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Jack comments on Justifiable Erroneous Scientific Pessimism - Less Wrong Discussion

14 Post author: Eliezer_Yudkowsky 08 May 2013 08:37PM

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Comment author: Jack 09 May 2013 03:18:16AM *  2 points [-]

*there would have been a parallax given assumptions at the time regarding the distance of the stars.

I've wondered though: if there were no planets besides Earth would we have persisted as geocentrists until the 19th century?

Comment author: SilasBarta 09 May 2013 07:12:48AM 0 points [-]

If there were no celestial bodies but Earth and the sun, we would have been just as correct as heliocentrists.

Comment author: Jack 09 May 2013 08:33:25AM *  3 points [-]

I don't think that's right.

Comment author: ArisKatsaris 09 May 2013 01:08:15PM 5 points [-]

The center of mass for the Earth-sun system is inside the sun; so, yeah, the heliocentrists wouldn't be "just as correct".

If the two masses were equal, then Earth and Sun would orbit a point that was equidistant to them; and in that scenario heliocentrists and geocentrists would be equally wrong....

Comment author: Kawoomba 09 May 2013 02:18:11PM -2 points [-]

Why privilege the center of mass as the reference point? Do we need to find the densest concentration of mass in the known universe to determine what we call the punctum fixum and what we call the punctum mobile?

As far as I can tell, most of the local universe revolves around me. That may be a common human misconception, seeing as I'm not a black hole, if we only go by centers of mass. But do we have to?

(Also, "densest concentration of mass" would probably be in the bible belt.)

Comment author: rocurley 09 May 2013 03:30:28PM *  1 point [-]

I think the center of mass thing is a bit of a red herring here. While velocity and position are all relative, rotation is absolute. You can determine if you're spinning without reference to the outside world. For example, imagine a space station you spin for "gravity". You can tell how fast it's spinning without looking outside by measuring how much gravity there is.

You can work in earth-stationary coordinates, there will just be some annoying odd terms in your math as a result (it's a non-inertial reference frame).

Comment author: SilasBarta 09 May 2013 05:26:14PM 3 points [-]

You can determine if you're spinning without reference to the outside world.

Technically, no you can't. Per EY's points on Mach's principle, spinning yourself around (with the resulting apparent movement of stars and feeling of centrifugal stresses) is observationally equivalent to the rest of the universe conspiring to rotate around you oppositely.

Einstein's theory further had the property that moving matter would generate gravitational waves, propagating curvatures. Einstein suspected that if the whole universe was rotating around you while you stood still, you would feel a centrifugal force from the incoming gravitational waves, corresponding exactly to the centripetal force of spinning your arms while the universe stood still around you.

The c.g. of the earth/sun solar system would likewise lack a privileged position in such a world.

Comment author: satt 10 May 2013 01:59:23AM *  1 point [-]

You can determine if you're spinning without reference to the outside world.

Technically, no you can't.

Is that correct? Spinning implies rotation implies acceleration, which I'd always thought could be detected without external reference points.

Per EY's points on Mach's principle, spinning yourself around (with the resulting apparent movement of stars and feeling of centrifugal stresses) is observationally equivalent to the rest of the universe conspiring to rotate around you oppositely.

Without taking a stance on Mach's principle or that specific question of observational equivalence, what about a spinning body in an otherwise empty universe? As an extreme example, my own body could spin only so fast before tearing itself apart. Surely this holds even if I'm floating in an otherwise utterly empty universe?

Comment author: SilasBarta 10 May 2013 04:54:52PM *  0 points [-]

Is that correct? Spinning implies rotation implies acceleration, which I'd always thought could be detected without external reference points.

This is addressed later in the article, very well IMHO. Let me just give the relevant excerpts:

If you tried to visualize [the entire universe moving together], it seems like you can imagine it. If the universe is standing still, then you imagine a little swirly cloud of galaxies standing still. If the whole universe is moving left, then you imagine the little swirly cloud moving left across your field of vision until it passes out of sight.

But then, ... you can't always trust your imagination. [...]

Suppose that you pick an arbitrary but uniform (x, y, z, t) coordinate system. [... Y]ou might say:

"Since there's no way of figuring out where the origin is by looking at the laws of physics, the origin must not really exist! There is no (0, 0, 0, 0) point floating out in space somewhere!"

Which is to say: There is just no fact of the matter as to where the origin "really" is. [...]

[...]

And now—it seems—we understand how we have been misled, by trying to visualize "the whole universe moving left", ... The seeming absolute background, the origin relative to which the universe was moving, was in the underlying neurology we used to visualize it!

But there is no origin!

Comment author: satt 10 May 2013 09:03:19PM 0 points [-]

I worry I'm missing something obvious, but that EY quote doesn't seem to address my belief (namely, that detecting accleration doesn't need an external reference point). It just argues there's no absolute origin to use as an external reference point.

Comment author: rocurley 10 May 2013 12:35:37AM *  0 points [-]

I agree that it's at least quite plausible (as per your post, it's not proven to follow from GR) that if the universe spun around you, it might be exactly the same as if you were spinning. However, if there's no background at all, then I'm pretty sure the predictions of GR are unambiguous. If there's no preferred rotation, then what do you predict to happen when you spin newton's bucket at different rates relative to each other?

EDIT: Also, although now I'm getting a bit out of my league, I believe that even in the massive external rotating shell case, the effect is miniscule.

EDIT 2: See this comment.

Comment author: SilasBarta 10 May 2013 11:55:27PM 0 points [-]

Are you sure you linked the right comment? That's just someone talking about centripetal vs centrifugal.

Comment author: rocurley 11 May 2013 01:53:21AM 0 points [-]

No, I didn't. It's fixed now, thanks.

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