The Universe accelerating and light

Click For Summary

Discussion Overview

The discussion revolves around the effects of the universe's accelerating expansion on our perception of the world and the implications of relative motion in different inertial frames. Participants explore concepts related to the expansion velocity of the universe, the nature of inertial frames, and how these relate to observations of distant galaxies.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how the accelerating expansion of the universe affects our perception, specifically whether it causes things to appear to move slower.
  • Another participant asserts that the world around us is unaffected by the universe's expansion, although observations of distant objects are impacted.
  • A clarification is made regarding the expression of expansion velocity as km/s/megaparsec, indicating that recession velocities vary based on distance.
  • Some participants discuss the concept of inertial frames, with one stating that receding galaxies are not in the same inertial frame as observers, thus time dilation does not apply.
  • There is a debate about the definition of "the same inertial frame," with differing interpretations of what it means in various contexts.
  • One participant expresses uncertainty about whether a receding galaxy is considered to be in an inertial frame, raising questions about how general relativity handles such scenarios.
  • Another participant suggests that the terminology used may lead to confusion, indicating that both perspectives on inertial frames could be valid.
  • A technical explanation is provided regarding the concept of reference frames, emphasizing the distinction between technical and non-technical uses of the term.

Areas of Agreement / Disagreement

Participants express differing views on the concept of inertial frames and their relevance in the context of general relativity. There is no consensus on the implications of the universe's expansion for perception or the definition of inertial frames.

Contextual Notes

Participants highlight the complexity of defining inertial frames, especially in the context of general relativity, where space-time is curved. The discussion reflects varying interpretations of how relative motion affects observations.

Jaami M.
Messages
28
Reaction score
1
How are we, and the universe effected from its accelerating expansion? Also how will our perception of the world around us be effected, since the universe is expanding and light is constant? <-Will things appear to move slower? I know that we only experience just a small fraction since the universe expanding velocity is 68 km/sec or 42.253 mi/sec. But is there any type of effect?
 
Space news on Phys.org
We are not... the world around us in unaffected, though what we see of large distances is affected.
"Universe expanding velocity" is a meaingless term.
 
Jaami M. said:
I know that we only experience just a small fraction since the universe expanding velocity is 68 km/sec or 42.253 mi/sec.

The expansion velocity is actually expressed as km/s/megaparsec. In other words, it's 68 kilometers per second per megaparsec. This means that objects 2 megaparsecs apart recede from each other at 136 km/s, double the velocity at one megaparsec. So the recession velocity is actually different for all objects based on their distance apart.
 
Jaami M. said:
... Will things appear to move slower?
No, things seem to move slower when they are moving relative to you in the same inertial frame of reference. Receding galaxies are not IN the same inertial frame of reference so time dilation is not applicable.
 
phinds said:
No, things seem to move slower when they are moving relative to you in the same inertial frame of reference. Receding galaxies are not IN the same inertial frame of reference so time dilation is not applicable.

What? What does "the same inertial frame" mean?
 
Drakkith said:
What? What does "the same inertial frame" mean?
I thought that was a well defined concept, no? If we are both on the same train and it is not accelerating then we are in the same inertial frame. If I'm on the platform and you're on the train and it's not accelerating (relative to me) then we are both in the same inertial frame. If I'm on the train OR the platform, a receding galaxy is not in an inertial frame relative to me.
 
I've never seen it used like that before. To me, the "same inertial frame" would be the two of us on the train or the two of us on the platform, not moving relative to each other. If you're on the train and I'm on the platform we will see things differently, so I wouldn't call it the same inertial frame. We're both in an inertial frame, but not the same one.

I'm also unsure about whether or not a receding galaxy is considered to be in an inertial frame. It's certainly not accelerating in its own frame, but I don't know how things like this are handled in GR.
 
I think maybe it's the way we use the words. If I'm on the platform and you're on the train (moving but not accelerating) you are moving in my inertial frame so I say we're in the same inertial frame, but I see what you mean by the way you use the term.
 
The same inertial frame is not a very meaningful concept in GR because space-time is generally curved. The only way to make proper sense of it is if you have two observers at the same event. Those inertial frames are local and not global.
 
  • #10
Orodruin said:
The same inertial frame is not a very meaningful concept in GR because space-time is generally curved. The only way to make proper sense of it is if you have two observers at the same event. Those inertial frames are local and not global.
Exactly. And if one of them is moving (but not accelerating) would you find it correct to say that they are "in the same inertial frame" as I do or do you hold with Drakkith's point of view that they are each in inertial frames but not the same one? Seems to me both points of view are right, actually.
 
  • #11
The "frame" of an object is usually (represented by or pictured as) the coordinate system attached to the object.
Two objects share the same reference frame if they are stationary with respect to each other.

However - it is common in physics to mix up technical and non-technical uses.
If two people are in the bath, moving about, then we say they share a bath ... we can put the grid on the bath and , similarly, in a non-technical sense, talk about the two people sharing the bath coordinate system ... but, in the technical use: that is the frame of the bath, not the people. If one were stationary in the bath, then that one would share the frame of the bath as well as being in the frame of the bath.

When a physicist writes about something moving in a frame, that's usually what is meant.
There are a lot of references about reference frames ... having trouble finding any that spell out when two objects share a frame.
 
  • #12
Thanks for that, Simon.
 

Similar threads

Undergrad How do you get to the conclusion that there is accelerating expansion?
  • · Replies 23 ·
Replies
23
Views
4K
High School Can the "creation" of space in the Universe affect the speed of light?
  • · Replies 7 ·
Replies
7
Views
4K
Undergrad What if the Universe expands faster than the speed of light?
  • · Replies 19 ·
Replies
19
Views
5K
Undergrad How would the laws of thermodynamics change in a contracting universe?
  • · Replies 5 ·
Replies
5
Views
3K
High School Conflict between the conservation of matter and the rapid expansion of our universe
  • · Replies 18 ·
Replies
18
Views
2K
Undergrad 3 questions about a paper on density fluctuations in the Universe
  • · Replies 1 ·
Replies
1
Views
3K
Undergrad How much does the Universe accelerate?
  • · Replies 18 ·
Replies
18
Views
3K
High School Clarification on how old the Universe is
  • · Replies 11 ·
Replies
11
Views
3K
High School Size of universe - rewinding big bang versus "flat" universe
  • · Replies 54 ·
2
Replies
54
Views
6K
Undergrad Density fluctuations in a homogeneous linearly expanding universe?
  • · Replies 7 ·
Replies
7
Views
3K