would cause the universe to infinitely expand then wouldn't this interfere with the collapse?
Dark matter has littlr to do with questions of cosmology. The dark matter hypothesis exists as a possible explanation for the rotation rate of many galaxies which requires a gravitayional force that greatly exceeds that which can be attributed to the observed mass. Ther is other evidence, including gravitational lensing effects that supports its existence.

dark energy, which is quite different from dark matter, is hypothesized yo explain the observed acceleration of the expansion of space. An increasing rate of expansion of space is inconsistent with a "big bounce".

9. Originally Posted by Janus

Observations were made which allowed us to compare the distance of a galaxy to it its observed speed of recession. Since the further away a galaxy is the further from its past the light we get from it left, we can use this to plot the expansion of the universe over time, and get the trend its expansion was following. Depending on the rate at which the expansion has slowed over time, we could tell it s eventual fate.

The surprise came when the data showed that the universe's expansion was actually speeding up over time. Something appeared to be actively pushing the universe apart. Just exactly what, was unknown, so the term "dark energy" was coined for it.
So the answer to as to "Why do we need dark energy?", is that it explains what we observe.

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Is it not true that we see certain specific objects accelerating away but bring the whole universe into our conclusions? If so, might not the extrapolation be stretching things a bit?

10. No, not certain specific objects, but every object we've measured, and at a rate consistent with a uniform stretching.

11. Can I ask how it is that we know the expansion is not just local to our observable region of the universe (or to some extent beyond it)? Is it possible for different areas of the universe outside of the observable region to have different expansion/contraction properties? How do we know that we dont just happen to be in a local bubble of expanding space, perhaps so large as to appear uniform through our current measurements? Also the most distant objects we have observed are still tens of billions of light years away from the estimated edge of the OU.

I suspect I have some misconceptions about observable universe in relation to the universe as a whole.

12. Originally Posted by MagiMaster
No, not certain specific objects, but every object we've measured, and at a rate consistent with a uniform stretching.
1. I confess to having thought that cosmic acceleration theory stemmed strictly from intensity measurements of certain fixed-luminosity stars. Have no quibble with just old expansion.
2. Have we any appropriate measurements pertaining to motion of any dark matter?

13. The dark matter has gravitational effect only. However, if this mass close to stars then stars should change it's own orbits trajectories close to this dark matter substance. Actually, it should be same behavior if stars close to black hole. I'm not sure the astrophysics see this. Therefore dark matter is a different class of objects.

My idea is the galaxies clusters can have different velocities because during interaction time their velocities may not follow by Newtons laws globally because these laws work locally during objects interactions. For example, my kind of research
shows the objects with same mass may not have same velocities because they have different type of motions after repulsive action. Each motion follows by it's own law of momentum conservation. However, it shows illusion of Newtons law incompatibility if law of momentum conservation is applying to whole object for one type of motion(translational motion). This could be applied for whole universe.

14. @harvestein, we know nothing beyond the observable universe. How could we? It's possible that there's more out there that behaves in unexpected ways, but it's pure speculation.

@dalemiller, ok, I should have said a uniform but accelerating stretching. And though completely unrelated, yes, there are measurements made of the clouds of dark matter moving away from galaxies after a collision (that have, in fact, already been posted in this thread, if you care to look).

15. Originally Posted by MagiMaster
@dalemiller, ok, I should have said a uniform but accelerating stretching. And though completely unrelated, yes, there are measurements made of the clouds of dark matter moving away from galaxies after a collision (that have, in fact, already been posted in this thread, if you care to look).
My query was meant to inquire as to whether any accelerated departure of dark matter from Earth has been detected. My two questions are related to my single quest for understanding how tenuous the conclusion might be that the entire universe is under acceleration in the outward direction.

16. @MagiMaster
The trouble with all this is that dark matter seems to be an excuse to preserve the validity of our equations which describe gravity. Scientists have basically just made it up to keep GR in working order. A much better avenue to go down is to question our fundamental understanding of gravity, after all, that is the main thing involved here. Gravity should depend on the shape of the body in question, and it surely must also depend on the relative positions of those bodies as well, things which haven't been well considered in current theories

17. Originally Posted by Waveman28
@MagiMaster
The trouble with all this is that dark matter seems to be an excuse to preserve the validity of our equations which describe gravity. Scientists have basically just made it up to keep GR in working order. A much better avenue to go down is to question our fundamental understanding of gravity, after all, that is the main thing involved here. Gravity should depend on the shape of the body in question, and it surely must also depend on the relative positions of those bodies as well, things which haven't been well considered in current theories
My face is getting sore from all the facepalms.

18. Originally Posted by Waveman28
@MagiMaster
The trouble with all this is that dark matter seems to be an excuse to preserve the validity of our equations which describe gravity. Scientists have basically just made it up to keep GR in working order. A much better avenue to go down is to question our fundamental understanding of gravity, after all, that is the main thing involved here. Gravity should depend on the shape of the body in question, and it surely must also depend on the relative positions of those bodies as well, things which haven't been well considered in current theories
That's...bold of you to state. Do you have any idea what current accepted theories state? At all?

And yes, face-palm.

19. Originally Posted by TheDr.Spo
That's...bold of you to state. Do you have any idea what current accepted theories state? At all?

And yes, face-palm.
Yes I do. And that was a wasted face-palm, save those for occasions which actually warrant them, that goes for you too MagiMaster.

20. Originally Posted by Waveman28
Originally Posted by TheDr.Spo
That's...bold of you to state. Do you have any idea what current accepted theories state? At all?

And yes, face-palm.
Yes I do. And that was a wasted face-palm, save those for occasions which actually warrant them, that goes for you too MagiMaster.
Your previous comment definitely warranted at least one. It's almost like you hadn't bothered to read anything anyone posted in this entire thread.

21. Originally Posted by MagiMaster
Your previous comment definitely warranted at least one. It's almost like you hadn't bothered to read anything anyone posted in this entire thread.
I read the thread from front to back. Have I missed something....?

22. It's not about reading the entire thread. Have you read and understood current theories? Assuming you have read any of them, you deduce that "dark matter" has been made up on some sort of scientific whim?

One of the earliest to hypothesize its existence as a universal repulsive force, something he called the "cosmological constant", was Einstein. Frustrated by the inability to include it in his theory if a stable universe, he tried to throw it away as a mistake, setting it to zero, but it wasn't that easy. It was brought up again later under another name, "vacuum energy". The vacuum energy is exactly what is sounds like it is, and is described by opposing pairs of particles or photons, such as an electron and a positron, being spontaneously created, circling each other and then annihilating, releasing energy. This all happens in a fraction of a second. I'll even go as far as to mention the fact that measurements of energy states of Hydrogen atoms are so precise, so sensitive, to such exquisite accuracy, that they are sensitive to vacuum energy.

