# All calculations are off?? **URL:** \ **Category:** Factual Questions\ **Created:** [July 6, 2013, 4:32am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773 "2013-07-06T04:32:25Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [July 6, 2013, 9:22pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/41 "2013-07-06T21:22:05Z")
> [@ZenBeam](#): > > So considering this, how is it that light travels more slowly through air? But does it really? IANAP, but this is one theory I’ve heard… A photon collides with an atom, the atom absorbs the energy, and then the atom emits a photon. This photon collides with \*another \*atom, the atom absorbs the energy, and then the atom emits a photon. And on and on. When the photon is traveling \*between \*atoms, it is traveling at a velocity of c. But when you measure the average velocity of the photon over a given distance, it is slower than c, because it takes time for each atom to adsorb and emit a photon. In other words, a photon is always traveling at c regardless of the medium. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 6, 2013, 9:39pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/42 "2013-07-06T21:39:50Z")
> [@Crafter\_Man](#): > > But does it really? > […] > When the photon is traveling between atoms, it is traveling at a velocity of c. But when you measure the average velocity of the photon over a given distance, it is slower than c, because it takes time for each atom to adsorb and emit a photon. In other words, a photon is always traveling at c regardless of the medium. Imagine you only ever measured it in air, and only ever got the slightly slower speed, and someone asked “What’s the true speed of light, the speed between atoms?” That’s analogous to what we’re asking about. The speed of light is only ever measured in the presence of virtual particles. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [July 7, 2013, 1:25am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/43 "2013-07-07T01:25:40Z")
> [@Crafter\_Man](#): > > A photon collides with an atom, the atom absorbs the energy, and then the atom emits a photon. This photon collides with \*another \*atom, the atom absorbs the energy, and then the atom emits a photon. And on and on. > > When the photon is traveling \*between \*atoms, it is traveling at a velocity of c. But when you measure the average velocity of the photon over a given distance, it is slower than c, because it takes time for each atom to adsorb and emit a photon. In other words, a photon is always traveling at c regardless of the medium. [The problems with this explanation](http://boards.straightdope.com/sdmb/showpost.php?p=16344434&postcount=4) came up in a different thread recently. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [July 7, 2013, 1:40am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/44 "2013-07-07T01:40:50Z")
> [@KarlGauss](#): > > Assuming that light (photons) can interact with all charged particles, and assuming that the composition of the “quantum vacuum” is everywhere changing, dynamic, and random with virtual (charged) particles “popping up and disappearing” everywhere and everywhen, would we not, therefore, _expect_ the speed of light to be inconstant and essentially random (albeit with most values clustered tightly around ‘c’)? This is exactly the sort of question that requires precise distinct between the physical constant _c_ and, separately, the speed that light travels through any particular medium. The speed that light travels might change, but the speed of light (i.e., the annoyingly named constant _c_) needn’t. Consider this experiment: Measure the energy released from the decay of a radioactive nucleus (calorimeter) and measure the mass difference of the nucleus before and after the decay (mass spectrometer). Divide those two numbers to obtain _c_[sup]2[/sup], since _E_=_mc_[sup]2[/sup], without any reference to light or the properties of the vacuum at all. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 7, 2013, 2:00am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/45 "2013-07-07T02:00:10Z")
> [@Crafter\_Man](#): > > But does it really? > > IANAP, but this is one theory I’ve heard… > > A photon collides with an atom, the atom absorbs the energy, and then the atom emits a photon. This photon collides with \*another \*atom, the atom absorbs the energy, and then the atom emits a photon. And on and on. > > When the photon is traveling \*between \*atoms, it is traveling at a velocity of c. But when you measure the average velocity of the photon over a given distance, it is slower than c, because it takes time for each atom to adsorb and emit a photon. In other words, a photon is always traveling at c regardless of the medium. Sometimes it is possible to describe light as if it were made of photons which behave like relativistic, subatomic ‘billiard balls’, but refraction really, really doesn’t lend itself to this model of light. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [July 7, 2013, 2:08am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/46 "2013-07-07T02:08:05Z")
