<p>Why is the speed of light 299,792,458 m/s, and not (for instance) 3,1 or 4,3 x 10^44 m/s?</p>

<p>The answer is that all those numbers are consequences of <em>arbitrary</em> choices of the units of measure. In fact there is nothing special about the number <em>299,792,458</em>, so much so that one can correctly write that the speed of light is:</p>

<ul>
<li>1079252848.8 [km/h]</li>
<li>0.3 [pc/year]</li>
<li>1 [lightyear/year]</li>
</ul>

<p>So much so, that in particle physics it is common to use the speed of light as the unit of measure of speed, so that it is effectively equal to unity. You would write that the speed of light is 1 [c], and that the speed of that particle was 0.99998 [c], while I typically get to work at a speed of 1. x 10^{-8} [c].</p>

CC BY-SA 3.0
2016-11-30T20:30:08.917Z
The numerical value of any physical constant with dimensions is not interesting. Only dimensionless ratios like the fine structure constant are important. We do not know why it is 1/137.
ProfRob
CC BY-SA 3.0
2014-02-14T12:00:18.843Z
In so-called "natural units" the speed of light is 1 (dimensionless)
Envite
CC BY-SA 3.0
2014-02-14T12:05:28.050Z
astabada, I think you missed the level of the question I was asking. I was asking whatever unit of measure you are using why is it that speed. For example if the meter and second stayed the same why is it 3E8 m/s and not 6E8m/s, Envite answered the quesiton nicely. Although, of course you can go on to ask why are electric and magnetic permeability the way they are...and even deeper question: why is any constant the value it is?!?! Is there a 'natural' optimization in the universe? or are these values related to the least amount of energy being used?
sci-guy
CC BY-SA 3.0
2014-02-17T09:54:31.560Z
@Envite I wanted to be clear, that is why I specified *1 [c]*. I agree that if c = 1 the expression is redundant, much like writing "8 x 1" rather than just "8"...
astabada
CC BY-SA 3.0
2014-02-17T09:58:27.487Z
@AndrewH yes, there is a reason for which all these universal constants have the value they have. It is called the Anthropic Principle: If they had any other value, we will not be here to observe them.
Envite
CC BY-SA 3.0
2014-02-17T09:59:20.827Z
@AndrewH I had already seen that you had accepted Envite 's answer. However, I stress that - as you have acknowledged - Envite's is not a full answer as it moves the question onto permeabilities. You might answer for example why the *ratio* between two speeds (e.g. light vs sound) is such and such, but not about a single value, without any comparison.
astabada
CC BY-SA 3.0
2014-02-17T10:01:01.163Z
@Envite I am not sure that life on Earth would have been impossible had the speed of light been  299,792,450 m/s.
astabada
CC BY-SA 3.0
2014-02-17T10:24:10.097Z
@astabada It seems you have a valid question. Please ask it as a Question on the site. It is pointless to discuss it via comments.
Envite
CC BY-SA 3.0
2014-02-14T10:03:25.893Z
2014-02-14T10:03:25.893Z
astabada
<p>Speed of light being finite is one of the fundamentals of our Universe.</p>

<p>If it were infinite, this would have a major implication in causality.</p>

<p>Besides, in non-quantum physics, light is just an electromagnetic wave. Eelectromagnetic field is described by Maxwell's equations, which predict that the speed $c$ of electromagnetic waves propagating through the vacuum depends on the dielectric permittivity $ε_0$ and the magnetic permeability $μ_0$ by the equation $c = {1\over\sqrt{ε_{0}μ_{0}}}$ so you can not have an infinite speed of light unless electric permittivity or magnetic permeability were zero, which in turn would cause all sorts of odd things to electromagnetical attraction (and thus, to matter existence beyond elemental particles).</p>

