299792459

[h.g.Whiz]

Could the velocity of light change in the future. I was thinking that it might travel faster. :jaw-dropp

[KingMerv00]

Will water stop being wet?

[paximperium]

Will chickens ever attack and destroy mankind?

[catbasket]

Will chickens ever attack and destroy mankind?
You have missed a very important memo.

[h.g.Whiz]

Will water stop being wet?


I suppose, if you can figure out how to freeze it. :cool:

[h.g.Whiz]

Will chickens ever attack and destroy mankind?


Why are you asking me, you're the one with the chicken obsession ? :cool:

[h.g.Whiz]

I think its odd that velocity of light is constant. Think about it, How can anything that travels be able skip acceleration and goes instantly to top speed.

[TheDaver]

299792459
867-5309!

[Thabiguy]

I think its odd that velocity of light is constant. Think about it, How can anything that travels be able skip acceleration and goes instantly to top speed.

Light does not go to "top speed"; it is at that speed whenever it exists. It doesn't skip acceleration; it never needs any.

[lionking]

It can't go faster. But it can certainly go slower. Like when it hits a brick wall.

Any other questions?

[KingMerv00]

I think its odd that velocity of light is constant.

Why? Photons act differently cannonballs and such but that doesn't mean it is "odd".

[KingMerv00]

I suppose, if you can figure out how to freeze it. :cool:

Then it wouldn't be water anymore. It would be ice. :p

[Mojo]

Then it wouldn't be water anymore. It would be ice. :p


And ice has memory: "An example may serve to clarify the concept here: if we take a little water and put it in the freezer, after a certain period of time it will freeze. On removing the water from the freezer, it will be observed that the block of ice, though now exposed to room temperature, will remain a block of ice for some time. Thus, there exists in water a property which enables it to "remember" for a certain amount of time that it has been kept in the freezer."
—Paolo Bellavite, M.D. and Andrea Signorini, M.D., The Emerging Science of Homeopathy: Complexity, Biodynamics, and Nanopharmacology, 2002, pp.68-69

[geni]

Could the velocity of light change in the future. I was thinking that it might travel faster. :jaw-dropp

Best we can tell the speed of light has not changed in at least thr last 2 billion years so we do not expect any other changes at this point.

[Ysidro]

867-5309!

Stop calling me!

[zooterkin]

Will water stop being wet?

More importantly, will waterwater?

[aggle-rithm]

I think its odd that velocity of light is constant. Think about it, How can anything that travels be able skip acceleration and goes instantly to top speed.

Isaac Asimov wrote a short story about that. It involved an inventor who came up with an anti-gravity machine. The first time he tried it out was at a public press event, where a billiard table with a hole in the middle was set up. As the cue ball approached the hole, it passed through an anti-gravity field that made it resist gravity in the only way possible--by giving it zero mass.

As soon as the ball entered the field, it immediately began moving at the speed of light. It began to slow down as soon as it exited the field, but was still moving at a very high speed. The result was a huge flash of light and a perfect billiard-ball shaped hole right through the chest of the inventor.

It was explained afterwards that anything with zero mass MUST move at the speed of light, and anti-gravity machines were rendered forever impractical.

[aggle-rithm]

Could the velocity of light change in the future. I was thinking that it might travel faster. :jaw-dropp

If light traveled faster, then the matter would change as well. It would take more energy to form matter, and more energy would be released when matter was converted back to energy.

Probably, the properties of gluons would change as well, and things might well fall apart.

All in all, I prefer things the way they are.

[Starthinker]

I think light has gotten away with speeding for far too long.

[ClassyElf]

It can't go faster.

Maybe I'm misinterpreting this, but it looks like you're wrong.

http://partners.nytimes.com/library/national/science/053000sci-physics-light.html

[Badly Shaved Monkey]

Light does not go to "top speed"; it is at that speed whenever it exists. It doesn't skip acceleration; it never needs any.

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

I know I have said that badly, can a grown-up please tidy it up for me?

