What is the relative speed of an object standing still on Earth?

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Discussion Overview

The discussion revolves around the relative speed of an object that is considered to be standing still on Earth, taking into account various cosmic motions such as Earth's rotation, revolution around the Sun, and the movement of the Solar System within the galaxy, as well as the expansion of the universe. Participants explore different frames of reference and the implications of the Cosmic Microwave Background (CMB) in defining these speeds.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the speed of an object at rest on Earth is dependent on the chosen frame of reference, questioning the meaning of "standing still."
  • Others argue that the Earth's motion relative to the CMB provides a more sensible context for discussing speed, with specific velocities mentioned (e.g., 390 km/sec relative to the Sun and 603 km/sec relative to the CMB).
  • There is a discussion about whether an absolute stationary point in the universe exists, with some participants expressing skepticism about the concept.
  • Some participants assert that the CMB can define a reference frame due to the homogeneity of the universe at the time of its emission, while others caution that it is just one of many possible reference frames.
  • The laws of physics are discussed in relation to reference frames, with some participants noting that they do not prefer any specific frame, while others question if nature aligns with this understanding.
  • There is a comparison made between the CMB reference frame and a hypothetical frame centered at the observer's location, with discussions on the convenience of using the CMB for cosmological calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of absolute reference frames or the implications of the CMB as a reference frame. Multiple competing views remain regarding the interpretation of speed and reference frames in the context of relativity.

Contextual Notes

Participants express uncertainty regarding the existence of an absolute center of the universe and the implications of using the CMB as a reference frame. There are also unresolved discussions about the relationship between different reference frames and the laws of physics.

yogiwp
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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.)
 
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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.
 
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.
 
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.
 
arildno said:
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.

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 traveling 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 traveling 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)
 
Tom McCurdy said:
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.
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:
Garth said:
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 traveling 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 traveling 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)
Cool!
I thought the question at present was unanswerable..:smile:
 
arildno said:
Cool!
I thought the question at present was unanswerable..:smile:

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.
 
THEY've probably known it a long time, I haven't..:wink:
 
  • #10
Garth said:
However when the Sun's motion around the Galaxy is also taken into account this translates into the fact that the Galaxy is traveling 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)

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
yogiwp said:
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?

there is no absolute centre of the universe.
 
  • #12
yogiwp said:
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?

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
Gonzolo said:
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.

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
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
jcsd said:
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).

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
Gonzolo said:
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?
Because we are moving relative to it?
 
  • #17
Gonzolo said:
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?
The CMB could possibly be in the same reference frame throughout the universe.

Vern
 
  • #18
Gonzolo said:
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?

It isn't different, that's what I've been saying, but it's convienent, because 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
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.
 

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