CC BY-SA 3.0
2016-11-30T14:40:06.830Z
I have the same question. Why a finite no. I was studying transformation of relativistic kinematics and realized light particles would not feel the journey if they were living.Everything would happen in an instance for them. the Value 299792458 comes from the permittivity and permeability that's okay. It's a cause that speed is finite. But why finite, to know that I think more advancement is needed.
Javi
CC BY-SA 3.0
2014-02-13T09:19:26.100Z
Ok, but why 300,000km/sec? I know km and sec are human inventions - but whatever your units of measure are - why that speed? what is so special about that number? and what about the other part of my question: if a photon experiences everything all at once - how can it be that it leaves the sun and hits earth in t=0 but to us t=8min20sec ?
sci-guy
CC BY-SA 3.0
2014-02-13T09:30:50.317Z
That speed (297000Km/s) comes from the formula I posted above: 1 over the square root of ε0μ0. About the photon experiencing everything at once, that is due to relativistic time dilation. For a photon, time is infinitely dilated so one instant extends infinitely long.
Envite
CC BY-SA 3.0
2014-02-13T09:32:17.790Z
oh ya.. that's the stuff. intellectual satisfaction. Thanks! I like that answer a lot.
sci-guy
CC BY-SA 3.0
2014-02-14T10:54:24.550Z
This answer is just a sophisticated version of "it is because it is"
adrianmcmenamin
CC BY-SA 3.0
2014-02-13T08:50:19.147Z
2014-02-19T16:28:05.517Z
2014-02-19T16:28:05.517Z
Envite
<p>Ok, I am majoring in physics (4th year) and I never understood this fundamental (kinda) question. Maybe I haven't explored it enough.</p>

<p>For example, why does it take 8min20sec for the light from the sun to get to us?
I know the answer to this question on a 'surface' scale. The sun is 1AU away, c=3E8 m/s, and d=v/t to get approx 8 min 20 sec. </p>

<p>my question is on a deeper level. </p>

<p>Say you could "ride a photon" I KNOW THIS IS IMPOSSIBLE, but just say you could. Or a better question: what does a photon experience? The photon, as per my understanding, would leave the sun and (if it is on the right trajectory) hit the Earth instantaneously. A photon leaving Alpha Centauri would see the universe all at once, in a infinitesimal small unit of time (if directed out to space). </p>

<p>If a photon sees everything all at once, why do we perceive it to have a speed? I am sure this has something to do with frames of reference, special relatively, Lorentz transforms? but just seems strange. why is the speed of light finite to us... if it was infinite would this be problematic? </p>

CC BY-SA 3.0
2015-01-16T20:30:20.397Z
It is theorized that other multi-verses may have different constants than the ones we have in this universe.

For example, the constant of gravity might be different, the speed of light might be different, etc.

These universes would look entirely different than our universe.
Scottie
CC BY-SA 3.0
2014-12-11T12:17:15.810Z
Better answers on Physics SE. e.g. http://physics.stackexchange.com/questions/150150/speed-of-light-definition , http://physics.stackexchange.com/questions/80365/what-is-so-special-about-speed-of-light-in-vacuum?lq=1 , http://physics.stackexchange.com/questions/143385/whats-so-special-about-the-speed-of-light?lq=1
ProfRob
CC BY-SA 3.0
2016-08-02T11:59:55.377Z
The speed of light is 299792458 meters per second because The universe works the way it does and also: the speed of light was defined much after the meter was. Hence in order to retain the original meter measure SI decided that the speed of light to be 299792458 m/s just in older to retain the old meter measure which earlier was defined to be equal to one ten-millionth of the distance from the equator to the pole measured on a meridian which passed through Paris France
Suhrid Mulay
CC BY-SA 3.0
2016-11-30T20:35:41.620Z
The actual value of a physical constant with dimensions is merely a product of our unit system. That there is a finite speed for light arises from some simple symmetry considerations - e.g. allowing the laws of physics to be unchanged by translations in space and time.
ProfRob
CC BY-SA 3.0
2018-02-21T13:48:50.987Z
If you need in-depth explanations, check Michael Stevens videos about lightspeed such as [Would Headlights Work at Light Speed?](https://youtu.be/ACUuFg9Y9dY). Beware that you will end up with more questions than before watching.
Sanya_Zol
CC BY-SA 3.0
2014-02-13T20:09:55.747Z
Since 1983, the meter has been defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second, so that number is exact by definition. Of course that doesn't answer your question, but it's interesting. https://en.wikipedia.org/wiki/Speed_of_light
Keith Thompson
CC BY-SA 3.0
2014-02-14T10:55:45.310Z
If you haven't read Feynmann's QED I'd really recommend it for more perspective on this.
adrianmcmenamin
<light><speed>
Why is the speed of light 299,792,458 meters/sec?
sci-guy
# Why is the speed of light 299,792,458 meters/sec?

