# Speed of light in water **URL:** \ **Category:** Factual Questions\ **Created:** [January 24, 2000, 12:18am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758 "2000-01-24T00:18:00Z")\ **Posts on this page:** 13\ **Page:** 3
**Author:** ![Zor](https://avatars.discourse-cdn.com/v4/letter/z/848f3c/32.png) [@Zor](https://boards.straightdope.com/u/Zor)\ **Post date:** [January 30, 2000, 2:46am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/41 "2000-01-30T02:46:00Z")
Geezer, I’m not all that familiar with BEC, so I can’t claim to know what the exact definition of it is. However, it seems like the folks at Boulder Colorado were indeed the first team to produce BEC in 1995. Their achievement is mentioned here… [http://www.sciam.com/explorations/102097nobel/hall.html#physics](http://www.sciam.com/explorations/102097nobel/hall.html#physics) The above article also discusses the 1997 Nobel Prize award, which was the one that made BEC popular I think. ---
**Author:** ![Geezer](https://avatars.discourse-cdn.com/v4/letter/g/49beb7/32.png) [@Geezer](https://boards.straightdope.com/u/Geezer)\ **Post date:** [January 31, 2000, 1:09am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/42 "2000-01-31T01:09:00Z")
> [@](#): > > jab1 > > I assume, therefore, one could see Cerenkov radiation if a particle passed through glass faster than light would? > > Geezer, could you remind me what the Moon Illusion is that you mentioned a while back? Thanks. Yes, Cerenkov radiation occurs in glass as well as water. The Moon Illusion is the the common illusion that the moon appears larger when seen near the horizon than when seen higher in the sky. It _is_ and illusion: the moon is not closer, and it is not magnified by the atmosphere. The illusion occurs because the eye/brain system perceives the high sky to be closer than the horizon sky. With no forefield references the high sky appears to you to be some distance away–on the order of fifty feet. The horizon sky looks farther away because you perceive the horizon is as distant, and the sky is even farther. Since the moon _appears_ to be more distant at the horizon than when at the zenith, the brain infers that it must be larger when seen at the horizon. Hence the illusion. The Ponzo illusion is similar. See [the IBM page](http://www.research.ibm.com/news/detail/moon_illusion.html). Or this [press release](http://eagle.almaden.ibm.com/almaden/moon_pr.html). > [@](#): > > Zor > Geezer, I’m not all that familiar with BEC, so I can’t claim to know what the exact definition of it is. However, it seems like the folks at Boulder Colorado were indeed the first team to produce BEC in 1995. I quote Fritz London in his book Superfluids, Volume II, originally published in 1954 by Wiley and reprinted by Dover in 1964: “Noticing this correspondence with respect to [calculations of BEC and observations of HeII], the author advanced the hypothesis (1) that the transition of liquid helium I into liquid helium II might be caused by the condensation mechanism of the degenerating Bose-Einstein gas…”(p. 59) The reference (1) is F. London, Nature, 141, 643 (1938); Phys. Rev., 54, 947 (1938). The problem with superfluid helium (helium II) as a BEC is simply that it isn’t a _pure_ condensate. So maybe you’re right that it’s a matter of definitions. The condensates being produced in magneto-optical traps using laser cooling are pretty close to the unadulterated real thing. Like I said: we made slush, those guys make ice cubes. So what they’re doing is special, and historically noteworthy. But I still think they’re stretching it a bit to say they’ve produced the “first” BEC. By the way, I think “slow glass” is an elegant literary device. But I agree with CurtC that the index of refraction problem would render it impractical. Still… tres cool idee. ---
**Author:** ![Geezer](https://avatars.discourse-cdn.com/v4/letter/g/49beb7/32.png) [@Geezer](https://boards.straightdope.com/u/Geezer)\ **Post date:** [January 31, 2000, 1:13am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/43 "2000-01-31T01:13:00Z")
Okay, try that again… > [@](#): > > jab1 > > I assume, therefore, one could see Cerenkov radiation if a particle passed through glass faster than light would? > > Geezer, could you remind me what the Moon Illusion is that you mentioned a while back? Thanks. Yes, Cerenkov radiation occurs in glass as well as water. The Moon Illusion is the the common illusion that the moon appears larger when seen near the horizon than when seen higher in the sky. It _is_ and illusion: the moon is not closer, and it is not magnified by the atmosphere. The illusion occurs because the eye/brain system perceives the high sky to be closer than the horizon sky. With no forefield references the high sky appears to you to be some distance away–on the order of fifty feet. The horizon sky looks farther away because you