Yet Another Speed of Light question: Gravity

Factual Questions
1

Physics question in two parts:

(1) What is the propagation speed of gravitational force? I’m no physicist, but it seems to me that if Gravitational energy moved only at light speed, orbits would not be possible because planets, moons, etc would be flung off. Unless I’m wrong (entirely possible), there would be an angular acceleration due to the gravitational acceleration vector pointing AHEAD of the <whatever you call the body that is being orbited–sun in our case> rather than at it, since gravity is accelerating toward where the <sun in our case> appeared to be when the light and gravity began travelling (~8 minutes for us on earth). According to my reading, there has been a lower boundary on gravitational propagation speed, on the order of 10[sup]8[/sup] times C. Backing up this idea is the fact that in physics classes we are told to assume gravity is instantaneous, otherwise the calculations don’t work.

(2) Everybody on this forum seems to get so furious whenever a poster suggests that information could be transmitted faster than C, you’d think they suggested the world was round. What next, is someone going to say it’s not the center of the universe, either? heh.

[sub]Aside: This is where I could get into the arrogance of modern science, claiming that anything we know now is “the way it is, period”, such as blindly clinging to the ideas that C is the ultimate speed limit, that no information can be transmitted faster than C, etc. After all, 700 years ago we KNEW the earth was flat, 300 years ago, we KNEW the atom was the smallest unit of matter, 30 years ago it was PROVEN that no human could run a mile in under 4 minutes, and so forth. But I won’t because that’s for GD, not GQ.[/sub]

But I don’t see any reason why information couldn’t at least theoretically be carried by gravitational variations via oscillation or whatever. Or at the very least, couldn’t the information that “there is a chunk of matter here” be transmitted faster than light speed, pending question (1)?

My crappy attempt at a diagram (sorry it’s not better):



        (instantaneous vector at t[sub]0[/sub])
(SUN[sub]actual[/sub])<--------------------------------------(EARTH)t[sub]0[/sub]
        -----\_________
                       \____________ (instantaneous vector at t[sub]1[/sub])
                                    \_________
                                              \
(SUN)<-----------------------------------------(EARTH)t[sub]1[/sub]
(apparent at t[sub]1[/sub])      (applied vector at t[sub]1[/sub])
2

Well, one 5 second search on http://www.google.com later…
http://math.ucr.edu/home/baez/physics/grav_speed.html

3

d’oh! thanks.

4

Regarding the speed of gravity: The brief answer is that, if gravity is a force directed between the center of masses of two bodies and if gravitational effects travel at the speed of light, then all orbits are unstable. Stable orbits are known; therefore one of the premises of that syllogism is incorrect. The first one is known to be incorrect, but it takes some fancy math to prove it.

In the case of relatively low warpage of space-time, you can approximate gravitation as a force field; when you work out the direction of the force vector used to approximate the real situation, you get a vector that is not parallel to the classic Newtonian force vector. And, guess what; it is non parallel by just the right amount and in just the right direction to compensate for the speed-of-light propagation of gravity!

Of course gravitation can be used to transmit information.

You do raise an interesting question of how proven Relativity is. Comparing it to proofs that no human can run a mile in under four minutes, or that bees can’t fly, is not really fair. Those proofs rested on shaky assumptions and inability to calculate or measure the real situation. Relativity is consistent with the results of many experiments and has made a staggering number of known-correct predictions. Relativity is also known to be incomplete; in some pathological situations, we need quantum gravity, which is not part of Relativity.

Back in the 60’s or 70’s Feynman pointed out that it’s really hard to come up with new theories in physics, because we have so many verified observations with which new theories must be consistent. It’s theoretically possible that Relativity will be replaced with a theory that allows faster-than-c transmission of information, but that new theory will have to be consistent with that big pile of observations. It’s more likely that Relativity will be extended or subsumed into another theory, and the predictions of extended Releativity or the new thory will be the same as the predictions of Relativity for all but the most pathological of situations.

5

Very interesting. Do you know of a place where I can get more background on this, that would make sense to somebody with 2 sems of college physics and math through Diff EQ?

