Simultaneity

[mmcirvin]

Measured in the heliocentric frame, the time to the Galileo probe's destruction is now less than the light-speed travel time between Earth and Jupiter. What that implies is that, according to special relativity, there are some inertial rest frames (corresponding to motion at a large fraction of the speed of light in the direction of Jupiter) in which Galileo is already gone as I write this, and others in which it still exists.

Comments

[sunburn.livejournal.com]

The part that confused me about all this is that I always figured NASA et al. would talk about what's happening by the spacecraft's clock, and that everyone would understandably take into account that we won't see evidence of whatever's going on for several hours as light plods back our way.

But I guess reporters are bozos and don't appreciate showing up just in time for something that they can't see for 2 hours, so they stick with local time.

But you'd think that given the relative closeness of Jupiter, compared to the spectacular distance of everything in the entire universe, it's still be safe to talk about what's going on in the spacecraft "at this moment" even though the concept of the current moment is subject to confusion when changing frames of reference. I guess the only time you can fudge like that is when the light-distance is comparable to the distance of a light-"this moment." But that just means the moon and the lagrange points, and maybe the occasional near-Earth asteroid rendezvous.

I believe we need longer moments. Much longer.

[mmcirvin.livejournal.com]

It all depends on the application. You need to be really careful when speaking of "this moment" even for earthly distances, if you're talking about the operation of GPS; the associated ambiguities are of a scale that's important for putting you on the right streetcorner. That's part of what gets some people so confused that they think GPS disproves relativity.

[mmcirvin.livejournal.com]

...And it's actually not as bad as I implied in my original post, since our spacecraft don't actually travel at a large fraction of the speed of light. So it's pretty reasonable to use something like a heliocentric rest frame when trying to define simultaneity. The resulting definition is not going to be far off from the rest frame of any spacecraft.

Though things get more complicated when you start to consider gravity and general relativity; then the whole notion of an inertial rest frame is only defined locally, and the arbitrariness of "this moment" spread over space becomes more theoretically stark.

But since we're now about half an hour inside the future light-cone of Galileo's plunge, we can quite unambiguously say that it's gone.

[mmcirvin.livejournal.com]

Correction: That should be "about one hour."

[mmcirvin.livejournal.com]

And so that I can sound even more insane by following up to myself repeatedly: NASA's Galileo impact page used UTC. If I recall correctly, this has surfaces of simultaneity defined in a nonrotating geocentric frame, though the lengths of the seconds are such that they correspond to seconds in a frame moving with the surface of the Earth as it rotates.

It probably doesn't make a great deal of difference, but my guess is that they're making the reasonable extension to Jupiter via taking half the round-trip light travel interval.

[ronebofh.livejournal.com]

Wait, some people think GPS disproves relativity? Who are these people and where is my aluminum bat?

[mmcirvin.livejournal.com]

One of Tom van Flandern's perennial claims is that the use of a nonrotating coordinate frame for GPS is some sort of means of covering up an "embarrassment" to relativity. Here's his head-breaking article on the subject (http://www.metaresearch.org/cosmology/gps-relativity.asp), and a good critique by Chris Hillman. (http://math.ucr.edu/home/baez/RelWWW/wrong.html#gps)

But van Flandern never actually comes out and states that GPS is evidence against relativity; it's all on the level of innuendo and burden-of-proof-shifting in the course of saying confusing things that could mostly be considered technically correct. Other critiques are less circumspect, if not more comprehensible. (http://egtphysics.net/GPS/RelGPS.htm)