Velocities in space relative to other objects

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Discussion Overview

The discussion revolves around the concept of velocities in space relative to other objects, exploring the idea of a reference frame that could be considered as "Dead Space" or a state of zero velocity. Participants examine various frames of reference, including the Cosmic Microwave Background (CMB), and the implications of different velocity vectors in cosmological contexts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose the existence of a "Dead Space" or a quantum material that travels at zero velocity relative to everything else, questioning how we can define motion in such a context.
  • Others mention the CMB as a reference frame, noting that the solar system's speed relative to the CMB is approximately 370 km/s towards the constellation Leo, which can be measured through the Doppler effect.
  • One participant clarifies that there is no single frame at rest with respect to all other frames, emphasizing that velocities are always relative to a chosen frame.
  • A hypothetical scenario is presented where the velocity vectors of a man on Earth's equator are summed with various cosmic motions, questioning if he could be considered at rest with respect to all those frames, while another participant counters that he would still only be at rest relative to his own frame or another on Earth.

Areas of Agreement / Disagreement

Participants express differing views on the concept of absolute rest and the nature of reference frames. There is no consensus on the existence of a universal zero velocity reference, and the discussion remains unresolved regarding the implications of these frames in understanding motion.

Contextual Notes

The discussion highlights the complexity of defining motion and rest in a universe with multiple reference frames, and the limitations of using the CMB as a universal standard. The assumptions underlying the proposed scenarios and the dependence on specific definitions of velocity are acknowledged but not resolved.

schonovic
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Forgive me if I sound stupid here but is there anything that we could call Dead Space? An essence or quantum material that is traveling at zero velocity to everything? Something that we could say we are all traveling at x velocity relative to. I know we can all say we are traveling at such and such velocity relative to light but Guth said that the speed of light is relative to the compact density of the universe after the Big Bang. So if space-time has different densities in different areas of the universe; how can we say we are traveling at such and such velocity relative to anything at all? There's got to be some concept here that I'm not getting.
 
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schonovic said:
Forgive me if I sound stupid here but is there anything that we could call Dead Space? An essence or quantum material that is traveling at zero velocity to everything? Something that we could say we are all traveling at x velocity relative to...

Sure, in cosmology there is the idea of being at rest relative to the soup of ancient light (the Cosmic Microwave Background or CMB) that we are immersed in. And we can say that the solar system is moving at a definite speed in a definite direction relative to CMB.

this was measured rather accurately back in 1990s, the keyword would be "CMB dipole" I think.
the solar system speed turned out to be 370 km/s and the direction is marked in the sky by the constellation Leo.

the way they tell is like this: the CMB light is amazingly uniform, same thermal mix of wavelengths, same "temperature" in all directions to within one part in 100,000 that is to within one thousandth of one percent.

except for a doppler hotspot around the constellation Leo! Because we are running towards the light it is detected as warmer in that direction! By how much?

Well, what percent of the speed of light is 370 km/s? The temperature is raised by exactly that percentage because of the doppler effect of moving towards the light.

speed of light is 300,000 km/s so the percentage is 370/3000= 0.12 percent
about a tenth of a percent warmer in the Leo direction

our motion increases temp by 1/8 of a percent. Once our own motion is subtracted from the data the variation is only 1/1000 of a percent. That is the way it would look to an observer at CMB rest.

So there is that idea of absolute rest and absolute motion, relative to the universe. It gets used a lot in cosmology.
 
Last edited:
Perhaps Marcus misunderstood your question. (Or I might be) There is no single frame that is at rest with respect to every other frame. Not even the CMB frame. That is just used as a convenient frame to measure everything against. We can ONLY say that we are moving at X velocity with respect to Y frame. Since Y frame is any frame of your choosing, your velocity X will be different depending on what you measure against.

Also, we do not measure our velocity with respect to light, as it will always be measured at the same speed, c.
 
What if you took the reference frame of a man sitting on the Earth's equator--

you find his velocity vector in 3-space resulting from the Earth's rotation

add to that the velocity vector of the Earth in its orbit

add to that the velocity vector of the sun in its orbit in the Milky Way

add to that the velocity vector of the MW in its orbit with M31

add to that the velocity vector of the Local Group towards the Great Attractor

Take the sum of all these vectors and from his chair, launch him at an equal and opposite vector in 3-space (thru the earth?:) ). Could he now be considered to be at rest with regard to all above, or what am I doing wrong?
 
chasrob said:
What if you took the reference frame of a man sitting on the Earth's equator--

you find his velocity vector in 3-space resulting from the Earth's rotation

add to that the velocity vector of the Earth in its orbit

add to that the velocity vector of the sun in its orbit in the Milky Way

add to that the velocity vector of the MW in its orbit with M31

add to that the velocity vector of the Local Group towards the Great Attractor

Take the sum of all these vectors and from his chair, launch him at an equal and opposite vector in 3-space (thru the earth?:) ). Could he now be considered to be at rest with regard to all above, or what am I doing wrong?

The man is only at rest relative to his own frame of reference or another one on Earth. After doing all of the above you would find the velocity vector of the man with respect to the Great Attractor.
 

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