If the sun was taken away would there be a time delay for the earth to loose its orbit?

Question

I asked this a minute ago (all answered instantly) but I'm reading a book on Feynman and I'll quote: Time delay had not been a feature of the original electromagnetic theory. In Maxwell's time, on the eve of relativity, it still seemed natural to assume, as Newton had, that forces acted instantaneously. An imaginative leap was needed to see that the earth swerves in its orbit not because the sun is there but because it was there eight minutes before, the time needed for gravity's influence to cross nearly a hundred million miles of space--to see that if the sun were plucked away, the earth would continue to orbit for eight minutes.
From Genius ~ Richard Feynman and Modern Physics.
The text is from the 1940's. Is this relativity? Is it the current theory?

Answer 1

Richard Feynman is correct. Gravity waves (the linearized version of gravity as described by general relativity) travel at the seed of light, so if you take the mass of the sun away, it would take about 8 minutes to be noticed here on earth.

And of course it is possible to synchronize clocks in two different coordinate systems like earth and on the sun sufficiently well to give practical meaning to the time difference of eight minutes. This has absolutely nothing to do with the issue that absolute time does not exist. What does exist is the well defined distance between two space-time events. In our case the earth changing orbit is a causal consequence of the sun being taken away. There is no coordinate system moving at any speed in which this causality can be eliminated. An observer moving at a high speed relative to these two events would see the time between them shortened, but they also could reconstruct what time difference would be measured in the eigentime of an observer resting relative to the sun or earth (which for our purposes is the same). And those are the eight minutes Feynman is talking about.

And yes, this is the current theory. We are looking very hard for one that could replace general relativity but haven't found one. So right now the score is

Newton, two centuries - Einstein, one century and counting.

Pretty good, in my books.

Answer 2

I am a physicist, and although my specialty is not relativity, electromagnetism, or classical mechanics, I have had plenty of graduate courses in those areas. First, I take issue with a book author who would say that "Time delay had not been a feature of the original electromagnetic theory". Maxwell, Farady, and others who developed the early theory, were well aware that the electromagnetic effects required finite time to transmit their fields and other effects, and the constant c was an integral part of the theory and was shown to be the equivalent of the speed of light. Remember, the book's author was not Feyman, but someone writing about Feynman. Was the author himself a physicist?

We learned in mechanics that physicists following Newton and working in classical (Newtonian) mechanics (and long before Einstein came along) were troubled by their ignorance of how the force of gravity propagated itself. The laws of dynamics did not say anything about a finite time for the gravitational force force of one object (e.g. the Sun) to flow from it and affect the motions of another object (e.g. the Earth). I do not know if Newton was troubled by this, or said anything about it, but we were told that others who came along were, but no one had an answer. They just solved the dynamical equations of motions as if the gravitational force were instantaneous. So gravity and E&M were different regarding assumptions they made about propagation of fields and forces, early E&M explicity including such effects.

In Relativity theory, as I understand it, the gravitational effect is caused by the warping of space-time by a massive body, as you stated. But it was my impression that from the start the theory included the concept that this warping would not be felt instantaneously by a distant body, but the distortion would have to propagate there, at the speed of light. I think this was the standard concept in physicists working in Relativity. Thus if the Sun were to disappear instantly (a physical impossibility of course), the space-time in the vicinity of Earth (and all other planets) would remain warped until the a new arrangement of space-time traveled (in 8 minutes) from the region where the Sun had been to the place where Earth now was (having moved a little in those 8 minutes).

Answer 3

Yes, according to both "old-fashioned" general relativity and the spin-2 massless graviton quantum field theory (which Feynman worked on in the 60s), gravity disturbances propagate at light speed. He did a bunch of lectures on this at Caltech that got turned into a book--Feynman lectures on gravitation--but I wouldn't tackle it until you've learned general relativity already and had some quantum field theory.

The current situation with particle physics and gravitation is a bit thorny. Theory predicts gravitational waves that propagate at light speed, but we can't measure them just yet to test that. That may change in the pretty near future. A problem which is unlikely to change any time soon is merging gravity with quantum field theory. At extremely high energies (which we may NEVER achieve experimentally), they contradict one another. Gravitational QFT can be made to agree with GR at low energies, but it is non-renormalizable, which means that it can't be perfectly correct at the end of the day. Resolving this theoretical issue is the reason for string theory.

Answer 4

Ug. I had to read a book by him for honors physics recently; very unclear...horrible syntax...

Anyway this is relativity, because newton's theory of gravitation was only valid under the principle that gravitation was instantaneous, but enstein said that gravity can not be faster than the speed of light which is the new standard in gravitational theory.

So yes, there is a time delay.

Answer 5

For a person standing on earth, the earth would lose its orbit instantly because the sun would disappear at the exact moment that gravity disappeared. There is no absolute time, so there would be no time delay for the person standing on earth.

However, lets imagine that someone can stand on the sun. They would see that the earth takes 16 minutes to lose its orbit, because it takes 8 minutes for the loss of gravity to affect earth, and then 8 minutes for the light to reach the observer back from earth.

Answer 6

Excellent question...the answer is...it depends on which theory of gravity you subscribe to.

If you believe in gravitons (string/M theory and quantum gravity theory), they are like all other messenger quanta, like photons, in that they travel at light speed.

If you believe in warped space (relativity), the warp and, therefore, gravitational force are instantaneous. As relativity was the dominant theory when your textbook was written, it is understandable that Feynman thought gravitational effects were instantaneous.

Answer 7

Yes, gravity travels at the speed of light.

Answer 8

gravity propagates at the same speed as the speed of light.

Answer 9

It would wouldn't matter, because you would freeze to death.
and not have to worry about "global warming." <}:-})