[Skwinty]

Maybe I'm misinterpreting this, but it looks like you're wrong.

http://partners.nytimes.com/library/national/science/053000sci-physics-light.html

Check this thread for an answer.

http://forums.randi.org/showthread.php?postid=4567070#post4567070

[Tim Thompson]

... How can anything that travels be able skip acceleration and goes instantly to top speed.
So it all comes down to common sense. Light does not behave the way you think it should behave, based on common sense, and that bothers you. But "common sense" comes from "common experience" with an essentially Newtonian (http://en.wikipedia.org/wiki/Newtonian_mechanics) world. The universe is not constrained by common sense. Rather, common sense is constrained by the universe. One of the great lessons of 20th century physics, and perhaps the greatest lesson, is that we now know the universe does not obey the rules of common sense on scales of time & space removed from the scale of common sense. The very tiny universe (quantum mechanics (http://en.wikipedia.org/wiki/Quantum_mechanics)) and the very large universe (general relativity (http://en.wikipedia.org/wiki/General_relativity)) both actually behave in bizarre fashions that are not at all compatible with our common sense.

Light is one of those things that ignores common sense. Light does not "accelerate" at all, it starts out at top speed. Not common sense, but that's the way it is. Is it a particle or a wave (http://en.wikipedia.org/wiki/Wave-particle_duality)? Yet another slap in the face of common sense. At some point we have to give up on the idea of being bugged by the fact that the universe does not work the way we think it should work, and simply settle for being able to figure out how it actually does work.

[Perpetual Student]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

I know I have said that badly, can a grown-up please tidy it up for me?

That's interesting. If we take the usual form of the Lorentz transformation for mass:

m_1 = \dfrac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}

using a little algebra we can tranform it to:

v = c{\sqrt{1-\dfrac{m^2_0}{m^2_1}}

Now, looking at m0, we can see that v --> c as m0 --> 0. However, I guess a complete picture would also include the transformation for time.

[Bob Klase]

Best we can tell the speed of light has not changed in at least thr last 2 billion years so we do not expect any other changes at this point.

If we can't determine that it's changed, would it make any difference if it had?

[aggle-rithm]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

I know I have said that badly,

Not at all. In fact, you just blew my mind!

[tsig]

Will chickens ever attack and destroy mankind?

No but they save the cows: http://www.albinoblacksheep.com/flash/cowswithguns

[geni]

If we can't determine that it's changed, would it make any difference if it had?

We can determine that it hasn't changed to any significant extent in the last 2 billion years. Useing measurements from distant gallaxies we can go back a few more billion years. It is posible that C was different in the early universe but the data is inconclusive.

Difference? Well obviously it would mess with photons and E=MC2. Other effects would depend on how other physical constants were effected.

[Tumblehome]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.


I like that. I don't know if it's true or not, but in the words of a wild-haired physicist, it's simple and elegant. :)

ETA: Wouldnt' that imply that space and time are constant?

[Thabiguy]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

This notion was popularized in Brian Greene's Elegant Universe.

The basic idea is that if you (in a fixed reference frame) take speed through three-dimensional space and, compensating for unit incompatibility, "speed through time", which Greene defines as the rate of change of proper-time with respect to time, calculation shows that the resulting velocity 4-vector does have a constant magnitude, which can be associated with "speed of light".

Now, I think this way of looking at special relativity is somewhat interesting, but not necessarily optimal. Here's what I think are the pros and cons of such approach:

Pros:

- It offers a relatively simple insight into the effects of time dilation. When looking at it this way, it becomes intuitively apparent that things moving faster will age slower, as they have "less speed left" to move through their "time". It even allows to calculate correct results of simple time dilation problems.
- It has a "ooh, I feel enlightened" quality. This may make one feel less scared of SR. And if it's the only thing one remembers about SR, it will probably be more "correct" than most other partial understandings of SR that people have.

