No vacuum anywhere vs the measurement of "c"

[Dangoe]

TL;DR

Question: We defined the second and hence the meter by the speed of light in vacuum "c", but there isn’t anywhere a real vacuum possible in the universe never (see Wikipedia). How and where was c measured then?

Hi to the community!

Glad to be able to ask 1-2 questions here so signed up. As an interested layman with some autodidactic efforts, still can’t see how this happened or got to the value:

Question: We defined the second and hence the meter by the speed of light in vacuum, but there isn’t anywhere a real vacuum possible in the universe (see Wikipedia). How and where was c measured then?

Would appreciate any comprehensive answers!

Regards,
Dangoe

 

[hutchphd]

Welcome.
Here's the deal. If you make a claim like " but there isn’t anywhere a real vacuum possible in the universe" then you need to document that and (see Wikipedia) is insufficient.
You need to up your game a bit if you wish serious discussion.

 

[Vanadium 50]

The index of refraction in the space between galaxies is about 1.000 000 000 000 000 000 000 000 000 2. Why should I worry about that?

 

[fresh_42]

My Wikipedia pages say, the pressure on the moon is about $0.000000001$ Pascal, and we have achieved a vacuum pressure below $0.0000000000000001$ Pascal. And the moon has a mean free path of about $10000$ km.

 

[Dangoe]

Welcome.
Here's the deal. If you make a claim like " but there isn’t anywhere a real vacuum possible in the universe" then you need to document that and (see Wikipedia) is insufficient.
You need to up your game a bit if you wish serious discussion.

Thanks for your reply hutchphd,

Of course you are absolutely right

Welcome.
Here's the deal. If you make a claim like " but there isn’t anywhere a real vacuum possible in the universe" then you need to document that and (see Wikipedia) is insufficient.
You need to up your game a bit if you wish serious discussion.

Thanks for your reply hutchphd,

Of course you are absolutely right, I just skipped that because so obvious presumably. Anyway, here we go: “Im bekannten Universum gibt es kein vollständiges Vakuum, und es ist mit bekannten technischen Mitteln auch nicht erzeugbar.” (https://de.wikipedia.org/wiki/Vakuum) =~ In the known universe there is no complete vacuum, and it is not producible with known technical means.

 

[Dangoe]

Sorry I have some issues with my device (iPad), keeps ghosting my keyboard, tried again maybe overlapping answers - I switch to another one now.

 

[Dangoe]

The index of refraction in the space between galaxies is about 1.000 000 000 000 000 000 000 000 000 2. Why should I worry about that?

Hi Vanadium 50,
Thanks for this number. How did you get it, and how being so sure?
Regards,
Dangoe

 

[Dale]

Summary: Question: We defined the second and hence the meter by the speed of light in vacuum "c", but there isn’t anywhere a real vacuum possible

The question isn’t if a vacuum meets some unobtainable ideal of a “real vacuum”. What matters is if the light interacts with matter.

There is a theorem called the optical extinction theorem that can be used to know if light of a specific wavelength traveling through a specific actual vacuum has interacted with matter.

While there are experiments and astronomical observations that are subject to criticism under the extinction theorem, there are many that are not. The requisite levels of vacuum for easily accessible sources are in fact achievable.

 

[Dangoe]

My Wikipedia pages say, the pressure on the moon is about $0.000000001$ Pascal, and we have achieved a vacuum pressure below $0.0000000000000001$ Pascal. And the moon has a mean free path of about $10000$ km.

Thanks fresh_42, but both not a vacuum anyway. Maybe enough you say? Regards, Dangoe

 

[Vanadium 50]

How did you get it

Looked up the density of gas in the IGM.

 

[hutchphd]

Of course you are absolutely right, I just skipped that because so obvious presumably.

The other motivator for me was that there are almost never real absolutes. (@Dale invocation's of the optical theorem notwithstanding !) But a number without an uncertainty usually lacks standing.

 

[Dangoe]

The question isn’t if a vacuum meets some unobtainable ideal of a “real vacuum”. What matters is if the light interacts with matter.

There is a theorem called the optical extinction theorem that can be used to know if light of a specific wavelength traveling through a specific actual vacuum has interacted with matter.

While there are experiments and astronomical observations that are subject to criticism under the extinction theorem, there are many that are not. The requisite levels of vacuum for easily accessible sources are in fact achievable.

Ah, ok, thanks Dale!

I am going to follow your path "optical extinction theorem" to understand it better. I still can't believe we got c in a "vacuum" just like that, implying the second, meter and etcetera...

Regards,
Dangoe

 

[Dale]

I am going to follow your path "optical extinction theorem" to understand it better.

A good resource is this page:

https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

It lists a lot of experiments on special relativity and indicates which ones are subject to criticism under the extinction theorem and which are not.

I still can't believe we got c in a "vacuum" just like that, implying the second, meter and etcetera...

Why not?

When scientists describe c as “the speed of light in vacuum”, there never was any implication by them that it was some sort of unobtainable ideal of vacuum. That is just something that you added.

When scientists refer to the speed of light in vacuum, they just mean that the light isn’t interacting with matter. The extinction theorem quantifies that.

What is so unbelievable about that?

 

[Dangoe]

While I haven't read about the extinction theorem for me "in vacuum" means in vacuum. Not complicated, what to not understand what I find unbelievable about.

 

[phinds]

While I haven't read about the extinction theorem for me "in vacuum" means in vacuum. Not complicated, what to not understand what I find unbelievable about.

You seem to be more interested in promoting, and insisting upon, your point of view than you are in learning about science. As should be clear from reading the responses, YOU are the only one proposing that there is a requirement for some perfect vacuum in order for the physics to make sense.

