Is the speed of light a constant? Or can it vary? If so, in what kinds of situations, or how much?

[sjastro]

I would consider one way speed of light as direct light. And the 2 way speed of light as reflective. We can’t measure the speed of direct light but we can measure the speed of reflective light. Einstein made the assumption that direct light was the same speed as reflective light since we could actual measure direct light speed. When asked how could he make that assumption he simply said it was his theory (Relativity) and he could make any assumption he wanted.

There was a logical reason why Einstein made the assumption as according to Maxwell’s equations formulated in the 19th century, light is an electromagnetic wave.
Like sound waves which required air to propagate through, a luminiferous ether was proposed which allowed light to travel through space.

If such an ether existed a one way anisotropic speed of light should have been observed as an interference pattern using interferometers with 180⁰ spaced mirrors when light in opposite directions is combined.
A null result is observed instead and even though interferometers test for two way light, the null result suggests one way light is the same in both directions vindicating Einstein’s assumption.

The absence for the evidence of an ether led to the development of special relativity where light can propagate through a vacuum, the speed of light is the same for all inertial observers, and one way light speed is isotropic in all directions making it indistinguishable from two way light.

 

[sjastro]

On the subject of the speed of light and interferometers one of the objectives of physics is to test whether the speed of light is constant down to quantum scales.
In special relativity, physical laws should be the same in all inertial frames of reference such as the speed of light being invariant (constant) for all observers moving at constant velocities. This also includes time dilation and length contraction defined by the Lorentz transformations.
The constancy of physical laws in inertial frames of reference is known as Lorentz invariance.

In quantum gravity and string theories however space-time may not be smooth and continuous but discrete where it is quantized or exists in a lattice type structure like a grid in which case Lorentz invariance could be broken and the speed of light is no longer invariant.
With the increasing sensitivity of interferometers, physicists are entering the stage where they may be able to probe the speed of light at quantum scales.

Here is the historical and future developments of interferometers described by GPT4-o.

Historical Interferometers

1. Michelson-Morley Interferometer (1887)
   • Application: Tested for differences in the speed of light due to Earth's motion through the aether.
   • Sensitivity: Could detect speed differences down to about 10⁻⁸ relative to the speed of light.
2. Kennedy-Thorndike Experiment (1932)
   • Application: Tested the dependence of the speed of light on the velocity of the apparatus.
   • Sensitivity: Not as sensitive as Michelson-Morley, detecting changes in speed down to about 10⁻⁶.

20th Century Interferometers

1. Modern Optical Cavities (1980s-2000s)
   • Application: Used resonant optical cavities to detect variations in the speed of light in different directions.
   • Sensitivity: Achieved sensitivities around 10⁻¹⁷.
2. Cryogenic Optical Resonators (2000s)
   • Application: Reduced thermal noise to provide highly stable and precise measurements of the speed of light.
   • Sensitivity: Improved sensitivity to the order of 10⁻¹⁸.

Modern and Future Interferometers

1. Advanced Optical Cavities (2010s-present)
   • Application: Continued advancements in optical cavities with even greater precision and stability.
   • Sensitivity: Sensitivities reaching around 10⁻¹⁹.
2. Space-Based Optical Cavities (Planned)
   • Application: Proposed space-based interferometers to eliminate terrestrial noise and achieve ultimate precision.
   • Expected Sensitivity: Targeting sensitivities potentially down to 10⁻²⁰.

These interferometers represent the key advancements in measuring the speed of light with increasing sensitivity, providing critical tests of Lorentz symmetry and the invariance of the speed of light.

 

[Platte]

There was a logical reason why Einstein made the assumption as according to Maxwell’s equations formulated in the 19th century, light is an electromagnetic wave.
Like sound waves which required air to propagate through, a luminiferous ether was proposed which allowed light to travel through space.

If such an ether existed a one way anisotropic speed of light should have been observed as an interference pattern using interferometers with 180⁰ spaced mirrors when light in opposite directions is combined.
A null result is observed instead and even though interferometers test for two way light, the null result suggests one way light is the same in both directions vindicating Einstein’s assumption.

The absence for the evidence of an ether led to the development of special relativity where light can propagate through a vacuum, the speed of light is the same for all inertial observers, and one way light speed is isotropic in all directions making it indistinguishable from two way light.

And that is one of the big problems with Science in general. When something (test results) is suggested, but far from proved, science takes and runs with and since the entire scientific community agrees with the suggestions we now have the suggestion treated as a fact since the scientific community is in agreement.

 

[sjastro]

And that is one of the big problems with Science in general. When something (test results) is suggested, but far from proved, science takes and runs with and since the entire scientific community agrees with the suggestions we now have the suggestion treated as a fact since the scientific community is in agreement.

