# Does the speed of light still = c...when NOT observed? **URL:** \ **Category:** Factual Questions\ **Created:** [January 9, 2026, 4:09am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499 "2026-01-09T04:09:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [January 9, 2026, 4:09am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/1 "2026-01-09T04:09:15Z")
There’s plenty of measurables which only take on a value when observed. Something with an indeterminate value due to superposition…until the act of observation collapses a wavefunction, thus locking in a value. can a photon or graviton have a…supervelocity…up until said quantum interacts with something? ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [January 9, 2026, 4:50am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/2 "2026-01-09T04:50:43Z")
In what theoretical environment would it not be interacting with anything? ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [January 9, 2026, 5:08am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/3 "2026-01-09T05:08:27Z")
I suppose one could imagine that a photon traveling for a light year though interstellar space at some point moved faster than the speed of light but then later moved slower than the speed of light such that it got to its destination in exactly one year, but then every photon would have to do so in a precise ratio that resulted in a speed exactly equal to the speed of light when it got to its final destination (which incidentally it did not know when it started). I have great difficulty imagining what model would allow such motion, and even if there was such a model it would be impossible to test since by definition the photon was not observed anywhere along its path where its speed could be checked. So Occam’s razor heavily favors the idea that photons in a vacuum move at the speed of light whether observed or not. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 9, 2026, 1:24pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/4 "2026-01-09T13:24:44Z")
So If a Photon Falls Superluminally in a Forest and Nobody Sees It, Does it Violate General Relativity? ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 9, 2026, 2:16pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/5 "2026-01-09T14:16:19Z")
There once was a man who said: "God Must think it exceedingly odd If he finds that this tree Continues to be When there’s no one about in the quad. ================ Dear Sir, Your astonishment’s odd. I am always about in the Quad. And that’s why the tree Will continue to be Since observed by Yours faithfully, God ---
**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:** [January 9, 2026, 2:54pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/6 "2026-01-09T14:54:08Z")
On a somewhat-related note, we have never measured the “one-way” speed of light. We have only measured the “two-way” (round trip) speed of light using a reflector. AFAIK it’s possible light has different speeds in each direction during the round trip. ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [January 9, 2026, 3:58pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/7 "2026-01-09T15:58:05Z")
The speed of light is the speed of light whether or not you are looking at it. If you are traveling at 90% of the speed of light and shine a beam of light out of the forward window, it is not traveling at 90% of the speed of light plus the speed of light, it is traveling at the speed of light, and that’s true whether it is being observed or not. ---
**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:** [January 9, 2026, 5:59pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/8 "2026-01-09T17:59:18Z")
Isn’t the constancy of the speed of light the base of relativity theory? Does quantum mechanics have anything to say about it? Yes, photons can be entangled, but aren’t the measurements after observation quantum properties, not speed? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [January 9, 2026, 6:14pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/9 "2026-01-09T18:14:57Z")
> [@Exapno\_Mapcase](#): > > Does quantum mechanics have anything to say about it? Oh yeah. Definitely. You should look up the debates between Einstein and Niels Bohr. Einstein had a major problem with quantum mechanics specifically because parts of it violate some of the fundamental principles of Einstein’s theory of relativity. Bohr claimed to have won the debates, and modern physics is built on the idea that quantum mechanics was right and Einstein was wrong, but if you really look into it, it looks like Bohr fundamentally misunderstood Einstein’s objections. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [January 9, 2026, 6:41pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/10 "2026-01-09T18:41:53Z")
> [@Exapno\_Mapcase](#): > > Isn’t the constancy of the speed of light the base of relativity theory? Does quantum mechanics have anything to say about it? Yes, photons can be entangled, but aren’t the measurements after observation quantum properties, not speed?   Right. The salient points being (a) the speed of light in a vacuum, equivalent to the speed of causality, is a universal constant and a fundamental property of spacetime, and (b) it has nothing to do with quantum superposition and the apparent collapse thereof upon detection/observation. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [January 9, 2026, 6:52pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/11 "2026-01-09T18:52:42Z")
> [@engineer\_comp\_geek](#): > > modern physics is built on the idea that quantum mechanics was right and Einstein was wrong To be perfectly clear, I assume what you’re saying is that modern **quantum mechanics** shows that Einstein’s skepticism was wrong about the probabilistic nature of quantum mechanics and things like quantum entanglement. Of course his special and general theories of relativity are still sound, though in extreme conditions like extreme gravity at quantum scales like black hole singularities there’s an unresolved disconnect between general relativity and quantum mechanics, so I guess it can be argued that both are incomplete theories. ---
