## Why does light move at the speed it does?

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

MHD
Posts: 630
Joined: Fri Mar 20, 2009 8:21 pm UTC
Location: Denmark

### Re: Why does light move at the speed it does?

SlyReaper wrote:A better question would be: why do relativistic effects happen such that "infinite speed" appears to be finite speed to an outside observer?

This is "asking the right question."
EvanED wrote:be aware that when most people say "regular expression" they really mean "something that is almost, but not quite, entirely unlike a regular expression"

WarDaft
Posts: 1583
Joined: Thu Jul 30, 2009 3:16 pm UTC

### Re: Why does light move at the speed it does?

If I was a betting person (and if I had enough money [and knew someone who would bother to take such a bet]) I'd wager a lot that it's strongly due to the anthropic principle.

We find ourselves on what appears to be a garden world. Perfectly tailored for life, so to speak. Not just 'okay' for life, some place that life can eek out in the cracks like a desert, practically perfect for life. Why? Because that's where most life is. (Well, there are better possible conditions for life, but you start needing even less and less likely circumstances)

Now relax the domain from the Earth to the Universe. We probably live in a universe with a high product of universe likeliness and in universe intelligent being count.

And that's as close to a why as you'll ever probably get.
All Shadow priest spells that deal Fire damage now appear green.
Big freaky cereal boxes of death.

Aelfyre
Posts: 358
Joined: Wed May 07, 2008 4:22 pm UTC
Location: 3 decades west of the teenage wasteland
Contact:

### Re: Why does light move at the speed it does?

WarDaft wrote:If I was a betting person (and if I had enough money [and knew someone who would bother to take such a bet]) I'd wager a lot that it's strongly due to the anthropic principle.

We find ourselves on what appears to be a garden world. Perfectly tailored for life, so to speak. Not just 'okay' for life, some place that life can eek out in the cracks like a desert, practically perfect for life. Why? Because that's where most life is. (Well, there are better possible conditions for life, but you start needing even less and less likely circumstances)

Now relax the domain from the Earth to the Universe. We probably live in a universe with a high product of universe likeliness and in universe intelligent being count.

And that's as close to a why as you'll ever probably get.

that may be so but that's no reason to not ask or continue to try and figure it out. This whole anthropomorphic principle is just lazy thinking IMO. It is this was because if it wasn't you couldn't ask the question.. absolute hogwash, not that I have anything against clean hogs mind you.

There must be some sort of underlying reason.. it *might* be completely ineffable or it might not but it's still there.. somewhere.
Xanthir wrote:To be fair, even perfectly friendly antimatter wildebeests are pretty deadly.

ibgdude
Posts: 33
Joined: Sun Oct 02, 2011 10:19 pm UTC

### Re: Why does light move at the speed it does?

Here's a better way to think about it: light goes at the speed of 1. The value 3*108 m/s is only generated by our units. It makes a lot more sense to define c as 1 and then go from there. Thus, I win the bet with WarDraft: c's value is 1 in all possible realities, and it has nothing to do with how nice the universe is for intelligent life. Our measurement of the speed of light is just based on our measurement system, and is not fundamental. In other words, there are two fundamental speeds: 0, the speed of something relative to itself, and 1, the speed of light. Everything else is relative to those values.
Hawknc wrote:This is the easiest question ever asked on the forum, and you guys turn it into a two-page debate.

...Don't ever change.

Six pages and still going.

Glass Fractal
Posts: 497
Joined: Thu May 13, 2010 2:53 am UTC

### Re: Why does light move at the speed it does?

ibgdude wrote:Here's a better way to think about it: light goes at the speed of 1. The value 3*108 m/s is only generated by our units. It makes a lot more sense to define c as 1 and then go from there. Thus, I win the bet with WarDraft: c's value is 1 in all possible realities, and it has nothing to do with how nice the universe is for intelligent life. Our measurement of the speed of light is just based on our measurement system, and is not fundamental. In other words, there are two fundamental speeds: 0, the speed of something relative to itself, and 1, the speed of light. Everything else is relative to those values.

That doesn't even manage to dodge the question. I can just ask why does light move a million times as fast as (some other thing)? No need for units.

