Doubt about the relativity of simultaneity

Click For Summary

Discussion Overview

The discussion revolves around the concept of the relativity of simultaneity, particularly in the context of measuring the cosmic microwave background (CMB) temperature from different reference frames. Participants explore how observers in relative motion perceive simultaneity and the implications for understanding the universe's expansion and isotropy.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants argue that the mean temperature of the CMB is the same for observers measuring it simultaneously, regardless of their relative motion.
  • Others suggest that observer B's conclusion about simultaneity being absolute is based on a misunderstanding of frame-dependent measurements.
  • It is proposed that if observer B uses a coordinate system where the telescopes are not activated simultaneously, he should not expect isotropy in the CMB measurements.
  • Some participants contend that while both observers can measure the same temperature, the interpretation of simultaneity varies depending on their reference frames.
  • There is a suggestion that the universe's expansion and isotropy may not be perceived the same way by different observers, leading to philosophical questions about the nature of simultaneity.
  • One participant raises the idea that special relativity does not differentiate between simultaneous and non-simultaneous events if they are not causally connected.

Areas of Agreement / Disagreement

Participants express differing views on the implications of simultaneity in the context of the CMB measurements. There is no consensus on whether simultaneity is absolute or relative, and the discussion remains unresolved regarding the interpretation of measurements from different reference frames.

Contextual Notes

Limitations include the dependence on the choice of reference frames and the assumptions about isotropy and expansion of the universe. The discussion highlights the complexity of interpreting measurements in different frames without reaching definitive conclusions.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in relativity, cosmology, and the philosophical implications of simultaneity in physics.

  • #31
Ideassimples said:
Since from the reference frame of observer B, radio telescope 1 is turned on first and then radio telescope 2, observer B expects that the measurement of the mean temperature of the cosmic microwave background of radio telescope 1 is higher than that of radio telescope 2. Without However this does not happen, both measures are identical.

So according to B the other beam travels for a very very very very long time, and then the beam is in a universe that is not very very old.

Is this perhaps a correct rephrasing of the problem?(Let's say B travels extremely fast relative to the telescopes, and also let's say the telescopes are not so far apart that expansion of universe matters.)
 
Last edited:
Physics news on Phys.org
  • #32
jartsa said:
the other beam

What "other beam" are you talking about?

The OP's scenario doesn't include any "beams". The radio telescopes are measuring the CMB at their location; they're not sending or receiving "beams".
 
  • #33
PeterDonis said:
What "other beam" are you talking about?

The OP's scenario doesn't include any "beams". The radio telescopes are measuring the CMB at their location; they're not sending or receiving "beams".
OP said: "Right in the center is a light source, with no relative motion relative to radio telescopes. The light source sends a beam of light to each radio telescope, hits the switch, turns them on."
 
  • #34
jartsa said:
OP said: "Right in the center is a light source, with no relative motion relative to radio telescopes. The light source sends a beam of light to each radio telescope, hits the switch, turns them on."

Ah, ok. But these "beams" have nothing to do with the actual measurements the radio telescopes are making. They're just a way of specifying in what frame the measurements made by the radio telescopes are simultaneous (the A frame, in which both telescopes are at rest).

Everyone appears to agree that in the B frame, the measurements are not simultaneous; explaining how that comes about is not the issue. The issue is how an observer at rest in the B frame explains the fact that both measurements still give the same result for the CMB temperature, which means that the "age of the universe" under the usual definition of that term (which is not the same as "coordinate time elapsed in the B frame") is the same at both measurements, even though they are not simultaneous in the B frame.
 

Similar threads

Undergrad Is Simultaneity Absolute or Relative in the Theory of Relativity?
  • · Replies 38 ·
2
Replies
38
Views
4K
Undergrad Why doesn't relativity of simultaneity lead to a contradiction?
  • · Replies 17 ·
Replies
17
Views
2K
Undergrad Understanding Relativity of Simultaneity in Resnick, Part I
  • · Replies 16 ·
Replies
16
Views
2K
Undergrad Understanding Relativity of Simultaneity in Resnick, Part II
  • · Replies 20 ·
Replies
20
Views
2K
Undergrad Relativity of simultaneity in actuality
  • · Replies 12 ·
Replies
12
Views
1K
High School Events vs Effect of Events in Relativity of Simultaneity
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad Relativity of simultaneity doubt
  • · Replies 221 ·
8
Replies
221
Views
15K
Undergrad Condition for Centering Starlight in a Water-Filled Telescope?
  • · Replies 6 ·
Replies
6
Views
2K
High School A Question about Cosmic Background Noise
  • · Replies 13 ·
Replies
13
Views
2K
High School Relativistic simultaneity and effects on time
  • · Replies 9 ·
Replies
9
Views
373