> [@theinscrutabledoctorknow](#): > > Yes Indeed it is 3.99999… !! > This obviously happened the moment they removed the penny from Canadian circulation. Welcome to the Straight Dope. I wasn’t sure about your first post, but with this one I think you’ll do just fine here. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 7, 2013, 2:14am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/47 "2013-07-07T02:14:53Z")
> [@iiandyiiii](#): > > welcome to the straight dope. I wasn’t sure about your first post, but with this one i think you’ll do just fine here. 4 ≠ 3.999…!! 😉 ---
**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:** [July 7, 2013, 3:26am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/48 "2013-07-07T03:26:58Z")
> [@Pasta](#): > > This is exactly the sort of question that requires precise distinct between the physical constant _c_ and, separately, the speed that light travels through any particular medium. The speed that light travels might change, but the speed of light (i.e., the annoyingly named constant _c_) needn’t. > > Consider this experiment: Measure the energy released from the decay of a radioactive nucleus (calorimeter) and measure the mass difference of the nucleus before and after the decay (mass spectrometer). Divide those two numbers to obtain _c_[sup]2[/sup], since _E_=_mc_[sup]2[/sup], without any reference to light or the properties of the vacuum at all. Thank you. You have a wonderful way of explaining things; the _essence_ of things. Much appreciated (and admired). ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [July 7, 2013, 5:27am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/49 "2013-07-07T05:27:59Z")
So, I think we still remain unclear about the question about the speed of photons in vacuum versus c, and as **ZenBeam** noted, also the speed of other massless particles. There are clearly some subtleties I had never thought about. And the wretched nomenclature does seem to get in the way of a clear discussion of the question. So, c is 1, is what we understand as c from SR. It is invariant. We have **ls** - the speed of photons in vacuum. We might have **gs** - the propagation speed of gravity (perhaps the speed of a graviton). Perhaps we can have **ts** - the speed of a thingyon - an as yet unknown massless particle. (Great shame it can’t be a neutrino anymore.) Does the existence of virtual particles in the vacuum mean that **ls** is intrinsically ever so slightly less than c? If so, how much? Could **gs** therefore be different to **ls** in a very subtle manner? Heck, if we ever get a theory of quantum gravitation might there be equivalent tricks in the vacuum that mean gravity propagates every so slightly less than c, or does this actually make no sense? ---
**Author:** ![Bremidon](https://avatars.discourse-cdn.com/v4/letter/b/ac91a4/32.png) [@Bremidon](https://boards.straightdope.com/u/Bremidon)\ **Post date:** [July 7, 2013, 3:57pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/50 "2013-07-07T15:57:54Z")
> [@Chronos](#): > > That’s a more subtle question than you might realize. It’s difficult to say something like “photons have no mass”, scientifically speaking, because we can never prove it observationally or experimentally. All anyone can ever do is find an upper bound for the photon’s mass, and then push that upper bound down further and further. So while most physicists believe that the photon is actually massless, it’s not really what you can consider a scientific truth. > > And regardless of their beliefs on the matter, I think you would find that the vast majority of physicists consider experiments of that nature to be a worthwhile endeavor, so long as one doesn’t devote one’s entire career to them. Even when we think we know something, we must always test it. > In technical contexts, c is just c, and is not referred to as “the speed of light”. Even if light really does travel at c (as seems quite likely), it’s not the only thing that does, and the significance of c is in most contexts unrelated to the fact that light, or anything in particular, happens to travel at that speed. > > In