WarDaft
Posts: 1583
Joined: Thu Jul 30, 2009 3:16 pm UTC

### Re: Why does light move at the speed it does?

Aelfyre wrote:that may be so but that's no reason to not ask or continue to try and figure it out. This whole anthropomorphic principle is just lazy thinking IMO. It is this was because if it wasn't you couldn't ask the question.. absolute hogwash, not that I have anything against clean hogs mind you.

There must be some sort of underlying reason.. it *might* be completely ineffable or it might not but it's still there.. somewhere.

The speed of light might indeed be derived from something else. But then the anthropic principle would apply to that something else. And I didn't say you *wouldn't* be there to ask about it otherwise, merely less likely. If there is a range of possible universes, most intelligent life will ask that question about their universes in universes that are both probable and highly conductive to intelligent life. You could ask for the deepest model, the one which determines the variable ranges, but without being able to observe anything but our own conditions, that will be very very hard if even possible to determine.
All Shadow priest spells that deal Fire damage now appear green.
Big freaky cereal boxes of death.

eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

### Re: Why does light move at the speed it does?

Glass Fractal wrote:
ibgdude wrote:Here's a better way to think about it: light goes at the speed of 1. The value 3*108 m/s is only generated by our units. It makes a lot more sense to define c as 1 and then go from there. Thus, I win the bet with WarDraft: c's value is 1 in all possible realities, and it has nothing to do with how nice the universe is for intelligent life. Our measurement of the speed of light is just based on our measurement system, and is not fundamental. In other words, there are two fundamental speeds: 0, the speed of something relative to itself, and 1, the speed of light. Everything else is relative to those values.

That doesn't even manage to dodge the question. I can just ask why does light move a million times as fast as (some other thing)? No need for units.

No, it does dodge the question because the ratio of speeds you'd measure would depend on reference frame and so doesn't say anything useful about the state of the universe. In natural units, c is always 1 because, other than 0, it is the only other fundamental speed in the universe and its value is determined solely by the units we use.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

ibgdude
Posts: 33
Joined: Sun Oct 02, 2011 10:19 pm UTC

### Re: Why does light move at the speed it does?

I think that question you're asking is really "why does light move about eight orders of magnitude faster than many of the things we see around us." In that case, the answer is that we move at the speed we do because of evolution based on the amount of energy available in a given area in the form of food, which is in turn based on our distance from the sun and its energy output, which are based on the phase of water and the heat released by hydrogen fusion. None of this has anything to do with light, per se. The fundamental fact is that "why does light move the speed it does" is the wrong question. The right question is " why does everything else move at a certain multiple of the speed of light" and that does have an answer, albeit a ridiculously complex one.
Hawknc wrote:This is the easiest question ever asked on the forum, and you guys turn it into a two-page debate.

...Don't ever change.

Six pages and still going.

SlyReaper
inflatable
Posts: 8015
Joined: Mon Dec 31, 2007 11:09 pm UTC
Location: Bristol, Old Blighty

### Re: Why does light move at the speed it does?

Well the sun's output does depend somewhat on the speed of light thanks to our old friend E=MC2.

What would Baron Harkonnen do?

Jplus
Posts: 1721
Joined: Wed Apr 21, 2010 12:29 pm UTC
Location: Netherlands

### Re: Why does light move at the speed it does?

I'm guessing that ibgdude would rather think of it as both the sun's output and the speed of light (and many other things) depending on a constant that we call c.
"There are only two hard problems in computer science: cache coherence, naming things, and off-by-one errors." (Phil Karlton and Leon Bambrick)

coding and xkcd combined

(Julian/Julian's)

ibgdude
Posts: 33
Joined: Sun Oct 02, 2011 10:19 pm UTC

### Re: Why does light move at the speed it does?

Both of the above are correct, my point was that the intervening interactions between the sun's output and our speed, through chemistry, biology and physics, are what determines the approximate relative speed of people to light. That is the only way to answer the question "Why does light move at the speed it does?"reasonably, or so I see it. The speed of light is fundamental, (almost) everything else changes based on it.
Hawknc wrote:This is the easiest question ever asked on the forum, and you guys turn it into a two-page debate.

...Don't ever change.

Six pages and still going.

soggybomb
Posts: 23
Joined: Wed Nov 09, 2011 2:46 pm UTC

### Re: Why does light move at the speed it does?

Dr.Buck wrote:There's one property of the speed of light that's always confused me a little (which, admittedly, isn't all that hard!), and that's how it has the same value to an observer, no matter what relative (to the light being measured) speed they are travelling at.
Example;
Spoiler:
Car A and car B are both travelling along the motorway, A at 60mph, B at 70mph. To a stationary person at the side of the road, car B is clearly travelling at 70mph, but to the occupants of car A, car B has a relative speed of 10mph.

Now, carry this example across to an experiment measuring the speed of light - the experiment while stationary measures the speed of light to be 3x108m/s. Fair enough. Now, place this experiment onto something travelling 21000m/s (quick Google search tells me this is the speed of a space probe) and it still measures the speed of light to be 3x108m/s. The light, we can assume, comes from the same source, which is stationary relative to the moving experiment (for example - it came frm the sun, with the experiment moving away from the sun at 21000m/s).
Can anyone offer me an insight into how this works?

One of the fundamental principles of physics is that of translational invariance. Essentially, if you perform an experiment in a location that is at rest (relative to a fixed point of course), then if you did the same experiment in a place that is in motion relative to that point (but has no forces acting on it) you expect the same result. This idea is manifested directly in the idea of a Galilean Transformation. Suppose you have a point fixed in space that we will call $x$, and another point that is moving at a constant velocity that we will call $x'$. So, to find out what $x'$ is, we say $x'=x+vt$.

It turns out that if you apply this idea to Maxwell's equations, it totally fails. ${\partial B \over \partial x } = -{1 \over c^2}{\partial E \over \partial t}$ becomes ${\partial B' \over \partial x'} =-{1 \over c^2}{\partial E' \over \partial t'} +{1 \over c^2} \left [ v{\partial E' \over \partial x'}-v{\partial B' \over \partial t'}+v^2{\partial B' \over \partial x'} \right ]$
whereas if translational invariance is true, then you would expect
${\partial B' \over \partial x' } = -{1 \over c^2}{\partial E' \over \partial t'}$
So, clearly a new type of transformation other than the Gallilean transformation was necessary, so this where the genius of Einstein came to the rescue and realized that another transformation was needed in order to derive the second equation, and therefore conserve translational invariance. Indeed, in order to make that work, the speed of light MUST be the same in all reference frames. Many people think that Einstein imposed this arbitrary condition on the problem, and that the theory of relativity comes from this axiom, but instead this axiom was derived from the transformation itself.

Myria
Posts: 15
Joined: Thu Jul 29, 2010 9:07 pm UTC

### Re: Why does light move at the speed it does?

I call "c" the "asymptotic speed limit", since that's what Lorentz transformations do to increasing speeds.

I call 1 divided by the Planck time the "frame rate of the Universe" for similar reasons.

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

Myria wrote:I call "c" the "asymptotic speed limit", since that's what Lorentz transformations do to increasing speeds.

It's not asymptotic if you happen to be massless.

Myria wrote:I call 1 divided by the Planck time the "frame rate of the Universe" for similar reasons.

Or you could call it the Planck angular frequency.

We don't really know what time is like at that scale. We need a theory of Quantum Gravity to properly talk about such matters. If spacetime is quantized, it most likely happens somewhere near that scale (as hinted by the Bekenstein-Hawking Entropy formula). But bear in mind that some sort of Heisenberg Uncertainty effect is likely, so a simple analogy with frame rate may not be so applicable.

Copper Bezel
Posts: 2426
Joined: Wed Oct 12, 2011 6:35 am UTC
Location: Web exclusive!

### Re: Why does light move at the speed it does?

Huh. The first metaphor for the uncertainty principle that made any sense to me was based on framerate, from a documentary with Jim Al-Khalili. His explanation was, say you have a video of a given, limited size; you can either have high resolution (information about position) or high framerate (information about motion) but not both. Is that too simplistic?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

she / her / her

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

Copper Bezel wrote:Huh. The first metaphor for the uncertainty principle that made any sense to me was based on framerate, from a documentary with Jim Al-Khalili. His explanation was, say you have a video of a given, limited size; you can either have high resolution (information about position) or high framerate (information about motion) but not both. Is that too simplistic?

That's fine when you're talking about stuff like electrons moving around. It's just that it's not clear how to apply it to the quantization of spacetime itself. You get all sorts of weird and wonderful possibilities: it may not be so easy to distinguish spatial directions from temporal ones, and the exact ordering in space and time of events at that scale may be fuzzy and dynamic. Also, if the compact ("rolled-up") dimensions of string theory are real, they will be significant at that scale, and on the same footing as the "regular" dimensions we're used to.

Remember that the Planck length / time scale is way smaller than the scale associated with the typical movements of atomic particles. It's hard enough to comprehend the atomic scale with our everyday intuitions, but atoms are pretty large on the Planck scale. Or as I put it recently in another thread here, relative to the Planck scale, humans are not that much bigger than atoms.

The Planck length = 1.616 199(97) × 10-35 m
The Bohr radius of hydrogen = 5.2917721092(17)×10-11 m
Another 10 orders of magnitude on top of 24 orders of magnitude is chickenfeed.

Copper Bezel
Posts: 2426
Joined: Wed Oct 12, 2011 6:35 am UTC
Location: Web exclusive!

### Re: Why does light move at the speed it does?

Yeah, that definitely puts it into perspective. = o Wow.

Thanks for the clarification. = )
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

she / her / her

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

I was idly wondering today if there might be a relationship between the mass of the universe and C (nice if there was, since we could then calculate with certainty what that mass is).
How can I think my way out of the problem when the problem is the way I think?

eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

### Re: Why does light move at the speed it does?

There isn't. The speed of light (or rather, the speed of massless particles) is simply due to the local geometry of spacetime (equivalently the geometry of spacetime in vacuum). All of this is independent of the total mass or distribution of mass within spacetime.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

eSOANEM wrote:There isn't. The speed of light (or rather, the speed of massless particles) is simply due to the local geometry of spacetime (equivalently the geometry of spacetime in vacuum). All of this is independent of the total mass or distribution of mass within spacetime.

Damn

Something in me just balks at the idea that something so fundamental could, essentially, be arbitrary (but I guess that's humans for you).

That said, isn't there a slight tautology? How do we know that the specifics of spacetime geometry aren't dependent on, say, the mass of the universe or some other similar value?
How can I think my way out of the problem when the problem is the way I think?

drachefly
Posts: 197
Joined: Thu Apr 23, 2009 3:25 pm UTC

### Re: Why does light move at the speed it does?

Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

But one what?
How can I think my way out of the problem when the problem is the way I think?

cyanyoshi
Posts: 418
Joined: Thu Sep 23, 2010 3:30 am UTC

### Re: Why does light move at the speed it does?

tomandlu wrote:
drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

But one what?

The speed of light is one light-year per year. It is also 299,792,458 meters per second. It is also roughly 1.8026175*10^12 furlongs per fortnight. Where do these different numbers come from?

Tyndmyr
Posts: 11443
Joined: Wed Jul 25, 2012 8:38 pm UTC

### Re: Why does light move at the speed it does?

Arkham wrote:I know science isnt aroudn to satisfy me, but i just find the answer "it just is" to be highly unsatisfying.

Which is fine...that sort of attitude is why we keep poking at stuff. We're working with best available knowledge in physics, not absolutely perfect knowledge. As mentioned by others, some of the stuff we've gotten so far is frigging crazy complicated to understand, let alone discover.

Lots of people do look for some sort of unifying theory that makes everything in physics connected and MUCH more satisfying, but that's kind of an impressive task to take on, and we really don't have such an answer yet.

Frenetic Pony
Posts: 177
Joined: Fri Jan 27, 2012 6:31 am UTC

### Re: Why does light move at the speed it does?

cyanyoshi wrote:
tomandlu wrote:
drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

But one what?

The speed of light is one light-year per year. It is also 299,792,458 meters per second. It is also roughly 1.8026175*10^12 furlongs per fortnight. Where do these different numbers come from?

Our heads, mathematical frameworks are arbitrary, those numbers are just units that we happened to have come up with and work for us. You could measure "the speed of light" in furlongs or acres* or whatever unit it is you want.

*Ok, an acre is a unit of area squared, but you get the idea

Hypothetically, the planck length, and thus all other planck units, are a "real" fundamental unit. But in reality no one can agree how long a planck length is, and thus any other unit derived from it, or even show that a planck length has any actual physical consequence whatsoever and isn't just another mathematical artefact. But then to be fair, a planck length is also really, really, REALLY (lots of really's) small, so it's pretty hard to test it. Though so far all tests that have been tried, such as measuring photon travel time from distant supernovae or whatever, have turned up nothing.

gmalivuk
GNU Terry Pratchett
Posts: 26836
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

### Re: Why does light move at the speed it does?

Acres per furlong-fortnight.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

drachefly
Posts: 197
Joined: Thu Apr 23, 2009 3:25 pm UTC

### Re: Why does light move at the speed it does?

I think the better way of rephrasing the original question is,

"Given that there is a speed built into this system, why does anything else have the speed that IT does?"

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

gmalivuk wrote:Acres per furlong-fortnight.

In Turkish Lira.
How can I think my way out of the problem when the problem is the way I think?

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

tomandlu wrote:
drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

But one what?

Just one: it's a pure number.

Speed is a ratio: (distance traversed) / (time taken), so in Newtonian mechanics speed has units of (length / time). But one of the fundamental principles of Special Relativity is that spatial distance and temporal duration are related geometrically. You can think of speed in SR as the slope of a right-angle triangle, so it's a pure number, with no associated dimensions or units. It's a triangle in Minkowski spacetime, so it's a little different to a Euclidean right-angle triangle. But it's still a right-angle triangle.

In SR, a distance of 1 light-second is the same size as a time interval of 1 second, so the ratio of (1 light-second)/(1 second) has magnitude (size) 1, corresponding to a 45° right-angle triangle.

For further details, please see the spoiler.

Spoiler:
Note that Special Relativity doesn't say that there's no difference between spatial distance and temporal duration. Given any two events (points in spacetime), we can connect those two points with a line segment through spacetime. That line segment is called a spacetime interval.

Relativity says that it's valid to use units of time or units of length to measure the magnitude of spacetime intervals. So it's valid to use units of time to measure distances; similarly, it's valid to use units of length to measure time periods (although that's less common).

There are 3 kinds of spacetime interval: space-like, light-like, and time-like. If two events are simultaneous but separated in space, then the interval between them is purely spacelike. If the two events occur at the same spatial location but are separated in time, then the interval between them is purely timelike. And if the two events lie on a line in spacetime that is (or could be) connected by a ray of light then the interval between them is lightlike.

In a Minkowski spacetime diagram (which is usually drawn with only one dimension of space to keep things simple (and easy to draw)), pure spacelike intervals are horizontal (0°), and pure timelike intervals are vertical (90°). And by using appropriate units for distance and time we get lightlike intervals at 45°. It's appropriate to do this because SR says that the magnitude of a timelike interval of 1 second is identical to the magnitude of a spacelike interval of 1 light-second, even though they are oriented at 90° with respect to each other.

An interval that's greater than 0° but less than 45° is called spacelike, since it's mostly spacelike, with a little bit of temporal displacement. Similarly, an interval that's greater than 45° but less than 90° is timelike.

Now for the important bit. All inertial observers will agree on the quality of the spacetime interval between two given events: whether it is space-like, light-like, or time-like. They will also agree on the magnitude of that spacetime interval, but observers in different rest frames will disagree on the exact mix of space and time in a given interval.

Note that for two events that are separated by a lightlike interval the spatial distance between the two events is equal in magnitude to the time separation between them, so the ratio of distance/time is 1, and all observers agree that it's 1. That's actually the point of all this geometrical shennanigans.

To calculate the magnitude of a spacetime interval from the temporal duration and the spatial displacement between the two events is simple: we use Pythagoras' theorem. But we need to use a modified form of the usual Euclidean formula, because we're working in Minkowski spacetime, not Euclidean space.

The formula is
T² = t² - s²,
where t is the time difference between the two events and s is the spatial distance.

If the interval is timelike, then T² is positive; if the interval is spacelike, then T² is negative; and if the interval is lightlike,T² is zero. For timelike intervals, T is the proper time experienced by an observer traveling between the two events.

For example, say we have a spaceship traveling with constant speed on a straight line. The ship emits two radio signals, 12 seconds apart (in the ship's rest frame); those two signals will be our events. Let's call the first signal E1 and the second signal E2. In the ship's frame, the ship is stationary (by definition), so the interval from E1 to E2 is purely timelike, with a magnitude of 12 seconds.

E1 and E2 are also observed by 3 observers (let's call them A, B & C) in different rest frames. Each observer records the spatial position of E1 and E2 and measures the time duration between them. To do this properly, they have to compensate for the light travel time, but that's just a minor technical issue that I'll henceforth ignore.

Observer A says the time between E1 and E2 is 13 seconds, and the distance is 5 light-seconds. So the interval E1-E2 is timelike in A's frame, with a magnitude of sqrt(13² - 5²) = 12 seconds. The ship's speed in A's frame is 5/13.

Observer B says the time between E1 and E2 is 15 seconds, and the distance is 9 light-seconds. So the interval E1-E2 is timelike in B's frame, with a magnitude of sqrt(15² - 9²) = 12 seconds. The ship's speed in B's frame is 9/15 = 3/5.

Observer C says the time between E1 and E2 is 20 seconds, and the distance is 16 light-seconds. So the interval E1-E2 is timelike in B's frame, with a magnitude of sqrt(20² - 16²) = 12 seconds. The ship's speed in C's frame is 16/20 = 4/5.

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

PM 2Ring wrote:
tomandlu wrote:
drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

But one what?

Just one: it's a pure number.

Well, yes, but it's still a specific speed, and a very special one given that it's the same in all frames of reference (in a vacuum). Is it really enough to just say "well, it had to be some speed," or is that just a place-holder for now with an underlying assumption that it's not just some random value picked from a hat?
How can I think my way out of the problem when the problem is the way I think?

Neil_Boekend
Posts: 3220
Joined: Fri Mar 01, 2013 6:35 am UTC
Location: Yes.

### Re: Why does light move at the speed it does?

As with most (or perhaps all) constants with units in physics it is just the result of our choice in units. If we had chosen the meter double as long then light speed would have been half as high.
Even some of the constants without units depends on our choice in units. If we had chosen a gram differently then Avogadro's number would have been different.
Mikeski wrote:A "What If" update is never late. Nor is it early. It is posted precisely when it should be.

patzer's signature wrote:
flicky1991 wrote:I'm being quoted too much!

he/him/his

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

tomandlu wrote:I was idly wondering today if there might be a relationship between the mass of the universe and C (nice if there was, since we could then calculate with certainty what that mass is).

(BTW, lightspeed should be written with a small "c", but you can italicise it if you wish).

The large-scale spacetime curvature of the universe is probably zero, or perhaps slightly negative. If so, it has infinite mass. Of course, a finite subset of the universe, eg the observable universe, has a finite mass, but such a subset is relatively arbitrary.

FWIW, if we express lightspeed in terms of rapidity rather than as a speed, then it, too, is infinite. But I guess that's not the sort of answer you're looking for.

tomandlu wrote:
drachefly wrote:Light-speed is only as arbitrary as the number 1.

In 'natural' units, that's its speed. No units, even. Just, use 1 as a velocity and there you go.

tomandlu wrote:But one what?

PM 2Ring wrote:Just one: it's a pure number.

Well, yes, but it's still a specific speed, and a very special one given that it's the same in all frames of reference (in a vacuum). Is it really enough to just say "well, it had to be some speed," or is that just a place-holder for now with an underlying assumption that it's not just some random value picked from a hat?

I guess it's a fair question to ask why 299792458 metres has the same magnitude as 1 second. It's not really an answerable question in current mainstream theory, but it might be once we have a working theory of quantum gravity, or at least some solid theory that unites all the fundamental interactions.

As thoughtfully mentioned on page 1, in some of the various multi-verse theories c is some random value picked from a hat. But even if that's the case c can't be totally random. As SlyReaper mentioned, if you try to build a universe with a c that's too different to our c you get a dud universe. Sure, you can partly compensate by juggling the other fundamental constants, but there's only so much leeway.

A bad set of constants impacts the stability of the universe or its contents. Lee Smolin speculates that for the vast majority of combinations of constants the universe re-collapses within a few Planck time units of its birth. With a slightly better combination of constants the universe is stable enough to exist, but its dimensions remain in a compactified state, so nothing much can happen there. With an even better set of constants spacetime expands like it did for us, but you don't get any stable particles of matter. Or you get stable particles, but you can't form stable atoms. Or you get stable atoms but they can't combine to form stable molecules. Or you can have stable molecules but space expands so fast that the matter doesn't have a chance to accumulate and form into stars & planets, or gravity is too strong & everything just gets stuck in a black hole. Or you can build stars, but they burn out too quickly, so you don't have time for complex stuff like life to evolve. Et cetera.

Smolin mentions in The Life of the Cosmos that there some crude simulations have been performed along these lines to get a rough idea of the permissible ranges of the various constants that lead to universes that are sufficiently stable to produce multiple black holes, but I don't know to what extent such simulations have improved in recent years. I suspect that it might be a long time before we have the computing power to adequately attack this question, but I still think it's cool that we can, at least in principle, validate (or invalidate) Smolin's Fecund Universes hypothesis via computer simulations.
Last edited by PM 2Ring on Thu May 07, 2015 1:04 pm UTC, edited 1 time in total.

eSOANEM
:D
Posts: 3652
Joined: Sun Apr 12, 2009 9:39 pm UTC
Location: Grantabrycge

### Re: Why does light move at the speed it does?

Saying that moving c changes things is a bit misleading. Only dimensionless constants are meaningful when it comes to things like distinguishing between universes. This includes things like the ratio of the proton-electron masses, things like the fine structure constant or even any quantity expressed in natural units (which is now a pure number). Provided those are kept constant, a change in the value of c would not be detectable.

Talking about changes in c hides the fact that it's these dimensionless constants which lead to different cosmologies and not changes in the dimensionful constants.
my pronouns are they

Magnanimous wrote:(fuck the macrons)

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

PM 2Ring wrote:
tomandlu wrote:I was idly wondering today if there might be a relationship between the mass of the universe and C (nice if there was, since we could then calculate with certainty what that mass is).

(BTW, lightspeed should be written a small "c", but you can italicise it if you wish).

<snip>

Many thanks - so we could consider c to be another aspect of the anthropic principle? (and then spend 10 pages arguing about that )
How can I think my way out of the problem when the problem is the way I think?

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

eSOANEM wrote:Saying that moving c changes things is a bit misleading. Only dimensionless constants are meaningful when it comes to things like distinguishing between universes. This includes things like the ratio of the proton-electron masses, things like the fine structure constant or even any quantity expressed in natural units (which is now a pure number). Provided those are kept constant, a change in the value of c would not be detectable.

Talking about changes in c hides the fact that it's these dimensionless constants which lead to different cosmologies and not changes in the dimensionful constants.

Yeah, ok. It is a bit misleading; I should've mentioned that stuff about the dimensionless constants. FWIW, I did read Martin_Rees' Just Six Numbers many years ago, so I know what you're getting at. My excuse is that I was trying to keep things (relatively) simple, and focus on the impact of changes to the real fundamental constants that would make detectable changes to c (when expressed in a way that makes it not equal to 1, eg using things like the Bohr radius as a unit of length). OTOH, to paraphrase Einstein, one should simplify things as much as possible, but no further.

sevenperforce
Posts: 658
Joined: Wed Feb 04, 2015 8:01 am UTC

### Re: Why does light move at the speed it does?

Arkham wrote:So if I'm understanding this right, The speed of light is c, while many other things are also set to c as well.

The SoL does not define C, C defines what the SoL is, as well as the speed of gravity and any other massless particle. We just call it the speed of light due to historical nomenclature.

Thus, C is really the universal constant.

Ok, so then what I guess I am not getting is, why is C the speed that it is? Why that specific number, and not any other arbitrary value?

This is echoing something someone else said before, but I think it bears repeating...

C is the conversion factor between space and time. Time is the fourth dimension of space; one second of time is equal to 186,000 miles of space. Asking "why is the speed of light 3e8 m/s?" is thus similar to asking "why are there 12 inches in a meter?" or "why are there 1000 millivolts in a volt?" or "why are there 2000 pounds in a short ton?" These are all things which are simply defined by reference to one another. If you measure the "length" of time you will find it to be 3e8 m per 1 second. It just so happens that massless particles like photons are the most straightforward tool we have to carry out this measurement.

Why is 1 second equal to 300,000 kilometers? I suppose that's where we have to punt to the anthropic principle.

Copper Bezel
Posts: 2426
Joined: Wed Oct 12, 2011 6:35 am UTC
Location: Web exclusive!

### Re: Why does light move at the speed it does?

c. Little c.

And there's a bit under thirty-nine and a half inches in a meter, actually.

I've heard that this "conversion rate" thing and the idea of tipping between dimensions is a heuristic. Is this description of simply tilting a bit into the time axis of a 4-D graph actually physically accurate?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

she / her / her

tomandlu
Posts: 1111
Joined: Fri Sep 21, 2007 10:22 am UTC
Location: London, UK
Contact:

### Re: Why does light move at the speed it does?

Copper Bezel wrote:c. Little c.

And there's a bit under thirty-nine and a half inches in a meter, actually.

I've heard that this "conversion rate" thing and the idea of tipping between dimensions is a heuristic. Is this description of simply tilting a bit into the time axis of a 4-D graph actually physically accurate?

This is the bit I don't get. To me, it doesn't look heuristic - in Minkowski space, everything seems to navigate between time and distance quite naturally. They appear to be the equivalent on NS and WE - the same 'thing'. My understanding was that everything travels at c - it's just that for most of us, we travel a little bit in space and a lot of bit in time, and the total is always c (whereas light, for example, only travels in space, not time).

However, in another thread about G, this has been qualified by people who actually know what they are talking about... (I think - eSOANEM might be able to clarify)

My vague understanding is that, although there is an implied 'sameness' to time and distance, the angle of the light-cones, and consequently how you translate between the two, is arbitrary, which if they were truly the same thing, would not be the case. Is that (sort of) right?
How can I think my way out of the problem when the problem is the way I think?

PM 2Ring
Posts: 3715
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

### Re: Why does light move at the speed it does?

Copper Bezel wrote:I've heard that this "conversion rate" thing and the idea of tipping between dimensions is a heuristic. Is this description of simply tilting a bit into the time axis of a 4-D graph actually physically accurate?

Well, all physics is models, so what does it even mean to ask "is it really like that?"?

Combining space & time into Minkowski spacetime is mathematically elegant, but if it offends your physical intuitions you can treat it as a convenient mathematical fiction. But IMHO that's a lot like saying that -1 doesn't actually have a square root, and that i is just a convenient mathematical fiction. Speaking of which, note that if we multiply either spatial distance or temporal duration by i in the Minkowski spacetime distance formula T² = t² - s², then we recover the standard Euclidean distance formula. Of course, visualising a right triangle with sides 5i, 3, 4i isn't particularly easy, but it's still a cute idea, IMHO.

As for the tilting of time & space axes, the tilting is an accurate geometrical representation of what the equations of SR say. But bear in mind that the tilting is happening in spacetime, so it works a little differently to simple spatial tilting.

doogly
Dr. The Juggernaut of Touching Himself
Posts: 5538
Joined: Mon Oct 23, 2006 2:31 am UTC
Location: Lexington, MA
Contact:

### Re: Why does light move at the speed it does?

tomandlu wrote:(whereas light, for example, only travels in space, not time).

Light moves on "null" paths, which have a proper separation of 0. That is, 0 = - (c t)^2 + x^2. (or all three space dimensions). So if c=1, you would say they move through "equal" amounts of space and time.
LE4dGOLEM: What's a Doug?
Noc: A larval Doogly. They grow the tail and stinger upon reaching adulthood.

Keep waggling your butt brows Brothers.
Or; Is that your eye butthairs?