# Thread: How fast are we going?

1. ## How fast are we going?

Hey, I'm sat in my study, I'm at rest relative to the earth. But the earth is spinning and I'm on the surface, so I must be going at some speed (unless I'm at a North or South Pole).
Also the earth is going around the Sun, One circuit every year and so I must be going some speed relative to the Solar System. And the Solar System is on the spiral arm arm of the Milky Way Galaxy which rotates around it's own center.

And I suppose the Galaxy is not stationary in the Universe as we know that the other galaxies are moving away from us (in increasing orders of magnitude)

So, is there a stopped place? Can we measure the Cosmic Background radiation and say we are moving away from a point in space where things are truly stationary? If so where is that place?

2. All we can ever talk about are relative speeds, there is simply no meaningful concept of absolute speed that science has ever developed, so far. This is the central tenet of relativity, an extremely successful theory.

If light-speed is the fastest anything can move, measure yourself against it.

4. Originally Posted by max8166
So, is there a stopped place? Can we measure the Cosmic Background radiation and say we are moving away from a point in space where things are truly stationary? If so where is that place?
I think the "cosmological principle" states that the cosmic background radiation should be the same from wherever you measure it. So things should be truly stationary from everywhere. So that place is everywhere. Unless there is an ether, that principle should hold. Now, the spinning issue presents some problems there, as per Mach's Principle.

5. You could reference 'The Galaxy Song' from Monty Python's "The Meaning of Liff" for some quick answers. But basically,

You are moving on the Earth at roughly 40,000 km/day times the cosine of your latitude, so roughly 0.3 km/sec

You are moving around the Sun at about 30 km/sec

The Sun and you and me, and all the stars that you can see are moving at a million miles a day (18 km/sec) around the center of the Milky Way.

How do all these vectors add up? It depends on the time of day and year, but as you can see the speed of the Earth around the Sun is the fastest of the three.

It is possible to compute our velocity relative to the CMBR, but I don't know this speed off the top of my head.

6. Order of Kilopi
Join Date
Apr 2007
Posts
4,430
Originally Posted by EvilEye

If light-speed is the fastest anything can move, measure yourself against it.
This doesn't work. Light will always be measured as c.

An example of Relativity: A large spaceship takes off from Earth and obtains a velocity of 0.9c relative to Earth. A second smaller spaceship (that was stored on board) then takes off from the first spaceship and obtains a velocity of 0.9c relative to the first spaceship and in the same direction. Then an even smaller third spacship that was stored on board the second spaceship takes off and obtains a velocity of 0.9c relative to the second spaceship and in the same direction. The third spaceship would still measure a passing light beam as c. Also, the third spaceship will still be measured as traveling less than c relative to the Earth. Actually, the measured speed of the third spaceship, realtive to the Earth, is easily calculated using the Lorentz Transformations equations.

7. You could probably measure your velocity against my brother, who never moves off of the couch.

8. Originally Posted by max8166
Can we measure the Cosmic Background radiation and say we are moving away from a point in space where things are truly stationary?
We are moving with respect to the Cosmic Background Radiation;
at about 600 kilometers per second.
From here
http://antwrp.gsfc.nasa.gov/apod/ap030209.html
The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?
Off the top of my head I seem to recall we are travelling towards the constellation Centaurus- but that might be my poor memory.

9. Originally Posted by eburacum45
We are moving with respect to the Cosmic Background Radiation;
at about 600 kilometers per second.
I always thought that the universe was supposed to look the same from all places, or that all places should appear to be the "center" of the universe. Doesn't that contradict this if we appear to be moving vis-a-vis the CBR? Though it's only a small part of the speed of light, so maybe it's not that significant?

10. ## Aside from small local variation...

Originally Posted by Jens
I always thought that the universe was supposed to look the same from all places...
It does...relatively speaking

11. Originally Posted by antoniseb
Y

The Sun and you and me, and all the stars that you can see are moving at a million miles a day (18 km/sec) around the center of the Milky Way.
Wiki:http://en.wikipedia.org/wiki/Sun_%28astronomy%29, has that as 217 km/sec and I used 250million years for a "Galactic year" and came out with 196 km/sec.

12. Originally Posted by mfumbesi
Wiki:http://en.wikipedia.org/wiki/Sun_%28astronomy%29, has that as 217 km/sec and I used 250million years for a "Galactic year" and came out with 196 km/sec.
The lyric would still scan if we said 'twelve million miles a day'. Maybe I misheard it. In any case *this* motion (not Earth orbital motion) dominates the velocity relative to the center of the galaxy. Thanks for the correction.

Sadly, the next line of the song is less fixable. 'forty-thousand miles an hour' has to get changed to 'half-a million miles an hour'.

13. Order of Kilopi
Join Date
Apr 2007
Posts
4,430
A little OT but not too much . . .

During news briefings about the status of interplanetary probes, it irritates me when they say the probe is going x mph, without stating the relativity point. x mph relative to the planet it's passing? x mph relative to Earth? x mph relative to the Sun?

14. Originally Posted by Tucson_Tim
A little OT but not too much . . .

During news briefings about the status of interplanetary probes, it irritates me when they say the probe is going x mph, without stating the relativity point. x mph relative to the planet it's passing? x mph relative to Earth? x mph relative to the Sun?
Find the fastest speed it will be travelling relative to any object and that will be the speed quoted. Journalists love picking the biggest number and pretending it is the most representative.

15. Originally Posted by Jens
I always thought that the universe was supposed to look the same from all places, or that all places should appear to be the "center" of the universe. Doesn't that contradict this if we appear to be moving vis-a-vis the CBR? Though it's only a small part of the speed of light, so maybe it's not that significant?
The universe does look the same from all places, but not at all velocities. Our movement relative to the Cosmic background seems to be a real movement, perhaps caused by random inequalities in the distribution of mass in the universe in our region of space. In particular, we may be moving to a very large concentration of mass called the Great Attractor
http://en.wikipedia.org/wiki/Great_Attractor

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•