How fast galaxies are moving?

[Darnok]

How fast galaxies are moving? And how we can check it?

[Bill Phil]

I think that any given point it's about 200 km/s as of now...

We can check it by looking at the "shifting" of light, which gives us a distance, and then later checking the distance again. AFAIK.

[theend3r]

How fast are they moving relative to what? Us?

[lajoswinkler]

How fast relative to what?

Ninja'd.

[Bill Phil]

How fast are they moving relative to what? Us?

The given point in space. Did you not see it?

"At any given point..."

[theend3r]

The given point in space. Did you not see it?

"At any given point..."

There isn't any absolute coordinate system so that doesn't really make any sense.

[Bill Phil]

There isn't any absolute coordinate system so that doesn't really make any sense.

You don't need one.

Just, you pop into a random point in space, and the galaxies move relative away from you at about 200km/s.

And btw, if there wasn't a coordinate system, how would we chart where Galaxies are in the sky? You give them points, and then specify distances. You can do the same here.

[ZetaX]

You don't need one.

Just, you pop into a random point in space, and the galaxies move relative away from you at about 200km/s.

And btw, if there wasn't a coordinate system, how would we chart where Galaxies are in the sky? You give them points, and then specify distances. You can do the same here.

There is no _inherent_ coordinate system. What you describe is using the one centered around you. And the answer is not 200km/s by the way; it strongly depends on where you "teleport" (not really well-defined) to: the farther away, the faster the galaxies move relative to you.

[BlueCosmology]

You don't need one.

Just, you pop into a random point in space, and the galaxies move relative away from you at about 200km/s.

And btw, if there wasn't a coordinate system, how would we chart where Galaxies are in the sky? You give them points, and then specify distances. You can do the same here.

None of that makes any sense. If you pick a point where galaxies are moving away from you at 200km/s, then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point.

The speed of galaxies is also very dependent on how far you are from them, and is dominated by peculiar velocities in near galaxies.

There is no absolute coordinate system, you go on to demonstrate there is by making a completely relative coordinate system, picking points and specifying distances between them is clearly entirely relative, not absolute.

[Darnok]

How fast are they moving relative to what? Us?

Exactly, my point is whether we have a reference point. If not we can't calculate galaxies speed, all we can observe is their movement related to our galaxy.

I have idea... what if galaxies are moving nearly at photons speed?

So fast that time is slowed down. That would explain why universe expansion is so fast from our perspective.

[YNM]

You're forgetting one thing: Universe's expansion, and that the expansion is due to the space itself (where the galaxies lies) expands, not that they're flying away from us. Your question might be not about the observed speed/velocity (only), but peculiar velocity. One way to define them is by setting a local standard of rest (the method that is also used to get the true velocity of stars in our own galaxy, noting that Sun also moves) on the scale of galaxies. Then you can start to see for their true peculiar velocity. It's very long mathematically, but the main idea is that if the velocities are truly random, their average should be zero (at rest).

[Bill Phil]

None of that makes any sense. If you pick a point where galaxies are moving away from you at 200km/s, then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point.

The speed of galaxies is also very dependent on how far you are from them, and is dominated by peculiar velocities in near galaxies.

There is no absolute coordinate system, you go on to demonstrate there is by making a completely relative coordinate system, picking points and specifying distances between them is clearly entirely relative, not absolute.

Where did you get " then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point."? You pick another point that's not moving.

It's just an average, anyways.

There is no way to have an absolute coordinate system, so all coordinate systems are relative to something. The origin, but what is it relative to? ALL coordinate systems are entirely relative, never absolute. And I was saying you don't need one, which is very true.

[ZetaX]

Where did you get " then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point."? You pick another point that's not moving.

It's just an average, anyways.

There is no way to have an absolute coordinate system, so all coordinate systems are relative to something. The origin, but what is it relative to? ALL coordinate systems are entirely relative, never absolute. And I was saying you don't need one, which is very true.

I am with BlueCosmology here: it is really hard to make sense from anything you said. That 200km/s was already debunked, the meaning of absoluteness versus relativeness was mentioned. And your last sentence sounds weird; if I understand it correctly, you claim that everything moves at 200km/s. Which makes really no sense. So please clarify as was already implied before...

