gre Posted May 2, 2009 Share Posted May 2, 2009 Why can't vacuum permeability/permittivity be measured? I've read that they can't be measured .. is this simply because "pure vacuum" isn't attainable? Link to comment Share on other sites More sharing options...
swansont Posted May 2, 2009 Share Posted May 2, 2009 I don't know that they can't be measured — do you have a reference for that? They aren't measured because they are defined values. It's not because of an unattainable vacuum; the "pure vacuum," i.e. absolute lack of anything, isn't the condition for which the terms are defined. Link to comment Share on other sites More sharing options...
gre Posted May 3, 2009 Author Share Posted May 3, 2009 At the end of this section: http://en.wikipedia.org/wiki/Magnetic_constant#Historical_origin_of_value_of_.CE.BC0 Link to comment Share on other sites More sharing options...
swansont Posted May 4, 2009 Share Posted May 4, 2009 I'm not sure if I entirely agree with the Wikipedia author's suggestion, since there's the possibility that in a configuration where there is an appreciable Casimir force you could decrease one or both of the terms (i.e. c changes) — what implications are there from that? Though I suppose you can argue that it's the magnetic and electric properties of the material and not space that you'd be measuring. But there's the idea that you can't measure something that is a vacuum property because you need to have some sort of probe there, which is decidedly non-vacuum. The actual values are by convention; you could choose them to be 1 in an appropriate coordinate system, but that means other constants would have to change. By choosing certain other constants (e.g. c), the permeability and permittivity have the values that they do. Link to comment Share on other sites More sharing options...
gre Posted May 5, 2009 Author Share Posted May 5, 2009 I'm not sure if I entirely agree with the Wikipedia author's suggestion, since there's the possibility that in a configuration where there is an appreciable Casimir force you could decrease one or both of the terms (i.e. c changes) — what implications are there from that? Though I suppose you can argue that it's the magnetic and electric properties of the material and not space that you'd be measuring. But there's the idea that you can't measure something that is a vacuum property because you need to have some sort of probe there, which is decidedly non-vacuum. The actual values are by convention; you could choose them to be 1 in an appropriate coordinate system, but that means other constants would have to change. By choosing certain other constants (e.g. c), the permeability and permittivity have the values that they do. Have there been any experiments done to measure the vacuum constants? I've been wondering about a variable magnetic and electric constant lately. Which wouldn't necessarily mean the speed of light changes, but could just be similar to red/blue shifts... What are the chances that gravity (or velocity) changes the magnitude of the vacuum constants (magnetic and electric). Link to comment Share on other sites More sharing options...
gre Posted May 8, 2009 Author Share Posted May 8, 2009 Here is a response I received on another forum. The permeability and permittivity of free space are now defined by other quantities, such as the length of a meter and the speed of light. Suppose you measured the capacitance of two one-square-meter plates separated by 1 millimeter in vacuum. Because the meter is defined by c, and c = 1/sqrt(u0e0), the measurement is not an independent measurement. the value of all the fundamental constants are based on a variety of interdependent quantities, every value is over determined, and least square fits are used to get the most probable values. In the 2002 table of the fundamental constants (which I am now looking at), both the permeability and permittivity of free space are "exact" defined quantities, meaning no uncertainty in value. So you cannot go out into intergallactic space and measure them, because of their dependence on other quantities. On the other hand, dimensionless quantities, like alpha, are dimensionless and measurable. Is this accurate? Link to comment Share on other sites More sharing options...
swansont Posted May 8, 2009 Share Posted May 8, 2009 The part about the terms being defined is. I'm not sure about the overdetermined claim. Link to comment Share on other sites More sharing options...
gre Posted May 8, 2009 Author Share Posted May 8, 2009 This is bugging me, now .. Lol ... Why can't someone just create a (partial) vacuum, and measure the permeability/permitivitty in it? Link to comment Share on other sites More sharing options...
gre Posted May 25, 2009 Author Share Posted May 25, 2009 Here's an answer from another forum Q: So is all the above correct? Vacuum constants can't be measured? I know they're derived from the speed of light... But wouldn't saying "the vacuum constants can't be measured" be similar to saying "the speed of light can't be measured"? A: No, because you can measure the speed of light directly without involving these constants, in the simplest case just by measuring the time it takes for a signal to travel between point A an B.Of course there are more sophisticated ways of doing it, but the point is that back when they were doing this (before the speed of light was defined to be constant, nowadays they instead measure the length of the meter) they used methods where the result only depended on a time and a length (and therefore indirecly on the definition of the meter, which back then was defined using an artifact). Constants like these are used to "glue" the SI system together. Some constants are always needed in a system and choice of which ones to include is to some extent arbitrary. In e.g the CGS system the permeability of vacuum is a dimensionless number equal to one. Hence, in equations where the CGS systems is used (nowadys mainly in the area of magnetism) there are fewer constants when dealing with magnetism (the fact that the equations are different depending on the system used can be quite confusing) swansont, is this acceptable? Link to comment Share on other sites More sharing options...
gre Posted May 31, 2009 Author Share Posted May 31, 2009 Maybe they do exist, but can't be measured. Link to comment Share on other sites More sharing options...
swansont Posted May 31, 2009 Share Posted May 31, 2009 Like the quoted part explained, the constants are an artifact of the unit system used. By convention, it has a certain value. Link to comment Share on other sites More sharing options...
Mr Skeptic Posted June 9, 2009 Share Posted June 9, 2009 However, there are unitless constants, and these are the same regardless of the units you use. Eg the fine structure constant. Link to comment Share on other sites More sharing options...
John Cuthber Posted June 9, 2009 Share Posted June 9, 2009 Since Mr Maxwell calculated the speed of em radiation from the known, measured, values of e0 and mu0 it must have been possible to measure them all those years ago. It still is. He then found that the speed he got tallied with the speed of light. On the other hand, since c and mu0 are fixed by definition, there's no point in measuring them or e0 because it can be calculated. The measurement of e0 is a high school experiment. Link to comment Share on other sites More sharing options...
swansont Posted June 9, 2009 Share Posted June 9, 2009 This is bugging me, now .. Lol ... Why can't someone just create a (partial) vacuum, and measure the permeability/permitivitty in it? Yes, you should. But I think one could make the argument that you're really measuring some other physical quantity in whatever apparatus you are using. It ties back into what constants you have defined and which ones you measure. Link to comment Share on other sites More sharing options...
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