[evan_au]

Has the speed of light been tested in a vacuum?

When it comes to the speed of light, air at sea level is a pretty good vacuum.

Light slows down in a medium, compared to its speed in a vacuum
- For example, in glass, light is slowed by a factor of about 1.5
- In air at sea level, light is slowed by a factor of 1.0003
- The amount of slowing is measured by the "Refractive Index"
- See the Refractive Index list at: https://en.wikipedia.org/wiki/List_of_refractive_indices#List

For non-magnetic materials, you can calculate the amount of slowing from the measured relative permittivity of the medium.
- For air at sea level, the relative permittivity is already close to 1 (almost the same as a vacuum)
- For achievable vacuums (eg at LHC), the relative permittivity is immeasurably close to 1
- See the list at https://en.wikipedia.org/wiki/Relative_permittivity

From a totally different viewpoint, gravitational waves also travel at "c" (commonly called "the speed of light in a vacuum", but its more fundamental than that).
- Gravitational waves travel almost unaffected through the high density of the Earth, the Sun and neutron stars
- The LIGO observatory observed a neutron star merger, which was accompanied by a gamma-ray burst starting 1.7 seconds later. The source was in a galaxy 130 million light years away.
- This suggests that light travels at a speed through intergalactic space at a speed that is reduced by at most 1.7 seconds in 130 million years, ie pretty much the speed of light in a vacuum.
- See description at: https://en.wikipedia.org/wiki/Neutron_star_merger#Observed_mergers

[alancalverd]

Or that gravitational waves travel slightly faster than light in a vacuum, or that the gamma ray burst really did occur slightly after the neutron star merger, or that the merger generated an intense local gravitational field that bent spacetime by 1.7 seconds.

[Kryptid]

people are telling me that if one of my hands is empty and the other has coins in it , that is the same thing

That's not what we are saying at all.

[Bobsey (OP)]

Has the speed of light been tested in a vacuum?

When it comes to the speed of light, air at sea level is a pretty good vacuum.

Light slows down in a medium, compared to its speed in a vacuum
- For example, in glass, light is slowed by a factor of about 1.5
- In air at sea level, light is slowed by a factor of 1.0003
- The amount of slowing is measured by the "Refractive Index"
- See the Refractive Index list at: https://en.wikipedia.org/wiki/List_of_refractive_indices#List [nofollow]

For non-magnetic materials, you can calculate the amount of slowing from the measured relative permittivity of the medium.
- For air at sea level, the relative permittivity is already close to 1 (almost the same as a vacuum)
- For achievable vacuums (eg at LHC), the relative permittivity is immeasurably close to 1
- See the list at https://en.wikipedia.org/wiki/Relative_permittivity [nofollow]

From a totally different viewpoint, gravitational waves also travel at "c" (commonly called "the speed of light in a vacuum", but its more fundamental than that).
- Gravitational waves travel almost unaffected through the high density of the Earth, the Sun and neutron stars
- The LIGO observatory observed a neutron star merger, which was accompanied by a gamma-ray burst starting 1.7 seconds later. The source was in a galaxy 130 million light years away.
- This suggests that light travels at a speed through intergalactic space at a speed that is reduced by at most 1.7 seconds in 130 million years, ie pretty much the speed of light in a vacuum.
- See description at: https://en.wikipedia.org/wiki/Neutron_star_merger#Observed_mergers [nofollow]

  I think that information is beyond the level of  my understanding and question . If I have this right you are saying that the speed of light through the spacing between particels in space is the speed of light ?

Then you are saying it slows down if it is't a vacuum like space ?

Thats all good but how can space be a vacuum when it has things in it ?

I'm so confused of your answers

[Bobsey (OP)]

Am I missing something here ?

Yes.

What am I missing ?

[Bobsey (OP)]

people are telling me that if one of my hands is empty and the other has coins in it , that is the same thing

That's not what we are saying at all.

You said the speed of light has been tested in a vacuum , by this I assume you meant a human made vaccuum where they suck out all the air ?

This vacuum hasnt got planets or stars in it like space , I can't understand how space is a vacuum ?

[Origin]

What am I missing ?

It appears that you are missing the ability to read and understand the replies you've received.  All the answers seem extremely straight forward and easy to understand yet appear to be way over your head. 