Cons:

- It has a "ooh, I feel enlightened" quality. It may seem like it conveys the fundamental basics of special relativity, and that if you understand this, you understand special relativity. But that's not true at all.
- It is a limited description of just one aspect of SR. While it offers an insight of time dilation, it offers no insight at all of other SR effects that are just as important as time dilation, such as length contraction or relativity of simultaneity. Also, it requires everything to happen in a fixed reference frame, offering no simple way to account for a change in reference frame - something crucial in SR.
(ETA: To illustrate just how limited the description is - it can't explain the twin paradox, the immediate consequence of time dilation. Which of the twins will be older at reunion, as they both see the other one as moving? SR readily answers that it's the inertial twin that will be older, while Greene's intuitive insight alone doesn't even explain what an inertial observer is.)
- It can reinforce misleading ideas. Specifically, the notion of "absolute" velocity of objects. But the whole idea of special relativity is that there is no absolute velocity. This way of looking at things does nothing at all to help convey this key aspect of SR.
- The idea is ill-worded. It uses the phrase "speed through time", but what it actually talks about would be better described as "speed through proper time". Although intuitively, "speed through time" will often be understood as the author intended, this actually relies on intuitive misinterpretation to describe a concept. That's not good. (When somebody will try to think about it in more depth, it will backfire.)
- The idea is often incorrectly expressed as "speed through spacetime is always the speed of light". But the 4-vector in the calculation does not actually deal with spacetime. What it deals with can at best be called "space-proper-time". That's an artificial construct significantly different from spacetime, and much less useful. When formulated as above, it seems that the idea tells you something about spacetime - while in fact it doesn't.

So these are my thoughts.

[Badly Shaved Monkey]

When I posted, I decided I couldn't handle the concept of proper time correctly.

Can you explain it briefly, but lucidly, please?

[Skwinty]

When I posted, I decided I couldn't handle the concept of proper time correctly.

Can you explain it briefly, but lucidly, please?

Einstein once said "Time is what clocks measure"
Our perception of reality is intertwined with our concept of time.
There is no past,as it is gone and there is no future as it hasn't happened yet.There is only now, a demarcation between past and present.
Thus, the real world is a collection of simultaneous events:)

[Thabiguy]

When I posted, I decided I couldn't handle the concept of proper time correctly.

Can you explain it briefly, but lucidly, please?

I'll try to explain with an example.

Let's take the basic setup of the twin paradox - you are on Earth, while your twin brother takes off on a rocket, flies about wildly, and then comes back to meet you.

You meet at the same point in space, and at the same point in time - it's not like you are there on Monday, and he's there on Tuesday, and you're wondering where the heck the other one is. No, your meeting will be simultaneous, there will be no difference in time.

Your difference will be in proper time. His clock will show less than yours, indicating that less time has passed for him. You will have eaten seven breakfasts, he will have eaten six. Each will have experienced different proper time between departing and rejoining.

I hope that helped; if not, Wikipedia (http://en.wikipedia.org/wiki/Proper_time) explains it more thoroughly, but perhaps less accessibly.

[Badly Shaved Monkey]

So it is "proper" in the sense that it is proper, i.e. specially pertaining, to the internal clock of the object under consideration. Could it, therefore, also be called subjective time, the time experienced by an object, but the semantic term 'proper' is more generalised and avoids the need or implication for that object to be a conscious entity?

[Thabiguy]

Yes, that sums it up rather nicely.

[JimBenArm]

I thought this was going to be one of those SafeLock commercials.

"Hi, I'm h.g. whiz, and my real social security number is..."

[Beerina]

I think its odd that velocity of light is constant. Think about it, How can anything that travels be able skip acceleration and goes instantly to top speed.

I always interpreted it as not being about light per se, but that we live in a 4-D spacetime continuum, in which you're always traveling at the speed of light, as measured by your x, y, z, and t axes. Hence the speed is fundamental and built into reality at the base level. "Light" travels at that speed because that's just the speed massless particles travel at, being massless.

[The Man]

I think its odd that velocity of light is constant. Think about it, How can anything that travels be able skip acceleration and goes instantly to top speed.


