# Speed of Light in a Vacuum **URL:** \ **Category:** Factual Questions\ **Created:** [December 3, 2000, 6:08am UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801 "2000-12-03T06:08:57Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [December 3, 2000, 6:08am UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/1 "2000-12-03T06:08:57Z")
The speed of light in a vacuum is constant and the apparent slowing down of light as it traverses a given medium is because of the time it takes for the light to interact (be absorbed and re-emitted) with the particles that makes up the medium. If the above is correct, here are my questions: 1. Is the light travelling through a medium interacting exlusively with the medium’s electrons? Or does light also interact, say, with its protons? 2. When we say that the speed of light in a vacuum is constant, are we referring to a _classical_ vacuum or a _quantum_ vacuum? 3. Assuming that light does interact with the virtual components that comprise the quantum vacuum, are all quantum vacuums the same in this regard? If not, does the speed of light vary (in a measurable way) depending on the quantum vacuum at hand? Thank you (and, as usual, I apologize if these questions are better considered as proof of my fundamental lack of understanding of the subject). ---
**Author:** ![Astroboy14](https://avatars.discourse-cdn.com/v4/letter/a/df705f/32.png) [@Astroboy14](https://boards.straightdope.com/u/Astroboy14)\ **Post date:** [December 3, 2000, 11:16am UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/2 "2000-12-03T11:16:02Z")
**KarlGauss** , I’m pretty sure that light slows down as it passes through a different medium is _NOT_ due to the photons being absorbed and then re-transmitted… but I can’t tell you why! Hoping that someone else will answer this, I’m sending it back to the top! ---
**Author:** ![galen](https://avatars.discourse-cdn.com/v4/letter/g/ecc23a/32.png) [@galen](https://boards.straightdope.com/u/galen)\ **Post date:** [December 3, 2000, 3:57pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/3 "2000-12-03T15:57:24Z")
When photons interact with atoms, it is because they are absorbed and re-emitted by electrons. I can’t think of any reaction where you would shine a light on an atom and have the photons react with protons. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 3, 2000, 7:11pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/4 "2000-12-03T19:11:30Z")
Photons can, in principle, interact with any charged particle, including protons. However, for light of reasonable energies, it’s going to interact with the electrons a lot more strongly than the protons. For high-energy light like gamma rays, the protons would be significant. Ordinarily, photons can only be absorbed at very specific energies, but there’s some slack allowed, depending on how long the state lasts between absorbtion and re-emission. If the photon is re-emitted quickly enough, it can have almost any energy. As for the question about the quantum vacuum, I don’t think that it’s meaningful to talk about different kinds of quantum vacuum. I could be mistaken on that, though. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [December 3, 2000, 7:37pm UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/5 "2000-12-03T19:37:32Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\* > As for the question about the quantum vacuum, I don’t think that it’s meaningful to talk about different kinds of quantum vacuum. \*\* The reason I wondered whether there might be different quantum vacuums is because there is a fundamental randomness, I suppose, in the virtual particles that pop in and out of existence (their numbers and type). So, the quantum vacuum should fluctuate in some sense, no? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 4, 2000, 5:46am UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/6 "2000-12-04T05:46:19Z")
Sure, it fluctuates, but by the time you get down to the scale where you could see it fluctuating, you’re going to have a very hard time even defining what you mean by the speed of light. Over large scales (like, say, the radius of an atomic nucleus), it all averages out. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [December 4, 2000, 6:46am UTC](https://boards.straightdope.com/t/speed-of-light-in-a-vacuum/43801/7 "2000-12-04T06:46:51Z")
If you start setting up things to interfere with the production of virtual pairs you probably also mess with light speed. Some devices to do that can get pretty big: [http://www.quantumfields.com/](http://www.quantumfields.com/)