 

[Dangoe]

I was just chatting before reading and learning more. Thanks again for the hint above. And yes, makes sense when clear, not before. Until later.

 

[Dale]

for me "in vacuum" means in vacuum

Yes, and it is measured in vacuum. An unrealizable stringent connotation to those words is not implied.

Not complicated, what to not understand what I find unbelievable about.

I understand your original confusion. What I don’t understand is why you found the clear answer unbelievable.

You are interpreting “in vacuum” with a meaning that is not intended by scientists. Why is that unbelievable? People miscommunicate in this fashion all the time. The listener hearing a word with a different meaning than the speaker intended. It is a daily experience for me. How can the idea that this has happened here be “unbelievable”? Have you really never had this experience before?

 

[sophiecentaur]

Maybe enough you say?

The phrase "near enough for Jazz" applies everywhere in Science. We know nothing with absolute accuracy but that doesn't prevent us from coming up with models that predict with useful accuracy. An example of this is the mass of a photon which is specified, not as zero but as less than a given value. Here, that value is quoted as possibly 3X10^-27eV but not zero.

 

[Drakkith]

There's a fairly simple procedure that would allay most doubts. Fill a chamber with high-pressure air and then measure the speed of light through that chamber. Next, reduce the air pressure and measure the speed of light through the chamber again. Then reduce the air pressure again and redo the measurement. Repeat until you can't get the pressure down any further or you reach the limits of precision with your experimental setup.

Now graph the calculated speed vs the air pressure. You'll find that it approaches a certain value as the pressure is reduced. This value also happens to closely match that calculated using measurements of the fine structure constant and many other methods.

How and where was c measured then?

See here for more info: https://en.wikipedia.org/wiki/Speed_of_light#Measurement
There have been many measurements of the speed of light throughout the last 100+ years, far too many to list individually. One could say that every single instance of telecommunications via satellites is a test, as are communications with space probes and rovers.

 

[Vanadium 50]

Well, there aren't any frictionless planes, stretchless ropes, perfect spheres, etc. either.At some point you have to ask "does the fact that this isn't ideal make any difference?"

Vacuum imperfections makes a difference of a part in 10²⁴. which is under a microsecond over the entire history of the universe. If that's really a problem, I don't know what to say.

It is also worth pointing out that a physical realization of a seconds standard - i.e. a clock - it a hundred million times worse. That is, if `I build two identical clocks as well as I can, over the life of the universe, they will drift apart by around a minute.

 

[berkeman]

Well, there aren't any frictionless planes

Obviously you've never seen me try to ice skate...

 

[malawi_glenn]

There are no infinite precision time- or position measurments. Since 1983 c is a defined quantity.

 

[Vanadium 50]

I think the OP's objection is "how can it be a defined quantity if the physical realization of the definition is imperfect in the 25th decimal place?"

This is essentisally the same question as "How can π have more than 25 digits if we can't make a bigger circle out of atoms?"

 

[fresh_42]

I guess the OP was surprised by the fact that meters and seconds are defined by some odd numbers with ten digits. If that involves the speed of light in a vacuum, the question is: Can we measure the speed of light up to ten digits without being able to create a perfect vacuum?

The answer is yes, and we can produce even better vacua than are necessary to obtain such precision. I further assume that this answer is satisfactory since it is a B-level thread. Measurement is never completely accurate, which is why those odd numbers were finally fixed in the definition of meters and seconds.

 

[Vanadium 50]

I guess the OP was surprised by the fact that meters and seconds are defined by some odd numbers with ten digits.

I don't think so. He starts with

Summary: Question: We defined the second and hence the meter by the speed of light in vacuum "c", but there isn’t anywhere a real vacuum possible

Based on what he writes, the objection seems to be that an ideal vacuum can't be realized. And there's nothing special about this - the ideal anything can't be realized. That's why we call it "ideal".

 

[fresh_42]

And there's nothing special about this - the ideal anything can't be realized.

Thank you! Nobody believes me when I tell them that there is no such thing like a circle.

 

[sophiecentaur]

Thank you! Nobody believes me when I tell them that there is no such thing like a circle.

Geometrical figures are all abstract and axiomatic. Saying there's 'no such thing' is bound to get you a hostile reaction. People tend not to get the distinction between maths and the real world.

 

[pinball1970]

Thank you! Nobody believes me when I tell them that there is no such thing like a circle.

A platonic one?

You mean in the real world though yes?

OOPS! EDIT. @sophiecentaur already answered

 

[jbriggs444]

It is important to get from the formal definition to a practical realization of the definition. The BIPM has something to say about that.

While the definition for the metre refers to light traveling in vacuum, in most cases the realization of the length unit is performed under atmospheric pressure. Then the exact value of the influence of the air on the speed of light is of major importance. Therefore, a distinction has to be made between c, the speed of light in vacuum and c′, the speed of light in general. Under atmospheric pressure, the air refractive index reduces the speed of light (c′ = c/n) with a relative effect of the order of $3 \times 10^{–4}$ corresponding to 300 µm per metre of measured length. Moreover, in the case of modulated light it is important to consider the group refractive index of air ng instead of the (phase-) refractive index, n. For example, for green light (λ  520 nm) ng – n is approximately $10^{–5}$ , i.e. considering n instead of ng causes an additional error of 10 µm per metre. This difference is significant and comparable in size to the variation of the phase refractive index of air over the entire range of visible light: n(380 nm) – n(780 nm) = $9 × 10^{–6}$ . Thus, determination of the exact speed of light to use in equation (1) is a significant consideration in realizing the metre through primary methods.

 

[hutchphd]

Don't abstractions exist?

Call the philosophy squad...