If Gravity were to race Light...

January 7, 2003 3:10 PM

In an interesting test of the theory of relativity they found that gravity travels at the same speed as light. Put simply if the sun were to disappear from existence it would take some 8 minutes before the Earth's orbit would be affected by the loss. Sadly this also means that FTL travel is becoming less and less likely to be possible.
posted by aaronscool (17 comments total)

I don't know why but it sorta freaks me out and makes me feel a little claustrophobic that we may never be able to have interstellar travel. At least the kind that would take less than several lifetimes to do...
posted by aaronscool at 3:14 PM on January 7, 2003

FTL travel is becoming less and less likely to be possible.

Au contraire my snowbird friend. Continental flies six flights a day to Fort Lauderdale, and that's just Philly.

And no, there's no way I'm going to be critically evaluating a link that tests the theory of relativity.

Or even reading it.
posted by luser at 3:29 PM on January 7, 2003

that we may never be able to have interstellar travel.

no one's been suggesting interstellar travel by way of going FTL in a direct straightforward way though (in fact the main reason for attempting FTL is for purposes of time travel as far as I understand it) - wormholes and warp speeds (literally changing the curvature of space) are the preferred theories. Though a lot of it does seem pretty unlikely...

what I don't get is how this is proving einstein as opposed to the quantum theories of gravity. If gravity is the result of traveling gravitons, it should have a speed; if it's the result of space-time being curved by mass, why couldn't it be instantaneous once the mass disappears?
posted by mdn at 3:29 PM on January 7, 2003

That's like proving that your video card can't render above 75fps ... with the refresh lock on and the refresh rate at 75.

It's going to look like 75fps no matter what because it's locked by the visual setting (speed of light in this case).

unlocking the rendering from the universal refresh rate causes nasty tearing though ... do not attempt at home.
posted by Dillenger69 at 3:44 PM on January 7, 2003

mdn - it's not an either/or thing, just an extra constraint that might affect a few models (my understanding of the article). and the rubber sheet analogy is a little misleading - relativity (broadly) works by specifying how each little piece of the universe curves, then you join all the bits up and work out what the global shape is. so the global shape is the result of a patchwork of local values; if you want to know how the global shape changes you need to start from how the local values change, and then it's more intuitive (maybe?) that a distant patch will not "know" about a change in mass (i'm waffling on the edge of my knowledge here, so this may be pure bullshit)

also, the apj letter questioning the result is interesting - if it's been accepted by apj then it's probably not obviously incorrect.

either way, it would have been much more interesting if the result had shown an infinite speed. this is a little like "dog bites man" (no criticism on the post - it was interesting).

i suspect (just guessing) that gravitation wave detectors will be able to do this kind of measurement eventually (eg by observing orbiting black hole pairs) - but they seem to be taking forever to develop.
posted by andrew cooke at 3:49 PM on January 7, 2003

oops. when i mention "rubber sheet" i'm referring to the tired old illustration of "bending space time" by placing a heavy ball on a rubber sheet. i thought mdn referred to it; they didn't. also, my explanation is waffly - much better to say that removing the ball causes waves in the sheet and those waves travel with a finite speed. similarly with gravity waves, and it's their velocity that is being measured here.
posted by andrew cooke at 3:56 PM on January 7, 2003

I may be veering off topic here, but similar to the "dog bites man" comment, I'm more interested in the click here to license content link. $750 to publish an excerpt once on the internet? I was under the impression that it was free to quote an article.

"Kopeikin said the observation sheds light on one of the last unsettled fundamental constants of physics — and could play a role in the continuing quest to develop a "theory of everything” that unites gravitational theory and quantum mechanics."

Yep, still free. Also, I think the above excerpt sums up the true importance of the article.
posted by BlueWolf at 5:12 PM on January 7, 2003

Just because the speed of light turns out to be a limit for gravity as well doesn't mean we're out of options; there are still a few things that travel faster. Right now I'm working on the Rumor of Impending Layoffs Drive. If that doesn't pan out there's always the Guy Just After His Girlfriend Asks 'What Are You Thinking' Drive.
posted by George_Spiggott at 5:13 PM on January 7, 2003

I still like Vernor Vinge's conjecture/plot-device - that there are different zones in the galaxy in which the laws of physics behave differently; i.e., Earth is in the Slow Zone, where FTL travel and certain technological accomplishments are impossible, but further out are zones where the laws of science are different, and even further out are civilizations that have hit the Singularity and are indistinguishable from God.

