October 19, 2017

22.10.09: Thank Relativity that We’re Here!

The next time you’re studying the Lorentz equation or are forced to account for Relativity on your Buzzard Ramjet trip to Sirius, thank Einstein that we’re here at all! Scientists Jacques Laskar and Mickael Gastineau at the Paris Observatory have been modeling orbital dynamics in our solar system and have come up with some “disturbing” results. It has long been known that Jupiter has a shepherding effect on the inner solar system, smoothing out planetary orbits while ejecting or sweeping up incoming debris. However, if you model the planetary orbits taking into account only classic Newtonian motion, the odds that Mercury goes out of whack in the Sun’s 10 billion year odd life span are about 60%. Throw in Einstein, and the effect shrinks to less than 1%. A careening Mercury would be a bad thing; if it impacted Venus, we would get showered with debris over a million year span, and if it hit us, well, it would just be a bad day. The best thing it could do is harmlessly impact the Sun. Even a near miss with the Earth could drastically alter our orbit, not to mention tinker with our stabilizing Moon. Fortunately, the tiny tweak that the Sun’s gravitational well gives Mercury’s eccentric orbit via General Relativity assures that a resonance keyhole with Jupiter’s orbit probably won’t happen. Keep in mind, we’re talking tiny effects that pile up over billions of years… every time an asteroid whizzes by, we launch a Space Shuttle, or LeBron performs a slam dunk, the Earth gets a tiny push. Over billions of years, tiny forces do add up (ever heard of the Butterfly Effect?) This is why astronomers cannot predict the positions of planets more than a million or so years into the future. Incidentally, the precession of Mercury’s orbit still stands as one of the great observational proofs of Relativity. One also wonders if such a perturbation might have been the fate of Theia, the Mars sized impactor that has been hypothesized to have struck a prehistoric Earth and created our Moon. So the next time you see gravity bend light at relativistic speeds, thank Einstein for protecting our home planet Earth!

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