Weird physics question - relativistic immobility

[Alinestra Covelia]

Imagine a portable object about the size of a remote control. Imagine that it is utterly indestructible, but could be carried around easily by a human.

Now, say that it suddenly became "fixed" with respect to some arbitrary point in the Universe. (Say the origin point of the Big Bang, if such a thing can be conceptually defined.) How fast would it travel relative to us? What direction?

The Earth itself travels through space at about 30 km/s relative to the Sun as it revolves, so the object is likely to move away from us at speeds not less than that. That's about 108 thousand km/hour. I have no real idea as to how fast the Solar System is moving relative to anything else (although presumably the Bow Shock ought to be able to give us an estimate of the amount of resistance the Solar System's getting as it moves through the interstellar medium).

Thoughts?


(This first started off when one of my friends described the magical item "Immovable Rod" from the DnD game to me. That item remains fixed in place, but only relative to the planet you're on.)

[Aeson]

How fast would it travel relative to us?

0

We would travel relative to it

[KrazyHorse]

Now, say that it suddenly became "fixed" with respect to some arbitrary point in the Universe. (Say the origin point of the Big Bang, if such a thing can be conceptually defined.) How fast would it travel relative to us? What direction?

No such thing. The nearest thing which can be defined is the rest frame of the local cosmic microwave background. Specifically the surface of last scattering at a distance (say, in comoving coordinates) such that light from it is reaching us at this instant. Our motion relative to this frame of reference can approximately be given by the dipole moment of the CMB (approximately since there is some intrinsic dipole moment, but this should be much smaller than the dipole due to Doppler shift). IIRC the answer you get is ~600 km/sec. There's a famous picture from COBE in which the dipole is not subtracted (experiments since then remove the dipole as a matter of course). IIRC the orientation of the dipole is toward the galactic south pole (a bit off; like ~20 degrees from due galactic south).

In other words, the Earth moves toward the galactic south pole at ~600 km/sec relative to the surface of last scattering.

[KrazyHorse]

Here's the COBE picture I was thinking about.

[KrazyHorse]

By the way, this is already corrected for movement of the satellite around the earth and the earth about the sun. It's a picture relative to the sun's rest frame. There is a lot of complicated and precision programming which went into correcting the raw datastream for these motions.

[KrazyHorse]

In COBE pic blue is hotter, so that indicates motion of Earth toward galactic south.

[Alinestra Covelia]

In other words, the Earth moves toward the galactic south pole at ~600 km/sec relative to the surface of last scattering.

Nice, this is probably the take-away sentence for a commoner like me. I did have a hunch that the Earth's velocity through space as relative to the Sun was overshadowed by its velocity relative to the larger scale.

I suppose the galactic dipole is the largest absolute scale of measurement we have so far?

[KrazyHorse]

It's not the "galactic dipole". It's the dipole moment of the cosmic microwave background. The 600km/sec is our movement relative to the CMB. The only reason I even mention the galaxy is that the pictures orient galactic north straight up, i.e. they are images of a certain projection of so-called "galactic coordinates" onto a flat surface. The center of our galaxy is right in the middle of the picture.

[KrazyHorse]

If it follows standard procedures the mapping is HEALpix (hierarchical equal area isolatitude)

That's a bit of a detail, though.

[KrazyHorse]

bonus points to any amateur astro people who can, without looking it up, figure out why the isolatitude part is really important.

[Kuciwalker]

Explain the surface of last scattering. I think I have an idea what it means, but am not sure.

[KrazyHorse]

At some moment in cosmological history, in a relatively short period of time the universe went from being ionized (a plasma of nuclei...i.e. hydrogen and electrons) to being composed mainly of neutral atoms. This is called "recombination" confusingly enough (since there is no "combination" prior to it...)

At some moment while the ionization fraction dropped the universe went very quickly from being optically dense to being transparent (specifically, the mean free path increased quickly enough to allow basically free transmission of photons). The specific moment is called "decoupling". From this moment on photons were out of thermal contact with the universe. BTW, decoupling happened on a very short timescale. Cannot remember precisely how short, but suffice it to say that you would be surprised.

The light which scattered last from the ionized hydrogen is the CMB. The point of origin of this light is the surface of last scattering.

EDIT: said galactic, meant cosmological. Too much thinking about galactic coordinates

[KrazyHorse]

BTW, ali gets off of my ignore list as this was an astute question.

[Kuciwalker]

stuff

Thanks, that's pretty much what I thought it meant (though in more detail than I remembered).

[Alinestra Covelia]

BTW, ali gets off of my ignore list as this was an astute question.

KrazyHorse likewise was removed from my ignore list as his answer was what I was looking for.