[ComradeWolfe]

I believe the answer was pretty well established by a small group of researchers in 1983. Here's a link to a video of their findings:

[Bill Phil]

I am with BlueCosmology here: it is really hard to make sense from anything you said. That 200km/s was already debunked, the meaning of absoluteness versus relativeness was mentioned. And your last sentence sounds weird; if I understand it correctly, you claim that everything moves at 200km/s. Which makes really no sense. So please clarify as was already implied before...

If a random point is selected in space, the average speed the galaxies will move away from you is 200 km/s. This an average of a large number of galaxies, with many outliers (close ones, super far ones, the like), so don't take it as an absolute.

Let me break it down:

I asked BlueCosmology where he/she got the statement , then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point."

I never mentioned it, and it seems to be an assumption on BlueComsology's part.

I said that when you pick a random point the speed of galaxies moving away from that point is 200km/s.

Then I said that it's just an average.

Then I said that an absolute coordinate system is impossible, since they're all relative to something. Coordinate systems just make life easier for us, but they can't be absolute.

Then I said that you don't need an absolute coordinate system to pick a random point.

That is all.

I never said that everything is moving at 200km/s.

EDIT:

Hold on, a bit of screw up here. If you're a few megaparsecs away from the nearest galactic cluster, then their average velocity travelling away from you is about 200 km/s.

Edited May 3, 2015 by Bill Phil

[theend3r]

If a random point is selected in space, the average speed the galaxies will move away from you is 200 km/s. This an average of a large number of galaxies, with many outliers (close ones, super far ones, the like), so don't take it as an absolute.

Let me break it down:

I asked BlueCosmology where he/she got the statement , then pick another point that is moving relative to the first point the galaxies will not be moving 200km/s relative to the new point."

I never mentioned it, and it seems to be an assumption on BlueComsology's part.

I said that when you pick a random point the speed of galaxies moving away from that point is 200km/s.

Then I said that it's just an average.

Then I said that an absolute coordinate system is impossible, since they're all relative to something. Coordinate systems just make life easier for us, but they can't be absolute.

Then I said that you don't need an absolute coordinate system to pick a random point.

That is all.

I never said that everything is moving at 200km/s.

So if I understand it right, if I pick an inertial frame of reference that is moving at 0.99 c relative to Earth then the average speed of all galaxies to my i.f.o.r. is 200 km/s? That is very unlikely.

[Bill Phil]

So if I understand it right, if I pick an inertial frame of reference that is moving at 0.99 c relative to Earth then the average speed of all galaxies to my i.f.o.r. is 200 km/s? That is very unlikely.

No, the point is stationary. I literally said that. Or I think I did.

Btw, that's a vector, not a point. Point implies stationary.

[ZetaX]

Ok, I understand now that you talk about averages only, thanks for clarifying that.

No, the point is stationary. I literally said that. Or I think I did.

Btw, that's a vector, not a point. Point implies stationary.

But what is a "point"¿ It cannot be a single point in spacetime because that would also mean being stationary in time, i.e. not being able to measure any speed, because speed is something requiring a time interval.

[theend3r]

Stationary to Earth? To Sun? To the average of vectors of all galaxies? To the Death Star? Stationary means it's relative velocity is 0 (there is no absolute velocity).

http://physics.stackexchange.com/questions/88152/what-does-it-mean-to-be-stationary

[Bill Phil]

Ok, I understand now that you talk about averages only, thanks for clarifying that.

But what is a "point"¿ It cannot be a single point in spacetime because that would also mean being stationary in time, i.e. not being able to measure any speed, because speed is something requiring a time interval.

[ZetaX]

A point in space, not in spacetime. A physical location.

This makes no sense, as I said: to measure speed or movement, you need time.

And those 200 km/s are probably intended to be the _current_ speed average, so you also need a specific moment in time where this statement is made.

[Bill Phil]

This makes no sense, as I said: to measure speed or movement, you need time.

And those 200 km/s are probably intended to be the _current_ speed average, so you also need a specific moment in time where this statement is made.

It's specifying the point in space, the point in time is now.

[BlueCosmology]

Absolutely everything you're saying is self contradictory. You accept that there is no absolute coordinate system but then say to pick a point that's not moving, but not relative to anything. All you're doing is picking a coordinate system where the average speed of galaxies is 200km/s.

[ZetaX]

It's specifying the point in space, the point in time is now.

So it actually is a point in spacetime.

But with only a point you cannot measure speed. What you actually need is a reference frame (i.e. a point moving through spacetime) to define speed.

[Wanderfound]

Of some relevance to this discussion: http://www.muppetlabs.com/~breadbox/txt/al.html