Actually there are some interesting points about this question. First how can you tell if anything is accelerating? We can perhaps observe its position at one point in time and then observe its position again at some other point in time. From that we have a delta X (change in position) over delta T (change in time) or a velocity (speed if you’re not concerned about the direction). However this can tell us absolutely nothing about acceleration. For acceleration we need a change in velocity (delta V) over a change in time (delta T). So we would need at the very least three observations of position with respect to time to observe an acceleration. We can determine the velocity between the first and second positions, then between the second thrid and use them to determine that there is a change in velocity. For any accelerating body it will start with some original velocity (VO) and end with some final velocity (VF). That VF of the previous segment becomes the VO for the observation of the next segment. Even if that acceleration is not uniform we know that the velocity we can determine between any two positions simply represents an average velocity (VA) or the constant velocity that would traverse the same distance in the same amount of time. Since we are considering the observation of an acceleration we can also know the finial velocity (VF) should be different then that average velocity (VA) and the original velocity (VO). In fact if you were to chart the finial velocity and the average velocity of an accelerating body based on special relativity you would find that as the final velocity (VF) approaches c the average velocity (VA) from rest (VO=0) to that VF also approaches c. So for light its average velocity is its finial velocity and both are c, so the very concept of acceleration simply becomes moot as you have no way of differentiating between any two positions VF from VA which also makes VO the same as well, all would be c.

[Thabiguy]

I always interpreted it as not being about light per se, but that we live in a 4-D spacetime continuum, in which you're always traveling at the speed of light, as measured by your x, y, z, and t axes.

I'm afraid this shows the misunderstandings that Greene's "insight" can lead to.

1. The notion of speed at which you can be "measured" to travel through "spacetime continuum". - In SR, there is no absolute velocity, and no measurement can be with respect to "spacetime continuum", but only to a specifically selected reference frame.

2. The notion that (in a fixed reference frame) the "always-at-speedlight" idea is about "velocity" in x, y, z and t, the axes of Minkowski spacetime. - While actually the idea is not about "velocity" in t, but in τ - proper time, as opposed to coordinate time of spacetime geometry. Greene's idea doesn't talk about spacetime, but about "space-proper-time".

[linusrichard]

It was explained afterwards that anything with zero mass MUST move at the speed of light, and anti-gravity machines were rendered forever impractical.

I want to live in the world in which this short story takes place - where someone invents an amazingly effective weapon, and it is considered "forever impractical."

[Roboramma]

I want to live in the world in which this short story takes place - where someone invents an amazingly effective weapon, and it is considered "forever impractical."

It would also be pretty good for power generation.

[h.g.Whiz]

Sol Invictus are you out there, are you viewing this ?

[Reality Check]

Could the velocity of light change in the future. I was thinking that it might travel faster.
Why do you think that it might travel faster? It might travel slower.

We now that the speed of light have been constant over the last few billion years. There is no reason to expect it to change.

[portlandatheist]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

I know I have said that badly, can a grown-up please tidy it up for me?
I'm not a grown up (well, not in the sense you are using the term) but I think you are onto something. From the photon's point of view, they are at point A, then they are at point B, and from their perspective, zero time has passed...there is no "time" for light, only space. Without time, there can be no acceleration.

[h.g.Whiz]

Why do you think that it might travel faster? It might travel slower.

We now that the speed of light have been constant over the last few billion years. There is no reason to expect it to change.


When the velocity in which our universe is expanding exceeds 299792458 metres per second I figured the speed of light might also exceed 299792458 metres per second.

[The Man]

Well if you are speaking of the expansion of the metric of spacetime or more specifically cosmological inflation then you should try to understand what that means.


http://en.wikipedia.org/wiki/Metric_expansion_of_space


Metric for spacetime
Points on the surface of the Earth can be specified by giving two coordinates. Because space-time is four dimensional, we must specify points in space-time by giving four coordinates. The most convenient coordinates to use for cosmology are called comoving coordinates. Because space appears to be Euclidean, on a large scale, one can specify the spatial coordinates in terms of x,y, and z coordinates, though other choices such as spherical coordinates are also commonly used. The fourth required coordinate is time, which is specified in comoving coordinates as cosmological time. Though large-scale space appears to be Euclidean, the same cannot be said for the metric of space-time. The non-Euclidean nature of space-time manifests itself by the fact that the distance between points with constant coordinates grows with time, rather than remaining constant.


