# How fast is standing still?

1. Aug 30, 2004

### yogiwp

Just curious, what is the speed of an object standing still on earth? (taking into account earth's rotation and revolution, the rotation of our galaxy, the fact that we are part of an expanding universe, etc.)

2. Aug 30, 2004

### Gonzolo

With each level, the former becomes negligeable. All you need is the radius and the time to complete a rotation or revolution (I don't have the numbers with me now).

Beyond the galaxy, you can only measure speed relative to another galaxy, or perhaps another cluster, there isn't a common reference to all galaxies.

3. Aug 30, 2004

### Tom McCurdy

It all depends on your frame of reference, it you negelt any accelerations just say we are standing still and everyone is moving around us.

4. Aug 30, 2004

### arildno

Fast in respect to what?
As Tom McCurdy implied, your question is meaningless without specifying a reference frame.
If you're asking "what is the velocity of an object at rest relative to the earth within the C.M. reference frame of the universe?", you have a somewhat better posed question.

5. Aug 30, 2004

### Garth

The discovery of the Cosmic Microwave Background (CMB) has indeed made the latter posed question a sensible one.

When the Earth's velocity around the Sun has been taken into account the Solar System is travelling at 390 +- 60 km/sec relative to the surface of last emission of the CMB. However when the Sun's motion around the Galaxy is also taken into account this translates into the fact that the Galaxy is travelling relative to the surface of last emission of the CMB, which probably defines the C.M. reference frame of the universe, at 603 km/sec or about 0.2%c! (Nature, Vol 270, 3 Nov 1977, pg 9)

6. Aug 30, 2004

### yogiwp

Let's say from the frame of reference of an absolute stationary point in the universe. (Does such point exist? The center of the universe maybe?)

Edit: Oops too late. yes, what arildno said.

Last edited: Aug 30, 2004
7. Aug 30, 2004

### arildno

Cool!
I thought the question at present was unanswerable..

8. Aug 30, 2004

### jcsd

They've known it for quite awhile, those 'maps' of the CMBR's anisotropy rely on knowing it, because movemnt relative to the CMBR's frame causes anistropy of it's own (which is removed from the nice pictures such as the ones that WMAP produced) which I guess in turn is just a measuremnt of the relative speed between the various probes and the CMBR.

9. Aug 30, 2004

### arildno

THEY've probably known it a long time, I haven't..

10. Aug 30, 2004

### yogiwp

Garth: thanks, that's exactly the answer I was looking.

So, according to SR, our earth time is somewhat slower than time at "some absolute stationary point in the universe" (say, CMB or something else). Is this correct?

11. Aug 30, 2004

there is no absolute centre of the universe.

12. Aug 30, 2004

### Gonzolo

If one believes CMB can define a reference frame, then the answer is yes. Personally though, I am not entirely convinced of this, although it sounds nice at first.

13. Aug 30, 2004

### jcsd

The CMB can define a refernce frame pretty much, due to the extermely homogenous nature of the universe when it was emitted, but it's just one refrence frame among an infinite number of reference frames. The key pont is that the laws of physics do not prefer any refrence frame; if stated corrcetly they are the same in all frames and not dependt on one particular frame. Howvere this doesn't stop nature from conspiring to create a refernce frame that is very convient, especially from the point of view of doing calculations in cosmology (i.e. the one defined by the CMBR).

14. Aug 30, 2004

### Garth

Perhaps if nature does give us a cosmic reference frame, the one in which the CMB is globally isotropic then (quote jcsd) "The key point is that the laws of physics do not prefer any refrence frame" may be incorrect. Certainly the present understanding of those laws i.e. SR and GR do not prefer any reference frame, but the question is, "Does nature 'agree' with them?"

15. Aug 30, 2004

### Gonzolo

How is the CMB reference frame different than the one defined by placing ourselves (or our galaxy) at (0,0,0) of a spherical coordinate system?

16. Aug 31, 2004

### Garth

Because we are moving relative to it?

17. Aug 31, 2004

### Vern

The CMB could possibly be in the same reference frame throughout the universe.

Vern

18. Aug 31, 2004

### jcsd

It isn't different, that's what I've been saying, but it's convienent, becuase the way the universe 'looks' (i.e. the FRW metric) in this frame of refrence makes it easier to do calculations on a cosmological scale.As a comaprison in special relativity quite often it's handy to do calculations in the centre of moamntum frame of a syetm where the total four momentum is (ETOT,0,0,0), but this is only 'prefered' by the person doing the calulations, not nature itself.

19. Aug 31, 2004

### Gonzolo

I see. The CMB is merely the "inside surface of the sphere" of which we are the center. But people in Andromeda would also see the same uniformity and define there own (0,0,0). So to answer yogiwp's 3rd post:

"So, according to SR, our earth time is somewhat slower than time at "some absolute stationary point in the universe" (say, CMB or something else). Is this correct?"

it is better to say that our time "is slower" relative to the center of the Earth, the Sun, and the center of our Galaxy (neglecting the high GR decribed G-field there), but not slower relative to the CMB frame we detect.