# 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:** 3
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [March 4, 2020, 10:55pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/41 "2020-03-04T22:55:29Z")
> [@Noel\_Prosequi](#): > > I was reasoning by analogy with other measures. Metres per metre as a metric of something is useless because the answer is always 1. Kilograms per kilogram will always be 1. Measuring something by reference to itself seemed to be trivially unproductive to my mind. If you use the same units in the numerator and the denominator, they cancel out. Actually, meters per meter makes sense in a variety of circumstances. If I’m traveling north and you’re traveling west at the same speed, the distance between us in meters increases by 1.41 meters per meter I’ve traveled. If a square with a fixed area is squeezed, the length increases by A/w^2 meters per meter of squeezing. For every meter than the equator moves (as the earth rotates), the city of London moves by about 0.62 meters (.62 meters/meters). Of course the “meters” part is arbitrary - it could be any unit of length, but it’s always length/length. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [March 5, 2020, 2:14am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/42 "2020-03-05T02:14:03Z")
> [@Whack-a-Mole](#): > > But does light really travel? This thread is very interesting to me but also way over my head. The OP posed a good question. Does light travel? It has a measurable velocity so it must right? At the risk of showing my extreme ignorance of the subject of light can I ask a couple of questions? 1. Is there such a thing as an individual photon? 2. Suppose a flashlight is floating “motionless” in space and it has a torque free on switch. If it is turned on would the photons leaving cause the flashlight to move backwards? (Similar time a gun’s recoil) ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [March 5, 2020, 2:16am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/43 "2020-03-05T02:16:43Z")
> [@Lucas\_Jackson](#): > > This thread is very interesting to me but also way over my head. The OP posed a good question. > > Does light travel? It has a measurable velocity so it must right? > > At the risk of showing my extreme ignorance of the subject of light can I ask a couple of questions? > > 1. Is there such a thing as an individual photon? > > 2. Suppose a flashlight is floating “motionless” in space and it has a torque free on switch. If it is turned on would the photons leaving cause the flashlight to move backwards? (Similar time a gun’s recoil) Yes and yes. Individual photons can be created and photons carry momentum and so can cause recoil. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [March 5, 2020, 2:35am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/44 "2020-03-05T02:35:00Z")
> [@Andy\_L](#): > > Yes and yes. Individual photons can be created and photons carry momentum and so can cause recoil. Thank you. So does momentum act as mass? I was expecting the answer to be no as photons have mass. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [March 5, 2020, 2:40am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/45 "2020-03-05T02:40:12Z")
> [@Lucas\_Jackson](#): > > Thank you. > So does momentum act as mass? I was expecting the answer to be no as photons have mass. Photons have no mass (meaning what physicists used to call “rest mass” but now prefer to just call “mass”), but have momentum proportional to their energy (momentum for a photon = Energy times speed of light (P=Ec)). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 5, 2020, 2:44am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/46 "2020-03-05T02:44:03Z")
> [@Lucas\_Jackson](#): > > This thread is very interesting to me but also way over my head. The OP posed a good question. > > Does light travel? It has a measurable velocity so it must right? > > At the risk of showing my extreme ignorance of the subject of light can I ask a couple of questions? > > 1. Is there such a thing as an individual photon? > > 2. Suppose a flashlight is floating “motionless” in space and it has a torque free on switch. If it is turned on would the photons leaving cause the flashlight to move backwards? (Similar time a gun’s recoil) **Andy L** already answered, “yes and yes” which is correct. See double slit experiment for examples of us [shooting single photons](https://www.youtube.com/watch?v=GzbKb59my3U) and getting…weird results. See [solar sails](https://en.wikipedia.org/wiki/Solar_sail) as a use of using light to push a spacecraft (and it would push your flashlight…only ever so slightly but assuming a very long lasting battery the effects add up over time and you would see it move). As for my post know that as you go faster length in the direction of travel contracts. Time slows down as you go faster and the distance between you and your destination gets shorter. At [light speed time stops and distance contracts to zero](https://www.youtube.com/watch?v=-NN_m2yKAAk). So, to a photon, it goes nowhere in zero time. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [March 5, 2020, 3:44am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/47 "2020-03-05T03:44:01Z")
> [@Lucas\_Jackson](#): > > Thank you. > So does momentum act as mass? I was expecting the answer to be no as photons have mass. Mass is zero, but the photon has momentum, which results in stress-energy hence spacetime bending, etc. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [March 5, 2020, 3:58am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/48 "2020-03-05T03:58:51Z")
> [@Whack-a-Mole](#): > > So, to a photon, it goes nowhere in zero time. Ok, but photons indisputably take paths through space-time, and these paths have well-defined lengths. And the particles move at the speed of light 🙂 ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 5, 2020, 4:13am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/49 "2020-03-05T04:13:06Z")