See also.

http://en.wikipedia.org/wiki/Cosmic_inflation



Space expands
To say that space expands exponentially means that two inertial observers are moving farther apart with accelerating velocity. In stationary coordinates for one observer, a patch of an inflating universe has the following polar metric:

This is just like an inside-out black hole metric—it has a zero in the dt component on a fixed radius sphere called the cosmological horizon. Objects are drawn away from the observer at r = 0 towards the cosmological horizon, which they cross in a finite proper time. This means that any inhomogeneities are smoothed out, just as any bumps or matter on the surface of a black hole horizon are swallowed and disappear.
Since the space–time metric has no explicit time dependence, once an observer has crossed the cosmological horizon, observers closer in take its place. This process of falling outward and replacement points closer in are always steadily replacing points further out—an exponential expansion of space–time.
This steady-state exponentially expanding spacetime is called a de Sitter space, and to sustain it there must be a cosmological constant, a vacuum energy proportional to Λ everywhere. The physical conditions from one moment to the next are stable: the rate of expansion, called the Hubble parameter, is nearly constant. Inflation is often called a period of accelerated expansion because the distance between two fixed observers is increasing exponentially (i.e. at an accelerating rate as they move apart), (but Λ can stay approximately constant see deceleration parameter.)

Then see.


The variable speed of light (VSL) concept states that the speed of light in a vacuum, usually denoted by c, may not be constant in some cases. In most situations in condensed matter physics when light is traveling through a medium, it effectively has a slower speed. Virtual photons in some calculations in quantum field theory may also travel at a different speed for short distances; however, this doesn't imply that anything can travel faster than light. While it is usually thought that no meaning can be ascribed to a dimensional quantity such as the speed of light varying in time (as opposed to a dimensionless number such as the fine structure constant), in some controversial theories in cosmology, the speed of light also varies by changing the postulates of special relativity. A fundamental change to relativity is needed if c is changing because relativity shows that space and time are equivalent.

So do not get too caught up in thinking about a simple expansion of three dimensional space or what one might commonly consider to be ‘expansion’, but remember that what we measure as time can also ‘expand’ or ‘contract’ based simply on relative motion in 'space' and the reference frame you choose. A metric expansion of spacetime is just that, an expansion of the unit of measure, metric or representation of distance in spacetime, which is referred to as separation (S) and takes the four dimensional form of S2 = (T*c)2 - X2 - Y2 - Z2.

Simple definition of metric.

http://en.wikipedia.org/wiki/Metric_expansion_of_space


Definition of a metric
A metric defines how a distance can be measured between two nearby points in space, in terms of the coordinates of those points. A coordinate system locates points in a space (of whatever number of dimensions) by assigning unique numbers known as coordinates, to each point. The metric is then a formula which converts coordinates of two points into distances.

More specific definition.

http://en.wikipedia.org/wiki/Metric_(mathematics)

In mathematical terms a ‘space’ does not just refer to the dimensions that we perceive as spatial but all dimensions in that ‘space’ being considered. Which in our case includes a temporal dimension that we can measure in the same units as our other 3 spatial dimensions, specifically by the consistency of the speed of light.

[Badly Shaved Monkey]

I'm not a grown up (well, not in the sense you are using the term) but I think you are onto something. From the photon's point of view, they are at point A, then they are at point B, and from their perspective, zero time has passed...there is no "time" for light, only space. Without time, there can be no acceleration.

Which leads to the next question that I've been pondering for a couple of days...

For massless particles, everything happens effectively all at once, not because they travel at infinite speed, but because to an observer their clocks have stopped.

If you model a universe that contains only massless particles travelling at the speed of light, what happens to causality? Or is this one of those questions like debating what happens at a singularity- once certain parameters tend to zero (or their reciprocal tends to infinity) does this just tell us that our model is broken? In this instance, modelling a universe of zero-mass particles is a nonsense model so don't be surprised that it gives nonsense answers.

On the other hand, though that model is unrealistic, nonetheless does the existence of mass dictate the variation in proper time for massive objects and are those variations of proper time necessary to define past and future and thus causal relations?

Way out of my depth now.

[Reality Check]

Which leads to the next question that I've been pondering for a couple of days...

For massless particles, everything happens effectively all at once, not because they travel at infinite speed, but because to an observer their clocks have stopped.
That is not quite right - you need to add where the observer is.
To an external observer, a clock attached to a massless particle (traveling at the speed of light) will not tick. To be even more exact: The external observer will not be able to observe the clock due to infinite redshift but can deduce using SR that time has dilated to its limit.
But an observer attached to the clock will see time ticking away at a normal rate. So according to that observer everything happens as normal.