23. Originally Posted by Waveman28
@MagiMaster
The trouble with all this is that dark matter seems to be an excuse to preserve the validity of our equations which describe gravity. Scientists have basically just made it up to keep GR in working order. A much better avenue to go down is to question our fundamental understanding of gravity, after all, that is the main thing involved here. Gravity should depend on the shape of the body in question, and it surely must also depend on the relative positions of those bodies as well, things which haven't been well considered in current theories
Alright, my bad. I was sure this had been mentioned in this thread, but it was mentioned in the other one: http://www.thescienceforum.com/viewt...=263022#263022.

Anyway, the point is, modifying gravity as an explanation for dark matter has been largely ruled out due to other measurements that have been made.

And again, GR won't suddenly fall apart if dark matter as a physical thing turns out to be false.

Also, I don't know how you think gravity doesn't depend on shape or position in current theories. That just makes absolutely no sense.

24. Originally Posted by MagiMaster
Anyway, the point is, modifying gravity as an explanation for dark matter has been largely ruled out due to other measurements that have been made.
Why wouldn't a different understanding of gravity's behaviour on such scales and speeds help us understand how the distance between stars in rotating galaxies remains constant? For example, in rotating objects, forces are surely subject to a rotational doppler effect. This will certainly affect the rotation rate of the stars in a galaxy and may explain why the outer stars move faster than the inner ones.

And again, GR won't suddenly fall apart if dark matter as a physical thing turns out to be false.

25. Originally Posted by Waveman28
Originally Posted by MagiMaster
Anyway, the point is, modifying gravity as an explanation for dark matter has been largely ruled out due to other measurements that have been made.
Why wouldn't a different understanding of gravity's behaviour on such scales and speeds help us understand how the distance between stars in rotating galaxies remains constant? For example, in rotating objects, forces are surely subject to a rotational doppler effect. This will certainly affect the rotation rate of the stars in a galaxy and may explain why the outer stars move faster than the inner ones.

And again, GR won't suddenly fall apart if dark matter as a physical thing turns out to be false.
It was a possibility when the only observations were the rotation curves. The halo of dark matter (measurable through gravitational lensing) moves away from the regular matter during galactic collisions though, which isn't explainable by modifying gravity. However, it is exactly what would be predicted for a cloud of invisible (doesn't-interact-with-photons invisible that is) matter around the galaxies.

26. Originally Posted by Waveman28
Originally Posted by MagiMaster
Anyway, the point is, modifying gravity as an explanation for dark matter has been largely ruled out due to other measurements that have been made.
Why wouldn't a different understanding of gravity's behaviour on such scales and speeds help us understand how the distance between stars in rotating galaxies remains constant?
The stars don't maintain a constant distance from each other.
For example, in rotating objects, forces are surely subject to a rotational doppler effect. This will certainly affect the rotation rate of the stars in a galaxy...
[i]1. Forces are not subject to rotational Doppler effect any more than they are to standard Doppler effect
2. The Rotational Doppler effect is something measurable by an observer looking along the axis of rotation and not sharing the rotation of the source. So even if it effected forces, it would have no effect on objects in the disc of the galaxy and sharing its rotation.
3. The Rotational Doppler effect is very weak even with light. It is subject to the rate of rotation and not the linear velocity of the individual parts. Since the rotational rates of galaxies are very low, in the range of one rotation per few 100 million years, even if we ignore the last two points, any rotational doppler shift effect on stars in a galaxy would be insignificantly small.
...and may explain why the outer stars move faster than the inner ones.
But they don't. What happens is that after you get a certain distance from the core of the galaxy, the orbital speeds start to even out; the outer stars speeds begin to equal that of ones nearer in. Inside of this distance you see what one would expect, inner stars speeds being greater than outer star speeds.

At this point all you've managed to do is betray your ignorance of the subject by displaying a misunderstanding of the actual dynamics of observed galactic rotation. You then only made things worse by throwing around a term (rotational Doppler effect) of which you had no real understanding.

Earlier in this thread someone asked you if you had any idea of what present theories said and you replied "yes".

But if you really did, then you would not have made the statements that you made in this post because it would have very quickly become clear that they were not meant to describe the type of galaxy rotation you describe in this thread.

That being the case, why would anyone care as to what your opinion on the subject is?

27. Originally Posted by TheDr.Spo
It's not about reading the entire thread. Have you read and understood current theories? Assuming you have read any of them, you deduce that "dark matter" has been made up on some sort of scientific whim?

One of the earliest to hypothesize its existence as a universal repulsive force, something he called the "cosmological constant", was Einstein. Frustrated by the inability to include it in his theory if a stable universe, he tried to throw it away as a mistake, setting it to zero, but it wasn't that easy. It was brought up again later under another name, "vacuum energy". The vacuum energy is exactly what is sounds like it is, and is described by opposing pairs of particles or photons, such as an electron and a positron, being spontaneously created, circling each other and then annihilating, releasing energy. This all happens in a fraction of a second. I'll even go as far as to mention the fact that measurements of energy states of Hydrogen atoms are so precise, so sensitive, to such exquisite accuracy, that they are sensitive to vacuum energy.
You are confusing dark matter and dark energy. Dark matter has nothing to do with rhe cosmologcal constant.

Dark energy does, There is no known connection between the two.

28. Originally Posted by Janus
1. Forces are not subject to rotational Doppler effect any more than they are to standard Doppler effect.
All forces are subject to doppler effects. Gravity, electric fields and magnetic fields all propagate at the speed of light. The density of the field affects the rate of acceleration, the higher the density, the greater the force. The density of the field in front of a moving source is higher than the density of the field behind it. Therefore, the field of force in front of a moving or rotating body, such as a star in this case, will be stronger. This is pretty basic stuff, I think you may be looking at this from the wrong point of view.

2. The Rotational Doppler effect is something measurable by an observer looking along the axis of rotation and not sharing the rotation of the source. So even if it effected forces, it would have no effect on objects in the disc of the galaxy and sharing its rotation.
Yes it would, it would have a "dragging" effect, because the rotation rates between the stars are slightly different. Not to mention that you are also ignoring that other forces such as electrostatic fields could be playing a small role as well.

3. The Rotational Doppler effect is very weak even with light. It is subject to the rate of rotation and not the linear velocity of the individual parts. Since the rotational rates of galaxies are very low, in the range of one rotation per few 100 million years, even if we ignore the last two points, any rotational doppler shift effect on stars in a galaxy would be insignificantly small.
Galaxies, as you know, are many millions of light years across. The stars within them, especially those on the outer spiral bars, have to travel huge distances to complete one complete rotation. In this case, the rotations and linear motions of the individual stars become more important.

