Scientific evidence solidly indicates that
Earth's ancient history has apparently involved a number of mass
extinctions, most notably the one at 65 million years ago where
the final dinosaurs died out along with many other creatures.
A number of rather speculative "theories" have been
publicized as trying to explain them, and especially regarding an
apparent 26 million year repetitiveness of mass extinctions. The
logic seems especially weak in the majority of such explanations.|
Consider this simplified representation of a top-view of the flattened, somewhat cone-shaped (tapering) Spiral Arm (straightened for simplicity here), with random stars within it. There are actually many billions of stars that should be in this picture. The Arm actually continues to the right, to join the Core of the Galaxy.
Now see that we have marked the Sun, which is known NOT to currently be on the central axis of the Arm. This is the situation that applies to most stars. All the individual stars in the Arm are gravitationally attracting the Sun. We know that the Sun is currently near the "inner edge" (bottom of our drawing) of the Arm, as shown. Do you see that very large numbers of stars are attracting the Sun upward in the drawing, toward the centerline of the Arm? When you add up the gravitational effects of all those billions of stars, there is a net gravitational attraction (within the Arm) pulling the Sun toward the centerline of the Arm (upward in our drawing).
The effect of this collective attraction is to cause the Sun (and everything else in the Arm) to weave back and forth across the width of the Arm. From where we are right now, the Sun will "fall" toward the centerline for millions of years, building up speed all the way. By the time the Sun crosses the Arm's centerline, it will be traveling rather quickly. Violent collisions with objects in that congested area, of all sizes, seem likely to be common. Standard gravitational calculations suggest that this full cycle takes around 52 million years to complete, so that centerline passages occurs about every 26 million years, twice in each cycle. This excellently matches the paleontological evidence of periodicity of mass extinctions.
Notice that is in total disagreement with the usual ASSUMPTION that the craters we see on the Moon are all 4.5 billion years old, from when the Moon was first formed!
The Sun and Earth happen to currently be near the "inner edge" of an Arm segment called the Orion Arm. There are virtually no nearby stars farther inward, but several billion of them that are nearer the center-line of the Arm are (currently) gravitationally pulling the Sun and earth "outward", toward that center-line of the Arm. Collectively, this gravitational attraction creates a relative net acceleration, ACROSS THE ARM, toward the center-line of the Arm.
Over a very long period of time, this crossways acceleration creates a crossways velocity, and the Sun will certainly move across the width of the Arm! Once it gets near the other side, all of those attracting stars in the Arm are behind it, which act to slow it down. It will eventually stop moving laterally outward and again "fall" back toward the center-line of the Arm, eventually getting back to where we are today, near the inner edge. This "weaving" action will continuously repeat for the Sun and every other star and other object within the Arm. The Sun therefore "weaves" continuously back and forth across the width of the Arm!
This sequence therefore represents a cycle that will repeat, with the Sun (and all other component stars of the Arm) weaving back and forth across the width of the Arm, in an incredibly complex "dance". This "inter-weaving" is prominent because of the massiveness of the component parts of the Arm.
Why is this significant?
First of all, preliminary computer analysis of the many gravitational forces acting on the Sun, and its resultant acceleration variations, suggest that the Sun will complete one whole cycle across the Arm and back, in roughly 52 million years. Second, we are currently near the inner edge of the Arm, where there are relatively few stars and other objects, and there is relatively low relative velocities between various objects out here. But near the centerline, there are two complicating circumstances. Any logical distribution suggests that there are at least several times as many stars and other smaller objects in that area. But worse, realize that the Sun (and Earth) enters that area with a substantial lateral velocity, around 23.8 km/sec (around 51,000 mph), and that there are OTHER stars and objects (half a cycle out of phase with the Sun) that are moving in the opposite direction at around the same velocity! These are tremendous differential velocities! Head-on collisions, even with relatively small objects, could have ferocious impacts!
Interestingly, the geologic record suggests that mass extinctions occurred on Earth at roughly those eras. A 26 million year pattern of repetition of mass extinctions was noted during the 1980s. A number of speculative theories were presented to try to explain the periodicity. Currently, the most popular one is that our Sun is actually a "double star" with the other star called "Nemesis", which allegedly is in a 26 million year long very elliptic orbit and only nears the Sun at such intervals, to disrupt a speculated "cloud" of comets (called the Oort Cloud), causing some of them to approach the inner solar system and do damage to the Earth and its creatures.
It is amazing that this idea has gathered the backing of many scientists! There is NO evidence whatever for the Sun being a double-star (other than the fact that many other stars are double), or for ANY observed gravitational effect or light from any object that could actually be such a star. In addition, there is actually no evidence that there even is an Oort Cloud of dormant comets. Further, the instigators of that concept must not have looked through the appropriate calculations!