A test result or measurement is not ‘suggested’ but an outcome of an experiment.
The experiment is repeated by both the individual test facility and independent groups to ascertain the measurements are both reproducible and repeatable.
When the technology is improved or an alternative test method is found, the reproducible and repeatable procedures are conducted again.
The speed of light has been measured many times over the centuries using different procedures and ever improving technology.

After 300 years of measuring the speed of light starting with Roemer in 1676, in 1983 the metre was formally defined as the distance light travelled in a vacuum in 1/299,792,458 of a second.

The example should also serve to illustrate science is never proven but only supported by evidence which depends on the technology of the day.

 

[Platte]

A test result or measurement is not ‘suggested’ but an outcome of an experiment.
The experiment is repeated by both the individual test facility and independent groups to ascertain the measurements are both reproducible and repeatable.
When the technology is improved or an alternative test method is found, the reproducible and repeatable procedures are conducted again.
The speed of light has been measured many times over the centuries using different procedures and ever improving technology.

View attachment 352159​

After 300 years of measuring the speed of light starting with Roemer in 1676, in 1983 the metre was formally defined as the distance light travelled in a vacuum in 1/299,792,458 of a second.

The example should also serve to illustrate science is never proven but only supported by evidence which depends on the technology of the day.

Thats exactly what I mean. The test results we can all agree upon, but it’s what they say the results suggest, thats what I’m talking about. Science will say the results suggest that one way light is same speed as 2 way. Then because the test results are agreed upon, they take what the suggested result mean to then as fact too. When in reality the results may suggest it, but are far far far from actually proving one way light = two way. But it’s what Science wants it to say, so that suggestion gets pushed as though it’s conclusive too. It’s done all the time in science to point to a narrative like mars having digital signatues that provide evidence that there is water in Mars. So now it’s just stated that there is water on Mars as if it’s a fact and if you suggest otherwise you are anti science. It’s really pretty cool how they do it to push their narrative.

 

[Carl Emerson]

Measuring the speed of anything (including light) involves a degree of confidence in the measurement of distance and time.

Time may or may not be a constant.

Einstein concluded that time was not absolute.

Interestingly enough when one graphs the ages of the Patriarchs the curve is very similar to 'e' the growth curve.

This would suggest the time is 'living' - before Abraham was I AM...

So maybe time and consciousness are related and if growing, this would explain why carbon dating very ancient specimens is inaccurate and therefor the earth 'scientifically' seems older than it is.

I read a paper in the 80's which suggested that phenomena observed in a large particle accelerator in CA was faster than light.

These are mere musings on the topic I claim no expertise.

We may be pondering the imponderable.

 

[sjastro]

@Platte.
It's obvious from your last couple of posts you do not understand how science works.
The one way speed of light is not a theory, nor a hypothesis, nor is it even classified as being science.
As I mentioned in post#14 the one way speed is of light is unfalsifiable, there are no tests or observations that can support the idea or disprove it.
If it not falsifiable it is not science and falls into other realms such as metaphysics or philosophy.

We live in a universe where the two way theory of light is supported by experiments and observations, in scattering experiments involving the collision of photons with particles, the speed c of the photons remains the same even though the energy of the photon will change in inelastic collisions, this may be an indication if there is a one way speed of light, it is the same in all directions and therefore indistinguishable from two way speed of light.

Unfortunately you cannot rely on a one way speed of light to show the age of the universe falls within the YEC time frame, it is a fallacious argument to do so.

 

[Platte]

@Platte.
It's obvious from your last couple of posts you do not understand how science works.
The one way speed of light is not a theory, nor a hypothesis, nor is it even classified as being science.
As I mentioned in post#14 the one way speed is of light is unfalsifiable, there are no tests or observations that can support the idea or disprove it.
If it not falsifiable it is not science and falls into other realms such as metaphysics or philosophy.

We live in a universe where the two way theory of light is supported by experiments and observations, in scattering experiments involving the collision of photons with particles, the speed c of the photons remains the same even though the energy of the photon will change in inelastic collisions, this may be an indication if there is a one way speed of light, it is the same in all directions and therefore indistinguishable from two way speed of light.

Unfortunately you cannot rely on a one way speed of light to show the age of the universe falls within the YEC time frame, it is a fallacious argument to do so.

My point was simply that we cannot measure the one way speed of light. Is it the same as 2 way? Maybe … probably. But we don’t know for sure. So I was actually doing the opposite of what you are saying. I was saying be careful when using light to falsify creation. Because that too is a fallacious argument.