...and may explain why the outer stars move faster than the inner ones.
But they don't. What happens is that after you get a certain distance from the core of the galaxy, the orbital speeds start to even out; the outer stars speeds begin to equal that of ones nearer in. Inside of this distance you see what one would expect, inner stars speeds being greater than outer star speeds.
Look, its all quite simple actually, its just a puzzle of interacting forces between different bodies, which can confuse people when large distances, speeds and structures are involved. My main point is that there is no need whatsoever to bring in another fabrication such as dark matter to complicate and distort the situation even further.

That being the case, why would anyone care as to what your opinion on the subject is?
Because at least my opinions about galaxy rotation are down to earth (sorry about the pun) and are less crazy than those proposed by others who resort to an invisible, non-existent substance they call dark matter. My opinion is in line with classical understanding, and doesnt require any new imaginative fabrication. All I am saying is that we just need to look at the situation logically, and we also need to do a complete review of the history of physics because clearly some fundamental errors have been made on the way which have amounted to the situation we are in now.

PS: we need an Ether!

29. Originally Posted by DrRocket
Originally Posted by TheDr.Spo
It's not about reading the entire thread. Have you read and understood current theories? Assuming you have read any of them, you deduce that "dark matter" has been made up on some sort of scientific whim?

One of the earliest to hypothesize its existence as a universal repulsive force, something he called the "cosmological constant", was Einstein. Frustrated by the inability to include it in his theory if a stable universe, he tried to throw it away as a mistake, setting it to zero, but it wasn't that easy. It was brought up again later under another name, "vacuum energy". The vacuum energy is exactly what is sounds like it is, and is described by opposing pairs of particles or photons, such as an electron and a positron, being spontaneously created, circling each other and then annihilating, releasing energy. This all happens in a fraction of a second. I'll even go as far as to mention the fact that measurements of energy states of Hydrogen atoms are so precise, so sensitive, to such exquisite accuracy, that they are sensitive to vacuum energy.
You are confusing dark matter and dark energy. Dark matter has nothing to do with rhe cosmologcal constant.

Dark energy does, There is no known connection between the two.
You're right. My bad, hahaha. Skimming over, I must have read "dark matter" and that translated to "dark energy"... even as I wrote it. I misspoke.

30. Originally Posted by Waveman28
Originally Posted by Janus
1. Forces are not subject to rotational Doppler effect any more than they are to standard Doppler effect.
All forces are subject to doppler effects. Gravity, electric fields and magnetic fields all propagate at the speed of light. The density of the field affects the rate of acceleration, the higher the density, the greater the force. The density of the field in front of a moving source is higher than the density of the field behind it. Therefore, the field of force in front of a moving or rotating body, such as a star in this case, will be stronger. This is pretty basic stuff, I think you may be looking at this from the wrong point of view.
Stuff and nonsense. Gravitational radiation and electromagnetic radiation propagate at the speed of light. The fields responsible for the forces do not. It not a matter of looking from the wrong point of view, its a matter of looking at thing from the side of the facts.

2. The Rotational Doppler effect is something measurable by an observer looking along the axis of rotation and not sharing the rotation of the source. So even if it effected forces, it would have no effect on objects in the disc of the galaxy and sharing its rotation.
Yes it would, it would have a "dragging" effect, because the rotation rates between the stars are slightly different.
More evidence that you don't know what "rotational Doppler effect" means; it's just a term you pulled out of the air because it sounded good. A dragging effect would work both ways. The mass in the arms of the galaxy is insignificant compared to the mass of the core, thus the core would dominate gravitationally. If such a dragging effect occurred, it would be the most pronounced closer in towards the core. Instead we find that the stars orbiting near the core follow a normal rotation curve. as opposed to the outer arms stars which due not. Do you ever bother to actually think out these ideas all the way before you blurt them out?
Not to mention that you are also ignoring that other forces such as electrostatic fields could be playing a small role as well.
Riiight. Astronomers who spend their entire careers examining the universe using the electromagnetic spectrum are going to ignore electrostatic interaction. Do you have any more beauties like this?

3. The Rotational Doppler effect is very weak even with light. It is subject to the rate of rotation and not the linear velocity of the individual parts. Since the rotational rates of galaxies are very low, in the range of one rotation per few 100 million years, even if we ignore the last two points, any rotational doppler shift effect on stars in a galaxy would be insignificantly small.
Galaxies, as you know, are many millions of light years across. The stars within them, especially those on the outer spiral bars, have to travel huge distances to complete one complete rotation. In this case, the rotations and linear motions of the individual stars become more important.
The orbital velocity of the Sun around the galaxy is 200km/sec. If we plugged that value into the Doppler effect formula, you would a shift of only 1.00067. Insignificant compared to what is needed to explain the rotation curves. And that does not even factor in that any Doppler shift would be due to relative velocity differences between stars which would be much smaller. Add in the fact that it in no way accounts for why the galaxies don't fly apart due to their high rates of rotation. This idea is wrong on so many levels that it is laughable.

...and may explain why the outer stars move faster than the inner ones.
But they don't. What happens is that after you get a certain distance from the core of the galaxy, the orbital speeds start to even out; the outer stars speeds begin to equal that of ones nearer in. Inside of this distance you see what one would expect, inner stars speeds being greater than outer star speeds.
Look, its all quite simple actually, its just a puzzle of interacting forces between different bodies, which can confuse people when large distances, speeds and structures are involved.[/quote]It certainly confuses you, but others are quite capable of handling it.

My main point is that there is no need whatsoever to bring in another fabrication such as dark matter to complicate and distort the situation even further.
You have no point. All you have is a bunch of hand-waving based on misconceptions. You have not presented one logical argument.

That being the case, why would anyone care as to what your opinion on the subject is?
Because at least my opinions about galaxy rotation are down to earth (sorry about the pun) and are less crazy than those proposed by others who resort to an invisible, non-existent substance they call dark matter. My opinion is in line with classical understanding, and doesnt require any new imaginative fabrication. All I am saying is that we just need to look at the situation logically, and we also need to do a complete review of the history of physics because clearly some fundamental errors have been made on the way which have amounted to the situation we are in now.
Your ideas are stuff and nonsense and is in no way "in line with classical understanding". For you to believe so just goes to show how out of touch with real physics you are.

PS: we need an Ether!
I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.

After all, dark matter would just simply be composed of particles that have mass and do not interact electromagnetically. Since it takes electromagnetic interaction to absorb, emit or affect light, it is undetectable by such means.

There nothing absurd about this. Just because we are used to dealing with matter that does interact with light, doesn't mean that all matter has to. In fact, we already know that such matter is possible and have an example in the neutrino.

The upshot is that dark matter is a more valid and supported hypothesis than any of your imagined and ill thought out mechanisms.

31. Stuff and nonsense. Gravitational radiation and electromagnetic radiation propagate at the speed of light. The fields responsible for the forces do not. It not a matter of looking from the wrong point of view, its a matter of looking at thing from the side of the facts.
Gravitational and electromagnetic fields of force do travel at the speed of light.

