Do we need Dark Matter?...

1. Feb 8, 2016

IsaacM

This is probably a stupid question with a very simple answer but I hope someone here can help me, I'll try to explain myself as best I can...
I was thinking about spacetime being elastic, if I place a large mass on the elastic sheet it bends around it, when i place a second mass on the sheet, as well as the sheet bending around the second mass doesn't this slightly impact on the effect of the first mass? I was thinking as you continue to add different masses their effect on each other locally becomes less.
I guess I was thinking that this describes the movements in spiral galaxies where masses move slower in the centre where the areas are more densely occupied. As an extra bonus, if the mass of the large black hole at the centre of our galaxy was estimated by measuring the speeds of stars orbiting it then this would also mean that the size had been massively underestimated which would help solve the problems of closely orbiting galaxies and the extra gravitational lensing effects that are all attributed to Dark Matter at the moment.
Again this probably has a very simple answer and I apologise if this is below the usual level here but I'm struggling to answer this myself.

2. Feb 8, 2016

Staff: Mentor

No, it doesn't. It actually becomes more.

If Newtonian gravity were exactly correct, then the effects of different masses would add linearly. But the extra corrections in GR due to nonlinear terms in the equations mean that the effects of different masses add more than linearly--the combined effect is greater than the sum of the two effects taken by themselves.

3. Feb 8, 2016

IsaacM

ok, think I've just taken the rubber sheet analogy and run with it much further than it was intended!!!

4. Feb 8, 2016

phinds

I recommend the link in my signature.

5. Feb 8, 2016

Orodruin

Staff Emeritus
Let this be a lesson for the future though. Popularised science is not science, it is scientists explaining in as close analogies they can find. Such explanations often have serious flaws and you really should not make the mistake of thinking you understand something at a level where you can draw further conclusions based on it.

6. Feb 8, 2016

ohwilleke

In fairness, your intuition that one could approximately reproduce many dark matter effects with some kind of gravitational mechanism in systems of the complexity that you are contemplating isn't wrong.

It simply isn't what Newtonian gravity, or textbook GR which isn't all that significantly different as applied to galaxy scale systems, actually predict when you run the numbers. To get a gravitational mechanism that has the effects necessary to reproduce phenomena attributed to dark matter, the tweaks relative to GR that are necessary need to take place in the very weak gravitational field regime, while your thought experiment was imaging effects basically attributable to strong gravitational field effects.

7. Feb 9, 2016

Laurie K

I just wonder why our latest total calculated mass (dark matter + ordinary matter) can be simplified to ordinary matter * 2 Pi, i.e. (dark matter + ordinary matter)/(2 * ordinary matter) = Pi +/- 3% (Planck 2013).

8. Feb 9, 2016

phinds

This is just boring and meaningless numerology.

9. Feb 10, 2016

Laurie K

In 2010 the ratio +/- was 5% so if future ratios continue to converge on 2 Pi would it still be meaningless numerology?

At least we know what 2 * Pi and ordinary matter are.

10. Feb 10, 2016

Orodruin

Staff Emeritus
Yes, it would still be meaningless numerology. This is not how science works, you do not go around looking for numbers which manipulated in a random way gives you a number you are familiar with. In order to claim any kind of correlation you would need to find a viable mechanism predicting this number. Anything else is meaningless numerology.

11. Feb 10, 2016

ChrisVer

well it won't become less than some percent...
$\frac{\text{DM} + \text{OM}}{2\text{OM}} =0.5 + \frac{\text{DM}}{2\text{OM}}$
now DM~5 OM (the one is 5% and the other is 25%).
So you get the result:
$\approx 0.5 + \frac{5}{2} =3.0$
and that's why you get the result pretty close to pi =3.14 ( with a % off of 0.14*100/3.14=5)... so yes it's just numerology due to the % of the DM and OM (and of course the right combination of OM and DM in your expression).

12. Feb 10, 2016

Orodruin

Staff Emeritus
Well, to be fair it is much closer than you would think from here. The Planck results have an error of around 1% and $2\pi$ is within those error bars (I checked). It is still numerology though.

13. Feb 16, 2016

Laurie K

Science contains, and uses, many constants (Physics. a number expressing a property, quantity, or relation that remains unchanged under specified conditions.) for complex conceptual elements so I don't really see why an analog that is also a constant in itself (and within the error bars) can be regarded as numerology unless you want to paint science with the same brush.

14. Feb 16, 2016

phinds

The kind of constants you are talking about are established by experiment and relate to physical reality. Your stuff is just numerology, as orodruin explained in post #10.

15. Feb 17, 2016

Laurie K

More like numerological.

https://en.wikipedia.org/wiki/Numer...d_on_possibly_coincidental_numerical_patterns

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