If you model a universe that contains only massless particles travelling at the speed of light, what happens to causality? Or is this one of those questions like debating what happens at a singularity- once certain parameters tend to zero (or their reciprocal tends to infinity) does this just tell us that our model is broken? In this instance, modelling a universe of zero-mass particles is a nonsense model so don't be surprised that it gives nonsense answers.

This model universe is fairly trivial: Everything is traveling at the speed of light. Thus the difference in velocity is zero. Thus there are no SR effects such as time dilation.

[aggle-rithm]

This model universe is fairly trivial: Everything is traveling at the speed of light. Thus the difference in velocity is zero.

???

Everything is moving in the same direction...?

Maybe you mean the difference in SPEED is zero?

[Thabiguy]

If you model a universe that contains only massless particles travelling at the speed of light, what happens to causality? Or is this one of those questions like debating what happens at a singularity- once certain parameters tend to zero (or their reciprocal tends to infinity) does this just tell us that our model is broken? In this instance, modelling a universe of zero-mass particles is a nonsense model so don't be surprised that it gives nonsense answers.

Photons move at the speed of light and they do not age. But electrons, for example, do not move at the speed of light, and they do not age either.

The important thing to realize is that observers - and physical processes in general - do not correspond to individual particles, but to complex systems built from interactions between particles. What special relativity predicts is that when such a system moves with velocity approaching the speed of light (with respect to some inertial reference frame), the rate of interactions, as observed in that reference frame, will be slowed down towards zero. It's about the systems, not about the constituents. If the particle doesn't interact with anything (including itself), then its proper time is inconsequential.

The real question about your universe is whether your massless particles could form bound systems (perhaps via some kind of attractive interaction). If they could, then these systems could develop in time. - But to describe such a universe, you would need more than just special relativity.

[lionking]

When the velocity in which our universe is expanding exceeds 299792458 metres per second I figured the speed of light might also exceed 299792458 metres per second.

I once thought that too.

[madurobob]

One of my favourite physics insights is that everything travels at the speed of light. It's just that zero-mass objects, like photons, travel only in space, but not in time, whereas objects having mass travel in time and space in proportions that vary with their mass and velocity.

I know I have said that badly, can a grown-up please tidy it up for me?
That's interesting and elegant.

But, everything travels at the speed of light relative to what? Relative to... everything else?

[Ocelot]

It's an intriguing question. How do we know that the speed of light is constant? How do we know that it wasn't different in the early universe or isn't different at some distant point.

Whilst absolute certainty can only be reserved for matters such as our own individual being and logical and mathematical proofs divorced from the world of perception we may have a degree of certainty about the things that we observe. The degree of certainty that the speed of light is contstant within the observable universe is so high that we feel safe in defining the metre with reference to this value. As such if we want to be nitpicky about it, the speed of light can't change. Under our current definition we'd have to change the size of the metre instead. However even that is not going to happen within any reasonable framework for probability.

We don't know this for certain, in the same way that we don't know that Russell's teapot (http://en.wikipedia.org/wiki/Russell's_teapot) doesn't exist. We assume it is so for it is the simplest assumption and our friend Ockham (http://en.wikipedia.org/wiki/Occam's_razor) indicates that this makes it most likley to be true. If we encouter observations best explained by an inconstant speed of light then and only then is it worth challenging that assumption.

What might such observations look like?

Maxwells equations allow us to derive the wave equation for electromangnetic waves. They tell us that the speed of light is related to two other physical constants.

http://upload.wikimedia.org/math/b/7/7/b77af05f45fd237598c44c6ca32c1b01.png

These are
http://upload.wikimedia.org/math/6/3/7/637e9aa8f9fc5c99c48feb86b1377aa0.png, the permittivity of free space (http://en.wikipedia.org/wiki/Free_space), officially the electric constant (http://en.wikipedia.org/wiki/Electric_constant).
http://upload.wikimedia.org/math/0/0/a/00a60ebc2590b45c2151ec6b12e3fd39.png the permeability of free space (http://en.wikipedia.org/wiki/Free_space), officially the magnetic constant (http://en.wikipedia.org/wiki/Magnetic_constant)

If these two are constant then c must be constant.