More evidence that you don't know what "rotational Doppler effect" means; it's just a term you pulled out of the air because it sounded good. A dragging effect would work both ways. The mass in the arms of the galaxy is insignificant compared to the mass of the core, thus the core would dominate gravitationally. If such a dragging effect occurred, it would be the most pronounced closer in towards the core. Instead we find that the stars orbiting near the core follow a normal rotation curve. as opposed to the outer arms stars which due not.
Gravity may not be linearly additive, which means that there could be a lensing effect which would result in gravity being stronger at longer distances in galaxies than what the inverse law suggests normally. That could be game over for dark matter.

The orbital velocity of the Sun around the galaxy is 200km/sec. If we plugged that value into the Doppler effect formula, you would a shift of only 1.00067. Insignificant compared to what is needed to explain the rotation curves. And that does not even factor in that any Doppler shift would be due to relative velocity differences between stars which would be much smaller. Add in the fact that it in no way accounts for why the galaxies don't fly apart due to their high rates of rotation. This idea is wrong on so many levels that it is laughable.
I am attempting to explain this in the same vein as "dark energy". Dark energy does not exist because "Space time" is not expanding, because it too doesnt exist. We can easily and simply explain the cosmological red shift by saying that galaxies are physically moving away from each other. That would result in a doppler shift without the need for the expansion of space. I am just trying to explain things in terms of mechanics, and I admit that a few things I have said so far about galaxy rotation could be wrong.

I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
2. It is not bizzare
3. It does not have much less evidence of existence of existence than dark matter.

Everything you just said in that whole paragraph was wrong, and wreaks of hypocrisy.

However, you did prove that some of my ideas about the behaviour of galaxies were wrong and I thank you for that.

32. Originally Posted by Waveman28
I am attempting to explain this in the same vein as "dark energy". Dark energy does not exist because "Space time" is not expanding, because it too doesnt exist. We can easily and simply explain the cosmological red shift by saying that galaxies are physically moving away from each other. That would result in a doppler shift without the need for the expansion of space. I am just trying to explain things in terms of mechanics, and I admit that a few things I have said so far about galaxy rotation could be wrong.
You must keep in mind that galaxies are not only moving away from each other but accelerating away from each other as well.

33. Originally Posted by TheDr.Spo
You must keep in mind that galaxies are not only moving away from each other but accelerating away from each other as well.
Indeed, but that doesn't really affect the possibility of the red shift being caused by simple relative motion. I am also interested in the "jerk" of the acceleration rate, that is, is the acceleration increasing or decreasing?

34. Originally Posted by Waveman28
Stuff and nonsense. Gravitational radiation and electromagnetic radiation propagate at the speed of light. The fields responsible for the forces do not. It not a matter of looking from the wrong point of view, its a matter of looking at thing from the side of the facts.
Gravitational and electromagnetic fields of force do travel at the speed of light.
Oh yeah, the bolded "do" makes all the difference. You are just exposing the fact that you do not know what a "field" is.

More evidence that you don't know what "rotational Doppler effect" means; it's just a term you pulled out of the air because it sounded good. A dragging effect would work both ways. The mass in the arms of the galaxy is insignificant compared to the mass of the core, thus the core would dominate gravitationally. If such a dragging effect occurred, it would be the most pronounced closer in towards the core. Instead we find that the stars orbiting near the core follow a normal rotation curve. as opposed to the outer arms stars which due not.
Gravity may not be linearly additive, which means that there could be a lensing effect which would result in gravity being stronger at longer distances in galaxies than what the inverse law suggests normally. That could be game over for dark matter.
Multiple attempts have already been made to modify gravity to explain away dark matter( by people a lot more versed in what it would take than you), and none of them could be made to match all the observations. The most recent observations have revealed that no mere modification of gravity can.

The orbital velocity of the Sun around the galaxy is 200km/sec. If we plugged that value into the Doppler effect formula, you would a shift of only 1.00067. Insignificant compared to what is needed to explain the rotation curves. And that does not even factor in that any Doppler shift would be due to relative velocity differences between stars which would be much smaller. Add in the fact that it in no way accounts for why the galaxies don't fly apart due to their high rates of rotation. This idea is wrong on so many levels that it is laughable.
I am attempting to explain this in the same vein as "dark energy". Dark energy does not exist because "Space time" is not expanding, because it too doesnt exist. We can easily and simply explain the cosmological red shift by saying that galaxies are physically moving away from each other. That would result in a doppler shift without the need for the expansion of space.
Don't you ever get tired of embarrassing yourself? Dark energy is just the label used for whatever the reason is for the acceleration of the expansion of the universe. It does not matter whether you consider this expansion is due to expanding space-time or just things flying apart. Your attempt to "explain" things are laughable because you don't have the vaguest idea of what your talking about.
I am just trying to explain things in terms of mechanics, and I admit that a few things I have said so far about galaxy rotation could be wrong.
A few?, try pretty much all.

I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible".

2. It is not bizarre
In wave mechanics the velocity of a wave is proportional to the rigidity of the medium. Since light travels faster in a vacuum than it does in, say, glass, the ether must be more rigid than glass. Yet a vacuum and the ether offers no resistance to physical object moving through it. So it is both rigid yet unsubstantial. Contradictory properties such as these is why many scientists considered the ether more trouble than it was worth even before Relativity.

3. It does not have much less evidence of existence of existence than dark matter.
How would you know? You have not shown the slightest inkling that you are even aware of what the evidence for dark matter is. As indicated above, you still cling to the idea that galaxy rotation is the only issue. Galaxy rotations are just one aspect of the Dark matter issue, and only one type of observation among varied independent observations which support DM.

Everything you just said in that whole paragraph was wrong, and wreaks of hypocrisy.
If anyone is being hypocritical here, it is you.

However, you did prove that some of my ideas about the behaviour of galaxies were wrong and I thank you for that.
Now if you could just learn not to pontificate on subjects you know little to nothing about.

35. Originally Posted by Waveman28
Originally Posted by TheDr.Spo
You must keep in mind that galaxies are not only moving away from each other but accelerating away from each other as well.
Indeed, but that doesn't really affect the possibility of the red shift being caused by simple relative motion. I am also interested in the "jerk" of the acceleration rate, that is, is the acceleration increasing or decreasing?

Indeed, the acceleration is increasing. The galaxies are accelerating away at an accelerating rate.

Seriously, at least Google this stuff. You've been called-out several times for misusing terms and failing to accurately explain their effects. How out-of-touch are you?

36. Originally Posted by TheDr.Spo
Seriously, at least Google this stuff. You've been called-out several times for misusing terms and failing to accurately explain their effects. How out-of-touch are you?
Ah. I see that you have met Waveman 28.