perceive the horizon is as distant, and the sky is even farther. Since the moon _appears_ to be more distant at the horizon than when at the zenith, the brain infers that it must be larger when seen at the horizon. Hence the illusion. The Ponzo illusion is similar. See [the IBM page](http://www.research.ibm.com/news/detail/moon_illusion.html). Or this [press release](http://eagle.almaden.ibm.com/almaden/moon_pr.html). > [@](#): > > Zor > Geezer, I’m not all that familiar with BEC, so I can’t claim to know what the exact definition of it is. However, it seems like the folks at Boulder Colorado were indeed the first team to produce BEC in 1995. I quote Fritz London in his book Superfluids, Volume II, originally published in 1954 by Wiley and reprinted by Dover in 1964: “Noticing this correspondence with respect to [calculations of BEC and observations of HeII], the author advanced the hypothesis (1) that the transition of liquid helium I into liquid helium II might be caused by the condensation mechanism of the degenerating Bose-Einstein gas…”(p. 59) The reference (1) is F. London, Nature, 141, 643 (1938); Phys. Rev., 54, 947 (1938). The problem with superfluid helium (helium II) as a BEC is simply that it isn’t a _pure_ condensate. So maybe you’re right that it’s a matter of definitions. The condensates being produced in magneto-optical traps using laser cooling are pretty close to the unadulterated real thing. Like I said: we made slush, those guys make ice cubes. So what they’re doing is special, and historically noteworthy. But I still think they’re stretching it a bit to say they’ve produced the “first” BEC. By the way, I think “slow glass” is an elegant literary device. But I agree with CurtC that the index of refraction problem would render it impractical. Still… tres cool idee. ---
**Author:** ![Saltire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saltire/32/553_2.png) [@Saltire](https://boards.straightdope.com/u/Saltire)\ **Post date:** [January 31, 2000, 4:39am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/44 "2000-01-31T04:39:00Z")
Cerenkov radiation is also known as ‘braking radiation’ because it is due to deceleration. The particle is going too fast, so it slows to below the local speed of light. To do this it must lose energy, which means it emits photons. We see this as Cerenkov radiation. While it is imprecise to use such ‘classical’ ideas as deceleration when speaking of quantum entities, in this case it works. ---
**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\ **Post date:** [January 31, 2000, 1:57pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/45 "2000-01-31T13:57:00Z")
Great exposition, Geezer! > [@](#): > > The simple answer to the question is that the photons do move between the atoms at the speed of light in vacuum ©. Just to confuse the issue a little farther, I was re-reading Feynman’s QED over the weekend, and he stated that quantum electrodynamics includes a non-zero amplitude for photons traveling faster than or slower than the speed of light in vacuum. However, just as light effectively travels in straight lines in most circumstances, these amplitudes cancel for travel distances of any practical size (he didn’t say exactly what distance). I would guess that interatomic distances are travel distances of practical size. I don’t know if this is still the state of the art … * * * jrf ---
**Author:** ![Geezer](https://avatars.discourse-cdn.com/v4/letter/g/49beb7/32.png) [@Geezer](https://boards.straightdope.com/u/Geezer)\ **Post date:** [February 1, 2000, 1:49am UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/46 "2000-02-01T01:49:00Z")
Careful here, Saltire. “Braking radiation” is Bremsstrahlung, a different animal from Cerenkov radiation. In brehmsstahlung, a fast, light charged particle–typically an electron–radiates a photon when it is suddenly accelerated by passing near a much heavier charged particle–typically a nucleus. Brehmsstrahlung is the typical method of producing medical x-rays, for instance. You beam electons at a metal target and they radiate x-rays. (CRTs do it, too!) In Cerenkov radiation, it is the atoms of the material that radiate as a result of having been stimulated by the passing of the fast charged particle. Cerenkov radiation depends upon the speed of the particle being faster than the phase velocity of light in the material (the c-in-water we’ve talked about). Bremsstrahlung does not require this condition–although it does require relativistically fast electrons (or whatever) to get a significant amount of radiation. Just to be perfectly clear about one other point: there is no relativistic prohibition on speeds greater than the speed of light in a material medium, only on speeds greater than the speed of light in vacuum. The relativistic particle doesn’t _have_ to slow down to less than the local speed of light. This