# Are some vacuums more slippery than others?

## Main Question or Discussion Point

Are some vacuums more "slippery" than others?

What is the proprogation of electric and magnetic fields near a neutron star, should there be a delay if a light signal passes near by? There is a delay, but the speed of light is the same! It took longer to get to other side, but the speed is the same! Mustn't that have been the opposite of a shortcut? Is the delay proportional to the path extension (note that notion of path is not simple-easy in GR)?

How do you calculate the permittivity and permeability of the vacuum as a function of radius from charged subatomic particles inside a plane of glass?

Couldn't the factor of a "lorentz contraction" or a like factor be the same as the factor by which $(electric\ permittivity*magnetic\ permeability)^{1/2}$ of a medium increases? It is utterly false, unconcievable, and unphysical that all lorentz contractions (as well as refractive indices) have to do with is the increase of $(electric\ permittivity*magnetic\ permeability)^{1/2}$?

Why is curvature of space time not considered as due to local variation of the local electric permittivity and magnetic permeability of the vacuum with respect to a distant observer? Note that angular momentum of a system of fundmental particles (including emitted photons) is conserved, with time dilation of the system relative to an external observer (affecting the relative rotations of particles) being compensated with the increase in inertial mass of the system of particles.

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## Answers and Replies

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Chris Hillman
Look up Shapiro time delay effect in any good gtr textbook http://www.math.ucr.edu/home/baez/RelWWW/reading.html#gtr [Broken] (I suspect that as Wikipedia editor, you will look it up in Wikipedia instead, but be aware that you have no reasonable expectation of getting good quality information there--- although you might get perfectly good information, you very well might get quite wrong information.)

Your remaining questions could easily lead us into the realm of fringe physics, which I decline to explore.

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turbo
Gold Member
Einstein did not contribute to Wiki, but he did write a book.

Relativity: The Special and General Theory
http://www.bartleby.com/173/

From Chapter 22:
In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its result hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light).

pervect
Staff Emeritus
There are a couple of different usages of the term velocity. Einstein illustrates an older usage, the idea of velocity as the rate of change of a position coordinate with respect to a time coordinate.

The velocity of light in this sense is not constant.

However, if you pick any small patch of space-time, and set up a local coordinate system with rulers and clocks, you'll find that the speed of light in your local frame-field is always constant, it doesn't vary with position, regardless of whether you are in empty space, or deep in a gravity well, or someplace in-between.

So in this sense, the velocity of light is constant.

Coordinate velocites really aren't all that useful, but they are a sort of historical artifact. They tend to generate a lot of totally unnecessary confusion, and to make the job of people writing posts more difficult, because they have to be careful to specify whcih sort of velocity they are talking about so that someone doesn't misintterpret the remark and use the wrong one.

Also, it seems that people fall asleep in the middle of long explanations of such distinctions, and don't get the point sometimes anyway, even when the author tries to be very careful :-(.

But rest assured, there is a sense in which the velocity of light is constant, this is the sense of "local velocity" measured in a "frame field", and it is very, very, very useful.

And please don't try to use quotes from Einstein to discredit or confuse this very useful notion of "local velocity" or its constancy.

Chris Hillman
And please don't try to use quotes from Einstein to discredit or confuse this very useful notion of "local velocity" or its constancy.
Ditto. The practice of quoting-out-of-context the snippet-of-the-day from some popular essay by Einstein is not only highly misleading but (given the effort pervect, myself, and others put into repeatedly trying to make sure everyone is aware of the points itemized in the post above) tends to appear a bit obnoxious.

However, if you pick any small patch of space-time, and set up a local coordinate system with rulers and clocks, you'll find that the speed of light in your local frame-field is always constant, it doesn't vary with position, regardless of whether you are in empty space, or deep in a gravity well, or someplace in-between.

So in this sense, the velocity of light is constant.
It is like saying: "if you pick any small patch of the Earth, and set up a local coordinate system with rulers and clocks, you'll find that it is flat. It doesn't vary with position, regardless of whether you are, on a great plain, on the slope of a mountain, or deep in a valley, or someplace in-between. So in that sense the earth is flat".

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jtbell
Mentor
If you're an ant, that's a perfectly reasonable perspective to take!

disregardthat
Einstein did not contribute to Wiki, but he did write a book.
:rofl:

I wonder, is this book published on the internet, free to read?

pervect
Staff Emeritus
It is like saying: "if you pick any small patch of the Earth, and set up a local coordinate system with rulers and clocks, you'll find that it is flat. It doesn't vary with position, regardless of whether you are, on a great plain, on the slope of a mountain, or deep in a valley, or someplace in-between. So in that sense the earth is flat".

IMO you're just trying to be a smart-ass (please don't take offense! I'm trying to be wry, not nasty) but yes, on a small enough scale, the Earth is flat.

The Earth also illustrates some common techniques and conventions on how to handle metrics, and velocities.

If you have a ship with a constant speed of 20 knots (nautical miles/ hour), it's speed it will travel 20 minutes of longitude per hour on the Earth's equator.

Lattiude and longitude are examples of coordinates. Furthermore, the rate of change of longitude would be an example of a "coordinate velocity". It's the rate of change of a coordinate (longitude) with time.

But if our hypothetical seagoing ship has a constant speed of 20 knots, it doesn't have a constant speed in minutes of longitude per hour at all lattitudes - the coordinate velocity changes with lattitude. Neart the north pole, it can travel a lot faster than 20 minutes of longitude per hour.

When people talk about the "velocity" of the naval ship, they usually talk about the velocity of the ship in a local frame-field, i.e. as measured on a scale small enough that the Earth appears flat. Or using "local rulers and local clocks" as I like to put it. This is what the velocity of 20 knots generally means operationally.

People generally do NOT talk about the velocity of the ship in terms of "minutes of longitude per hour", which would vary with lattitude.

Someone who is "stuck" on coordinate velocites might try and come up with bizarre explanations for why ocean ships travel "faster" the north pole, in terms of the physics being "different" at the North pole than it is on the equator.

Someone who perfers to use local velocities would say that the ships velocity is indepdent of it's lattitude, and points to the assignment of coordinates to the Earths' surface as the explanation of why the coordinate velocity (minutes of longitude per hour) of the ship varies with its lattitude.

IMO you're just trying to be a smart-ass (please don't take offense! I'm trying to be wry, not nasty) but yes, on a small enough scale, the Earth is flat.
No offense taken.

Someone who is "stuck" on coordinate velocites might try and come up with bizarre explanations for why ocean ships travel "faster" the north pole, in terms of the physics being "different" at the North pole than it is on the equator.
Very true.

So do you think that light bends sometimes?

pervect
Staff Emeritus