37. Originally Posted by Janus
Oh yeah, the bolded "do" makes all the difference. You are just exposing the fact that you do not know what a "field" is.
As a clichéd example, if the sun suddenly disappeared right now, the earth would still orbit for about 6 and a half minutes because the now non-existent gravitational field takes time to reach earth and moves at the same speed as light.

Multiple attempts have already been made to modify gravity to explain away dark matter( by people a lot more versed in what it would take than you), and none of them could be made to match all the observations. The most recent observations have revealed that no mere modification of gravity can.
Just because none of the modifications have been successful so far doesn't mean that another one wont be in the future.

Don't you ever get tired of embarrassing yourself? Dark energy is just the label used for whatever the reason is for the acceleration of the expansion of the universe. It does not matter whether you consider this expansion is due to expanding space-time or just things flying apart.
Yes but in that case, the term "dark energy" could be being used inappropriately to describe something which already exists and that we know of which could be causing the acceleration. Dark energy implies there is something "new" and separate from what we already know, which there may be, but if there isn't (which is more likely) then dark energy loses its meaning.

What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible"
We perceive the Ether all the time. We see light waves, we feel forces between us and other objects and even our bodies themselves are waves of which the Ether is the medium. So this is another example of a false idea that you use to justify the rejection of the Ether. You never actually understand that the Ether does not have the properties of the so-called "Luminiferous Aether" that was postulated centuries ago. The properties of that medium were a mess because the fundamental underlying assumptions about things such as light were wrong. Thus, that Aether was made to try and suit the theories of the time which were wrong in the first place.

In wave mechanics the velocity of a wave is proportional to the rigidity of the medium. Since light travels faster in a vacuum than it does in, say, glass, the ether must be more rigid than glass. Yet a vacuum and the ether offers no resistance to physical object moving through it. So it is both rigid yet unsubstantial. Contradictory properties such as these is why many scientists considered the ether more trouble than it was worth even before Relativity.
That is the worse false premise you have towards the Ether: "Since light travels faster in a vacuum than it does in glass, the Ether must be more rigid than glass". That statement is invalid. For one, rigidity is not even a property of the Ether, it is elastic. Furthermore, you say an Ether cannot exist because it would cause resistance to moving bodies such as the earth. This, not surprisingly, is plain wrong. You then say the Ether has contradictory properties, which is wrong again. You think that the Ether has contradictory properties, because you do not understand how an Ether might work. Waves and mediums are clearly outside of your area of expertise.

38. Originally Posted by Waveman28
You never actually understand that the Ether does not have the properties of the so-called "Luminiferous Aether" that was postulated centuries ago.
Those terms are interchangeable unless you are purposing some new kind of Ether, one for gravitons, which seems to be the case. I suggest using a different name since the properties of Ether were aimed at avoiding affecting the orbits of planets and stars.

I do wonder why you are so afraid of a new concept such as Dark Matter. Instead, you would like to see a modification of our understanding of gravity. Something not far off from, say, that gravity gets weaker with distance and then stronger due to a lensing effect. What sort of lensing effect are you proposing for gravity? How does it occur? From my knowledge, trying to modify the fundamentals of gravity in such a way would make things convoluted rather than simple, which seems to be your motive for being so fervently against Dark Matter and Dark Energy.

As a side note: Dark Matter, although you view it as an unnecessary fabrication, has actually produced some accurate predictive power. One could argue the other way, saying that modifications of gravity are fabrications used to preserve current theories, if, that is, any modifications of gravity were sufficient.

39. Originally Posted by TheDr.Spo

I do wonder why you are so afraid of a new concept such as Dark Matter. ..
I would say the reason I am scared of it is because unobservable quantities are difficult to compare with experiment/observation, and I don't want to see cosmology just become a big guessing game. I mean sure: it's fine to guess when forming hypothesis, but after it's formed that's when the guessing should stop. Confirmation should be found in something that can be seen for certain, a concrete reality.

A modified version of gravity would have the benefit of being comparable with observation on a very specific level. One after another, instances can be compared with it and we can see for ourselves whether each one exactly matches the outcomes of the formulas. If the formulas are wrong, then you'd have to be lucky enough to be continually guessing the right number time after time after time in order for a discrepancy not to emerge. DM doesn't give us that. We only know how much DM is present after we've applied the theory. How do you get narrow predictions out of that?

Originally Posted by Janus
I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible".
I think it's kind of funny that you reject unobservable Aether so adamantly, and then turn around and embrace unobservable Dark Matter with open arms. ( .....though..... I guess you could similarly chuckle at me for entertaining a version of the Aether theory and disliking DM. :?)

DM doesn't play a different role than Aether used to play. It's a miraculous substance that makes it possible for certain correlations to emerge between observable matter and gravitational behavior.

Originally Posted by Waveman28
Originally Posted by Janus
Oh yeah, the bolded "do" makes all the difference. You are just exposing the fact that you do not know what a "field" is.
As a clichéd example, if the sun suddenly disappeared right now, the earth would still orbit for about 6 and a half minutes because the now non-existent gravitational field takes time to reach earth and moves at the same speed as light.

Changes in the field propagate at C. That means the C limit only becomes important when something changes. If it stays the same, then propagation is not an issue.

40. Originally Posted by kojax
I would say the reason I am scared of it is because unobservable quantities are difficult to compare with experiment/observation, and I don't want to see cosmology just become a big guessing game. I mean sure: it's fine to guess when forming hypothesis, but after it's formed that's when the guessing should stop. Confirmation should be found in something that can be seen for certain, a concrete reality.
Dark Matter has shown evidence of its existence in numerous galactic observations of Low Surface Brightness galaxies, Spiral galaxies and Elliptical galaxies for instance. The case of the Bullet Cluster is strong observational evidence of Dark Matter due to its independence from Newtonian Laws. Also, observations of gravitational lensing show very promising results for the existence of Dark Matter. I would also like to add that these observations allow for calculations by statistical means, which aren't to be grouped with a guessing game. So, the quantities are not unobservable or unable to be obtained by experiment and calculation.

41. @Kojax, as Dr.Spo pointed out, dark matter is not unobservable. It is only unobservable in the electromagnetic spectrum. There are many other ways to observe something.

Actually this has been pointed out numerous times already, yet you keep ignoring that.

42. Originally Posted by kojax
Originally Posted by TheDr.Spo

I do wonder why you are so afraid of a new concept such as Dark Matter. ..
I would say the reason I am scared of it is because unobservable quantities are difficult to compare with experiment/observation, and I don't want to see cosmology just become a big guessing game. I mean sure: it's fine to guess when forming hypothesis, but after it's formed that's when the guessing should stop. Confirmation should be found in something that can be seen for certain, a concrete reality.