 

[essentialsaltes]

I was saying be careful when using light to falsify creation.

Fortunately, if we're talking about some sort of standard YEC, I don't think the fossil record, evolution, genetics, geology, or radiometric dating depend on the isotropy of the speed of light to any degree.

 

[Carl Emerson]

Fortunately, if we're talking about some sort of standard YEC, I don't think the fossil record, evolution, genetics, geology, or radiometric dating depend on the isotropy of the speed of light to any degree.

Yes it would if time has changed it's rate.

 

[sjastro]

My point was simply that we cannot measure the one way speed of light. Is it the same as 2 way? Maybe … probably. But we don’t know for sure. So I was actually doing the opposite of what you are saying. I was saying be careful when using light to falsify creation. Because that too is a fallacious argument.

Seriously now?
What is meaning behind this post from the How Old is the Earth thread?

Platte wrote,

also It is interesting that we can’t see any planets further than 6000 light years away. Planets are seen with reflective light which we do know is 186,000 miles per second.

It seems very much like an advocation for YEC if the one way speed of light should be conveniently adjusted to avoid the starlight problem and come up with a 6000 year old universe.

I had in mind creationist/scientists such as Jason Lisle mentioned earlier in this thread, who has abandoned his credentials as a scientist for ignoring the evidence the universe is nearly 14 billion years old.

 

[Platte]

Fortunately, if we're talking about some sort of standard YEC, I don't think the fossil record, evolution, genetics, geology, or radiometric dating depend on the isotropy of the speed of light to any degree.

I’m not YEC but light is often used, I’m sure you'd agree, even more so than the other examples you mentioned IMHO.

 

[Platte]

It seems very much like an advocation for YEC if the one way speed of light should be conveniently adjusted to avoid the starlight problem and come

I never said to adjust anything. I simply stated a fact which is we are unable to measure the one way speed of light. My point wasn’t to use light in support of Creation - but to point out that since we don’t know than it shouldn’t be used to debunk it. It seemed to me that most people didn’t realize that and I havnt really seen it be mentioned (that the One way speed of light can’t be measured). I’m not an advocate for YEC. God creating the world young wouldn’t make much sense if He wanted the Earth to be hospitable and functional for Adam from the beginning.

I had in mind creationist/scientists such as Jason Lisle mentioned earlier in this thread, who has abandoned his credentials as a scientist for ignoring the evidence the universe is nearly 14 billion years old.

Have no idea who that is.

 

[Platte]

Seriously now?
What is meaning behind this post from the How Old is the Earth thread?

Yes it was discussed there. The light topic was moved here so more science minded people could confirm what I said - that the one way speed of light cannot be measured.

You didn’t find that interesting? I find little things like that very interesting


Platte wrote,
also It is interesting that we can’t see any planets further than 6000 light years away. Planets are seen with reflective light which we do know is 186,000 miles per second.

 

[Hans Blaster]

Platte wrote,
also It is interesting that we can’t see any planets further than 6000 light years away.

Planets are very tiny things. The furthest resolved planet is only 1/10 that distance away:

CT Chamaeleontis - Wikipedia

Planets are seen with reflective light which we do know is 186,000 miles per second.

Reflected light is not special relative to direct light. It does not have a special speed. If reflected light travels at a particular speed (even with the silly notion of speed anisotropy) then so does direct light.

 

[Platte]

Reflected light is not special relative to direct light. It does not have a special speed. If reflected light travels at a particular speed (even with the silly notion of speed anisotropy) then so does direct light.

I was referring to Reflected light as 2 way light...and direct light as 1 way light.

There is no proof that reflected light and direct light travel at the same speed. Maybe it does....but it has not proven. The 1 way speed of light has not been measured - only the 2 way speed of light has been measured. That makes Reflected light special relative to direct light.

 

[Platte]

Planets are very tiny things. The furthest resolved planet is only 1/10 that distance away:

There are planets bigger than stars

 

[Hans Blaster]

I was referring to Reflected light as 2 way light...and direct light as 1 way light.

There is no proof that reflected light and direct light travel at the same speed. Maybe it does....but it has not proven. The 1 way speed of light has not been measured - only the 2 way speed of light has been measured. That makes Reflected light special relative to direct light.

Reflected light is light. There is no difference in photons that have been reflected or scattered and those that haven't.

 

[Hans Blaster]

There are planets bigger than stars

Name one.

 

[essentialsaltes]

Yes it would if time has changed it's rate.

That's entirely separate from the one-way versus two-way speed of light.

I’m not YEC but light is often used, I’m sure you'd agree, even more so than the other examples you mentioned IMHO.

I do not agree that 'light' is frequently used to dispense with creationism.