Can you actually measure an object's velocity?

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Discussion Overview

The discussion revolves around the measurement of an object's velocity and the nature of the speed of light, particularly whether it can be considered absolute. Participants explore concepts related to relative motion, the constancy of the speed of light in different mediums, and the implications of these ideas in the context of physics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that absolute velocity cannot be measured due to the relative nature of motion in the universe.
  • Others propose that the speed of light is absolute because it remains constant in a vacuum, independent of the motion of the source or observer.
  • A participant suggests a hypothetical scenario where a tunnel could lower the speed of light, leading to different observed constants.
  • Warren asserts that all observers agree on the speed of light, countering claims that it can be lowered.
  • Another participant challenges Warren's assertion, demanding evidence for the claim that light's speed can vary.
  • There is a discussion about the propagation of photons in materials, with some stating that while light slows down in non-vacuum conditions, it still travels at c between absorptions.
  • Concerns are raised about the implications of over 50% of the universe not being a vacuum, questioning the constancy of the speed of light.
  • A participant expresses skepticism about the reliability of certain articles discussing variable speed of light, emphasizing the importance of established scientific contributions.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of velocity measurement and the constancy of the speed of light. There is no consensus on whether the speed of light can be considered absolute or if it can vary under certain conditions.

Contextual Notes

Limitations include the dependence on definitions of velocity and the conditions under which light is measured. The discussion reflects unresolved mathematical and conceptual challenges related to the nature of light and motion.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in physics, particularly in the realms of relativity, the nature of light, and the philosophical implications of measurement in a relative universe.

Brainv2.1beta
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Since everything in the universe is moving relative to the other, how can one measure an absolute velocity? Therefore, how can the speed of light be absolute?
 
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Brainv2.1beta said:
Since everything in the universe is moving relative to the other, how can one measure an absolute velocity?

We can't and we don't

Therefore, how can the speed of light be absolute?

Light has the exceptional property that its speed (in vacuum) is independent of the speed of both source and receptor, this is why we always measure it to be c.
 
The universe has only one velocity that all observers will agree upon, and it happens to be c. If you ask "why is it that way?", we cannot give you an answer.

- Warren
 
What if there is a tunnel that lowers the speed of light, observers would observe 2 different "constants".
 
The speed of light cannot be lowered. All observers always agree on the speed of light.

- Warren
 
Ah, but it CAN.
 
No, it can't. Please provide evidence, or retract the claim.

- Warren
 
  • #10
When people refer to "the speed of light," they mean the speed of light in vacuum, which never changes. The decrease in propagation velocity in materials is due to frequent absorption and re-emission. In between those absorptions, the photons still travel at c.

- Warren
 
  • #11
1effect said:
You didn'r read, did you?

"Photons move at a speed less than c, unless they are traveling in vacuum. "
Did you notice the "vacuum" in my post?

Admittedly, I did not notice. However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.
 
Last edited:
  • #12
Brainv2.1beta said:
Admittedly, I did not notice.

Now you do, try to remember it next time you post such stuff.

However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.

You are writing pure nonsense.
 
  • #13
True, but it sounded good at the moment so I posted. :D
 
  • #14
Brainv2.1beta said:
http://en.wikipedia.org/wiki/Variable_speed_of_light
I would be suspicious of that article. There's a lot I don't understand, but the author made a big deal out of the variability of G (among other constants) without referencing any of the people who have so long worked on that (e.g. George Gillies). That would be like an article on QED that leaves out Schwinger - you have to ask why?
 
  • #15
Brainv2.1beta said:
Admittedly, I did not notice. However, there still is the problem that if over 50% of the universe can not be converted into a vacuum then the speed of light is not officially a constant.

1effect said:
You are writing pure nonsense.

Agreed. The question has been asked, good answers have been given (and unfortunately mostly ignored by the OP). I'm locking the thread, it serves no further purpose.

If people are having trouble sorting the wheat from the chaff, I will point out that answers given by science advisors and staff are generally more reliable than answers given by non-science advisors. However, answers given by SA's and staff should not be taken as gospel, and we definitely encourage people to read standard textbooks and peer reviewed papers. We do not encourage "making it up as you go along" sorts of answers, though, that's one reason I'm locking the thread, we've seen too much of that here.

The internet presents some special problems with repsect to relativity. There are some fine resources on the topic in the WWW, there are also many incorrect and downwright wrong websites as well.
 

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