Measuring Speed of Light: Reference Guide

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Discussion Overview

The discussion revolves around the measurement of the speed of light and the implications of relative motion on time perception, particularly in the context of special relativity. Participants explore the concept of time dilation, reference frames, and the relativity of simultaneity, while questioning how these ideas apply to various scenarios involving moving observers and clocks.

Discussion Character

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

Main Points Raised

  • Some participants emphasize the importance of specifying the reference frame when discussing the speed of a moving object relative to the speed of light.
  • There is a discussion about the symmetrical nature of time dilation, where each observer perceives the other's clock as running slow.
  • A participant raises a question about how to determine the reference frame when discussing the age of the universe, suggesting a co-moving observer as a possible reference.
  • Concerns are raised about the wording used when discussing time and motion, with a focus on the necessity of clarifying relative motion.
  • Participants discuss the perception of clock rates, noting that a moving observer will not notice any change in their own clock's ticking rate, while an observer at rest relative to them will see their clock ticking slower.
  • One participant points out the complexity of synchronizing clocks in different reference frames, highlighting that what is synchronized for one observer may not be for another.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of specifying reference frames in discussions of motion and time. However, there are multiple competing views on how to interpret time dilation and the implications of relative motion, leaving the discussion unresolved in some aspects.

Contextual Notes

Limitations include the potential for misunderstanding the implications of relative motion on time perception and the complexities involved in synchronizing clocks across different frames of reference.

Passionate Eng
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I don't know what's the appropriate title for my question but here it is:
What's the reference we consider to measure the speed with respect to when we say that as the speed of a moving object approaches the speed of light the time becomes slower in the frame of this moving object.
 
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Passionate Eng said:
I don't know what's the appropriate title for my question but here it is:
What's the reference we consider to measure the speed with respect to when we say that as the speed of a moving object approaches the speed of light the time becomes slower in the frame of this moving object.
When someone says "as the speed of a moving object approaches the speed of light" they're being a bit careless with the wording; to be precise they have to say "as the speed of a moving object relative to <something else> approaches the speed of light". Thus, the reference we're considering is whatever that something is.

This has several implications. First, you may be moving very slowly or even at rest relative to one thing (perhaps the chair that you're sitting in) but very rapidly relative to something else (perhaps an astronomer on Mars watching you through a telescope). Second, the time dilation that you mention ("the time slows down") is symmetrical. If you are moving at a speed close to ##c## relative to me, I will find that your clocks are running slow compared with mine, but you will find that it is my clocks that are running slow relative to yours - and we're both right. This apparent paradox (how can both clocks be slower than the other?) is explained by the relativity of simultaneity - if you aren't already familiar that, it should be the next thing you learn about.
 
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Maybe the secondary question is 'how do you know what that reference is?' The answer to that is: it can be anything; you get to choose. Just try to make the choice useful.
 
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Nugatory said:
When someone says "as the speed of a moving object approaches the speed of light" they're being a bit careless with the wording; to be precise they have to say "as the speed of a moving object relative to <something else> approaches the speed of light". Thus, the reference we're considering is whatever that something is.

This has several implications. First, you may be moving very slowly or even at rest relative to one thing (perhaps the chair that you're sitting in) but very rapidly relative to something else (perhaps an astronomer on Mars watching you through a telescope). Second, the time dilation that you mention ("the time slows down") is symmetrical. If you are moving at a speed close to ##c## relative to me, I will find that your clocks are running slow compared with mine, but you will find that it is my clocks that are running slow relative to yours - and we're both right. This apparent paradox (how can both clocks be slower than the other?) is explained by the relativity of simultaneity - if you aren't already familiar that, it should be the next thing you learn about.
Thanks! I will read about the relativity of simultaneity as soon as I can.
I have another question just crossed my mind.
If I am moving fast and I have a mechanical or digital clock, will they TIK TOK more rapidly? If so, Why?
 
Passionate Eng said:
I have another question just crossed my mind.
If I am moving fast and I have a mechanical or digital clock, will they TIK TOK more rapidly? If so, Why?
That wording is problematic. *You* will notice no change in the rate of passage of time because time is essentially carrying you with it, so locally nothing changes when you change your speed. But another observer who is stationary with respect to the frame you measured your speed in will notice your clock appearing to tick slower than theirs.
 
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(Sorry, my questions may look irrelevant but complete my thoughts)
So, every moving object has its own time frame which appears to move slower than every other time frame. i.e. there is no absolute point in time.
So, which frame do we refer to when we say that the universe is 14 billion years old?
 
Passionate Eng said:
So, which frame do we refer to when we say that the universe is 14 billion years old?
Passionate Eng said:
If I am moving fast...
You did it again - you said "moving" without saying what it is relative to. You absolutely have to break yourself of that habit before relativity will start to make sense for you.
 
Passionate Eng said:
So, which frame do we refer to when we say that the universe is 14 billion years old?
A co-moving observer. Google it.
 
Passionate Eng said:
So, which frame do we refer to when we say that the universe is 14 billion years old?
When in doubt, the answer is often "ours". With respect to the Hubble Flow or cosmic microwave background might be more accurate but since our speed with respect to that is on the order of a thousandth of the speed of light there isn't much difference.
 
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Passionate Eng said:
If I am moving fast and I have a mechanical or digital clock, will they TIK TOK more rapidly? If so, Why?

The relevant question: What is the speed of that clock relative to you?

If the clock is not moving relative to you then the clock ticks at the normal rate.

All clocks in motion relative to you will run slower than that clock, but that claim is easy to misunderstand because it doesn't imply that for someone at rest relative to one of those clocks your clock will tick too fast!

To understand this, realize that you need two clock readings to make the necessary comparison of elapsed times. But since the two clocks are in relative motion, they can't both be at the same location for both of those clock readings. You will therefore necessarily need to be able to determine what time it is on a clock that is not at your location. This is no trivial matter, but rather lies at the heart of the issue. To determine what time it is at some distant location you have to synchronize clocks that are separated, and as it turns out what's synchronized for you is not synchronized for someone moving relative to you.
 
  • #11
Passionate Eng said:
If I am moving fast and I have a mechanical or digital clock, will they TIK TOK more rapidly? If so, Why?
To you, the vlock is not moving so ticks normally.

To an observer who sees you moving at 0.87c it ticks at half the rate you see. But your heart rate and all your biological processes including your brain functions run at half speed too. So the observer is unsurprised that you notice nothing out of the ordinary.
 

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