lay contexts, there’s really no hope of a change, until and unless it’s actually proven that light doesn’t travel at c (and probably not even then). “The speed of light” has just gotten too firmly embedded into the popular lexicon. I’ve seen attempts to relabel it as “Einstein’s constant”, but I fear such attempts are doomed to failure. > > EDIT: And **Exapno** goes and sneaks in such an attempt while I’m posting. I really do applaud the effort… Thanks guys. I was aware that the question is subtle, and I agree that the tests are worthwhile; I just was surprised at how often I run across tests of this nature. As to “c” and “C” and “speed of light”: I suppose you’re right. Disappointing, really. It would help to remove a lot of the misconceptions out there if “light” were not treated as something ultra-special. ---
**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:** [July 7, 2013, 4:18pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/51 "2013-07-07T16:18:05Z")
> [@Asympotically\_fat](#): > > 4 ≠ 3.999…!! 😉 Of course not. 3.999…!! = 8. Or 6.204484e+23, depending on how you interpret the double factorial. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [July 7, 2013, 9:28pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/52 "2013-07-07T21:28:56Z")
> [@ZenBeam](#): > > Would there be any reason to expect the measured speed of light in a vacuum, the measured speed of gravity in a vacuum (if/when such measurements could be made sufficiently accurately), and the value of _c_ in special relativity to all be precisely equal? (For example, is there some kind of cancellation occurring that drops the perturbation to zero?) I guess I would think of the SR value to be the fundamental one, with the other two possibly slightly different, due to higher order contributions, entirely analogous to the speed of light being slightly smaller in air. But I wouldn’t take the _measured_ value of the speed of light to be fundamental, over the constant of special relativity. The vacuum in SR is Minkowsky space, and in that space, light propagates at c; if you introduce the Casimir plates, you really just create a different vacuum, in which light may propagate faster. The Casimir vacuum isn’t really ‘vanilla SR’ anymore, and that you can obtain a different speed for light might not be any more shocking than the same thing happening in a non-inertial frame of reference. (But I’m not really sure about this point.) The speed of light in (Minkowski) vacuum in QED must equal c, because you put it into the theory by construction; likewise, the speed of gravity, if gravity is described by a relativistic QFT (or some generalization true to the same principles, like string theory) must be c. That’s because all effects at spacelike separation commute, which is an assumption you include into the theory from the start. ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [July 7, 2013, 11:12pm UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/53 "2013-07-07T23:12:43Z")
> [@Try2B\_Comprehensive](#): > > What force is acting on photons to slow them down? There isn’t any ‘ether’. What makes you so sure? 😉 (signed) **dropzone** , the SDMB’s sole proponent of ether. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [July 8, 2013, 12:18am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/54 "2013-07-08T00:18:11Z")
> [@dropzone](#): > > What makes you so sure? 😉 > > (signed) > **dropzone** , the SDMB’s sole proponent of ether. Knock, knock. Who’s there? Ether. Ether who? Ether Bunny. Knock, knock. Who’s there? Ether. Ether who? Ether Bunny. Knock, knock. Who’s there? Ether. Ether who? Ether Bunny. Knock, knock. Who’s there? Orange. Orange who? Orange you glad it’s not another Ether Bunny? ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [July 8, 2013, 12:34am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/55 "2013-07-08T00:34:35Z")
> [@Exapno\_Mapcase](#): > > Knock, knock. > Who’s there? > Ether. > Ether who? > Ether Bunny. > Knock, knock. > Who’s there? > Ether. > Ether who? > Ether Bunny. > Knock, knock. > Who’s there? > Ether. > Ether who? > Ether Bunny. > Knock, knock. > Who’s there? > Orange. > Orange who? > Orange you glad it’s not another Ether Bunny? Knock, knock. Who’s there? Cargo. Cargo who? Cargo “beep beep”, run over Ether Bunny. Knock, knock. Who’s there? Nome. Nome who? Nome ore Ether Bunny. My parents used to tell me all of these when I was a kid…they claimed that they’d heard them on Jack Paar’s Tonight Show. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [July 8, 2013, 12:39am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/56 "2013-07-08T00:39:50Z")
