# Why does light travel? **URL:** \ **Category:** Factual Questions\ **Created:** [March 1, 2020, 8:45am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744 "2020-03-01T08:45:29Z")\ **Posts on this page:** 20\ **Page:** 4
**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:** [March 6, 2020, 8:36pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/61 "2020-03-06T20:36:35Z")
A neutron-antineutron reaction will release much more energy than fusion, but a sizeable fraction of that energy will be in neutrinos, which don’t have much effect. What’s left will still be significantly more energetic than fusion, though. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [March 6, 2020, 9:57pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/62 "2020-03-06T21:57:24Z")
> [@Jim\_Peebles](#): > > Light is a wave. A wave is a type of motion. You can’t have a wave without motion. Things that are moving travel. This is obviously wrong if you’ve ever seen a standing wave, which is what an electron is when it’s in an atomic orbital, or when a guitar string is vibrating in one of its vibrational modes. It’s the exact same kind of math, except the electron orbitals are spherical harmonics. Really, thinking in terms of particles is kind of 19th Century; the central object of study is fields, which vibrate, and which sometimes vibrate in a vibrational mode and sometimes don’t. [This has direct relevance to quantum physics](https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/): > [@](#): > > Analogy time (and a very close one mathematically); think about a child’s swing. If you give it a shove and let it go, it will swing back and forth with a time period that is always the same, no matter how hard was the initial shove you gave it. This is the natural motion of the swing. Now compare that regular, smooth, constant back-and-forth motion to what would happen if you started giving the swing a shove many times during each of its back and forth swings. Well, the swing would start jiggling around all over the place, in a very unnatural motion, and it would not swing smoothly at all. The poor child on the swing would be furious at you, as you’d be making his or her ride very uncomfortable. This unpleasant jiggling motion — this disturbance of the swing — is different from the swing’s natural and preferred back-and-forth regular motion just as a “virtual particle” disturbance is different from a real particle. If something makes a real particle, that particle can go off on its own across space. If something makes a disturbance, that disturbance will die away, or break apart, once its cause is gone. So it’s not like a particle at all, and I wish we didn’t call it that. When you put waves first, the Heisenberg Uncertainty Principle becomes trivially easy to understand: It’s exactly analogous to the fact a sharp drum hit has no defined frequency, whereas a pure tone with a defined frequency must occur over a span of time and so has no sharply-defined moment when it occurred. Think about frequency as momentum and duration as position and you’ve just recovered a real quantum conjugate pair. ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [March 6, 2020, 10:15pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/63 "2020-03-06T22:15:45Z")
Having studied liberal arts I have nothing to add to this discussion. But it does make me look back fondly on the days of [airplanes and treadmills](http://boards.straightdope.com/sdmb/showthread.php?t=348452). ---
**Author:** ![dougrb](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dougrb](https://boards.straightdope.com/u/dougrb)\ **Post date:** [March 6, 2020, 10:40pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/64 "2020-03-06T22:40:39Z")
> [@Derleth](#): > > When you put waves first, the Heisenberg Uncertainty Principle becomes trivially easy to understand: It’s exactly analogous to the fact a sharp drum hit has no defined frequency, whereas a pure tone with a defined frequency must occur over a span of time and so has no sharply-defined moment when it occurred. Think about frequency as momentum and duration as position and you’ve just recovered a real quantum conjugate pair. That’s the best explanation I’ve ever seen. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [March 6, 2020, 11:43pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/65 "2020-03-06T23:43:08Z")
> [@Jim\_Peebles](#): > > Light is a wave. A wave is a type of motion. You can’t have a wave without motion. Things that are moving travel. :dubious: You’ve never heard of a standing wave? It oscillates in time but whose peak amplitude profile does not move in space. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [March 6, 2020, 11:53pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/66 "2020-03-06T23:53:55Z")