But we can ask what if they weren't what would we expect to see that would be different. If http://upload.wikimedia.org/math/6/3/7/637e9aa8f9fc5c99c48feb86b1377aa0.png were different then that would affect the solution to Shrodinger's equation (http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/hydsch.html) for the hydrogen atom. That elegent but complex equation that accurately predicts the spectrum of hyrdogen.

Any variation in http://upload.wikimedia.org/math/6/3/7/637e9aa8f9fc5c99c48feb86b1377aa0.png would show up as a variation in the spectrum of hydrogen measured in the most distant stars, galaxies and pulsars. Significant variation in http://upload.wikimedia.org/math/6/3/7/637e9aa8f9fc5c99c48feb86b1377aa0.png would might even stop the H atom from being stable in the first place. No Hydrogen, no fusion, no stars.

So what about http://upload.wikimedia.org/math/0/0/a/00a60ebc2590b45c2151ec6b12e3fd39.png? Same sort of problem. When we see the results of novae and the spectra of the elements they produce we can tell the the same or remarkably similar laws of physics are in play with what must be almost exactly the same constants. Where there is carbon being produced the fine structure contstant must be extremely close to what it is here. As the fine structure constant is related to both the speed of light http://upload.wikimedia.org/math/6/3/7/637e9aa8f9fc5c99c48feb86b1377aa0.png and http://upload.wikimedia.org/math/0/0/a/00a60ebc2590b45c2151ec6b12e3fd39.png we end up knowing that these values are all "constant."

In fact the presence of stars requires such fine tuning of the fundamental constants that if the universe had been only marginally different we wouldn't be here to know about it. Some see that as argument for intelligent design, others look for multiverses with all manner of fundamental constants only rare combinations of which produce the building blocks for life as we know it. As such there may well be arguments for different so called constants elsewhere in the "multiverse" but within the observable universe, for as long as there've been stars doing their thing, we know that the speed of light has been the same as it is today.

That's good enough reason for me to say that as certainly as the sun will rise tomorrow, the light it emits will travel at 299792459 metres per second.

[aggle-rithm]

I just watched "The Elegant Universe" (the Nova production) and noticed something interesting.

They showed a chart of the twenty constants which, if any of them were different, would result in the Universe coming apart at the seams.

I looked at the chart carefully, and the speed of light was not among them.

Is this because the chart was written from a quantum mechanical perspective, or because the speed of light really isn't that important?

[aggle-rithm]

That's good enough reason for me to say that as certainly as the sun will rise tomorrow, the light it emits will travel at 299792459 metres per second.

I think the likelihood of the sun not rising is greater than that of a universal constant changing.

[rlr]

When the velocity in which our universe is expanding exceeds 299792458 metres per second I figured the speed of light might also exceed 299792458 metres per second.

Congratulations on being the first and only person to give the correct speed of light in this thread. Because the thread title is off by 1 m/s and nobody noticed.

[Cuddles]

Congratulations on being the first and only person to give the correct speed of light in this thread. Because the thread title is off by 1 m/s and nobody noticed.

Is this where we should point out that the person you are praising for getting it right is in fact the same person who got it wrong in the title? And is in fact the only person who has typed it out at all, the rest of us realising that doing so would be as pointless as quibbling about the exact number when everyone reading understands what the point is anyway.

[Modified]

That's interesting and elegant.

But, everything travels at the speed of light relative to what? Relative to... everything else?

In the sense we are using "travels" here, relative to any inertial reference frame.

[madurobob]

In the sense we are using "travels" here, relative to any inertial reference frame.
OK, my brain hurts a little less, then (but not much). So, basically, from the frame of reference of the observer, wherever that observer may be and whatever his state of motion.

[Tubbythin]

I just watched "The Elegant Universe" (the Nova production) and noticed something interesting.

They showed a chart of the twenty constants which, if any of them were different, would result in the Universe coming apart at the seams.

I looked at the chart carefully, and the speed of light was not among them.

Is this because the chart was written from a quantum mechanical perspective, or because the speed of light really isn't that important?

