'That WHY? question is just not answerable, pretty much whenever it is asked in the context of Physics.Johnnyallen said:Question: Why is the speed of light what it is? Could it have been another velocity?
The Guardian posed the question soliciting answers from its readers. Going back to their site, I couldn't find the forum, so I came here thinking that this forum would be the best place to post the question.sophiecentaur said:'That WHY? question is just not answerable, pretty much whenever it is asked in the context of Physics.
When you say you were unable to follow it up, have you been anywhere else but Physics Forums? The speed of light is fundamental to the way things interact and its value (in a vacuum) was measured with greater and greater accuracy over the centuries. JC Maxwell studied electromagnetic theory and he predicted that the speed of light relates directly to the Electric and Magnetic properties of empty Space. This is not an answer to 'why?' but it contributes to a very well founded model of the World - including Special Relativity. All we can help for is to find models that are closer and closer to what we can measure.
Try this wiki article. (Resist the urge to skim through it - it is heavy stuff.)
I take the (on line) Guardian and it is a fine newspaper. I do think they should have thought twice about presenting that question to their general readership because it's far to specialised for them to get (or be able to recognise) good answers.Johnnyallen said:The Guardian posed the question soliciting answers from its readers.
BUT ... be aware that the early statementsophiecentaur said:Try this wiki article.
INCORRECTLY implies that conventional matter CAN travel at c, whereas really only massless particles / EM waves / gravitational waves can travel at c and conventional matter cannot.According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of https://en.wikipedia.org/wiki/Physical_information in the universe can travel.
That's Wiki for you. But the whole article is not a bad overview of the whole topic. You can look in a lot of references and find the same gaff about massive particles and c.phinds said:BUT ... be aware that the early statement
INCORRECTLY implies that conventional matter CAN travel at c, whereas really only massless particles / EM waves / gravitational waves can travel at c and conventional matter cannot.
The value of c is not physically relevant. The value is merely an artifact of the units used.Johnnyallen said:Why is the speed of light what it is? Could it have been another velocity?
The meter is defined to be the distance that light travels in 1/299792458 seconds (or equivalently 1/299792458 of the distance that light that light travels in one second) so the speed of light pretty much has to be 299792458 m/sec. If I were ever to get a different answer when I tried to measure the speed of light, I would have to conclude that one or both of my clock and my meter stick were somehow defective.Johnnyallen said:Question: Why is the speed of light what it is? Could it have been another velocity?
Actually, it's 1/137Nugatory said:In this particular problem we want the "fine structure constant" (Google for it), which is equal to 1/237
Oops - of course you're right. I'm fixing it above and thanks for the catch.phinds said:Actually, it's 1/137
I realize that our systems of measurement are arbitrary or subjective. So consider the following:Nugatory said:The meter is defined to be the distance that light travels in 1/299792458 seconds (or equivalently 1/299792458 of the distance that light that light travels in one second) so the speed of light pretty much has to be 299792458 m/sec. If I were ever to get a different answer when I tried to measure the speed of light, I would have to conclude that one or both of my clock and my meter stick were somehow defective.
So asking whether the speed of light could be anything other than 299792458 meters/sec doesn't make much sense - it will always be that unless we've made a mistake. To get an interesting "what would be different?" question we need to work with something whose value is unrelated to the way that we define our units. In this particular problem we want the "fine structure constant" (Google for it), which is equal to 1/137 no matter what units we use - meters, furlongs, miles, smoots, feet, fathoms, whatever - and relates the speed of light to various other quantities.
So your question comes down to: Why is the fine structure constant equal to 1/137? Physics, being an experimental science, offers a rather unsatisfying answer: Because that's how the universe we live in works. It has to have some value, that's the value it has, and everything else follows from that.
We often accept that "because that's how he universe we live in works" answer without even noticing when it's consistent with our intuition and life experience. For example, Newton's law of gravity (##F=Gm_1m_2/r^2##) completely and magnificently explains why the planets move the way they do. But if we were to ask why it's ##1/r^2## instead of, for example, ##1/r^3## (which would lead to a completely different solar system)... Well, that's how the universe we live in works.
[Edit: corrected the value and thanks to @phinds for the catch]
Some scientists are confused then. That is a physically meaningless proposal.Johnnyallen said:I read that some physicists propose that the speed of light was different in the past.
