Breaking Science News
This just came across the Reuters wire:
Einstein was right -- again.
Satellites that have been pulled slightly off their orbits show
that the Earth is indeed twisting the fabric of space-time as
it rotates, scientists said Thursday.
They said their findings are the first to directly measure
and prove an important aspect of Albert Einstein's General
Theory of Relativity -- that a rotating body warps and twists
the "fabric" that combines the three dimensions of space and
the fourth dimension of time.
"As the Earth turns, it is actually twisting space-time
with it. Near Earth, the twisting is greater," said Michael
Salamon, a physicist at NASA in Washington.
"This twisting of space-time, which is also referred to as
frame-dragging, has never been directly observed before,"
Salamon told reporters in a telephone briefing.
"This is the first real, solid, direct evidence we have for
the twisting of space-time caused by the spinning of a rotating
body."
Einstein was right -- again.
Satellites that have been pulled slightly off their orbits show
that the Earth is indeed twisting the fabric of space-time as
it rotates, scientists said Thursday.
They said their findings are the first to directly measure
and prove an important aspect of Albert Einstein's General
Theory of Relativity -- that a rotating body warps and twists
the "fabric" that combines the three dimensions of space and
the fourth dimension of time.
"As the Earth turns, it is actually twisting space-time
with it. Near Earth, the twisting is greater," said Michael
Salamon, a physicist at NASA in Washington.
"This twisting of space-time, which is also referred to as
frame-dragging, has never been directly observed before,"
Salamon told reporters in a telephone briefing.
"This is the first real, solid, direct evidence we have for
the twisting of space-time caused by the spinning of a rotating
body."
posted by cvo @ 2:50 PM
9 Comments:
At 2:53 PM,
cvo said…
Somebody decipher this for me.
Does this mean that time-travel may actually be possible someday?
Or does that have nothing to do with this story.
Signed,
Scientifically Challenged
At 3:09 PM,
Unknown said…
It means that an orbiting spacecraft's position at any given time is not precisely what would be predicted on the basis of the curvature of spacetime (normally referred to as the "gravitational field") due to an earth that doesn't spin.
According to general relativity, the mass of the earth is the primary source of curvature for spacetime near the earth, but there is a small distortion of spacetime due to the earth's rotation.
This small distortion has just been measured for the first time.
At 3:13 PM,
Unknown said…
You should check out my StumbleUpon blog. I have some summary information and a couple of interesting links related to this subject there. Right now, one of the relevant entries is about third from the top. You might need to go back to previous pages to see the other entry.
At 3:17 PM,
Unknown said…
I think that a blog (like my SU blog) is really best for a single person who is doing all of the logging. That's why I have written in the other thread about using a mailing list. We have several persons involved, and I think that a mailing list with a good Web interface would be better for fostering dialogue in an efficient and convenient manner.
At 6:46 PM,
Unknown said…
Back to the original topic of this thread.
There is a subtlety that is interesting to point out. Even if one does not consider general relativity, the spinning of the earth influences the predicted location of an object in orbit around the earth.
The earth's mass density is not spherically symmetric, and so the gravitational field produced by the earth is not spherically symmetric. As the earth spins on its axis, the irregular field spins with it, and every orbiting satellite has its trajectory perturbed by the spinning inhomogeneities in the earth's gravity field.
The point of the new results (and of results yet to come from Gravity Probe B), however, is that even if the earth *did* have a spherically symmetric mass distribution, general relativity predicts that the earth's rotation should, by distorting the the symmetric spacetime curvature that would be due to a non-rotating earth, thereby distort the trajectory of a satellite.
At 9:15 PM,
R said…
I need to take my mind off baseball... I didn't realize this was NOT a GPB story. Was this measurement made by a LAGEOS-like constellation? That was a competitor to GPB for years. It's hard to believe GPB is actually flying; they've been working on it longer than some of us have been alive. It's produced literally 100s of Stanford Ph.Ds. They had some thruster problems early in the mission that some folks in my group were asked to consult on, but I think the GPB folks solved it on their own.