is not to say that there isn’t any “braking” involved in Cerenkov radiation. Clearly, as the particle loses energy to the surrounding atoms (which radiate it away as Cerenkov radiation), the particle must decelerate. But there’s also no reason that the energy can’t be continuously replaced by an applied electric field, maintaining its superluminal speed. Thank you for the compliment, JonF. I appreciate hearing it. As for Feynman and QED, well, um, it’s been too long. I think his point is that in order to get the speed of the photon to come out right the other possibilities have to cancel. It’s like in the sum-over-paths where all the unrealistic paths interfere with each other leaving only the “real” path. Virtual particles can disobey all kinds of laws because they don’t live long enough to get caught! ---
**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\ **Post date:** [February 1, 2000, 1:10pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/47 "2000-02-01T13:10:00Z")
The answer has already been given several times, but it may be worth trying to clarify a bit more. The situation you describe is not symmetrical. In order for the two clocks to start together, then move apart, then come back togeteher, there must be acceleration. The acceleration allows us to detect which one will run slower. If there was no acceleration, and at some time the clocks happened to be in the same place at the same time and indicate the same time reading, then they would be separated at all other times and could not be compared without getting into the transmission of a signal. * * * jrf ---
**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\ **Post date:** [February 1, 2000, 1:13pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/48 "2000-02-01T13:13:00Z")
Damn, replied in the wrong topic. * * * jrf ---
**Author:** ![gEEk](https://avatars.discourse-cdn.com/v4/letter/g/ed8c4c/32.png) [@gEEk](https://boards.straightdope.com/u/gEEk)\ **Post date:** [February 1, 2000, 4:32pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/49 "2000-02-01T16:32:00Z")
> [@](#): > > Originally posted by Alan Smithee: > **Oh, yeah, gEEk, the link you posted referred to an article in Nature, but your quote was from Scientific American. What issue did you find that in, so I can read it myself?** Unfortunately, I don’t have the full citation for that quote, I found it buried in another blurb that I have since lost. However, the June 1995 issue apparently has a BEC related article, and given the above date, may in fact be the one in question. Hope that helps. gEEK ---
**Author:** ![Geezer](https://avatars.discourse-cdn.com/v4/letter/g/49beb7/32.png) [@Geezer](https://boards.straightdope.com/u/Geezer)\ **Post date:** [February 2, 2000, 7:11pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/50 "2000-02-02T19:11:00Z")
Zor, I hope I’m not beating a dead horse here, but I don’t want to look like a crackpot who rejects the latest finings of reputable scientists. Here’s a [link](http://amo.mit.edu/~bec/intro/intro.html#bec) to one of the active groups working on BEC. It clears things up pretty well, with an acknowledgement of London’s early theoretical work, but also showing why what they are doing now is new and noteworthy. ---
**Author:** ![Zor](https://avatars.discourse-cdn.com/v4/letter/z/848f3c/32.png) [@Zor](https://boards.straightdope.com/u/Zor)\ **Post date:** [February 2, 2000, 11:41pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/51 "2000-02-02T23:41:00Z")
Hey, no problem here. Looks like another matter of who grabs the credit to me. It’s a choice between London, you, or the folks who just got it at 1995. I guess the squeeky wheel gets the oil then 🙂 ---
**Author:** ![howardsims](https://avatars.discourse-cdn.com/v4/letter/h/e8c25b/32.png) [@howardsims](https://boards.straightdope.com/u/howardsims)\ **Post date:** [February 2, 2000, 11:57pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/52 "2000-02-02T23:57:00Z")
Speaking of credit… HURRAY for me creating a topic that has gone into 2 pages!!! Who’s your daddy!!! ---
**Author:** ![Geezer](https://avatars.discourse-cdn.com/v4/letter/g/49beb7/32.png) [@Geezer](https://boards.straightdope.com/u/Geezer)\ **Post date:** [February 3, 2000, 2:19pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water/6758/53 "2000-02-03T14:19:00Z")
Well, Zor, _I_ certainly don’t deserve any credit. There were a zillion people doing superfluid helium in the fifties, sixties, and seventies (including Feynman). The point is that we were studying it because it represents a macroscopic quantum state, namely the Bose-Einstein condensate. I turned out to be a little more complicated than we’d hoped, though. But Howard, you can take all the credit you want! You do ask interesting questions. What’s next? [Previous page](https://boards.straightdope.com/t/speed-of-light-in-water/6758.md?page=2)