Dark matter? Maybe not...

There's a fascinating article in Scientific American about the ongoing search for dark matter; a small but vocal group has suggested that it's the theory of gravity itself that needs to be modified to account for the observed discrepancies:
"Once you convince yourself that the universe is full of an invisible substance that only interacts with ordinary matter through gravity, then it is virtually impossible to disabuse yourself of that notion. There is always a way to wiggle out of any observation."
The article talks about how researchers have continually altered their parameters as all attempts to detect dark matter have failed:
After each non-detection, McGaugh says, theorists continually redefine the interaction cross-section of WIMPs to safely undetectable levels. This kind of behavior, he adds, can spark a never-ending game of leapfrog between experimental physicists and theoreticians, allowing them to continue business as usual without ever revising their cosmology. 
I have to admit, the idea sounds compelling. Think about it: we've spent the last century attempting to reconcile classical theories of gravity with quantum mechanics and it just doesn't work. And there are some large-scale phenomenon in which classical equations are not producing accurate results:
Stars at the very edges of spiral galaxies, for instance, rotate much faster than can be explained by Newtonian gravity alone; the picture makes sense only if astrophysicists either modify gravity itself or invoke additional gravitational acceleration due to an unknown source of mass such as dark matter.

"The mass of visible matter falls very short of what is needed to account for the gravity shown by these systems," Milgrom says. "The mainstream assumes it is due to the presence of dark matter, while others, like me, think that the theory of gravity has to be modified."
This reminds me somewhat of the idea of the "aether" that was proposed during Newtonian-era physics. It proposed that space was filled with some sort of mysterious substance through which light traveled. As failures to detect it mounted, physicists constructed more elaborate models of it. Eventually Einstein came along with his radical idea of special relativity, and the aether was gone for good. Maybe dark matter is a modern version of the aether, and our classical theories of gravity are still incomplete.

I suspect that if dark matter continues to elude detection and physicists have to keep pushing back the parameters, we will indeed need a fundamental reworking of our understanding of gravity – a bold new evolution in physics.


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