Would it be correct to describe this result as due to the fact that curvature in the metric makes coordinate rotations only local, not global as they are in a flat metric? I have been trying for some time to get my mind around G.R., but seem to never get past the Christoffel symbols. Once you start having derivatives in space with a nonuniform metric, I get lost in all the commas in the notation.
At 7:05 AM,
Unknown said…
About LAGEOS, yes. See, for example, 'http://physicsweb.org/articles/news/8/10/12/1'.
I agree about the confusion with the symbols.
In my general relativity class in graduate school, the most practical thing we ever did was to show that in the weak-field limit the gravitational field of a spherical mass worked out to be just about what Newtonian theory says it should be (action-at-a-distance and all). So we did some (zero-angular-momentum) Schwarzschild-metric stuff. We also did the usual cosmological problems with the Robertson-Walker metric.
I don't remember if we ever even *looked* at the Kerr metric seriously. So I don't have any physical insight or intuition into the Lense-Thirring effect. If you find a good link, then post it or send it via e-mail.
Curtis was astute in his seeing the connection to time travel, as for a sufficiently massive and fast-spinning body (i.e., not the earth) there may exist---so I have read---closed time-like paths in the vicinity of the body.
At 10:53 AM,
Unknown said…
As to Russell's question about curvature:
I don't know if this is really an answer, but we might start thinking about the problem by backing out to a higher level. General relativity assumes that spacetime is a differentiable manifold. I think that calling spacetime "a differentiable manifold" is the same thing as saying (or at least it implies) that spacetime is locally Euclidean.
Only in a Euclidean space can a Cartesian coordinate system exist. So in a sufficiently small region of spacetime, one can always set up a familiar Cartesian coordinate system. (By "Cartesian coordinate system", of course, I mean the typical coordinate system of special relativity, which still has a -1 in the metric, but this is still essentially Cartesian.)
Anyway, one question is whether, in the vicinity of a large, spinning mass, a small object with zero spin angular momentum might, as it moves along a geodesic, end up changing its orientation with respect to some distant frame because of spacetime curvature. That is, might the local Cartesian body frame have a different transformation (to a far-away reference frame) when the body, retaining zero spin angular momentum, is at a different point in the orbit?
Another question is whether such a small object with initially zero spin angular momentum might, as it moves along a geodesic, obtain spin angular momentum from the large mass only by means of non-tidal gravitational (curvature) interaction. (That is, assume that the object in question is a very small rigid sphere of uniform density.)
Another question is whether the small object might simply move faster---or slower, if the stallite's orbital angular momentum is directed opposite to the spin angular momentum of the large mass---around its orbital trajectory than could normally be explained by the conservation of energy. In this case, the idea is that space itself is flowing like a whirlpool around the large spinning mass. Normally an object is considered to move through space as the object orbits a large mass, but if space were flowing around a large spinning object, then the small object would be like a boat in a river. The boat moves through the water, but the water itself is also moving.
This latest case is similar to Hubble expansion, in which sufficiently distant light traveling in our direction (traveling through local space at the speed of light toward us) is nevertheless being dragged away from us by the expansion of space itself.
I was imagining that the frame dragging reported in the news lately is of the space-flowing (translational) kind, but I suppose that there might instead (or additionally) be components of the spinning kind. Unfortunately, no one has stepped down to my level of understanding to explain it to me in detail.
At 2:10 PM,
R said…
I was thinking of a simpler case where the large mass is not (necessarily) spinning. We presume in attitude determination that there is Newtonian inertial frame against which motions of a rigid body can be measured, by gyros or whatever. If the underlying metric of the space-time in which that body is embedded reflects a curvature, then it would seem to me that you would see differences in the Euler angles that relate a set of axes fixed in the body to inertial, because the inertial frame is not really orthogonal like you are expecting it to be. As the body moves along a closed path like an orbit, these differences could look like precession.
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