> [@Qadgop\_the\_Mercotan](#): > > :dubious: > You’ve never heard of a standing wave? It oscillates in time but whose peak amplitude profile does not move in space. whoops, ninja’ed long ago, but the board crashed when I tried to edit. ☹ ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 7, 2020, 1:22am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/67 "2020-03-07T01:22:52Z")
> [@Jim\_Peebles](#): > > Light is a wave. A wave is a type of motion. You can’t have a wave without motion. Things that are moving travel. Hang onto this thought! I think you’re on to something here! You can’t have a wave without motion, yes. And you can’t have motion without something moving! Congratulations! You’ve just invented [_luminiferous aether_](https://en.wikipedia.org/wiki/Luminiferous_aether)! On a more constructive tangent: **Derleth** posted, a few posts above, a snippet quoted from Professor Matt Strassler, who has a fairly extensive web site of articles discussing particle physics issues in mostly lay terms that even **Jim Peebles** , and sometimes even **Senegoid** might be able to understand! For any lay person, or more-or-less lay person, these pages are a great beginner’s _must-read_ tutorial into these mysterious and mystical topics. Some good pages to get started there: [His home page.](https://profmattstrassler.com) [About This Site and How to Use It](https://profmattstrassler.com/about/about-this-site-and-how-to-use-it/) — Contains a bunch of links to other pages to get started with. **Derleth** ’s quote, with the analogy about the child’s swing, comes from the following page (third paragraph). In this page, Prof. Strassler laments that virtual particles were ever called that in the first place, since they’re not much like actual particles at all and he feels it’s just a confusing misnomer to call them that: [Virtual Particles: What are they?](https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/) ---
**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:** [March 7, 2020, 9:53am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/68 "2020-03-07T09:53:18Z")
> [@Derleth](#): > > This is obviously wrong if you’ve ever seen a standing wave, which is what an electron is when it’s in an atomic orbital, or when a guitar string is vibrating in one of its vibrational modes. It’s the exact same kind of math, except the electron orbitals are spherical harmonics. Not to be too nitpicky, but I think it’s a bit misleading to say that that’s what an electron _is_ when it’s in an orbital. The idea that the wave function directly describes the particle was considered early on in quantum mechanics, but (to the dismay of many) found not to work. Rather, the square of the wave function describes the probability of finding the electron—as a particle—in a certain location. Whether the wave function describes something real—known as the ‘psi-ontic’ view—or just our knowledge/expectations about the system—‘psi-epistemic’—is very much an open debate, and at any rate, the intuitive picture of an electron as a standing wave encircling the nucleus went out with Bohr’s model of the atom. > [@](#): > > When you put waves first, the Heisenberg Uncertainty Principle becomes trivially easy to understand: It’s exactly analogous to the fact a sharp drum hit has no defined frequency, whereas a pure tone with a defined frequency must occur over a span of time and so has no sharply-defined moment when it occurred. Think about frequency as momentum and duration as position and you’ve just recovered a real quantum conjugate pair. It makes the Uncertainty Principle trivially easy to understand, but only at the cost of sweeping what’s difficult to understand about it under the rug entirely. A sound wave (or another classical wave) can have a certain duration and a certain range of frequencies associated with it—in that case, these simply are its properties, and upon measurement, that’s what we’ll find. But a quantum system, like an electron, upon being measured, will always yield a sharp position, or a sharp momentum—which is, for a classical object, in direct contradiction to the spread-out wave picture. How uncertainty can be associated with something that, every time we look, has sharply defined properties—that’s the question. The frequency/duration analogy illustrates the mathematical relationship, but not why that math should apply in the first place, and that’s the tricky bit. To get the analogy right, you’d have to imagine beating a drum in such a way that, whenever it’s listened to in one way (for duration), will be a sharp crack, while when it’s listened to for tone, will be a pure frequency, without any change in the way the drum is struck. When you hear it such that the frequency is sharply defined, then sure, it has no clear duration; likewise, when you hear it such that its duration is sharp, then of course, it’s got no clear frequency. Moreover, if you listen to the same drum beat that had a defined frequency again, but listen for duration, it will now be a sharp crack—with no clearly defined frequency. But how can you hear the same ‘drum beat’ in both these contradictory ways? That’s the question, not how the mathematics of waves and Fourier transformation relates the two quantities. That’s just a relationship that was found to work, and isn’t mysterious at all. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [March 7, 2020, 12:34pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/69 "2020-03-07T12:34:31Z")
