Pionier Anomaly and MOND

[Max Pain]

Hi all,

currently I am reading "The Trouble with Physics" by Lee Smolin. In this book I stumpled across the so called "Pionier anomaly".

In short the Pioner anomaly "is the observed deviation from predicted trajectories and velocities of various unmanned spacecraft visiting the outer solar system, most notably Pioneer 10 and Pioneer 11." (Wikipedia)

This anomaly seems to appear when the sunward acceleration, caused by the sun's gravity, get near to the magnitude where stars in galaxies don't move like predicted by Newton's laws. For the explanation of this strange behaviour, of galaxies, we invented the idea of dark matter.

However alternative theories, like "MOND", don't need dark matter for the explanation of the galaxies movement. This theories are a modification of Newton's laws which becomes notable at the acceleration mentioned before (a0=1.2×10−10 ms−2).

I'd like to know what you guys think about a modification of Newton's laws (and of course, therefore the theory of general relativity). However this wouldn't exclude dark matter in general, which is a better explanation for other stuff.

Greetings

 

[RisingFury]

I'm waiting for the results of the LHC before I comment on this one...

All I'm gonna say is that there are way more theories then this one.

 

[Eagle]

I'd like to know what you guys think about a modification of Newton's laws (and of course, therefore the theory of general relativity). However this wouldn't exclude dark matter in general, which is a better explanation for other stuff.

As long as an experimental testable rationale beyond 'it fits the observed curve better' can be presented. Newton's laws, General & Special Relativity, and Quantum mechanics are all wrong, and replacing them with something better is the goal of many physicists. That being said, they're still astoundingly accurate and are the most correct theories we have to date.

I'm not a physicist, but I have several (many likely incorrect) opinions. I'm not a fan of 'dark matter' in general. Mostly the silly assumption that dark matter is somehow non-interacting with regular matter. My (probably improper) use of Occam's Razor leads me to think that dark matter isn't fundamentally different from normal matter, its just in low temperature clumping that make it difficult to detect.

I don't see an excellent physical rationale for the MOD theory, however I might change my mind if they can find some harmonics or other natural patterns to relate it to.

 

[Nerull]

The problem is that is that while we cannot detect dark matter, or whatever it is, directly, we can see the effect it has on other matter, and the observed effects do not work if you just use normal matter that is hard to spot. Occam's Razor, which has nothing to do with simplicity, despite movie misquotes, only works to remove entities if the explanation with fewer entities fits the data.

 

[Linguofreak]

As long as an experimental testable rationale beyond 'it fits the observed curve better' can be presented. Newton's laws, General & Special Relativity, and Quantum mechanics are all wrong, and replacing them with something better is the goal of many physicists. That being said, they're still astoundingly accurate and are the most correct theories we have to date.

And that's one of the problems with MOND. It's easy to modify Newtonian gravity to fit the observed data. Modifying General Relativity to fit the observed data is considerably harder, and we already know that GR corresponds to how the universe works better than Newtonian gravity.

I'm not a physicist, but I have several (many likely incorrect) opinions. I'm not a fan of 'dark matter' in general. Mostly the silly assumption that dark matter is somehow non-interacting with regular matter.

First of all, "dark matter" is simply matter we can't see with current instruments, except through how its gravity affects other things. It can be normal matter or something as yet unknown. But we can place limits on how much of the dark matter in the universe is made of stuff we're familiar with based on the observations we've made.

Secondly, how is non-interaction except by gravity silly? To me it seems fairly benign. I think I used to be a bit more skeptical of dark matter, but we already know of one particle (the neutrino) that pretty much fits the bill except for being far too light. It doesn't interact through the strong or electromagnetic forces, and just barely interacts through the weak force. You'd need a light year of lead just to stop half of a given beam of neutrinos. Supernovae put out 100 times more energy in neutrinos than in electromagnetic radiation, but most of them go right through the Earth without ever being detected, so the amount of energy we detect in neutrinos from a supernova is far less than what we detect in visible light. So neutrinos are part of the dark matter in the universe, but there's a problem.

The problem with neutrinos being the majority of dark matter is that they're light enough that they don't clump together well gravitationally (because they're so light, they move quite quickly even at very low temperatures). Dark matter has to be made of particles heavy enough to clump from gravitational attraction on galactic scales.

My (probably improper) use of Occam's Razor leads me to think that dark matter isn't fundamentally different from normal matter, its just in low temperature clumping that make it difficult to detect.

This is definitely a component of dark matter (for one thing, we know that most stars are red dwarves, and red dwarves are difficult to detect beyond a few tens of light years), but it can't explain everything. For one thing, there was a period between the first three and twenty minutes after the big bang when conditions across the universe were equivalent to the interior of a star now, and nuclear fusion was occurring. If all of the mass we observe in the universe were normal matter, then this period of fusion, referred to as "Big Bang nucleosynthesis," would have produced much less deuterium than we observe today, and there aren't any processes other than Big Bang nucleosynthesis that can produce deuterium in large amounts. Therefore, the majority of the mass in the universe has to be either neutrinos, or something else that is like neutrinos but heavier, and since we know that neutrinos wouldn't clump enough, that leaves us with "something else". (And neutrino clumping brings up another point about why normal matter can't account for all of "dark matter." Because it interacts through the strong and electromagnetic forces, normal matter would clump much *more* than has been observed if it accounted for all of the dark matter in the universe.)

So we know that dark matter consists of some neutrinos, some normal matter, and a whole lot of "something else".

 

[docabn]

“Dark matter”, “Big Bang”, or “Great sneeze”; the cause is not so important as the effect. Newtonian dynamics or modified Newtonian dynamics, or something better. Laws of the universe are written to describe observed behavior in a useful and meaningful way. Weather it is dark matter that causes these phenomenon or not the fact is that the phenomenon exists.
Personally I think we are missing a form of energy that binds things together, kind of like water tension. But the cause of such phenomenon is of less significance that how we can use it to our advantage.
The laws and theory of aerodynamics are of significance to me because an airplane based on those models will transport needed supplies to me.
I am all for rewriting or modifying any previous laws or theories so long as the new laws or theories accurately describe and predict the actions of the universe around us. The result of maintaining an accurate account of how the universe behaves allows us to successfully modify our actions to be more successful within it.
On the other hand arguing the existence of a substance which we can neither prove nor disprove at this time does not change the fact that something fundamental throughout all of observable space causes objects to move in an unexpected manner, meaning our current model has some errors and must be modified accordingly.

Rant decreased to 25% as I go to get more coffee.

 

[TSPenguin]

Here is a nice talk by Ian Morison, titled "The Invisible Universe".
In chapter 4 he talks a bit about MOND.

Here is a direct link to FORA.tv.