All of this makes me sad that our planet probably has an intergalactic "Stay Away, They're Fucking Nuts" sign on it, eliminating the possibility of the Qeng Ho making contact to trade goods and tech.
posted by GriffX at 5:31 PM on January 7, 2003

George_Spiggot, you're awesome.
posted by swerdloff at 5:43 PM on January 7, 2003

makes me feel a little claustrophobic that we may never be able to have interstellar travel

Well, interstellar travel isn't really a problem without FTL; it just means you have to accept certain limitations.

If all you want to do is get somewhere fast, relative to you, you're laughing; just travel at some suitably high velocity towards it and time dilation will kick in. For you, you'll pass lightspeed and get there in a week. For everyone else you might still take a century or two, but so what, you're the one who matters ;)

This, of course, somewhat depends on you being able to provide enough energy to reach these velocities, while taking care of the rather abrasive nature of the interstellar medium.

If not, there are plenty of other options; sleeper ships, generational ships, robotic ships (maybe with your mind downloaded into it, or perhaps just your DNA and some very impressive cloning techniques), etc.
posted by Freaky at 5:44 PM on January 7, 2003

Doesn't this all become academic if you don't look at space and time as two distinct things? Everything in space-time(x) is relative to everything else in space-time(x). If you double space-time(x)'s space, the time it takes to go from point A to point B also doubles within space-time(x), but so does the speed of an object travelling between A and B.
But within space-time(x), space, time and speed all seem to stay the same, relative to each other.

But if space-time(x) doubles and space-time(y) doesn't, then everything in space-time(y) looks half-size from space-time(x)'s point of view, and objects travelling from point A to point B in space-time(x) at the speed of light seem to be going at twice the speed of light from space-time(y)'s point of view.

So to go faster than light, you only need to access a space-time where *its* sub-light speed is faster than your native space-time's speed of light. When you go a distance, then you return to your space-time and the distance you will have travelled will be equivalent to having travelled faster than light.
posted by kablam at 6:26 PM on January 7, 2003

But Rael...the extraterrestrials..it MUST BE POSSIBLE!
posted by ParisParamus at 6:38 PM on January 7, 2003

Given the wonders humans have spread across the planet as they developed sufficient technology to traverse large distances in less and less time, I've always felt it was no coincedence that star systems are conveniently spaced apart sufficiently to prevent any kind of similar interstellar effect.

Thats my take on "The Prime Directive" of Star Trel fame.
posted by Fupped Duck at 9:39 PM on January 7, 2003

[slightly offtopic]
I'm currently reading Hawking's "A Brief History of Time" (finally!) and find his ability to explain these concepts useful.
[/offtopic]

It really isn't surprising that the forces of gravity coincide with the speed of light.
posted by nofundy at 6:11 AM on January 8, 2003

Actually, according to several scientists, with whom I agree, they did not prove that the propogation of the force of gravity is limited only to the speed of light. Instead they proved that the speed of light is limited to the speed of light.

Hideki Asada explains the results as an effect of the light-cone effect here.

At the very best (which I still doubt) they may have proven that gravity waves travel at/near the speed of light.

Not the first case in science where they measured the wrong thing by accident. But leave it to the media to take it at face value, without waiting for the peer review process to complete.
posted by Lafe at 7:20 AM on January 8, 2003

For you, you'll pass lightspeed and get there in a week. For everyone else you might still take a century or two, but so what, you're the one who matters ;)

the problem is, even at light speed most things in the galaxy are lifetimes away.

and objects travelling from point A to point B in space-time(x) at the speed of light seem to be going at twice the speed of light from space-time(y)'s point of view.

one of einstein's discoveries, which has been held as true ever since, is that relativity does not apply to light. You can be going at 99% the speed of light and light will still look like it is going at the speed of light (186,000 miles per second or whatever it is). I don't really see what you're getting at with "space-time x doubling and space-time y staying the same" (are these separate universes? how are they interacting? what makes one of them 'double'?) but the speed limit is seen as absolute, not relative.

andrew cooke, I know it's not either/or but there has been some disagreement between those expecting an answer through particles and those who expect a geometric unity in the end, and this seemed to be more compatible with the particle understanding to me. I can imagine it being the travel of information of this disappearance, but i'm not exactly sure why the curving of space-time wouldn't be essentially a property of the mass rather than a message from it, so to speak, so that if we posit the magic disappearance of the mass, I could imagine the effect going with it.

thanks for the link, lafe.
posted by mdn at 10:29 PM on January 8, 2003