A modified version of gravity would have the benefit of being comparable with observation on a very specific level. One after another, instances can be compared with it and we can see for ourselves whether each one exactly matches the outcomes of the formulas. If the formulas are wrong, then you'd have to be lucky enough to be continually guessing the right number time after time after time in order for a discrepancy not to emerge. DM doesn't give us that. We only know how much DM is present after we've applied the theory. How do you get narrow predictions out of that?
We know what types of properties DM has. From that we know how it should behave under given circumstances, and in what ways this will differ from any modified version of gravity. Then you look to see if you find instance of this type of behavior.
Example: There is a cluster of galaxies in which tidal forces are strong enough to tear the galaxies apart. Some of the galaxies are being ripped apart, but others aren't. They are subject to the same tidal forces, but something is holding them together. Just modifying gravity won't solve the problem, since it would have to apply to all the galaxies and not just some. This restriction does not apply to DM, as nothing prohibits one galaxy from having more DM than another. Thus you have a situation where DM provides a solution and modified gravity can't. Just because it would be neater to be able explain things by new gravity theory, doesn't mean that it is possible to.

Originally Posted by Janus
I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible".
I think it's kind of funny that you reject unobservable Aether so adamantly, and then turn around and embrace unobservable Dark Matter with open arms. ( .....though..... I guess you could similarly chuckle at me for entertaining a version of the Aether theory and disliking DM. :?)
The difference is that the Aether is the invisible elephant that leaves no traces whatsoever of it existence, and DM is the invisible elephant that leaves footprints. In fact, in the case of the Bullet cluster, it leaves footprints where there is nothing else to do so.

DM doesn't play a different role than Aether used to play. It's a miraculous substance that makes it possible for certain correlations to emerge between observable matter and gravitational behavior.
Except that the Aether is superfluous and DM actually performs a function and explains observations that cannot be explained otherwise. And DM is not "miraculous". Simply put, it has mass and does not interact electromagnetically. It behaves perfectly naturally for something with those properties. And its not as if that combination of properties doesn't have precedent. The neutrino exhibits them. To insist that the neutrino can be the only example seems a little silly.
I really fail to see what gets people's undies into a twist about it.

43. Originally Posted by Janus
Originally Posted by kojax
Originally Posted by TheDr.Spo

I do wonder why you are so afraid of a new concept such as Dark Matter. ..
I would say the reason I am scared of it is because unobservable quantities are difficult to compare with experiment/observation, and I don't want to see cosmology just become a big guessing game. I mean sure: it's fine to guess when forming hypothesis, but after it's formed that's when the guessing should stop. Confirmation should be found in something that can be seen for certain, a concrete reality.

A modified version of gravity would have the benefit of being comparable with observation on a very specific level. One after another, instances can be compared with it and we can see for ourselves whether each one exactly matches the outcomes of the formulas. If the formulas are wrong, then you'd have to be lucky enough to be continually guessing the right number time after time after time in order for a discrepancy not to emerge. DM doesn't give us that. We only know how much DM is present after we've applied the theory. How do you get narrow predictions out of that?
We know what types of properties DM has. From that we know how it should behave under given circumstances, and in what ways this will differ from any modified version of gravity. Then you look to see if you find instance of this type of behavior.
Example: There is a cluster of galaxies in which tidal forces are strong enough to tear the galaxies apart. Some of the galaxies are being ripped apart, but others aren't. They are subject to the same tidal forces, but something is holding them together. Just modifying gravity won't solve the problem, since it would have to apply to all the galaxies and not just some. This restriction does not apply to DM, as nothing prohibits one galaxy from having more DM than another. Thus you have a situation where DM provides a solution and modified gravity can't. Just because it would be neater to be able explain things by new gravity theory, doesn't mean that it is possible to.

Originally Posted by Janus
I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible".
I think it's kind of funny that you reject unobservable Aether so adamantly, and then turn around and embrace unobservable Dark Matter with open arms. ( .....though..... I guess you could similarly chuckle at me for entertaining a version of the Aether theory and disliking DM. :?)
The difference is that the Aether is the invisible elephant that leaves no traces whatsoever of it existence, and DM is the invisible elephant that leaves footprints. In fact, in the case of the Bullet cluster, it leaves footprints where there is nothing else to do so.

DM doesn't play a different role than Aether used to play. It's a miraculous substance that makes it possible for certain correlations to emerge between observable matter and gravitational behavior.
Except that the Aether is superfluous and DM actually performs a function and explains observations that cannot be explained otherwise. And DM is not "miraculous". Simply put, it has mass and does not interact electromagnetically. It behaves perfectly naturally for something with those properties. And its not as if that combination of properties doesn't have precedent. The neutrino exhibits them. To insist that the neutrino can be the only example seems a little silly.
I really fail to see what gets people's undies into a twist about it.
There is little point in arguing this topic with people who lack the requisite understanding for meaningful discussion.

Dark matter is really just a name for something that is not understood, although the evidence that it is truly a form of matter is strong and mounting. It is not a slam dunk that such will be the ultimate answer -- speculation on the cutting edge of physics has been notably vaporous as for example with the prediction of immenent experimental verification of supersymmetry for over a decade.

NOBODY KNOWS. That is what makes physics a vibrant research subject. What is most certainly true is that the solution will require both a deep knowledge of established physics and a bit of insight of the genius variety which will not come from some delisional idiot on the internet.

Cosmology like the rest of science is a guessing game -- one cannot avoid that fact, That is how reseaech is done. The key to good science is goof guessing -- followed by hard theoretical and experimental work to show that the guess is correct. Most times the guess is not correct, so then you put aside the incorrect guess and guess again. It takes a lot of knowledge and education and deep intuition to be a good guesser -- which is why there are few Nobel laureates and fewer outstanding ones.

44. Although, DrRocket, I think you forget that a big point remains, it's fun to argue.

Debating certainly does educate the debaters. However, personalizing the argument is what causes the argument lose its point. As for me, I try not to do that, but it's quite hard to avoid. It's such an easy trap to fall into! I thought that if I were to be careful, I would be immune to the urge to slander posters' intelligence. That's simply not always the case, and I get frustrated with myself more often than I would like to for falling victim to temptation . :x

As for the educational elements of this debate, when they are accurate, I encourage the debate itself to go on as long as no one gets angry.

45. Originally Posted by TheDr.Spo
Although, DrRocket, I think you forget that a big point remains, it's fun to argue.

Debating certainly does educate the debaters. However, personalizing the argument is what causes the argument lose its point. As for me, I try not to do that, but it's quite hard to avoid. It's such an easy trap to fall into! I thought that if I were to be careful, I would be immune to the urge to slander posters' intelligence. That's simply not always the case, and I get frustrated with myself more often than I would like to for falling victim to temptation . :x

As for the educational elements of this debate, when they are accurate, I encourage the debate itself to go on as long as no one gets angry.
Discussing interesting points of science is fun -- often the fun increases with the level of sophistication of the ideas under consideration.