Do you have a link to that article? Without that link, we can't tell whether you misunderstood the article, or whether the writer of the article misunderstood the underlying proposal - although I'd bet it's the latter if you were reading a popularization instead of a peer-reviewed paper in a serious professional journal.Johnnyallen said:I read that some physicists propose that the speed of light was different in the past. perhaps going back to the early history of the universe after the Big Bang. The article I read also said that as of now, no research has produced any evidence of this.
It's easy to lose the distinction between ##\le## and ##\lt## in natural language. I expect that it's worded that way because any attempt at more precise wording ("the smallest speed that soomething with non-zero rest mass cannot reach in an inertial frame"?) would just be clumsy and confusing.sophiecentaur said:That's Wiki for you. But the whole article is not a bad overview of the whole topic. You can look in a lot of references and find the same gaff about massive particles and c.
Using the word "article" may be a misnomer. It was Wikipedia's "Speed of Light" ( https://en.wikipedia.org/wiki/Speed_of_light )Nugatory said:Do you have a link to that article? Without that link, we can't tell whether you misunderstood the article, or whether the writer of the article misunderstood the underlying proposal - although I'd bet it's the latter if you were reading a popularization instead of a peer-reviewed paper in a serious professional journal.
Stuff like this is the reason Physics Forums has its rule about acceptable sources.
Johnnyallen said:Question: Why is the speed of light what it is? Could it have been another velocity?
The latter, but as Jedishfru stated, "Why is it this speed and no other we just don’t know." And Nugatory said, "So your question comes down to: Why is the fine structure constant equal to 1/137? Physics, being an experimental science, offers a rather unsatisfying answer: Because that's how the universe we live in works. It has to have some value, that's the value it has, and everything else follows from that."DrStupid said:What does "speed of light" mean in this question? The speed of light in general? The speed of light in vacuum? Or the invariant speed according to special relativity?
If you read the references you will see that they are actually discussing variations in the fine structure constant rather than variations in c. The scientists are not confused, but the people summarizing the scientists are.Johnnyallen said:However, it has been suggested in various theories that the speed of light may have changed over time.
Johnnyallen said:The latter, but as Jedishfru stated, "Why is it this speed and no other we just don’t know." And Nugatory said, "So your question comes down to: Why is the fine structure constant equal to 1/137? Physics, being an experimental science, offers a rather unsatisfying answer: Because that's how the universe we live in works. It has to have some value, that's the value it has, and everything else follows from that."
From a layman's point of view no question is a dumb question. The fact that the question was asked shows, in my opinion, a curiosity in how the Universe works.
Nugatory said:Do you have a link to that article? Without that link, we can't tell whether you misunderstood the article, or whether the writer of the article misunderstood the underlying proposal - although I'd bet it's the latter if you were reading a popularization instead of a peer-reviewed paper in a serious professional journal.
Stuff like this is the reason Physics Forums has its rule about acceptable sources.
Albrecht said:Regarding the constancy of the speed of light I think we should be aware that the measured value is constant. The real value may be a different story. Because we know that our measurement tools change when we move, e.g. a clock runs more slowly. And by use of these changed tools we yield the constancy. That looks like being a different story.
I thought that had to do with the fact that we live in 3-dimensional space, so that the total intensity of a field from a point source at distance ##r_1## (integrated over the surface area of a sphere with radius ##r_1##) has to be the same as the intensity at distance ##r_2## (integrated over a sphere with radius ##r_2##). Since it's dependent on the surface area of the sphere, the intensity at any given point goes as ##1/r^2##.Nugatory said:But if we were to ask why it's 1/r21/r21/r^2 instead of, for example, 1/r31/r31/r^3 (which would lead to a completely different solar system)... Well, that's how the universe we live in works.
ZapperZ said:This last claim is very puzzling, and frankly, an assault on experimental physics.
First of all, this claim implies that all inertial reference frame are not equal, that by "moving", one is measuring something that is different than it should be. Secondly, we have a theoretical foundation for what we measure, and this understanding allows us to interpret the data. After all, why do you think we need relativistic correction for time measurements in GPS satellites?
Thirdly, what is a "real value" here? If the "real value" is not accessible via any experimental measurement, then the claim that it exists is purely unverified speculation.
And finally, many of the earlier claims of varying fine-structure constants, especially those made by J.K. Webb et al., have been superseded by significantly more accurate measurements that did not support the initial findings. So there is no experimental support at all for such a claim. I always find it puzzling when theorists ignore experimental evidence, or jump too quickly on something that hasn't been verified (Can we say superluminal neutrinos of OPERA?).
Zz.