[alancalverd]

I can't understand how space is a vacuum ?

You seem to be confusing "space" with the universe, which is mostly space with a few bits of matter scattered through it. The matter either emits or absorbs light, which travels between the bits at c. 

[vhfpmr]

I always thought a vacuum was a process of extracting the air from a containment .

That would make a bottle of helium a vacuum.

What am I missing ?

A bit of common sense.

I'm so confused of your answers

No you aren't, you're just being deliberately obtuse.

[Bobsey (OP)]

I always thought a vacuum was a process of extracting the air from a containment .

That would make a bottle of helium a vacuum.

What am I missing ?

A bit of common sense.

I'm so confused of your answers

No you aren't, you're just being deliberately obtuse.

I arent been deliberately anything

In mathematics, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero.[1] Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set, while in other theories, its existence can be deduced. Many possible properties of sets are vacuously true for the empty set.

Any set other than the empty set is called non-empty.

In some textbooks and popularizations, the empty set is referred to as the "null set".[1] However, null set is a distinct notion within the context of measure theory, in which it describes a set of measure zero (which is not necessarily empty). The empty set may also be called the void set.

{}≠{1}

According to the math , space can't be a vacuum because it isnt a empty set

The number of elements of the empty set (i.e., its cardinality) is zero:

[Bobsey (OP)]

What am I missing ?

It appears that you are missing the ability to read and understand the replies you've received.  All the answers seem extremely straight forward and easy to understand yet appear to be way over your head.

If space is a vacuum it isn't a very good vacuum because I can see lot of matter
space.jpg (143.21 kB . 853x480 - viewed 544 times)

[Bored chemist]

According to the math , space can't be a vacuum because it isnt a empty set

The empty set is a mathematical entity, not a physical one, so that makes no sense.
The set theory has absolutely nothing to do with where the speed of light has been measured.

[Bored chemist]

If space is a vacuum it isn't a very good vacuum because I can see lot of matter

You can only see the matter because there's a vacuum between you and it.


BTW, you keep refusing to answer this.

Do you understand that a vacuum is impossible?

[Origin]

If space is a vacuum it isn't a very good vacuum because I can see lot of matter

Seriously?

Just out of curiosity...

You seem to agree that we can create a vacuum in a container on earth.  According to your understanding, if we had a marble in the container when we drew the vacuum would that container still be under a vacuum?

[Bobsey (OP)]

If space is a vacuum it isn't a very good vacuum because I can see lot of matter

Seriously?

Just out of curiosity...

You seem to agree that we can create a vacuum in a container on earth.  According to your understanding, if we had a marble in the container when we drew the vacuum would that container still be under a vacuum?

If you place a marble in a container then that container wouldn't be devode of all matter . People are telling me space is a vacuum devode of all matter , I can see that space isn't devode of all matter . Are they suggesting what we see doesn't exist ? In a vacuum it suppose to be empty , nothing to see and no atmosphere .
I understand thier stance , the spacing between planets is empty of atmosphere . But the space isn't empty of particles or other things . Space has lots of space dust from the stars and planets and stars

[Bobsey (OP)]

If space is a vacuum it isn't a very good vacuum because I can see lot of matter

You can only see the matter because there's a vacuum between you and it.


BTW, you keep refusing to answer this.

Do you understand that a vacuum is impossible?

Impossible ? No i do't know this . I know we can make partial vacuums

[Kryptid]

This vacuum hasnt got planets or stars in it like space , I can't understand how space is a vacuum ?

Steve, it's the space between the planets and stars that is a vacuum.

[Bobsey (OP)]

According to the math , space can't be a vacuum because it isnt a empty set

The empty set is a mathematical entity, not a physical one, so that makes no sense.
The set theory has absolutely nothing to do with where the speed of light has been measured.

My speed of light question was answered but the thread developed into a what i s a vacuum thread .

There is no reason a mathematical entity can't be applied to a physical process

Volume 1 is set A and Volume 2 is set B


Set A is a empty set (vacuum)

Set B has elements (matter)

[Bored chemist]

There is no reason a mathematical entity can't be applied to a physical process

There is if it gives the wrong answer.
So please stop trying to do it.

[Bored chemist]

I know we can make partial vacuums

On a big enough scale, the universe is a pretty good partial vacuum.