Is the Speed of Light Governed by an Ether?

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Discussion Overview

The discussion revolves around the concept of whether the speed of light is governed by an "Ether" and how the speed of light is measured. Participants explore the implications of the constancy of the speed of light in relation to moving observers and the historical context of measuring light speed, touching on concepts from special relativity and the definitions of time and distance in physics.

Discussion Character

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

Main Points Raised

  • Some participants assert that there is no Ether and that the speed of light is constant, independent of the motion of the source or observer.
  • One participant proposes a thought experiment involving comoving observers to illustrate how the speed of light can be measured.
  • Another participant explains that historically, the speed of light was measured by timing light signals over known distances, but now it is defined in terms of distance light travels in a given time.
  • There is a discussion about the definition of a second and its relation to the speed of light, with some participants clarifying that it is based on the frequency of radiation from a caesium atom.
  • Some participants mention the evolution of the definition of the meter and other SI units, noting that the meter is now defined by the distance light travels in a specific time.
  • There are corrections and clarifications regarding the definitions of time and distance, with references to the historical context of measurement standards.

Areas of Agreement / Disagreement

Participants generally agree that the classical concept of Ether is incorrect and that the speed of light is a constant. However, there are multiple views on how to conceptualize the implications of this constancy and the methods of measurement, indicating that the discussion remains somewhat unresolved.

Contextual Notes

Participants express uncertainty about the implications of measuring the speed of light and the definitions of time and distance, highlighting the complexity of these concepts in physics.

JB321
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From everything that I have read, it seems fairly non-controversial that there is no Ether and that the only thing that remains constant (or that can be consistently measured against) is the speed of light. But how do we measure the speed of light? If the speed of light is constant, isn't there some type of "Ether" by which the speed of light is governed? Or is the speed of light always c in relation to the speed at which an object is traveling? There is probably a very simple answer to this question that I am overlooking...

Thanks in advance,
JB
 
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JB321 said:
From everything that I have read, it seems fairly non-controversial that there is no Ether and that the only thing that remains constant (or that can be consistently measured against) is the speed of light. But how do we measure the speed of light? If the speed of light is constant, isn't there some type of "Ether" by which the speed of light is governed?
I think to measure the speed of light, you can do a "little" experiment here.
You have to have two parties which are comoving. Comoving is speeding at the same direction and at the same speed. And for all practical purpose, you can call the parties stay relatively to each other. Doppler effect can confirm that. And you measure the distance, then you beam a light from A to B and reflects the light back to A and divide the time taken by two.
JB321 said:
Or is the speed of light always c in relation to the speed at which an object is traveling? There is probably a very simple answer to this question that I am overlooking...
Yes, But how can you measure it anyway?
All you can do is this simple thought experiment here.
B and C are comoving and traveling from A at some speed.
A beams light to B, and after the light reach B, then the light will reach C at some time later. Then C will record the time when C see the light and compare it to the time recorded by B. And they will confirm the speed of light is c, some 300000km/second.
Of course this would be in B and C frame, from A frame, well... the distance between B and C is contracted. And again A will record that the time difference when (from A point of view) the time reaches B and when (from A point of view) the time reaches C is somewhat shorter that what was recorded by B and C.

JB321 said:
Thanks in advance,JB
You're welcome.
 
JB321 said:
Is there an Ether or not?
No.
... isn't there some type of "Ether" by which the speed of light is governed?
No. I just said no. The answer is no.

Or is the speed of light always c in relation to the speed at which an object is traveling?
That is correct. It doesn't matter how fast something is traveling relative to you, it always sees light as traveling at c relative to it. Google "Special Relativity"

EDIT: this is, by the way (and fairly obviously) totally counter intuitive. If a passing spaceship, traveling at .5c relative to you, shines a laser at you, the spaceship sees the light leave at c. You see the light arrive at c, even though the source is moving relative to you. You DO see it red-shifted, so the frequency is different but the speed is still c.
 
JB321 said:
But how do we measure the speed of light?
Historically, by bouncing a light signal over a known distance and finding the travel time. Today, the speed of light is defined so you do not measure it. Instead, the above procedure would calibrate the distance. This is because the most accurate way of defining a length unit is in terms of how far light travels in a given time.
 
Orodruin said:
Historically, by bouncing a light signal over a known distance and finding the travel time. Today, the speed of light is defined so you do not measure it. Instead, the above procedure would calibrate the distance. This is because the most accurate way of defining a length unit is in terms of how far light travels in a given time.
or to calibrate clock?
I just realize now. How to measure distance? If you have a very accurate clock then we can calibrate distance, by bouncing light. But..., how to have a very accurate clock?
I know this is just a technical problem. Is there a solution for this problem?
Thanks.
 
Stephanus said:
or to calibrate clock?
No. The definition of a second is not based on the speed of light.

Stephanus said:
how to have a very accurate clock?
The definition of a second is defined using the frequency of the radiation from a particular caesium atom.
 
Orodruin said:
No. The definition of a second is not based on the speed of light.The definition of a second is defined using the frequency of the radiation from a particular caesium atom.
Yes, thanks. And from then we get the definition of a meter. Which was stored in Paris? The standard meter.
 
Stephanus said:
Yes, thanks. And from then we get the definition of a meter. Which was stored in Paris? The standard meter.
The meter used to be defined using a standard rod. This is no longer the case. It is now defined in terms of how far light travels in a given time.

The only SI unit currently defined using an artefact is the kilogram.
 
Orodruin said:
The only SI unit currently defined using an artefact is the kilogram.

Even that will go away soon and be tied to Planck constant. Till then, don't breath around the kg standard.

Zz.
 
  • #10
ZapperZ said:
Till then, don't breath around the kg standard.
And clean it very carefully. (note Figures 3 through 12!)
 
  • #11
JB321 said:
From everything that I have read, it seems fairly non-controversial that there is no Ether and that the only thing that remains constant (or that can be consistently measured against) is the speed of light. But how do we measure the speed of light? If the speed of light is constant, isn't there some type of "Ether" by which the speed of light is governed? Or is the speed of light always c in relation to the speed at which an object is traveling? There is probably a very simple answer to this question that I am overlooking...

Thanks in advance,
JB

It is uncontroversial that the mechanical, 19th century "ether" concept was wrong. In particular there is no frame in which the laws of nature are preferred. "Ether" may simply correspond to the concept that there is something in nature that determines such things as the speed of light, which is the limit speed for everything. This speed is independent of the speed of the source, and independent of the speed of a target at which it ends. Other words that are used try to capture that idea are for example vacuum, space and spacetime. In science we can only describe what we measure (observe); and we cannot measure anything else but particles and fields.
Oops, correction: funny enough I forgot to mention radiation. But then, many people include that in "particles"; and if I'm not mistaken, QFT includes everything in "fields". Only the phenomena themselves are uncontroversial.
 
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