**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:** [January 9, 2026, 7:16pm UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/12 "2026-01-09T19:16:13Z")
> [@engineer\_comp\_geek](#): > > Oh yeah. Definitely. What does QM say about the constant speed of light? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [January 10, 2026, 1:31am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/13 "2026-01-10T01:31:49Z")
You are asking about “virtual photons” in quantum field theory. Indeed, such a particle does not satisfy the usual relationship between energy and momentum; it has have arbitrarily high momentum, or negative energy, and you can consider virtual photons with a definite momentum, and, therefore, an uncertain position filling all of space… however, all of this stuff to compute particle interactions does not make any real light or information travel faster than the speed of light. ---
**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:** [January 10, 2026, 1:41am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/14 "2026-01-10T01:41:25Z")
The speed of light isn’t an “observable”, in the sense that you’re referring to. It’s not something that can have different values in different experiments; it’s always what it is. ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [January 10, 2026, 1:57am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/15 "2026-01-10T01:57:12Z")
I believe that when they made measurements of the speed of light, they did it through a series of mirrors, but they also did it at different times of the day. That is, the earth would have at least rotated on its axis and also orbited the sun by some amount, so the light could never have been traveling in the same direction universe-wise. They always got the same answer, so it’s hard to see how the simple direction of the light beam could have been a factor. If I understand correctly. ---
**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:** [January 10, 2026, 1:59am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/16 "2026-01-10T01:59:36Z")
QM is based, in part, on special relativity. So there is no conflict in that respect. Einstein was pointing out the problem with causality as expressed in the EPR paradox. Bohr never seemed to grasp that there was a problem. Bohr was a curious personality at best. If one was being charitable, he seemed to be in awe of his own genius. One could be forgiven for being less charitable. That he didn’t get EPR is not in any way a reflection of the remainder of physicists. The path integral approach leads to one interpretation of QM, the sum of histories model. That particular view makes the paths of the virtual particles real entities, including off-shell super-luminal particles, rather than a mathematical fiction. But, in general, path integrals are just one of a range of mathematical tools that cover the ground and provide correct answers. There isn’t any good reason to claim that it is the one true representation of reality. Popular descriptions of QM seem to latch onto the idea of sum of histories as being the accepted canonical truth, which it isn’t. Far from it. ---
**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:** [January 10, 2026, 2:02am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/17 "2026-01-10T02:02:47Z")
What @Crafter_Man is saying is strictly speaking true, because you always have to have the light go out from your timing device and then come back to the same device. One can construct a model where the light is always going out faster and then back slower for the same total time, or the like. But those models are stupid. Any universe that worked that way would be exactly equivalent to one where the speed of light is the same in every direction. In fact, you could convert such a model to one where the speed of light is the same in every direction, just by changing how you define distance and time. It’s not possible to define those things without definitional assumptions about how the speed of light in different directions compares (most simply, by assuming it’s the same). ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [January 10, 2026, 5:00am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/18 "2026-01-10T05:00:06Z")
In any case’ “the speed of light” is a misnomer for the speed limit of the universe; rather it is that light, having no mass, is one of the few things permitted to move at that speed. While I’m no expert past that, I think gravity is the only other force/entity/whatever that can too? ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [January 10, 2026, 5:14am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/19 "2026-01-10T05:14:45Z")
Well, yes, but it’s still seems to be a bit of a misunderstanding. If ‘c’ is not the value we believe when not observed, it doesn’t change a lot. Stuff outside our observable universe is outside our observable universe. A simple reading of the OP, which may or may not be correct, seems to imply that “observation” is understood to be some sort of direct, explicit measurement as in QM experiments, so “unobserved” is implied to be stuff that goes on when we’re not actively watching (say behind our backs or something). But this is not really what we mean by observable or not when it comes to ‘c’. ---
**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:** [January 10, 2026, 5:52am UTC](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499/20 "2026-01-10T05:52:53Z")
We use the speed of light really as a matter of accidents of history. Light was what was measured, and that gave us the name. Unification of EM in special relativity made it a natural thing to use. Mostly it doesn’t matter, but occasionally causes confusion. The better answer consonant with relativity is the speed of _causality_. By a lucky coincidence ( or accident of history) we use _ **c** _ to denote this speed already. Personally I would love to have a push to use c = causality everywhere. Any particle with no mass is required to travel at exactly c. So photons. Gravitons if you must. The Higgs field tends to wreck things for others. [Next page](https://boards.straightdope.com/t/does-the-speed-of-light-still-c-when-not-observed/1026499.md?page=2)