Neither I think.
I'm guessing alpha (the fine structure constant) was in the list. A change in c would lead to a change in alpha (assuming it wasn't accompanied by a change in one of the other constants that make up alpha to counteract this).

[Modified]

OK, my brain hurts a little less, then (but not much). So, basically, from the frame of reference of the observer, wherever that observer may be and whatever his state of motion.

Yes, the proportion of "travel" through space vs. time will be different for different reference frames, but the "speed" will be the same.

[sol invictus]

I just watched "The Elegant Universe" (the Nova production) and noticed something interesting.

They showed a chart of the twenty constants which, if any of them were different, would result in the Universe coming apart at the seams.

I looked at the chart carefully, and the speed of light was not among them.

Is this because the chart was written from a quantum mechanical perspective, or because the speed of light really isn't that important?

It doesn't sense to talk about the speed of light changing until you specify the units. For example measured in meters/second it can't change, because the meter is defined by the speed of light!

Now you might be thinking, "so what - just use different units". But the point is that the change in a quantity with dimensions must always be quantified in units of something else with the same dimensions. In order for the speed of light to really change, or rather in order for that change to have any physical effect, it must change in units of some other speed.

So you can simply set the speed of light to 1 - which means measure all speeds in units of c - and then it cannot change by definition (but any other speed, like the speed of gravity, could). That's what's often done, and that might be why it didn't appear on that list.

[Perpetual Student]

It doesn't sense to talk about the speed of light changing until you specify the units. For example measured in meters/second it can't change, because the meter is defined by the speed of light!

Now you might be thinking, "so what - just use different units". But the point is that the change in a quantity with dimensions must always be quantified in units of something else with the same dimensions. In order for the speed of light to really change, or rather in order for that change to have any physical effect, it must change in units of some other speed.

So you can simply set the speed of light to 1 - which means measure all speeds in units of c - and then it cannot change by definition (but any other speed, like the speed of gravity, could). That's what's often done, and that might be why it didn't appear on that list.

I don’t get that! If we defined the meter as “the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice” and the second as “the duration of
9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom,” would we not be able to measure changes in the speed of light?

[Reality Check]

I don’t get that! If we defined the meter as “the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice” and the second as “the duration of
9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom,” would we not be able to measure changes in the speed of light?
The definition of speed of light (http://en.wikipedia.org/wiki/Speed_of_light#Speed_of_light_set_by_definition) was set in 1983 to be 299,792,458 metres per second by defining the meter and second to be to be:

The metre is the length of the path traveled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second.
...
The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

[JoeTheJuggler]

How can anything that travels be able skip acceleration and goes instantly to top speed.
Lack of mass.

[dudalb]

Will chickens ever attack and destroy mankind?


Remember Bill Cosby's "Giant Chicken Heart" routine?

[sol invictus]

I don’t get that! If we defined the meter as “the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice” and the second as “the duration of
9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom,” would we not be able to measure changes in the speed of light?

Yes, and it would boil down to measuring the change in a dimensionless ratio (in that case the speed of light divided by the length of that bar over the duration of those radiation cycles).

Suppose both the speed of light and the length of the bar changed by the same factor (relative to something else) - the speed of light measured in your units would be constant. So it's often easier to just set the speed of light to 1 - i.e. measure everything with dimensions length/time in units of the speed of light.

[Perpetual Student]

Yes, and it would boil down to measuring the change in a dimensionless ratio (in that case the speed of light divided by the length of that bar over the duration of those radiation cycles).

Suppose both the speed of light and the length of the bar changed by the same factor (relative to something else) - the speed of light measured in your units would be constant. So it's often easier to just set the speed of light to 1 - i.e. measure everything with dimensions length/time in units of the speed of light.

OK, I understand that, but if we wanted to find out if the speed of light is changing, would we not need such a method? Or perhaps several such methods -- in case we wanted to account for measuring "rods" also changing?
Another related question: Could a standard length be defined as a number of diameters of a specific atom at a specified temperature (energy state)? Would that kind of definition be sufficient to assure that our lengths were not changing?

[The Man]

Remember Bill Cosby's "Giant Chicken Heart" routine?

Yep, side two of his 1966 album ‘Wonderfulness’ (http://en.wikipedia.org/wiki/Wonderfulness).