> [@dropzone](#): > > What makes you so sure? 😉 > > (signed) > **dropzone** , the SDMB’s sole proponent of ether. [The most famous negative experimental result in the history of physics.](http://Galileo.phys.Virginia.EDU/classes/109N/lectures/michelson.html) (Well, a series of negative results, because a good experimentalist never suspects the theory’s wrong after the first go.) (I realize you’re joking, but I happen to like that website.) ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [July 8, 2013, 12:50am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/57 "2013-07-08T00:50:13Z")
I think that every time you use your GPS, you are implicitly measuring the speed of light. And it isn’t changing since the GPSs are still working. I once heard a talk by a physicist who said that if general relativity weren’t taken into account, the GPS would drift by close to miles per day. General relativity causes the clocks in orbit to run slightly more slowly and I imagine that is some miserable fraction of 1%. The OP claims that the speed of light is now more than 1% slower than it was in 1958. Absurd! Incidentally, the Michaelson-Morely experiment, which I think was done about 125 years ago was carried out with an interferometer. ---
**Author:** ![TubaDiva](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tubadiva/32/507_2.png) [@TubaDiva](https://boards.straightdope.com/u/TubaDiva)\ **Post date:** [July 8, 2013, 1:26am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/58 "2013-07-08T01:26:55Z")
\*\*UPDATE: Pasta did not actually do this deliberately – he cut and pasted from Google and did not see the offensive portion of the link. He did nothing wrong at all. We did not understand what was going on here, now we do and we’re working on the problem. Warning is of course rescinded.\*\* > [@Pasta](#): > > [The problems with this explanation](http://boards.straightdope.com/sdmb/showpost.php?p=16344434&postcount=4) came up in a different thread recently. **MODERATOR WARNING** I have edited your link to remove the offending portion. The first rule of the Straight Dope is “don’t be a jerk.” Playing stupid games like altering quote links to display in a way that is offensive to some people is being a jerk. **This is a formal warning.** ---
**Author:** ![Try2B\_Comprehensive](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/try2b_comprehensive/32/65_2.png) [@Try2B\_Comprehensive](https://boards.straightdope.com/u/Try2B_Comprehensive)\ **Post date:** [July 8, 2013, 1:43am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/59 "2013-07-08T01:43:39Z")
> [@dropzone](#): > > What makes you so sure? 😉 > > (signed) > **dropzone** , the SDMB’s sole proponent of ether. Heh, I did put ether in quotes. There is [vacuum energy](http://en.wikipedia.org/wiki/Vacuum_energy), which has some resemblance to a medium of space, only it doesn’t really act like ether now does it? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 8, 2013, 2:07am UTC](https://boards.straightdope.com/t/all-calculations-are-off/662773/60 "2013-07-08T02:07:19Z")
> [@Half\_Man\_Half\_Wit](#): > > The vacuum in SR is Minkowsky space, and in that space, light propagates at c; if you introduce the Casimir plates, you really just create a different vacuum, in which light may propagate faster. The Casimir vacuum isn’t really ‘vanilla SR’ anymore, and that you can obtain a different speed for light might not be any more shocking than the same thing happening in a non-inertial frame of reference. (But I’m not really sure about this point.) You’re not correctly distinguishing between c and the speed of light here. The Lorentz equations use **c**. Light propagates at **ls** , to use **Francis Vaughn** ’s notation. > [@](#): > > The speed of light in (Minkowski) vacuum in QED must equal c, because you put it into the theory by construction; likewise, the speed of gravity, if gravity is described by a relativistic QFT (or some generalization true to the same principles, like string theory) must be c. That’s because all effects at spacelike separation commute, which is an assumption you include into the theory from the start. You can’t just assume c = **ls** here, because that is what we are questioning. Perhaps the correct answer is **ls** = c \* (1 - 1e-30), but you’re currently unable to measure that difference. Your precision isn’t good enough to distinguish between c and **ls**. [Previous page](https://boards.straightdope.com/t/all-calculations-are-off/662773.md?page=2) [Next page](https://boards.straightdope.com/t/all-calculations-are-off/662773.md?page=4)