(Asked by: username_0 on 2014-02-13)

Ok, I am majoring in physics (4th year) and I never understood this fundamental (kinda) question. Maybe I haven't explored it enough.

For example, why does it take 8min20sec for the light from the sun to get to us?
I know the answer to this question on a 'surface' scale. The sun is 1AU away, c=3E8 m/s, and d=v/t to get approx 8 min 20 sec.

my question is on a deeper level.

Say you could "ride a photon" I KNOW THIS IS IMPOSSIBLE, but just say you could. Or a better question: what does a photon experience? The photon, as per my understanding, would leave the sun and (if it is on the right trajectory) hit the Earth instantaneously. A photon leaving Alpha Centauri would see the universe all at once, in a infinitesimal small unit of time (if directed out to space).

If a photon sees everything all at once, why do we perceive it to have a speed? I am sure this has something to do with frames of reference, special relatively, Lorentz transforms? but just seems strange. why is the speed of light finite to us... if it was infinite would this be problematic?

**Tags:** light, speed

> **Comments:**
> **username_5**: Since 1983, the meter has been defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second, so that number is exact by definition. Of course that doesn't answer your question, but it's interesting. https://en.wikipedia.org/wiki/Speed_of_light  
> **username_6**: If you haven't read Feynmann's QED I'd really recommend it for more perspective on this.  
> **username_2**: Better answers on Physics SE. e.g. http://physics.stackexchange.com/questions/150150/speed-of-light-definition , http://physics.stackexchange.com/questions/80365/what-is-so-special-about-speed-of-light-in-vacuum?lq=1 , http://physics.stackexchange.com/questions/143385/whats-so-special-about-the-speed-of-light?lq=1  
> **username_1**: It is theorized that other multi-verses may have different constants than the ones we have in this universe.  For example, the constant of gravity might be different, the speed of light might be different, etc.  These universes would look entirely different than our universe.  
> **username_3**: The speed of light is 299792458 meters per second because The universe works the way it does and also: the speed of light was defined much after the meter was. Hence in order to retain the original meter measure SI decided that the speed of light to be 299792458 m/s just in older to retain the old meter measure which earlier was defined to be equal to one ten-millionth of the distance from the equator to the pole measured on a meridian which passed through Paris France  
> **username_2**: The actual value of a physical constant with dimensions is merely a product of our unit system. That there is a finite speed for light arises from some simple symmetry considerations - e.g. allowing the laws of physics to be unchanged by translations in space and time.  
> **username_4**: If you need in-depth explanations, check Michael Stevens videos about lightspeed such as [Would Headlights Work at Light Speed?](https://youtu.be/ACUuFg9Y9dY). Beware that you will end up with more questions than before watching.  

---

## 2 answers

### Answer by username_8 on 2014-02-13. Score: 10 | Accepted answer

Speed of light being finite is one of the fundamentals of our Universe.

If it were infinite, this would have a major implication in causality.

