Is it dark matter, or a previously unrecognized failure of Newton?


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Dark matter?

The uncertainty of science: Using new data gathered by the 10-meter Keck telescope in Hawaii, astronomers have found that the outer stars of elliptical galaxies exhibit the same behavior as the outer stars of spirals, suggesting once again the existence of dark matter.

One of the most important scientific discoveries of the 20th century was that the spectacular spiral galaxies, such as our own Milky Way, rotate much faster than expected, powered by [the] extra gravitational force of invisible “dark matter” as it is now called. Since this discovery 40 years ago, we have learned that this mysterious substance, which is probably an exotic elementary particle, makes up about 85 percent of the mass in the Universe, leaving only 15 percent to be the ordinary stuff encountered in our everyday lives. Dark matter is central to our understanding of how galaxies form and evolve – and is ultimately one of the reasons for the existence of life on Earth – yet we know almost nothing about it.

“The surprising finding of our study was that elliptical galaxies maintain a remarkably constant circular speed out to large distances from their centers, in the same way that spiral galaxies are already known to do,” said Cappellari. “This means that in these very different types of galaxies, stars and dark matter conspire to redistribute themselves to produce this effect, with stars dominating in the inner regions of the galaxies, and a gradual shift in the outer regions to dark matter dominance.”

What is most fascinating about this press release, however, is that it also noted that dark matter is only one explanation for the data, and that the failure of Newtonian physics at large distances, instead of dark matter, might also provide an explanation.

However, the [solution] does not come out naturally from models of dark matter, and some disturbing fine-tuning is required to explain the observations. For this reason, the [problem] even led some authors to suggest that, rather than being due to dark matter, it may be due to Newton’s law of gravity becoming progressively less accurate at large distances. Remarkably, decades after it was proposed, this alternative theory (without dark matter) still cannot be conclusively ruled out.

Physicists call this other theory MOND, for modified Newtonian dynamics. It is not a very popular theory, however, and is almost always ignored, even though it appears to work as well as dark matter to explain the motion of stars in galaxies. Instead, most scientists favor dark matter.

For this press release to mention it as suggests the new data favors it over dark matter, which would make this a significant discovery.

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9 comments

  • mpthompson

    I’m an engineer, not a scientist, but thinking about this issue it seems to me that on galactic scales Newtonian dynamics may be a poor model for motion on such scales. Newtonian dynamics provides a very precise model for guiding space probes with great accuracy across our solar system, but the explored region of solar system is small enough to be traversed by light on the order of a handful of hours. It takes 100,000’s of years for light to propagate across galactic distances — a scale billions of times larger.

    Given that Einstein demonstrated a century ago that Newtonian dynamics is just a simplified model of the true motion of objects in our solar system, is it surprising that it may not provide such a good model for motions on a galactic scale? At such scales gravitational forces between stars is very weak and interactions occur on a vastly different time scale. This provides a tremendous opportunity for errors within a simplified model to multiply several fold and yield much different results than what is actually observed.

    In the 19th century the idea of the luminiferous ether was so firmly established that it was taken as a given by many of the leading scientist of the time. Even though supporting evidence of its existence had yet to be found. I wonder if dark matter will become the 21st century equivalent.

  • jburn

    A fascinating premise. I can’t help but ponder if an inverse sizing relationship might also produce unique characteristics. Would extraordinarily small bits of matter also equally stray from Newtonian dynamics.

  • I’m not convinced. Newtonian physics has been tested for centuries, and works very well in the macroscopic environment. Einstienien physics describes high-acceleration properties well, but humans are a ways from experiencing those effects. Your postulate would assume that scale somehow alters perception. How does this occur? I submit that any coherent model must work at all scales given a consistent manifold. Newtonian physics satisfies that requirement.

  • mpthompson

    Given that Newtonian physics failed to accurately model the orbital motion of Mercury with the error significant enough for 19th century technology to easily measure, the breakdown of Newtonian physics doesn’t seem to be that far outside of our direct experience. It just happens that Newtonian physics is just an approximate model that happens to work well enough for scales that humans are mostly concerned with.

    When scales are increased by many orders of magnitude and forces between bodies are reduced by several orders of magnitude, there seems to be an opportunity for the Newtonian model to again breakdown so that surprising results are observed just as the orbital motions of Mercury were surprising and unexplained to 19th century astronomers.

    The fact we observe the stars on the outskirts of spirals and elliptical galaxies to have unexpected motion, does not in and of itself establish dark matter as fact. It is just conjecture that with further evidence proving to be pretty elusive. Perhaps the net should be widened for other ideas.

  • Edward

    It seems to me that dark matter better fits the Occam’s Razor test, although Occam’s Razor admits that a more complex explanation could still be the correct answer.

    On the other hand, dark matter also seems to require the invention of dark energy in order to explain observations in the expansion of the universe, so maybe the “dark side” of physics isn’t the answer.

    The universe is a wondrous and mysterious place. I wonder if we will ever solve all its mysteries.

  • Actually, Edward, dark matter and dark energy are completely separate mysteries involving solutions that are not required to be related, though they might be. Dark matter has to do with the unexplained velocities of the outer objects in galaxies, apparently caused by mass that is as yet unseen. Dark energy relates to the acceleration of the expansion of the universe on very vast scales, much greater than that seen with dark matter.

    In one, the scale is in the size of galaxies and their neighbors. In the other, the scale involves half the size of the entire universe. They are very unrelated problems.

  • LocalFluff

    But what about dark matter gravity lenses, like the Bullet galaxy cluster? Isn’t it now very firmly established that there exists specific regions in space which are curved, as if there were some invisible massive matter located there? How could this be explained by some general modification of the theory of gravity?

    I’m afraid that there could be more than one cause, which makes it harder to disentangle. QM and Relativity are extremely successful theories, but they obviously have huge blind spots. They didn’t predict Big Bang or that most of the matter is “dark”. What else do those theories fail to reveal to us?

  • LocalFluff

    Dark energy is less of a problem, theoretically. It is simply the energy in vacuum. The discovery that the expansion of the universe is accelerating, doesn’t mean any change in the theory, it is only a more precise measurement of a parameter in relativity which has been there for 100 years. Einstein picked a value that fit with the steady state universe then observed.

    Dark matter is however really ugly. It challenges the first beginnings of astrophysics. Kepler’s laws from 400 years ago are not applicable to galaxies. Dark matter is adding to the observations to make them fit the theory. Funny enough, it seems to hold up well. If the incompatibility of QM and relativity was not bad enough, now there’s dark matter as a third independent phenomenon.

  • Edward

    Thanks for the clarification, guys. I have not been a fan of what I like to call the “dark side” of physics (or astrophysics), but that is because I really don’t understand it well enough to explain it or justify it to anyone.

    It is worse than quantum mechanics (if you know how fast you are going, you don’t know where you are — or when you will get there) and relativity (I’m not fat, I’m just going really, really fast).

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