Arguing with the lkes of Waveman28 and Kojax is necessary for the benefit of lurkers ad general intellectual integrity, but is hardly fun. From the perspective of logic and science it is akin to shooting fish in a barrel. There is no hope of educating them, it would be easier to teach quantum field theory to my Labrador retriever, but perhaps debunking them might help some young person. But fub? Hardly.

With the lack of participation by salsaonline as a result of his recent job change, there has come a huge vacuum in discussion of interesting math and physics. Perhaps you can help to fill the void a bit, but to do that you will have to meet a very high standard of accuracy and sophistication.

Much of this thread has been a case study in inanity, posts by you Magimaster and Janus being glimpses of reason. Some of the others should have to pay for air. The educational elements were exhausted long ago -- there is not enough known to provide for prolonged discussion without invoking physics that is FAR beyond the grasp of those who are arguing.

This thread is unfortunately typical. It would help immensely if the participants would, or could, read an actual serious book or two. As it stands too few are equiped to understand even simplified, correct, explanations.

46. Originally Posted by Janus
Originally Posted by kojax
Originally Posted by TheDr.Spo

I do wonder why you are so afraid of a new concept such as Dark Matter. ..
I would say the reason I am scared of it is because unobservable quantities are difficult to compare with experiment/observation, and I don't want to see cosmology just become a big guessing game. I mean sure: it's fine to guess when forming hypothesis, but after it's formed that's when the guessing should stop. Confirmation should be found in something that can be seen for certain, a concrete reality.

A modified version of gravity would have the benefit of being comparable with observation on a very specific level. One after another, instances can be compared with it and we can see for ourselves whether each one exactly matches the outcomes of the formulas. If the formulas are wrong, then you'd have to be lucky enough to be continually guessing the right number time after time after time in order for a discrepancy not to emerge. DM doesn't give us that. We only know how much DM is present after we've applied the theory. How do you get narrow predictions out of that?
We know what types of properties DM has. From that we know how it should behave under given circumstances, and in what ways this will differ from any modified version of gravity. Then you look to see if you find instance of this type of behavior.
Example: There is a cluster of galaxies in which tidal forces are strong enough to tear the galaxies apart. Some of the galaxies are being ripped apart, but others aren't. They are subject to the same tidal forces, but something is holding them together. Just modifying gravity won't solve the problem, since it would have to apply to all the galaxies and not just some. This restriction does not apply to DM, as nothing prohibits one galaxy from having more DM than another. Thus you have a situation where DM provides a solution and modified gravity can't. Just because it would be neater to be able explain things by new gravity theory, doesn't mean that it is possible to.
You do realize that both MOND and DM could be right, don't you? Just because DM exists doesn't mean every galaxy has large amounts of it, or that it is the cause for the flat rotation curves.

The problem with situations where nothing prohibits a value from assuming any number of a wide range of possible quantities, is that it's a double edged sword. It can serve as a bridge to validate good theories in the face of anomalous circumstances, but it can just as easily preserve theories that shouldn't be preserved.

Originally Posted by Janus
I find it ironic that someone who believes so strongly in the Ether, something far more intangible, more bizarre and with much less evidence of existence, has such a problem with dark matter. But it just goes to show how irrational some people can be with their beliefs.
I find it ironic that you have a go at me for hand waving, and now your essentially doing the same thing towards the Ether.
1. The Ether is not intangible.
What else do you call something that cannot be perceived by the senses or measured in any way? That pretty much the definition of "intangible".
I think it's kind of funny that you reject unobservable Aether so adamantly, and then turn around and embrace unobservable Dark Matter with open arms. ( .....though..... I guess you could similarly chuckle at me for entertaining a version of the Aether theory and disliking DM. :?)
The difference is that the Aether is the invisible elephant that leaves no traces whatsoever of it existence, and DM is the invisible elephant that leaves footprints. In fact, in the case of the Bullet cluster, it leaves footprints where there is nothing else to do so.

DM doesn't play a different role than Aether used to play. It's a miraculous substance that makes it possible for certain correlations to emerge between observable matter and gravitational behavior.
Except that the Aether is superfluous and DM actually performs a function and explains observations that cannot be explained otherwise. And DM is not "miraculous". Simply put, it has mass and does not interact electromagnetically. It behaves perfectly naturally for something with those properties. And its not as if that combination of properties doesn't have precedent. The neutrino exhibits them. To insist that the neutrino can be the only example seems a little silly.
I really fail to see what gets people's undies into a twist about it.
Clearly the Aether wasn't superfluous in its day, or it never would have been proposed. Theories evolve. I think one of the biggest mistakes we can make is to become convinced that we're at the end of the evolutionary chain. Then things stagnate.

That said: I really can't say anything bad about the Bullet Cluster analysis. It's an amazingly creative approach, and nothing looks to be wrong with it. I'm actually working my way through the ARVIX paper right now. It's a little bit out of my range, but I'm extending my range to make sense of it. There doesn't appear to be any explanation other than some unseen matter moving with the non-interacting matter.

http://arxiv.org/PS_cache/astro-ph/p.../0608407v1.pdf

http://arxiv.org/PS_cache/astro-ph/p.../0608408v1.pdf

47. While I usually like arguing, and hope that some of these people might actually learn something from my arguments, I'm starting to agree with DrRocket. I know Waveman's beyond reason WRT ether, and now I'm starting to think Kojax is in the same boat.

If you're going to start mashing things up, you're just going to end up running into the wrong end of Occam's razor. If one works, there's no reason for two.

Also, ether was superfluous when it was proposed, they just didn't know it at the time. Read a history book, but basically, at the time, no one could comprehend how something could propegate without a medium. They called the hypothetical medium for light ether (just like we call the stuff responsible for those rotation curves dark matter, or the stuff responsible for the acceleration of the universe dark energy). Later, experiments designed to find the ether failed. Every experiment designed to detect it came up negative. That is what makes it superfluous. You can assume it's there somewhere, but it's just an assumption with no effect either way. It has no measurable properties left.

Experiments to find dark matter have succeeded though (but not quite enough to show things conclusively). The maps of gravitational lensing around collinding galaxies match the predictions, supporting dark matter. If these experiments had failed, dark matter would be abandoned.

(I haven't heard much about experiments involving dark energy.)

48. I don't think tying multiple effects to the same cause is an accurate application of Occam's Razor. It's not necessarily bad for us to do that sometimes, but I don't think it's what Occam tells us to do.

It just ends up too much like religion. All the evil in the world has to come from one source: the devil. It couldn't come from a bunch of separate causes. All good has to come from God. We can't spread the credit around, because that doesn't make as great a story. Imagine if a police officer in a given city wanted to determine that all the murders committed throughout the entire month in his city were committed by one guy. Would simply citing Occam's Razor be validation enough for that view (because blaming one person does make things wonderfully simple), or would we want a lot of specific evidence tying each murder separately to that one guy?