No, relativity tells us that our measurement tools work exactly the same in different inertial systems.Albrecht said:But on the other hand relativity tells us that our measurement tools change and work differently in different inertial systems.
The clock that was moved is not an inertial reference frame.Albrecht said:If we move a clock and let it come back then it indicates a different time than a clock which was not moved.
Inertial reference frames maximize proper time.Albrecht said:This slower clock is used when the speed of light is measured in a state of motion.
Actually, the value of c is an artifact of whatever system of measurements you use. it is the fine structure constant that is the universal (dimensionless) constantrootone said:This is similar to asking why the value of Pi happens to be what it is.
It simply is a universal constant, something which is not disputed.
Arguably Pi, c, and other constants might differ in some interpretation of a 'multiverse', (not something I am keen on),
However. even if that was the case, it makes no difference to the Universe we exist in.
Theorists are people and people are only human. We all like a bit of magic in our lives and think of the kudos of your theory turning out to be right.ZapperZ said:I always find it puzzling when theorists ignore experimental evidence, or jump too quickly on something that hasn't been verified (Can we say superluminal neutrinos of OPERA?).
They are also very clear to point out that it is physically nonsense to talk about a variable speed of light and that what they are actually describing is a variation in the fine structure constant.Albrecht said:There is a paper of A. Albrecht and J. Magueijo about this: "A time varying speed of light as a solution to cosmological puzzles", Physical Review D, Band 59, 1999, 043516
An unmeasurable “real” value is physically silly.Albrecht said:I think we should be aware that the measured value is constant. The real value may be a different story.
No, relativity tells us exactly the opposite. It is the first postulate of relativity.Albrecht said:But on the other hand relativity tells us that our measurement tools change and work differently in different inertial systems.
No.Albrecht said:Speed of light at earlier times: can we really conclude so much from the behavior of alpha? Isn't that model dependent?
According to the other contributions here, both statements are equivalent. And the topic of the paper is clearly the speed of light. - The assumption of a variation has fascinating consequences. It explains directly the (otherwise not understood) cosmological inflation (said by the authors). And even better: The problem of Dark Energy, which is such a mystery now, is immediately explained if a decrease of c with time is assumed. We can be very sure that the question of Dark Energy will never be solved on the basis of present understanding, i.e. a constancy of c.Dale said:They are also very clear to point out that it is physically nonsense to talk about a variable speed of light and that what they are actually describing is a variation in the fine structure constant.
We know that our measurement tools a cheating us (by dilation and contraction). So it makes sense to ask for the truth. - This was also an argument of Lorentz when he once discussed relativity with Einstein.An unmeasurable “real” value is physically silly.
The principle of relativity says that we observe the same physical laws in any inertial system. Contraction and dilation are on the other hand essential parts of Einstein's SRT. Which is not a logical conflict.No, relativity tells us exactly the opposite. It is the first postulate of relativity.
No, because dilation and contraction influence our tools.TeethWhitener said:No, relativity tells us that our measurement tools work exactly the same in different inertial systems.
A clock moving with a (different) inertial frame is delayed. Take the following experiment:The clock that was moved is not an inertial reference frame.
What does "maximization of time" mean?Inertial reference frames maximize proper time.
Albrecht said:We know that our measurement tools a cheating us (by dilation and contraction). So it makes sense to ask for the truth. - This was also an argument of Lorentz when he once discussed relativity with Einstein.
You really have a serious misunderstanding of time dilation. You should fix that before you dig a hole any deeper for yourself.Albrecht said:No, because dilation and contraction influence our tools.
No.Albrecht said:No, because dilation and contraction influence our tools.
The information moved between two inequivalent inertial frames; I.e., did not follow a geodesic; I.e., did not maximize proper time. The stationary clock followed a geodesic.A clock moving with a (different) inertial frame is delayed. Take the following experiment:
You have a clock at rest in your system. Then move another clock by linear motion along this clock, and in the moment of passing by the moving clock is synchronized to the clock at rest. Then after a time of motion, another clock may move linearly along this clock into the direction of the clock at rest. And when passing the other clock the new clock shall be synchronized to the first moving clock. Then when the new clock passes the clock at rest compare both clocks. The moving one will show a delayed time, so dilation. All clock in this experiment move linearly, so they are all in inertial frames.
This shows that also clocks in linear motion are dilated.
Minimize the arc length in Minkowski spacetime = maximize proper time. Edit: in the interest of complete transparency, this is true only for timelike separations. (Mentors: This statement is still I-level, right?)What does "maximization of time" mean?