> [@DPRK](#): > > Ok, but photons indisputably take paths through space-time, and these paths have well-defined lengths. And the particles move at the speed of light 🙂 From you sitting there watching them…yeah. But again, from the photon’s perspective there is no time and no distance. Your room and the universe are the same to it. We can get close to seeing this in action. Of course, we can never get to light speed but we can imagine it getting pretty close and see how this works: > [@](#): > > **The Oh-My-God Particle** > > \ > > On the night of October 15, 1991, the Fly’s Eye detected a proton with an energy of 3.2±0.9×1020 electron volts.[1,2] By comparison, the recently-canceled Superconducting Super Collider (SSC) would have accelerated protons to an energy of 20 TeV, or 2×1013 electron volts—ten million times less. The energy of the Oh My God particle seen by the Fly’s Eye is equivalent to 51 joules—enough to light a 40 watt light bulb for more than a second—equivalent, in the words of Utah physicist Pierre Sokolsky, to “a brick falling on your toe.” The particle’s energy is equivalent to an American baseball travelling fifty-five miles an hour. > > \ > > How fast was it going? Pretty fast. > > \ > > v = 0.9999999999999999999999951 c > > \ > > Recall also that time passes more slowly in a moving reference frame… > > \ > > Thus, given that the particle travels with essentially the speed of light, an observer traveling along with the particle would perceive the flight time from the following objects to the Earth. > > - Alpha Centauri – 4.36 [light years] – 0.43 milliseconds [perceived travel time] > - Galactic nucleus – 32,000 – 3.2 seconds > - Andromeda galaxy – 2,180,000 – 3.5 minutes > - Virgo cluster – 42,000,000 – 1.15 hours > - Quasar 3C273 – 2,500,000,000 – 3 days > - Edge of universe – 17,000,000,000 – 19 days > > \ > > Finally, let’s consider the length contraction in the direction of motion which results from the Lorentz transformation—objects in the direction of travel are seen to contract in that direction… > > \ > > - Earth’s diameter – 12,756 km [Rest Frame Thickness] – 0.0399 mm [Particle Frame Thickness] > - Solar system – 80 AU – 37 metres > - Sun/Alpha Centauri – 4.3 light years – 127 km (79 miles) > - Milky Way galaxy – 30 kiloparsecs – 2,895,000 km, about ten times the distance from the Earth to the Moon > > SOURCE: [The Oh-My-God Particle](https://www.fourmilab.ch/documents/OhMyGodParticle/) ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [March 5, 2020, 12:11pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/50 "2020-03-05T12:11:57Z")
Agreed that if a photon is emitted in Andromeda, and is observed here, the space/time interval between these events is zero. But, that does not mean that we are in Andromeda (although, as you say, if you go fast enough relative to Earth, Andromeda is not very far at all). The only thing you can’t do is go so fast that the light seems to stay put (or even slow down). ETA since the light travels a path of length 0 (= 0 time!), that is one way to look at it, yes… ---
**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:** [March 5, 2020, 12:36pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/51 "2020-03-05T12:36:55Z")
Since time does not pass for a photon, doesn’t that mean that a single photon can be everywhere simultaneously and that maybe there’s only one photon in the entire universe? I know that a [single-electron universe](https://en.wikipedia.org/wiki/One-electron_universe) was considered by Wheeler. Could a similar line of reasoning apply to a photon? ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [March 5, 2020, 1:30pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/52 "2020-03-05T13:30:11Z")
> [@KarlGauss](#): > > Since time does not pass for a photon, doesn’t that mean that a single photon can be everywhere simultaneously and that maybe there’s only one photon in the entire universe? I know that a [single-electron universe](https://en.wikipedia.org/wiki/One-electron_universe) was considered by Wheeler. Could a similar line of reasoning apply to a photon? Part of the one-electron universe is the idea that positrons (anti-electrons) are electrons moving back in time. Since there is no distinct anti-photon, that idea might not work. Or it might work just as well. It’s kind of a bizarre theory anyway. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [March 5, 2020, 3:53pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/53 "2020-03-05T15:53:26Z")
> [@Derleth](#): > > Since there is no distinct **anti-photon** , that idea might not work. Or it might work just as well. Anti-photons! So _that’s_ what all the dark matter is. 😃 ---
**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 5, 2020, 10:12pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/54 "2020-03-05T22:12:09Z")
Key word there: There is no _distinct_ anti-photon. That’s not because there are no anti-photons, just because they aren’t distinct. An anti-photon is the same thing as a photon. ---
**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 5, 2020, 10:19pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/55 "2020-03-05T22:19:35Z")
> [@Chronos](#): > > Key word there: There is no _distinct_ anti-photon. That’s not because there are no anti-photons, just because they aren’t distinct. An anti-photon is the same thing as a photon. What happens when a neutron meets an anti-neutron? ---
**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 5, 2020, 11:23pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/56 "2020-03-05T23:23:10Z")
Neutrons and anti-neutrons are distinct: They have the same charge, but charge isn’t all that matters. And so when they meet, there’s an energetic event that initially releases three pions, which in turn eventually (in a fairly short time) decay into some combination of photons and neutrinos. Note that it’s not quite the same as an electron and positron (anti-electron), which go straight to photons, because neutrons are more complicated particles, composed of quarks, and subject to the strong interaction in addition to electromagnetism. ---
**Author:** ![Dana\_Scully](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@Dana\_Scully](https://boards.straightdope.com/u/Dana_Scully)\ **Post date:** [March 5, 2020, 11:53pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/57 "2020-03-05T23:53:23Z")
How can any kind of object have no mass? ---
**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:** [March 6, 2020, 12:22am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/58 "2020-03-06T00:22:07Z")
Why should it have mass? It isn’t an easy question. ---
**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, 12:43am UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/59 "2020-03-06T00:43:08Z")
> [@Chronos](#): > > Neutrons and anti-neutrons are distinct: They have the same charge, but charge isn’t all that matters. And so when they meet, there’s an energetic event Thanks for the explanation, it was interesting. One more question, though. How energetic is that event? Exceeding nuclear fusion style events, particle for particle reaction? Or tamer than that? Not that I have anything nefarious in mind, of course. 😉 ---
**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 6, 2020, 5:08pm UTC](https://boards.straightdope.com/t/why-does-light-travel/848744/60 "2020-03-06T17:08:30Z")
Light is a wave. A wave is a type of motion. You can’t have a wave without motion. Things that are moving travel. [Previous page](https://boards.straightdope.com/t/why-does-light-travel/848744.md?page=2) [Next page](https://boards.straightdope.com/t/why-does-light-travel/848744.md?page=4)