> [@Dana\_Scully](#): > > How can any kind of object have no mass? One form/definition of “mass” is “inertial mass”. From good old _F_ = _ma_, if you apply a force to something and see it accelerate a certain amount, you can figure out its mass. But light _can’t_ be accelerated. It’s _always_ going the same speed. So the idea of inertial mass is irrelevant. (We hope that inertial mass is the same thing as gravitational mass. And photons have funny properties when it comes to gravitational effects as well. Best overall to just put them into a separate category from particles with mass.) ---
**Author:** ![Jim\_Peebles](https://avatars.discourse-cdn.com/v4/letter/j/5f9b8f/32.png) [@Jim\_Peebles](https://boards.straightdope.com/u/Jim_Peebles)\ **Post date:** [March 7, 2020, 1:21pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/70 "2020-03-07T13:21:19Z")
> [@Senegoid](#): > > Hang onto this thought! I think you’re on to something here! > > You can’t have a wave without motion, yes. And you can’t have motion without something moving! Congratulations! You’ve just invented [_luminiferous aether_](https://en.wikipedia.org/wiki/Luminiferous_aether)! > > On a more constructive tangent: > > **Derleth** posted, a few posts above, a snippet quoted from Professor Matt Strassler, who has a fairly extensive web site of articles discussing particle physics issues in mostly lay terms that even **Jim Peebles** , and sometimes even **Senegoid** might be able to understand! > > For any lay person, or more-or-less lay person, these pages are a great beginner’s _must-read_ tutorial into these mysterious and mystical topics. > > Some good pages to get started there: > > [His home page.](https://profmattstrassler.com) > > [About This Site and How to Use It](https://profmattstrassler.com/about/about-this-site-and-how-to-use-it/) — Contains a bunch of links to other pages to get started with. > > **Derleth** ’s quote, with the analogy about the child’s swing, comes from the following page (third paragraph). In this page, Prof. Strassler laments that virtual particles were ever called that in the first place, since they’re not much like actual particles at all and he feels it’s just a confusing misnomer to call them that: > > [Virtual Particles: What are they?](https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/) Virtual particles are more ridiculous than ether. I am betting on ether. There is nothing wrong with the absolute reference frame that entails. Newton demonstrated that with his Bucket Argument, which you probably have the materials to try at home: > **[Bucket argument](https://en.wikipedia.org/wiki/Bucket_argument)** > > Isaac Newton's rotating bucket argument (also known as Newton's bucket) was designed to demonstrate that true rotational motion cannot be defined as the relative rotation of the body with respect to the immediately surrounding bodies. It is one of five arguments from the "properties, causes, and effects" of "true motion and rest" that support his contention that, in general, true motion and rest cannot be defined as special instances of motion or rest relative to other bodies, but instead can be... The Michelson-Morley experiment doesn’t rule out the ether moving like air particles do in air currents. Light is a wave, it is never a particle. Treating light as a particle allows you to work out the math of certain problems to a certain degree, such as the photoelectric effect. But I believe there is always a wave analysis which will yield the same or better results. For the photoelectric effect see the 1968 paper by Willis Lamb, Jr. and M.O. Scully, “The Photoelectric Effect Without Photons.” It’s not my fault theoretical physicists went through a phase of picking the most ridiculous answers to a whole slew of questions. (And as for the standing wave, which I consider a minor point: a standing wave has no bearing on what I said before: all you are seeing is two ether perturbations which have apparent motion add up to give a perturbation which does not appear to be moving.) ---