For the record, I accept relativity in the sense that its implications match our observations. But, along with quantum mechanics, (based on my admittedly limited understanding of both) I accept it as an accurate model for making predictions, not necessarily a correct explanation. This is analogous to the model the ancients had of the solar system’s orbital mechanics. They had planets going back and forth across the sky in odd-shaped patterns that exactly fit observations, but didn’t necessarily make sense. A deeper understanding of the universe gave us the heliocentric model, which also fit, but went a step further and explained the observations.

6

No, not really. Understanding why the speed of light is unbreakable as a speed is sort of akin to understanding why nothing can be colder than absolute zero.

Our experience wants us to believe that the only physical limits on the universe are * practical* limits. The only reason we don’t have cars moving at, say, 1000 miles per hour is the impracticality of creating that much thrust. We want to believe that speed is a function of thrust alone.

If you knew NOTHING about what causes “heat” and “cold” you might be tempted to think that “hott” and “cold” were just scales that ran on for ever, however, once you realize that heat is a function of molecular motion, and that the lower limit of motion is of course “stillness,” then there must be a lower limit of heat and of temperature.

Likewise, if you know nothing about Relativity, you might assume that speed is merely a function of how fast it takes you to get somewhere, and that there is no upper limit to the scale of speed. However, speed DOES NOT work like this. There have been numerous, better explanations of how “the speed of light” works, and i would recommend the reader to these, but suffice it to say that acceleration (thrust) does NOT only effect the speed of an object, but its time and size(mass) as well. Speed, time, and size are things that cannot be measured without a reference. These words have no meaning unless you compare them to something else. Speed, size, and time all change in a moving frame of reference such that from any frame of reference light always is measured at the SAME speed. The description of the speed of light as unbreakable is not merely a theory; it is not an explanation of observed events, it IS itself an observed event, and thus a law.

Ask youself this: How does one go faster than the observed speed of light(in reference to anything else) and maintain a constant speed of light from your moving reference? It can’t be done, since this would require the object to have an infinite mass, and to also be moving at an infinitely slow time (motionless). Since time itself can’t go any slower than a standstill, you CAN’T accelerate faster than the speed of light from your current reference frame. It’s not merely some number someone pulled out of the air, its a function of the nature of reality, exactly like absolute zero is.

7

This is not true. We have known the world was (roughly) spherical since before the time of Christ. Eratosthenes even calculated its size, with pretty accurate results, around 230 BCE.

Columbus never thought the world was flat (nor did any person of his time, at least the sane ones); he thought it was small. And he was wrong. Luckily there was a large land mass and some islands in his path, or he and his entire crew would’ve died at sea.

8

Joe_Cool wrote:

This is true, but I want to point out that the ancient greeks actively debated heliocentrism. (Aristotle mentions debating the subject.) Ironically, it was regarded with skepticism because the stars would have to be an absurdly long distance away in order to appear stationary w.r.t. a rotating earth. This is a fairly rational objection (at least, much more so than later objections would be) but was wrong- the stars are a ludicriously long distance from earth.
jayron 32 wrote:

nitpick: actually, it’s moving AT the speed of light that requires an object to have infinite mass and experience no time. Objects moving faster than light we don’t have any data on, for obvious reasons. Hypothetical objects moving faster than light might have finite mass and travel backward in time, but the resulting violations of causality would be tachy. :slight_smile:

9

Well, you can try Aberration and the Speed of Gravity. The only way I could get the paper was to pick “Other Formats” and then the “Create PDF” button.

You can also browse 5000 Lines of Fascinating Stuff, Mostly by Steve Carlip.

At the end of Does Gravity Travel at the Speed of Light?, Carlip lists T. Damour, in “Three Hundred Years of Gravitation”, S.W. Hawking and W. Israel, editors (Cambridge Univ. Press, 1987) as a good technical reference.

10

OK, show of hands, who thinks that (Tim) should be taken out and shot for that pun? As to FTL, the current theories (GR and SR) actually do allow for the possibility, but with some rather weird consequences that most folks are not willing to accept, namely negative (or even imaginary) mass and bidirectional causality. We do not, however, have any actual scientific evidence as yet that nonpositive mass or bidirectional causality are impossible. Therefore, the possibility of FTL remains open. I’ll let you’uns know if I ever figure out the answer :slight_smile: