Velocity

[The Holy Mol]

Does anyone know the velocity the Earth is travelling in relation to a fixed point in space?

 

In other words....if an object could be released that was absolutely fixed in place, at what rate would I move away from it?

[Ophiolite]

There are no fixed points in space. Velocity can only be measured relative to something else. Here are some examples:

 

Earth's average orbital velocity around the sun: 29.8 km/second relative to the sun

 

Astronomers sometimes find it convenient to imagine that there is a sort of fixed point in space as you have described. For example they will refer to the Local Standard of Rest (LSR). This is an imaginary circular orbit around the galaxy lying within the Galactic plane. At our distance from the galactic centre a star that followed that orbit exactly would be travelling at 220 kms per second relative to the galactic centre.

The Solar system's orbit is not exactly circular (nor does it remain in the galactic plane). So, it has a motion, relative to the LSR, towards the constellation of Hercules, of between 17 and 22 kms/sec.

 

Our closest large neighbour galaxy is the Andromeda galaxy. We are approaching it at about 140 kms/second and will eventually collide in around 3 billion years.

 

Hope that helps.

[The Holy Mol]

That was exactly the type of information I was looking for...it's just too bad that an exact speed from a fixed point could not be figured out.

 

It would be interesting to know how fast we're moving...as an argument to say that even though we can move 1,000's of mph, that's actually pretty slow.

 

Is Earth (in it's spot in our galaxy) in a rotation moving towards Andromeda? So are we moving at the combined speed of the rotation plus the direction of the galaxy?

 

That way I could say that either we're moving at a max of 777,600 mph or we could be moving a snail's pace of 172,800 mph.

[Martin]

There are no fixed points in space. Velocity can only be measured relative to something else. Here are some examples:

 

....

 

here's another to add to Ophi's list of speeds relative to various things

 

the whole-sky average temperature of the CMB (cosm. microwave background) is

some 2.725 kelvin

but in direction of constellation Leo the CMB is 3.5 millikelvin hotter and in opposite direction it is 3.5 millikelvin colder

this is exactly the doppler effect one would anticipate if the earth and sun were moving in the direction of Leo at a speed of 1.28 thousandths of the speed of light (relative to the expansion of space)

[Ophiolite]

Is Earth (in it's spot in our galaxy) in a rotation moving towards Andromeda? So are we moving at the combined speed of the rotation plus the direction of the galaxy.

Good question. I don't know the answer. The figure I gave you was actually derived by adding (or subtracting) the appropriate velocity of the Solar System' date=' and the Earth within the System, from the closing velocity of the Earth with stars in Andromeda, based on their blue shift. Try Googling some combination of [i']velocity Andromeda[/i] "blue shift" collision.

 

Just to re-emphasise a point. Our inability to measure a velocity relative to a fixed point has nothing to do with failure of technique. As we understand the Universe today there really are no fixed points.

[Martin]

...

 

Is Earth (in it's spot in our galaxy) in a rotation moving towards Andromeda? So are we moving at the combined speed of the rotation plus the direction of the galaxy?

....

 

hello Holy

you asked Ophi but I will take the liberty of answering too.

 

yes, the sun is orbiting the galactic center and currently its orbit motion direction is in the constellation Cygnus which is rather much in the same direction as the constellation Andromeda, where Andromeda galaxy is.

(that orbit motion is not an absolute but is relative to the center of Milky)

 

 

So at present we are getting closer to Androm. galaxy for two reasons----because the two galaxies (Androm and Milky) are getting closer----and because for the time being our orbital motion around Milky's center is also taking us towards Andromeda

 

but this is probably not such a good way to visualize motions in space.

 

astronomers measure the motion of our whole little group of galaxies---the Local Group----relative to the expansion of space (or socalled hubble flow which is the flow of distant galaxies away from us and defines a preferred frame of reference coinciding with the CMB frame.

 

A year or two ago ago I saw a measurement of our Local Group velocity, that it was roughly toward HydraCentaurus at about 600 km/second. this is in the southern hemisphere. A more precise direction for the milkyway is in the constellation Crater (also southern, near Hydra and Centaurus)

 

Our local group is really moving! It challenges all the other speeds in the picture. Andromeda is TAILING US and gradually moving closer. She is very slowly catching up.

 

We are like a bug riding on a Frisbee. the Frisbee is going south (say) but because it is whirling and we are on the side that is going backwards, well you have to add the speed directions and there is some cancelation---not entirely simple---Ophi described it earlier

 

Suppertime, will get back later and check the numbers

 

I havent found good sources for this. here is one that is tangential but has some information anyway

http://arxiv.org/astro-ph/0210165

 

they unless there is some other explanation for the dipole in the CMB, the local group of galaxies is moving 627 +/- some km/s (basically in direction of constel. Crater) as always with respect to something, in this case it is the surface of last scattering of the CMB---the CMB defines a common preferred frame for working cosmologists (the metric they use called FRW has a preferred frame so they have to have an external standard of rest in order to position the metric)

 

 

I gather that Milky is now considered more massive than Andromeda (they are the two main bodies in the Local Group) so Milky is basically the leader of our local fleet of galaxies and our direction---relative to the CMB---corresponds approximately to the Local Group direction.

[The Holy Mol]

Ophiolite, I didn't think that there were any fixed points....that's why I used the idea of dropping a theoretical object (that was not affected by any force...could a neutrino accomplish this?) and measuring the speed it appeared to move away.

 

Can space be compared to an empty room? I mean does it exist no matter what, it's just the universe inside of space that changes? So in the empty room a balloon would be the known universe...is that accurate?

 

 

Martin, that is pretty fast! The information provided at that link is above my head, but I can makes sense of some if it.

 

 

Thanks for the help to get me to understand Ophiolite and Martin.

[Martin]

...

 

Can space be compared to an empty room? I mean does it exist no matter what' date=' it's just the universe inside of space that changes? So in the empty room a balloon would be the known universe...is that accurate?

 

....[/quote']

 

here I am butting in again, this question was directed to Ophi.

 

well any and all are allowed to answer, i guess.

 

I say No, it is not accurate. In General Relativity, which is the basis for standard mainstream cosmology, THERE IS NO ROOM, THERE IS ONLY THE BALLOON.

 

The expanding "balloon" we all realize is only figurative. We are talking about an expanding 3D space with some as yet unknown topology which might be very simple-----like ordinary infinite 3D Euclidean space or like a 3D analog of a sphere. It is probably good to imagine it simple because there are so far no observational hints that it is not.

 

the Einstein eqn of Gen Rel describes the dynamic evolving geometry of this space, without imagining any higher dimension space "Room" in which it is contained.

the geometry is described intrinsically from inside viewpoint.

there is no outside (in the standard theory that working cosmologists use).

 

one can make up higher dimensional "Rooms" but so far it does not seem to help, only makes theory more cluttered and speculative (less provable) without explaining anything new

 

the aim of LQC (loop quantum cosmology) is to quantize this theory (the cosmology based on Gen Rel) as it stands, with the least possible extra equipment and assumptions. Currently LQC is making some progress and beginning to attract notice-----like the piece in NATURE this week about Martin Bojowald.

 

maybe in the end there will need to be some higher dimensional surround, but Gen Rel has been used successfully since 1915 and so far there is no proof that it needs more than just the 4 dimensions we have

 

 

the math technique for describing the geometry of space, and working with it, from within

were developed around 1850 by a protestant minister's son named Riemann.

it involves having a math gadget called a "metric" that tells you distances and angles between things in the space from the inside, and you can infer

curvature and stuff from that. You pick 3 points that make a triangle and you see if the angles add up to 180 degrees and if they dont you say hmmm there must be some curvature here. After a while you get accustomed and it gets easy and you never feel the need to imagine a surrounding Room.

By serendipity it turned out that what Riemann invented circa 1850 was just what Einstein needed in 1915 to make a workable theory of gravity and the shape of spacetime, which made surprisingly accurate predictions (much more accurate than Newton's theory). And the 1915 theory was so good that cosmologists are still using it (.... the big bang and dark energy and the cosmological constant and gravity, and slowing down binary pulsars, and the timing used in the GPS system, and the Microwave Background, and the redshifts Hubble observed and all that good stuff arise all or partly in the Gen Rel context...)

 

It is an impressive theory and has legs

but people have not yet been able to quantize it

so that is a very interesting current problem

which may eventually force Gen Rel to evolve into something new

(that somehow includes the classical theory but lends itself better to

being quantized)

[Martin]

Ophi and Holy! I have put some discussion and a link to the authoritative report on sun's motion relative to CMB here

http://www.scienceforums.net/forums/showthread.php?p=122364#post122364

 

there is also the CMB temperature skymap

and an online calculator for converting galactic coordinates into ordinary

(rightascension/declination) celestial coordinates.

 

what we have to reconcile is the local group motion "The simplest explanation for the dipole is a Local Group velocity v = 627 km/ sec toward (l, b) = (276 ±3, 33 ±3) with respect to the CMB rest frame."

 

with the sun's motion which is 370 km/sec in a slightly different direction

(l, b) = (264 degrees, +48 degrees)

 

these are galactic coordinates given in the 1996 COBE report.

you see that the sun's motion is off by 12 degrees in one coordinate and 15 degrees in the other

 

so the sun is not going in the direction of the Local Group but rather a direction a few degrees off.

 

this would be because of Milky's motion in the local group (which may be negligible since it is the most massive) PLUS the sun's motion relative to Milky center.

I worked this out some time ago with sines and cosines and vector addition and stuff and I found that the estimated orbital motion of the sun was just right to cancel out some of the velocity and to account for the few degrees difference.

 

the Local group direction (276,33) is what Lloyd Knox says in

http://arxiv.org/astro-ph/0210165

and the Sun direction (264, 48) is what Smoot et al says in the COBE report

http://arxiv.org/astro-ph/9601151

http://arxiv.org/PS_cache/astro-ph/pdf/9601/9601151.pdf

 

"The Dipole Observed in the COBE DMR Four-Year Data"