**Author:** ![Jim\_Peebles](https://avatars.discourse-cdn.com/v4/letter/j/5f9b8f/32.png) [@Jim\_Peebles](https://boards.straightdope.com/u/Jim_Peebles)\ **Post date:** [March 7, 2020, 2:21pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/71 "2020-03-07T14:21:03Z")
> [@ftg](#): > > One form/definition of “mass” is “inertial mass”. From good old _F_ = _ma_, if you apply a force to something and see it accelerate a certain amount, you can figure out its mass. > > But light _can’t_ be accelerated. It’s _always_ going the same speed. So the idea of inertial mass is irrelevant. > > (We hope that inertial mass is the same thing as gravitational mass. And photons have funny properties when it comes to gravitational effects as well. Best overall to just put them into a separate category from particles with mass.) Light CAN be accelerated. And it is REAL BAD if you believe light is a particle. Because when light leaves air (where the speed of light is v1) and enters a vacuum (where the speed of light is v2), its speed goes from v1 to v2 INSTANTANEOUSLY. There is no acceleration that can make a particle go from speed v1 to v2, where v1 \< v2 INSTANTANEOUSLY. But it is no big deal for a wave. It is well understood how a sound wave can go from the speed of sound in water to the speed of sound in air INSTANTANEOUSLY when it leaves water and hits air, as it is recognized as compressions and rarefactions of it carrying material, in this case water or air. To sum up: instantaneous jump from one speed to another: business as usual for a wave. Instantaneous jump from one speed to another for a particle: all sorts of mental contortions to explain it. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [March 7, 2020, 2:28pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/72 "2020-03-07T14:28:38Z")
For those just reading along to learn, those last two posts are just totally wrong. Seriously, ether? Jeez. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [March 7, 2020, 2:36pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/73 "2020-03-07T14:36:50Z")
Ok, anybody want to speak up and advocate for the phlogiston theory? How about alchemy? Flat earth? Young earth creationism? How about the Great Green Arkleseizure? ---
**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:** [March 7, 2020, 2:40pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/74 "2020-03-07T14:40:34Z")
And virtual particles do, in fact, behave like particles. Every particle that we’ve ever detected has, in fact, been a virtual particle. Now, those are the virtual particles that behaved _most_ like real particles, and some virtual particles behave much less like real particles… but there’s no sharp cutoff anywhere where you can say that THESE virtual particles actually exist and are particles, but THOSE virtual particles are just a computational convenience (or inconvenience). ---
**Author:** ![jonpluc](https://avatars.discourse-cdn.com/v4/letter/j/fbc32d/32.png) [@jonpluc](https://boards.straightdope.com/u/jonpluc)\ **Post date:** [March 7, 2020, 7:21pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/75 "2020-03-07T19:21:36Z")
My basic answer is becuse all light is created from an explosion, so of course its moving. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [March 7, 2020, 7:26pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/76 "2020-03-07T19:26:21Z")
> [@jonpluc](#): > > My basic answer is becuse all light is created from an explosion, so of course its moving. Cite? ---
**Author:** ![jonpluc](https://avatars.discourse-cdn.com/v4/letter/j/fbc32d/32.png) [@jonpluc](https://boards.straightdope.com/u/jonpluc)\ **Post date:** [March 7, 2020, 10:18pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/77 "2020-03-07T22:18:25Z")
As particles are created or destroyed they release photons as a byproduct. So you have colliding particles emitting photons. ---
**Author:** ![RioRico](https://avatars.discourse-cdn.com/v4/letter/r/f04885/32.png) [@RioRico](https://boards.straightdope.com/u/RioRico)\ **Post date:** [March 8, 2020, 1:17am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/78 "2020-03-08T01:17:51Z")
> [@jonpluc](#): > > My basic answer is becuse all light is created from an explosion, so of course its moving. That’s HOW it moves (the mechanics) but not WHY (the motivation). What is light’s emotional impetus? Boredom? Anger? Curiosity? Horniness? ---
**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:** [March 8, 2020, 1:21am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/79 "2020-03-08T01:21:56Z")
Even if that were true, which it’s not, that’s not the same thing as “explosions”. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 8, 2020, 6:09am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/80 "2020-03-08T06:09:26Z")
Ah yes, WHY did the photon cross the road? Now you can’t just say “To get to the other side, of course!” because it’s _already_ at the other side! (From the photon’s POV of course.) No, the photon crosses the road so it can look wistfully back at the side from whence it came, and sigh “Those were the good old [del]days[/del] nanoseconds!” (ETA: Okay, the photon crosses the road because the wave length is always greener on the other side.) [Previous page](https://boards.straightdope.com/t/why-does-light-travel/848744.md?page=3)