In an orbit that would have a 26 million year period, the semi-major axis would be around 800 AU, and the premise offered is that it spends most of its time at the outer end of a highly elliptic orbit, which means it is usually about 1,600 AU from the Sun, about 50 times as far out as Neptune, essentially out alone in space. All of science knows that the Sun is traveling about 20 km/sec toward the direction of the Constellation of Hercules. Many of the relatively distant stars of Orion are therefore about 600 light years "behind" us. Simple calculation shows that the Sun was in that vicinity, with all those stars that are presently 600 light years away, only 10 million years ago. The point is, the Sun's motion (and that of other stars) causes us to pass a LOT of stars in any 26 million year period, many of which are massive enough to disrupt the orbit of a "Nemesis" star that is so far out. To believe that such a Nemesis could persist in a stable orbit for hundreds of millions of years is beyond comprehension! Even if it actually existed, its orbital period would certainly regularly be altered by near passes of other massive stars.
Finally, there is no compelling evidence that a comet interacting with or colliding with the Earth would necessarily do much damage. In 1910, the Earth passed through a tail of Halley's Comet, with many people fearful, but nothing happened. In my education as a Physicist, I was taught that a worthy theory must be based on actual evidence, and there doesn't seem to be any regarding the very popular claims for the Nemesis Death Star. (The recent "Chain of Pearls" sequence of collisions involving the planet Jupiter involved a much more massive object than a normal comet.)
There is substantial evidence that periods of bursts of evolutionary development have occurred at very similar times to the mass extinctions. There may be a simple and obvious explanation for this. If a large proportion of dominant life forms have just died off in a mass extinction, there can be a vacuum of sorts, for existing surviving life forms to adapt into roles of greater dominance. Nothing external would be needed for such a situation.
However, in connection with the situation described above, there might be some interesting new possibilities. At roughly the same times as the mass extinctions, the Sun and the planets are passing through a very congested area of stars and miscellaneous other materials near the center-line of the Arm. Astrophysics has discovered quite an assortment of simple organic molecules out in distant open space. What if, as the Earth was passing through that congested area in the Spiral Arm, our atmosphere picked up any of a wide assortment of such organic molecules?
This speculation has several possible results. The "new" organic molecules could be incorporated into existing creatures or plants. That could result in materially modified DNA and distinctly different living organisms, essentially almost instant evolution. Given normal statistics, probably half of the changes might be positive and the other half negative. This would result in some "new" species having competitive advantages and quickly multiplying, while others would be degraded and soon die off. That could explain both the mass extinctions and evolutionary bursts, even without violent collisions. Another possibility would be that the molecules picked up from space might actually already be (dormant) simple life forms, like viruses or bacteria. Once in the Earth atmosphere, the effects could again be rapid and extensive, from a great devastation of an Ebola virus to some selective advantage of a digestion-aiding bacteria. Finally, such molecules might just somehow upset the existing balance among Earthly organisms. Thousands of such possibilities could exist, such as a self-replicating organic material that stayed at the ocean's surface and was impermeable to light, killing all the plants and animals under it, causing the water to absorb more heat and therefore evaporate more into the atmosphere, etc.
The general premise suggested here just implies that the Sun and Earth would pass through such congested areas at roughly 26 million year intervals. These speculations regarding potential "contamination" by extra-Solar molecules, are just that, speculation. Maybe some other researcher can figure out some way of either confirming or denying it.
Interesting enough, these results were not the initial goal of my research. An attempt at explaining the persistence of the existence of Spiral Arms in galaxies, based purely on Newton's Laws, was progressing with computer simulations that were meant for determining if the Spiral Arm had structural integrity (it does!) and whether its shape and edge delineation were stable (they are!). Standard applications of Newton's Law of Gravitation were used, with no exotic assumptions involved. During those researches, in tracking the location of the Sun in the computer simulations, it was noted that the Sun was oscillating back and forth across the width of the Arm, much like the tone arm on an old phonograph after a song had played.
The appearance of a 26 million year periodicity of passing through the Arm's centerline, and our known current position near the inner edge of the Arm we are in, seemed like amazing coincidences, in presenting seemingly very obvious explanations for the known mass extinctions. This essay is presented with that intent, as a possible explanation for some events that are difficult to otherwise scientifically explain. No assumption of any exotic form was involved in arriving at these results.
There could also be a very traditional explanation for repetitive mass extinctions. Please see An Old Explanation for Mass Extinctions An Old Explanation for Apparent Periodicity of Mass Extinctions.
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago