Why, why, why, Mr. Anderson, WHY?

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Discussion Overview

The discussion revolves around the speed of light, specifically why it is defined as approximately 186,000 miles per second (MPS) and the implications of this value. Participants explore theoretical, observational, and conceptual aspects of the speed of light, including its constancy, the choice of units, and the nature of physical constants.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why the speed of light is specifically 186,000 MPS and suggest that it could theoretically be different.
  • One participant explains that the speed of light is defined based on the meter, which is set to make light travel at that speed, indicating that it is a matter of unit choice.
  • Another participant notes that the speed of light is constant regardless of how light is generated, which is a foundational aspect of special relativity.
  • Some contributions discuss the idea that changing physical constants, like the speed of light, is more complex than it seems, as it relates to fundamental principles and experimental results.
  • A participant mentions that while the speed of light is constant in a vacuum, it can vary when light travels through different media, leading to phenomena like refraction.
  • There are references to the implications of changing the speed of light on technology and the laws of physics, with some participants speculating on hypothetical scenarios.
  • One participant expresses confusion about the nature of physical constants and how they are determined, emphasizing the experimental basis for constants like the speed of light.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of the speed of light and its implications. There is no consensus on why the speed of light has its specific value, and multiple competing perspectives are presented regarding the nature of physical constants and their definitions.

Contextual Notes

Some discussions touch on the limitations of current understanding regarding physical constants and the dependence on definitions and experimental results. The conversation also highlights the complexity of changing fundamental constants and the potential effects on physical laws and technology.

Bible Thumper
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Why does light have to necessarily go 186K MPS? Why not 100K MPS? Or 300K MPS?
Why strictly 186K MPS even tho there are numerous, different ways to generate light?

Why, Mr. Anderson? Why?
 
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That's because the meter is defined so that it travels at exactly that speed. If you choose your unit of distance as the lightsecond and your unit of time as second, then light travels at speed 1. So it all depends on the choice of units. Also, the speed of light doesn't depend on how it is generated. It is constant, no matter where or how it is emitted. This is an observational fact, and also the basis of special relativity. It doesn't have any deeper explanation at the moment.
 
Bible Thumper said:
Why does light have to necessarily go 186K MPS? Why not 100K MPS? Or 300K MPS?
Why strictly 186K MPS even tho there are numerous, different ways to generate light?

Why, Mr. Anderson? Why?
In some ways it's meaningless to imagine a "change" in a physical constant that has definite units like the speed of light, although you can imagine changes in "dimensionless" physical constants that are pure numbers (because they are ratios of various physical constants with units)--see this article explaining why, as well as this page summarizing all the dimensionless constants.
 
Who is Mr. Anderson?
 
atyy said:
Who is Mr. Anderson?
Keanu Reeves (it's a line from 'The Matrix')
 
JesseM said:
Keanu Reeves (it's a line from 'The Matrix')

Ah, should have known that - thank you!
 
JesseM said:
In some ways it's meaningless to imagine a "change" in a physical constant that has definite units like the speed of light, although you can imagine changes in "dimensionless" physical constants that are pure numbers (because they are ratios of various physical constants with units)--see this article explaining why, as well as this page summarizing all the dimensionless constants.

Thanks for the very useful references, JesseM.
Forgive my denseness, but it appears that whenever something is massless (or becomes massless), the speed of light is always the inevitable result. And as your reference points out, the author is also bewildered (as I am) about the fine structure constant, and why the fine structure constant is the quantity it is.
I know stuff like Celsius and Fahrenheit are arbitrary and meaningless, but absolute zero is not. Neither is the phenomena of Brownian motion--the latter being an effect of the raising of these arbitrary and meaningless temperature scales.

So I guess I'm still stuck on this 186K MPS. Create any scale you wish (or go with 300 KPS if you wish), the light will still go that speed. So will anything that's massless.
It gets odder when we introduce how light is generated. For example, we can make them by running an excessive current thru wire. Or, exposing some elements to U/V radiation (spontaneous emission). Or, by running electrons close to one another (florescent tubes). Or, by using a blender (radio emission). There are too many different ways at obtaining this same velocity.

Why, why, why?! Vagaries of perception. Temporary constructs of a feeble human intellect trying desperately to justify an existence or something!
 
  • #10
Bible Thumper said:
Why does light have to necessarily go 186K MPS? Why not 100K MPS? Or 300K MPS?
Why strictly 186K MPS even tho there are numerous, different ways to generate light?

Why, Mr. Anderson? Why?
The speed of light is exact. And exactly the same for all inertial observers. If this were not so, human thought as electric current would not generate a magnetic field, and the Matrix could not exist.

http://physics.nist.gov/cgi-bin/cuu/Value?c
http://en.wikipedia.org/wiki/Magnetoencephalography
http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation
 
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  • #11
Why does light have to necessarily go 186K MPS? Why not 100K MPS? Or 300K MPS?

No one knows! We are unable to calculate all physcial constants based on fundamental principles. Hence the speed of light, the charge of the electron, the strength of the strong nuclear force, the gravitational constant G are all determined experimentally. Another way to make an equivalent statement is that the standard model of particle physics has so far been unable to determine any of the basic constants in our universe...they are based on experimental results, not theory. String theory has the abilty to produce many constituents...too many, in fact. So far in string theory you can pick almost any value you like for many of the basic forces and particles...nobody knows how to make the approximate perterbation solutions match only what we observe in this universe.
 
  • #12
Bear in mind that this is the speed of light IN A VACUUM. When light travels through a medium such as air or water, it slows down to a speed of .. well, it depends on the medium.
This slowing down is basically (without going into details) what causes refraction.
 
  • #13
If the speed of light is globally changed would anyone notice?

Obituary: I see BibleThumper finally thumped-out. A slow motion, inevitable tragedy--a car crash in slow motion. It seems the contra-thumper faithful got thumped out.

The universe just made itself from nothing. I know the line.
 
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  • #14
The fact that the speed of light is that particular number IS known. It is because the meter is defined in such a way as to make the speed of light exactly equal to that.

Nor is it peculiar that the speed of light is a constant in the usual sense. It is a wave and the speed of a wave, relative to its "carrier", depends on the properties of "carrier". Light is an electromagnetic wave carried by space itself and it appears that the properties of space area always the same, hence always to same speed, relative to space.

But that should mean that we, moving through space, should then be able to use light to measure our speed "relative to space" and so get an absolute speed. Ever since the Michaelson-Moreley experiment we have known we can't do that and all of relativity is to determine why that is so. "Why is the speed of light constant relative even to moving reference systems" is the question relativity was designed to answer.
 
  • #15
Phrak said:
If the speed of light is globally changed would anyone notice?
I'd be pretty upset, since my WiFi router would crap out and I wouldn't be able to surf PF anymore!
 
  • #16
russ_watters said:
I'd be pretty upset, since my WiFi router would crap out and I wouldn't be able to surf PF anymore!

how? I'd be inclined to think that the laws of physics are invariant under a rescaling of c.
 
  • #17
Phrak said:
how? I'd be inclined to think that the laws of physics are invariant under a rescaling of c.
I also assumed you meant that the laws of physics would remain unchanged, just the value of C changed. But if you change the value of C, then things (such as radio frequencies) that depend on that value also change. Since we're speaking hypothetically, it is possible that both my router and wifi card would change the same way if their antenna and electronics are similar, but that still doesn't mean the new frequency would be able to find its way through my house.

Now, my current router only has one antenna, but my last one had two and the spacing of the antennas was based on half the wavelength of the signal being transmitted. Change the wavelength and the system loses its constructive interference.

Anything that depends on the frequency of em radiation could potentially be affected. And then there's the concern about the sun no longer working or that nuclear plant 5 mile from my house exploding...
 
  • #18
It would be interesting if the speed of light were slower than the speed of sound. For example, you would hear the thunder before you saw the lightning flash.
In Terry Pratchett's Discworld novels, the speed of light is very slow, making for some interesting physical effects.
 
  • #19
Phrak said:
how? I'd be inclined to think that the laws of physics are invariant under a rescaling of c.
Because physical distances would NOT change. Circuits would behave differently, GPS signals would behave differently, human bodies would behave differently. Heck, molecules and atoms might behave differently.

[EDIT] D'oh. Active thread.
 
  • #20
DaveC426913 said:
Because physical distances would NOT change. Circuits would behave differently, GPS signals would behave differently, human bodies would behave differently. Heck, molecules and atoms might behave differently.

[EDIT] D'oh. Active thread.

I'll try to simplify. Under a rescaling of c, if all velocites didn't rescale by the same factor, you would have a point. But I don't think this is physically missible.

what experiment can you preform that would yield different results under a global rescaling of c?

Film an experiment taking place. The measured values that the experimenters obtain are independent of whether the tape is payed back at normal speed or speed-up.
 
  • #21
Only changes in dimensionless physical constants (constants that are pure numbers with no units) would be noticeable. Now, all the dimensionless constants can be stated in terms of ratios of various constants that do have units, like the fine structure constant (also known as the 'alpha constant' or the 'electromagnetic coupling constant') or like the ratio of the electron mass to the Planck mass. You can see a list of all known dimensionless constants here...most of the ratios which define them would include c somewhere. The problem is that if various dimensionless constants changed, even if you could interpret this as a change in the value of c, you could probably also equally well interpret it as a change in other constants like Planck's constant with no change in c, and get exactly the same changed values for the dimensionless constants. Likewise, if you imagine changing a combination of constants with units in such a way that all the ratios which define the dimensionless constants stay exactly the same, then this "change" would be physically meaningless since there'd be absolutely no measurable difference in any experiment. This is basically the reason that most physicists would say it's meaningless to talk about changes in physical constants with units, as opposed to dimensionless constants, I think.
 
  • #22
Hi Jesse. I anticipated an argument about units which is why I left the question sufficiently ambiguous, though I don't see how this would preclude global invariance over c, either.

The expression G/hc is unitless.
So then, according to what you're saying, a global variation in c would neccesitate that G/hc is constant, right?

We may ask, instead, would experiments yield different results over a global change in G/hc?

A single example is all that would be needed for either G/hc=const, or G/h=const.
 
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  • #23
Phrak said:
The expression G/hc is unitless.
So then, according to what you're saying, a global variation in c would neccesitate that G/hc is constant, right?
No, we can certainly imagine a dimensionless ratio like G/hc changing. My point was that if it did change, how would you know it was changing because c increased, as opposed to because G decreased or h increased? You could leave c unchanged and fiddle with the other constants to produce the same change in the dimensionless ratios. In such a situation, I don't see how it's physically meaningful to ask which of the constants with units are "really" changing and which aren't, you can only talk about which dimensionless ratios changed and by how much.
 
  • #24
JesseM said:
No, we can certainly imagine a dimensionless ratio like G/hc changing. My point was that if it did change, how would you know it was changing because c increased, as opposed to because G decreased or h increased?
That's what I said.


I see no examples counter dimensionful of dimensionless changes.
 
  • #25
Phrak said:
That's what I said.I see no examples counter dimensionful of dimensionless changes.
Can you clarify what is meant by the phrase "dimensionful of dimensionless changes"?
 

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