I already asked this (and nobody answered), but I'll ask again: what trait of DM is expected to cause it to be spread out throughout a galaxy instead of being concentrated in the center like most of the visible mass is? If it didn't have this interesting trait then it wouldn't explain the rotation curves at all. What tells us that it would have this trait if it exists?

Why can't MOND explain the rotation curves, and DM explain the bullet cluster (as well as those galaxies that aren't pulling apart when they should, which Janus mentioned earlier)?

49. Originally Posted by MagiMaster
Experiments to find dark matter have succeeded though (but not quite enough to show things conclusively). The maps of gravitational lensing around collinding galaxies match the predictions, supporting dark matter. If these experiments had failed, dark matter would be abandoned.

(I haven't heard much about experiments involving dark energy.)
Not quite right.

There is plenty of observational data telling us that the dark matter problem is real. It is also strongly suggestive that the solution is some kind of matter that has mass, but does not feel the electromagnetic force, only the weak force. It does not appear to be anything covered by the Standard Model, which is a problem. There have been suggestions that it might be the lightest supersymmetric particle, if such a thing exists. So far there is zero experimental evidence for supersymmetry, but hope seems to spring eternal. So,basically there is no good empirical data that would identify dark matter -- only indirect evidence of its existence. In short no one knows what dark matter might possibly be, and lacking that knowledge "existence" is a rather elusive notion. What certainly exists is a mystery.

The situation with dark energy is worse. The existence is suggested by the observational evidence for an accelerating expansion of space, which is modeled by a positive cosmological constant in general relativity. No one knows the source of this positive cosmological constant. The zero point energy of the vacuum in quantum electrodynamics would create a positive cosmological constant. Unfortunately estimates based on QED overestimate the observed constant by 120 ORDERS OF MAGNITUDE, which is ludicrous. Nobody has a clue how to solve this problem. This once again illustrates that physics is a vibrant research subject.

50. Originally Posted by kojax
I don't think
yep

51. Originally Posted by DrRocket
Originally Posted by kojax
I don't think
yep
Blatant flaming.

52. Originally Posted by kojax

I already asked this (and nobody answered), but I'll ask again: what trait of DM is expected to cause it to be spread out throughout a galaxy instead of being concentrated in the center like most of the visible mass is? If it didn't have this interesting trait then it wouldn't explain the rotation curves at all. What tells us that it would have this trait if it exists?
The answer is: The very same "trait" that makes it "dark"; the fact that it does not interact electromagnetically.

If you have some gravitational bound gas of visible matter. The individual particles are constantly bumping into each other and bouncing around.
But what actually is happening when to particles "bump" into each other? They are just getting close enough to each other so that they interact electromagnetically. In some cases this causes them to repel each other, and in others they "stick" together. In either case, they undergo acceleration. And accelerating the charges from which the particles are made causes to them radiate away energy as electromagnetic radiation. The loss of energy comes from the particles own kinetic energy and they slow down, causing the gas to condense to a smaller volume. Things like net angular momentum will determine just how the collapse takes place.

Now take a gravitationally bound mass of DM particles. They do not interact electromagnetically, so they do not "bump" into each other. Like neutrinos, they pass right through visible matter and themselves as if they were not there. They do interact gravitationally, so close passes will cause a deflection or acceleration of the particles, but again, having no charge means they have no way to radiate energy away via electromagnetic radiation.

That being said, gravitational interaction does lead to energy losses through gravitational radiation. However gravity, as a force, is some 10^40 times weaker than electromagnetism, and the same holds for the energy of gravitational radiation. Thus you while you would expect some clumping of DM(galactic halos, etc) since the beginning of the universe, it wouldn't be to the same degree or manner as visible matter.

IOW, the very reason we wouldn't see it leads to the distribution it appears to have.

53. Originally Posted by MagiMaster
While I usually like arguing, and hope that some of these people might actually learn something from my arguments, I'm starting to agree with DrRocket. I know Waveman's beyond reason WRT ether, and now I'm starting to think Kojax is in the same boat.
I am not beyond reason when it comes to the Ether, I am beyond doubt.

If you're going to start mashing things up, you're just going to end up running into the wrong end of Occam's razor. If one works, there's no reason for two.
The ether doesn't mash things up. It explains them, and all using the one simple mechanism. It is like cellular automata, very simple rules can still result in complex patterns and behaviour. And that is all the universe is, a pattern of waves. If your intelligence allows you to doubt this, it also demands that you verify it, instead of dismissing it without investigation.

If you took the time to investigate the Ether, you would also understand that Occam's razor favours the Ether enormously over current theories.

Also, ether was superfluous when it was proposed, they just didn't know it at the time. Read a history book, but basically, at the time, no one could comprehend how something could propegate without a medium.
It is not superfluous. It has a purpose and a function which can be used to explain physical phenomena.

Later, experiments designed to find the ether failed. Every experiment designed to detect it came up negative.
That is incorrect. An experiment designed to detect motion relative to the Ether failed, but to argue that the Ether can be ruled out simply because absolute motion cannot be detected is completely absurd and invalid.

Also, whenever I have a conversation with Janus about the Ether, he quickly retracts into his shell because he knows the Ether is a valid theory and doesn't want to admit it. I know that you are one of the more open-minded members on this forum, along with Kojax, so don't just become a disciple of Dr. Rocket who refuses to even discuss matters related the the Ether at all.

54. I would sorely like to see the excat mechanism upon which the galaxy simulators has been made.

55. Originally Posted by Waveman28
, so don't just become a disciple of Dr. Rocket who refuses to even discuss matters related the the Ether at all.
Wrong.

If you read my posts you will find rather clear and correct statements regarding the validity of the ether hypothesis, and the equivalence of the Lorentz ether theory with special relativity. This discussion also shows the total irrelevance of the ether in any correct theory,

It is hardly my fault that you lack the intelligence and background to understand. But it is clearly your fault that you deliberately lie about the fact that I and others have addressed and debunked your absurd statements.

56. Originally Posted by Waveman28
Originally Posted by MagiMaster
While I usually like arguing, and hope that some of these people might actually learn something from my arguments, I'm starting to agree with DrRocket. I know Waveman's beyond reason WRT ether, and now I'm starting to think Kojax is in the same boat.
I am not beyond reason when it comes to the Ether, I am beyond doubt.
Same thing.

Look, go to the new ideas forum and post some actual math, and then I might can give you some more constructive criticism, but as it is, all you have are assertions that you're right and we should believe you. Assertions don't count in science, only evidence/experiment/observation does.

57. Originally Posted by Waveman28
I am not beyond reason when it comes to the Ether, I am beyond doubt.
What is beyond all doubt is that you have no idea what in the hell you are talking about.

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