Yes, considering that we're talking flat Minkowski spacetime (that's a sufficient condition, not a necessary one).TeethWhitener said:(Mentors: This statement is still I-level, right?)
You really should read more than just the title. The topic is the speed of light but the actual content of the paper is as I described. It acknowledges that only changes in dimensionless constants are physically meaningful, specifically identifies the fine structure constant as the thing measured, and then simply by fiat attributes any measurable variation in the fine structure constant to a change in c.Albrecht said:And the topic of the paper is clearly the speed of light
You mean that it would explain those things if it (the fine structure constant) did vary. But it doesn’t vary so that isn’t the explanation.Albrecht said:The assumption of a variation has fascinating consequences. It explains directly the (otherwise not understood) cosmological inflation (said by the authors). And even better: The problem of Dark Energy,
I am not very sure about that. I am more certain that believing a counterfactual assumption will not solve the question either.Albrecht said:We can be very sure that the question of Dark Energy will never be solved on the basis of present understanding, i.e. a constancy of c.
I know very well where this line of thought comes from. Please read the forum rules.Albrecht said:We know that our measurement tools a cheating us (by dilation and contraction). So it makes sense to ask for the truth. - This was also an argument of Lorentz when he once discussed relativity with Einstein.
The reason is well known. It has that specific value because the BIPM committee members voted for it to have that value.FactChecker said:Or is it just what it is, with no known reason for that specific value?
I hope we are talking about the same thing. I am not asking about the units of measurement so much as the actual physical movement of the light. So it sounds like there is nothing in physics to set limits or a preference for the particular motion that light has now. So if the light always went twice as far per unit time as it does now (regardless of the units used), that would not have violated any fundamental law of physics as we know it.Dale said:The reason is well known. It has that specific value because the BIPM committee members voted for it to have that value.
If you are not asking about the units then you are asking about dimensionless constants, specifically the fine structure constant.FactChecker said:I am not asking about the units of measurement so much as the actual physical movement of the light.
Yes, there is nothing in known physics which determines the fine structure constant. It is considered to be fundamental.FactChecker said:So it sounds like there is nothing in physics to set limits or a preference for the particular motion that light has now.
rootone said:This is similar to asking why the value of Pi happens to be what it is.
It simply is a universal constant, something which is not disputed.
Arguably Pi, c, and other constants might differ in some interpretation of a 'multiverse', (not something I am keen on),
However. even if that was the case, it makes no difference to the Universe we exist in.
This statement is correct for dimensionless physical constants like the fine structure constant.AgentSmith said:There is a difference between mathematical constants like Pi, and physical constants like c. The latter are experimental, while mathematical constants not.
Good point. But even ##\pi##, if it is simply defined as ##\pi = \frac {\text{circle circumference}} {\text{circle diameter}}##, can be experimental and depend on curvature.AgentSmith said:There is a difference between mathematical constants like Pi, and physical constants like c. The latter are experimental, while mathematical constants not.
You can slow down light. You can't change c. The named constant "c" denotes the speed of light in a vacuum. The numeric value of its speed depends on your choice of units. Our conventional choice of units is such that c has a defined value that is not experimentally determined.Gary_T2018 said:You most certainly can slow it down by juggling either medium or temperature.
jbriggs444 said:You can slow down light. You can't change c. The named constant "c" denotes the speed of light in a vacuum. The numeric value of its speed depends on your choice of units. Our conventional choice of units is such that c has a defined value that is not experimentally determined.
Nugatory said:its value was chosen by a committee.
Because the logic of the Special theory of Relativity is based on distortions of space and time, it applies to everything that moves, not just light. There are two systems of geometry which are compatible with some simple assumptions: Galilean and Einsteinian. (see https://arxiv.org/pdf/physics/0302045.pdf ). When experiments showed that the speed of light in a vacuum was always measured the same, the Galilean theory was ruled out. That left only the Einsteinian theory, which includes formulas for adding velocities. With those formulas for adding velocities, nothing will be measured as moving faster than c. Light and all electromagnetic waves move at the maximum measureable velocity, c.Gary_T2018 said:Why at this moment there is no solid theory which supports the possibility of moving faster than the speed of light in vacuum?
You have an additional assumption here, namely that the length of that prototype meter stick is constant.suremarc said:We could measure the discrepancy in the speed of light by comparing the length of a light-second to the prototype meter stick.
