my hypothesis: dark matter observations are relative

[swansont]

1 hour ago, Maartenn100 said:

Ask yourself these questions:

What is a straight line for observers in different, curved spacetime environements? Is it the same straight line for all observers? Or do we disagree about our ruler? 

For every observer, his particular idea of straight uncurved ruler (a straight line) is an individual idea of space, like he has an individual clock (time rate passage). 

Whatever an observer will see as a straight line, in his own referenceframe, will be curved somewhere else by heavier masses.

Whatever an observer will see as a straight line in his own reference frame, will be seen as an expanded space somewhere else in less heavy environements (in intergalactic space f.e.).

We have individual clocks, but we also have individual rulers. We all have a particular idea of straight uncurved and unexpanded spaces in our own reference frames.

 

If you are unwilling to calculate the effect, you aren’t doing science. There’s no conversation to be had without quantifying this.

[Strange]

7 hours ago, Maartenn100 said:

So, my point is: wherever you are as an observer, you use your standard idea of normal timeflow to determine the 'stetch or contraction' of time elsewhere by relatively more/less heavy massive bodies and relatively higher or lower speeds. The amount of 'curvature of spacetime' (the stretch or contraction of time f.e.) depends on your standard clock and standard ruler, wherever you are.

And it is up to you to show that this effect produces the observed galactic rotation curves. Can you do that?

[Markus Hanke]

21 hours ago, Maartenn100 said:

Please, explain this in terms of observers and their clocks and rulers.

A clock near a black hole ticks at “1 second per second”, a ruler near a black hole is “one meter per meter” long.
A clock on Earth ticks at “1 second per second”, a ruler near Earth is “one meter per meter” long.

Locally in their own small enough neighbourhoods, neither observer determines anything special to be happing. As the name already implies, relativistic effects become apparent only when you compare clocks and rulers in different frames - they are relationships between frames, not things in and of themselves that somehow happen to clocks and rulers. No clock will ever tick at anything other than “1 second per second”, and no ruler will ever measure anything other than “1 meter per meter”, within their own local frames. Time does not “slow”, space is not “stretched”, they merely become local concepts. It is crucially important to understand this point.

So, in order to determine the geometry of spacetime, you need to either compare clocks and rulers at different events, or (equivalently) observe what happens when they traverse extended regions of spacetime. What you find then is that different observers may disagree on specific measurements of space and time, but they will always agree on how those measurements are related to one another. In other words - all observers agree on the geometry of spacetime. 

Mathematically speaking, the object that quantifies relevant aspects of the geometry of spacetime (in GR that is the Einstein tensor) is something that all observers agree on, regardless of where and when they are, or how they measure and determine it. The components of the tensor vary as you go from one observer to another, but they vary in such a way that the overall tensor remains the same one - that is the meaning of “covariance”.