Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 07/10/2015 20:50:01
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Phillip Nance asked the Naked Scientists:
Newton's "Gravity" failed to explain the 'perihelion precession' of the planets, most noticeable in Mercury's orbit due to its nearness to the Sun. Einstein's space-time distortion (again best observed under 'high' gravity) explained the discrepancy.
What Newton never knew (or would have guessed?) is that "gravitation' acts at the 'constant' speed of light, and known to Einstein. While his Maths would likely have been incapable of the necessary calculations (a snap for computer in the simplest case), had he have known what Einstein knew, would he have made the following observation to explain Mercury's 'perihelion precession' (again of which he was unaware) - the maximum "Gravitational attraction" between Mercury and the Sun occurs some 3 mins after Mercury's perihelion (closest approach to the Sun). Thus, it would be 'dragged' inward from its orbit and explain the precession.
What do you think?
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Would Einstein have reached his conclusions if Kepler had not determined his laws of planetary motion?
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the maximum "Gravitational attraction" between Mercury and the Sun occurs some 3 mins after Mercury's perihelion
I can see a couple of potential problems with this hypothesis:
1. Effect on other Planets
Mercury's perihelion is 46 million km from the Sun, or 2.6 light-minutes (call it 3 minutes).
Earth's perihelion is at 147 million km, or 8.2 light-minutes.
If the gravitational attraction were to propagate at lightspeed (but only at perihelion), then you would expect that the precession of Earth's orbit would be larger than that of Mercury, and this deviation from Newton's theory would have been recognized a lot earlier than 1859. The impact on Jupiter would be even greater. However, the measured precession of Earth's orbit is about 1/10 that of Mercury.
An alternative hypothesis (thanks to Einstein) is that the precession of Mercury's perihelion (http://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury) is greater than that of Earth or Jupiter, because Mercury is deeper in the Sun's gravitational well. The prediction for Earth's precession is in good agreement with the measured value.
2. How long does it take the Sun's Gravitational Field to Propagate?
Precisely how General Relativity relates to propagation of quantum-level gravitons is one of the unresolved mysteries in physics.
However, Mercury does not suddenly "appear", for then we might ask how long before it "feels" the Sun's gravitational attraction. Gravitational attraction is present throughout Mercury's 88-day "year" (modulated by Mercury's 20% orbital eccentricity), so there should be no delay for propagation time.
In fact, the Sun's gravitational field has been in place for roughly 5 billion years, so it shouldn't take an extra 2.5 minutes for this gravitational field to propagate out to where Mercury is now.
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If gravity does in fact trap itself behind an event horizon then its speed is <= c. If the speed of gravity > c then there is a problem. Since a singularity will produce a gravitational force that approaches an infinite value then at some point inside the horizon gravity is still trapped. This leads to the conclusion that it is more likely that the speed of gravity is <= c. In the case where the speed of gravity is less than the speed of light, even by only a slight amount, then its effectiveness dies off in the vicinity and outside of the event horizon. This could explain the survival of the G2 gas cloud. The energy of the gravitational field is lower than that of the electromagnetic field by an appreciable amount. This may make a difference to the speed of propagation. With much longer wavelengths, the gravitational field has an altogether different profile.