Besides, in non-quantum physics, light is just an electromagnetic wave. Eelectromagnetic field is described by Maxwell's equations, which predict that the speed $c$ of electromagnetic waves propagating through the vacuum depends on the dielectric permittivity $ε\_0$ and the magnetic permeability $μ\_0$ by the equation $c = {1\over\sqrt{ε\_{0}μ\_{0}}}$ so you can not have an infinite speed of light unless electric permittivity or magnetic permeability were zero, which in turn would cause all sorts of odd things to electromagnetical attraction (and thus, to matter existence beyond elemental particles).

> **Comments:**
> **username_0**: Ok, but why 300,000km/sec? I know km and sec are human inventions - but whatever your units of measure are - why that speed? what is so special about that number? and what about the other part of my question: if a photon experiences everything all at once - how can it be that it leaves the sun and hits earth in t=0 but to us t=8min20sec ?  
> **username_8**: That speed (297000Km/s) comes from the formula I posted above: 1 over the square root of ε0μ0. About the photon experiencing everything at once, that is due to relativistic time dilation. For a photon, time is infinitely dilated so one instant extends infinitely long.  
> **username_0**: oh ya.. that's the stuff. intellectual satisfaction. Thanks! I like that answer a lot.  
> **username_6**: This answer is just a sophisticated version of "it is because it is"  
> **username_9**: I have the same question. Why a finite no. I was studying transformation of relativistic kinematics and realized light particles would not feel the journey if they were living.Everything would happen in an instance for them. the Value 299792458 comes from the permittivity and permeability that's okay. It's a cause that speed is finite. But why finite, to know that I think more advancement is needed.  

---

### Answer by username_7 on 2014-02-14. Score: 4

Why is the speed of light 299,792,458 m/s, and not (for instance) 3,1 or 4,3 x 10^44 m/s?

The answer is that all those numbers are consequences of *arbitrary* choices of the units of measure. In fact there is nothing special about the number *299,792,458*, so much so that one can correctly write that the speed of light is:

* 1079252848.8 [km/h]
* 0.3 [pc/year]
* 1 [lightyear/year]

So much so, that in particle physics it is common to use the speed of light as the unit of measure of speed, so that it is effectively equal to unity. You would write that the speed of light is 1 [c], and that the speed of that particle was 0.99998 [c], while I typically get to work at a speed of 1. x 10^{-8} [c].

> **Comments:**
> **username_8**: In so-called "natural units" the speed of light is 1 (dimensionless)  
> **username_0**: username_7, I think you missed the level of the question I was asking. I was asking whatever unit of measure you are using why is it that speed. For example if the meter and second stayed the same why is it 3E8 m/s and not 6E8m/s, username_8 answered the quesiton nicely. Although, of course you can go on to ask why are electric and magnetic permeability the way they are...and even deeper question: why is any constant the value it is?!?! Is there a 'natural' optimization in the universe? or are these values related to the least amount of energy being used?  
> **username_7**: @username_8 I wanted to be clear, that is why I specified *1 [c]*. I agree that if c = 1 the expression is redundant, much like writing "8 x 1" rather than just "8"...  
> **username_8**: @AndrewH yes, there is a reason for which all these universal constants have the value they have. It is called the Anthropic Principle: If they had any other value, we will not be here to observe them.  
> **username_7**: @AndrewH I had already seen that you had accepted username_8 's answer. However, I stress that - as you have acknowledged - username_8's is not a full answer as it moves the question onto permeabilities. You might answer for example why the *ratio* between two speeds (e.g. light vs sound) is such and such, but not about a single value, without any comparison.  
> **username_7**: @username_8 I am not sure that life on Earth would have been impossible had the speed of light been  299,792,450 m/s.  
> **username_8**: @username_7 It seems you have a valid question. Please ask it as a Question on the site. It is pointless to discuss it via comments.  
> **username_2**: The numerical value of any physical constant with dimensions is not interesting. Only dimensionless ratios like the fine structure constant are important. We do not know why it is 1/137.  

---

