# Why is there a speed of light? **URL:** \ **Category:** Factual Questions\ **Created:** [October 5, 2015, 8:30pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639 "2015-10-05T20:30:01Z")\ **Posts on this page:** 20\ **Page:** 2
**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:** [October 6, 2015, 2:24pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/21 "2015-10-06T14:24:34Z")
OK, so how did you set the size of your rod? If I got a transdimensional e-mail from another universe that said that “We measured the speed of light, and found that it traveled 2e8 times the length of our standard platinum rod in each second”, then I would conclude that their standard platinum rod was half again as long as ours. > [@](#): > > Quoth **RadicalPi** : > > And also, is there anything constraining the fine-structure constant to be what it is? Not to our knowledge. Personally, I suspect that it can be mathematically derived from more fundamental principles, but that’s just a purest guess, with no evidence behind it, and many other physicists would guess otherwise. As for what would change, it would depend on how much the value changed, and if it were increased or decreased. But off the top of my head, you’d change the stability of many of the heavier isotopes (either causing some currently unstable ones to become stable, or vice-versa), change the magnetic anomaly of the electron, which would effect many macroscopic magnetic effects, and change the cross-section of many electromagnetic processes, like the scattering of light by light. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [October 6, 2015, 2:26pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/22 "2015-10-06T14:26:46Z")
But if we’re in a different universe, we don’t have access to the platinum rod in Paris. We can make another platinum rod–or we could, if this universe has platinum–but we can’t compare the rod in one universe to another rod in another universe. We could measure the Earth, or the distance from the Sun to the Earth, but if we’re in another universe on another planet that planet is going to have a different diameter and a different distance to its Sun. So if we find that a quasi-Platinum rod that is a certain fraction of the diameter of Quasi-Earth has a certain relationship to quasi-c, what have we measured? Have we discovered that quasi-c has a different value in the quasi-universe? Or have we measured that quasi-Platinum and Quasi-Earth have different physical values in the quasi-universe? If we define a second as a certain number of vibrations of rubidium, and we calculate how far a particle traveling at c would travel in that time, and then go to another universe and do the same experiment using quasi-rubidium and get a different result, what have we measured? Is c different or is quasi-rubidium different than rubidium? ---
**Author:** ![D18](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@D18](https://boards.straightdope.com/u/D18)\ **Post date:** [October 6, 2015, 2:51pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/23 "2015-10-06T14:51:22Z")
One explanation I found helpful was the way Brian Greene put it in The Elegant Universe: everything in fact travels the speed of light through all four dimensions of space-time. [This](http://blog.buberel.org/2008/07/brian-greene-ex.html) isn’t from Greene directly, but sums up his explanation: > [@](#): > > The crux of his explanation is that all things are constantly traveling in one of four dimensions - three of which are spatial and one is the time dimension. So an object that appears to be at rest in the three spatial dimensions is actually moving forward at the speed of light, but only in the time dimension. With this idea as the foundation, the remainder is easy to explain: When an object is traveling through one of the three spatial dimensions, it has to ‘borrow’ some of that speed from the time dimension. If you add up the total speed of an object through the 3 spatial and 1 time dimensions, that number always adds up to the speed of light. So it’s not that we can’t travel the speed of light, we just can’t travel the speed of light through the three spacial dimensions. And we can turn it around and have a laugh at light: it can’t travel the speed of us through time! Ha! At any rate, if anyone who knows what they’re talking about would like to comment on this take on the situation, it would be interesting to have that perspective. ---
**Author:** ![Snarky\_Kong](https://avatars.discourse-cdn.com/v4/letter/s/a183cd/32.png) [@Snarky\_Kong](https://boards.straightdope.com/u/Snarky_Kong)\ **Post date:** [October 6, 2015, 5:34pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/24 "2015-10-06T17:34:11Z")
Does the mass of an atom depend on the speed of light? If not, make a rod of pure platinum of a given mass with given aspect ratios and define that as your unit of measurement. Why are we positing different universes? Why not posit that tomorrow the speed of light was different as measured by travel time through the same length of fiber optic cable or whatever. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [October 6, 2015, 6:03pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/25 "2015-10-06T18:03:13Z")
But if tomorrow we notice that it takes light Y vibrations of rubidium to travel X distance measured by a certain platinum rod, whereas yesterday it too Z vibrations, what has changed? The speed? The frequency of vibrations? The distance? We’d be just as safe arguing that the platinum rod changed size, or that rubidium now vibrates at a different rate. ---
**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:** [October 6, 2015, 6:28pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/26 "2015-10-06T18:28:41Z")
Occams razor would say the rod had been changed. Or your measuring equipment was misbehaving. After that the question becomes akin to “what if tomorrow fairies appeared in everyone’s living rooms”. Everything else requires an unimaginable rethink of the basics of physics, so within the current understanding of the universe, the question cannot even be framed sensibly, let alone answered. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [October 6, 2015, 7:49pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/27 "2015-10-06T19:49:17Z")
What if I took the speed of light ©, multiplied it with the mean half life of a free neutron (t) and divided it with the mean square radius of a free neutron ®. I will get a dimensionless number (ct/r) ---
**Author:** ![Baffle](https://avatars.discourse-cdn.com/v4/letter/b/dec6dc/32.png) [@Baffle](https://boards.straightdope.com/u/Baffle)\ **Post date:** [October 6, 2015, 11:15pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/28 "2015-10-06T23:15:17Z")
> [@Snarky\_Kong](#): > > Does the mass of an atom depend on the speed of light? Mass and light are inextricably linked by Einstein’s equation E=mc^2. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [October 7, 2015, 12:19am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/29 "2015-10-07T00:19:02Z")
