HomeDiscussion ForumThe speed of light in a vacuum?

The speed of light in a vacuum?

On http://en.wikipedia.org/wiki/Speed_of_light, it says:
“The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning “swiftness”.
“In metric units, c is exactly 299,792,458 metres per second (or 1,079,252,848.8 km/h).”
What is meant by “vacuum” here? Certainly not a perfect vacuum, as “space can never be perfectly empty. A perfect vacuum, known as “free space”, with a gaseous pressure of absolute zero is a philosophical concept with no physical reality, not least because quantum theory predicts that no volume of space is perfectly empty in this way.” http://en.wikipedia.org/wiki/Vacuum
The article on vacuum continues:
“Physicists often use the term “vacuum” slightly differently. They discuss ideal test results that would occur in a perfect vacuum, which they simply call “vacuum” in this context, and use the term partial vacuum to refer to the imperfect vacua realized in practice.”
My question is: is the speed of light given above the theoretical speed light would have in a perfect vacuum? If so, is this not nonsense? If a space contains light, then it isn’t a perfect vacuum, right?
But Ares, if there were a perfect vacuum (which is impossible, as then there would be nothing, which is not the case), then light could not propagate there, right?


  1. The “speed of light in a vacuum” is a term in physics that merely means the unimpeded speed of electromagnetic propagation. Vacuum here means a hypothetical perfect vacuum. Transparent substances slow the propagation of electromagnetic radiation to a ratio of the speed of light called the refractive index of the substance.

  2. Vaccuum there is meant as perfect vaccuum.
    As it says the science is based on ideal vaccuum which is an assumption of a perfect vaccuum. It does not mean that that number is exact for a real case, but it is a very close approximation. Even in the densest parts of the oceans, light does not travel much slower than it would in a perfect vaccuum. It is simply scattered faster and thus is dimmed and travels less distance in a concentrated enough form to be visible.

  3. A vacuum can’t be perfect, because it has, at minimum, the various fields for the various particles, even if those fields are in their lowest-energy state. This is in the “quantum field theory” paradigm. Obviously if you’ve got light, you must have some positive excitation of the electromagenetic (or photon) field, although this excitation can be made arbitrarily small.
    What is meant by “speed of light in a vacuum” is the speed light would have in a volume with no particles in it. It is possible to make a laboratory vacuum of a cubic millimeter or so that has no particles in it. That volume would still contain all the zero-energy fields and virtual particles that go along with them. You could extrapolate your measurement at various pressures down to zero pressure, since at low pressure the speed of light is a linear function of pressure. This is not a very hard thing to do, because the needed correction is very small—after all the speed of light in air is only slightly smaller than the speed of light in a vacuum.

  4. Light propagates just fine in a vacuum. It’s an electromagnetic wave and doesn’t need any medium for propagation.
    Propagation through glass and many other media can slow it down, but nothing known can speed it up. The speed is a fundamental property of the interaction between the electric and magnetic fields.
    The metric definition of c is really a definition of the meter, using the speed of light as the reference.

  5. What you hitting at is a very important aspect of physics. In class we talk about perfect conductors, perfect vacuum, etc, while in reality these systems never exist.
    When we measure the speed of light there is always a small measure of experiential error. While we can’t make a perfect vacuum we can come close are the error caused by the imperfect vacuum is taken into account. If you look in textbooks they say the speed of light is 299,792,458 m/s but if you look at the papers where experimenters actually measure c, they include an error term which takes into account things like imperfect vacuums.


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