> [@Chronos](#): > > OK, so how did you set the size of your rod? Please tell me you at least chuckled a little to yourself when you posted that. ---
**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:** [October 7, 2015, 1:18am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/30 "2015-10-07T01:18:59Z")
No, but I’m laughing out loud now. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [October 7, 2015, 1:41am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/31 "2015-10-07T01:41:21Z")
> [@Saint\_Cad](#): > > Or maybe Why is the speed of light what it is? > So why can’t massless particles travel infinite velocity? Also, what is it in the structure of the universe (assuming you are one of those that don’t believe in the aether) that makes c the velocity it is? Why is it not 37 ft/sec or 903 km/hr? So the massless particle travels at infinite velocity (from it’s perspective it arrives at the same time it leaves), and… from our perspective, the particle which travels at ‘c’ is not massless – it has no rest mass, but from our perspective it has mass, momentem, and energy: 1/2 m V2. v is velocity is c, energy is hf. Which means (without meaning anything at all), that ‘c’ has to be that value given the energy and momentum of light, and the scale is set by h. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 7, 2015, 2:16am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/32 "2015-10-07T02:16:19Z")
I still don’t get why Chronos is so hung up on measuring scales. Does light travel a set velocity i.e. covers a certain distance in a certain time no matter the scale? Why does it travel at that velocity instead of covering that distance in half the time? ---
**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:** [October 7, 2015, 2:28am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/33 "2015-10-07T02:28:18Z")
I’m so hung up on the measuring scales because c is used to set those scales. How, then, can we determine if it’s changing? ---
**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:** [October 7, 2015, 2:50am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/34 "2015-10-07T02:50:59Z")
Time travels at one second per second, or one week per week, one year per year. c = 1. If you want to scale c to some other measuring system, you introduce a set of arbitrary constants that actually cancel out if you fully expand what it is you are trying to measure. 3x10[sup]8[/sup]ms[sup]-1[/sup] needs you to define a metre. If it is a metal bar the damned thing is eventually determined by a whole mess of things that all cancel out leaving you with a time. 3.333x10[sup]-9[/sup]s. If you defined it by cycles of a wavelength of light - it is pretty obvious how everything cancels out to leave you with a time. If you want to determine the exact set of parameters that determine the wavelength involved, you get c, h, α. But that is just a set of numbers that eventually get you a unitless constant to multiply 1 by. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 7, 2015, 3:38am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/35 "2015-10-07T03:38:07Z")
> [@Chronos](#): > > I’m so hung up on the measuring scales because c is used to set those scales. How, then, can we determine if it’s changing? I see a car travel a distance over a time period. I see a second car traveling slower over that same distance. Do I really have to know the units to say car 2 travels slower than car 1? It’s like that part in Phantom Tollbooth where someone wants to travel by inches because it’s quicker and another wants to travel the same disance by miles because it’s shorter. Light travels a set velocity does it not? Why is it not faster or slower than that? Or how bout this. You have a stick 1 mile (not defined by speed of light) long. Light travels from one end to the other in X seconds. Why X seconds and not Y seconds? ---
**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:** [October 7, 2015, 6:15am UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/36 "2015-10-07T06:15:28Z")
Perhaps you could phrase the question - since c = 1, what is it about the rest of the universe that scales our experience to place our everyday experience of speeds as being so low? ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 7, 2015, 1:50pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/37 "2015-10-07T13:50:07Z")
OK I’ll try again. light travels 186282.4 mps. Using the same mile and second as in calculating that velocity, why does light travel at that particular velocity and not slower or faster. ---
**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:** [October 7, 2015, 2:32pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/38 "2015-10-07T14:32:11Z")
Because one hour is equal to 186,282.4 miles. And if that weren’t true, then it would mean that the definition of one or both of those units was different. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 7, 2015, 4:04pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/39 "2015-10-07T16:04:45Z")
> [@Chronos](#): > > Because one hour is equal to 186,282.4 miles. And if that weren’t true, then it would mean that the definition of one or both of those units was different. That is not the question!!! Why does light travel at the rate it does. It has nothing to do with units of measures and everything to do with the concept of “faster” and “slower” which independent of units of measures. This is a physics question - not a metrology question. Why does light travel at the velocity it does and not a different velocity. And don’t give me the crap about “it could if you change the units.” I am asking why is light not faster or slower than it is now. If you are talking about units and defining meters and seconds defined as vibrations that would scale with c or anything like that you are missing the point of the question. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [October 7, 2015, 5:03pm UTC](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639/40 "2015-10-07T17:03:38Z")
> [@Francis\_Vaughan](#): > > Perhaps you could phrase the question - since c = 1, what is it about the rest of the universe that scales our experience to place our everyday experience of speeds as being so low? I guess it’s because of these fairly weak electromagnetic and gravitational forces we have in this universe. We end up with these bound clumps of matter that are basically motionless with respect to each other (10000km/h being nearly motionless in the grand scheme of things), because any clump with any significant kinetic energy has already escaped. Since our planet is made up of very low-energy matter, we are made of very low energy matter, and that affects our sense of what “fast” is. [Previous page](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639.md?page=1) [Next page](https://boards.straightdope.com/t/why-is-there-a-speed-of-light/733639.md?page=3)