Talk:Speed of light

Animation — light from furthest observed galaxy

There's a nice animation on the webpage The Distance Scale of the Universe which might be useful in the section Speed_of_light#Astronomy which discusses light from faraway galaxies. From the copyright information at that website, it appears that the animation can be copied to Wikimedia Commons for use in Wikipedia. So there it is, if anyone would like to move it into Wikimedia Commons and then into the article. --Bob K31416 (talk) 15:28, 6 December 2011 (UTC)

It doesn't belong to this article IMO. If we really think that some reader would take distance to mean anything else than ‘light travel distance’ in that table (which I don't; they would have knowledgeable enough to be aware that other distance measures exist and stupid enough to fail to realize they are not what is meant here), then we'd better not have such an entry at all. ― A. di M.​  16:02, 6 December 2011 (UTC)

I'm not sure you understood my last message, which wasn't about the entry that was recently removed from the table, but rather about an animation for the section Speed_of_light#Astronomy. --Bob K31416 (talk) 16:11, 6 December 2011 (UTC)

I feel that this is entirely off-topic for the article: this would amount to illustrating the findings of cosmology. I don't think it is appropriate to have more than the brief mention that is already in the section. It'll raise far more questions for the average reader than it'll answer. On a side-note, the illustration gives a sense that things are distorted by the idea of the light "trying to catching up", but little more. Quondum^talk_contr 16:19, 6 December 2011 (UTC)

Wow, do we see things differently! I could respond but I sense there is so much resistance here that it's not worth it for me to continue discussing it, and if I did the only thing I think I would accomplish is to annoy people. I'll wait to see if anyone finds the animation worthwhile for the article. [Note added 17:58, 6 December 2011 (UTC): Be sure to read the section Speed_of_light#Astronomy before deciding whether the animation would be useful there.]--Bob K31416 (talk) 16:52, 6 December 2011 (UTC)

Sorry, I do not mean to antagonize anyone or to dampen your enthusiasm. I suppose I feel articles are best concise and focused, not pedagogical or full of information covered in other articles. Judging by the style of many articles, mine may be a minority view. I'll also wait for other views. Quondum^talk_contr 17:29, 6 December 2011 (UTC)

Maybe that animation, with a very short caption, could be added to the penultimate paragraph of Section “Faster-than-light observations and experiments”, but even that would be a stretch. (The article is already 108 KB, about 5% more than when it was promoted to FA, and even then people agreed that it was already comprehensive enough.) ― A. di M.​  14:46, 7 December 2011 (UTC)

A. di M., I see why you considered that penultimate paragraph somewhat appropriate for the animation, and I understand your concern about the size of the article. However, the Astronomy section presently discusses the light travel time of 13 billion years from faraway galaxies and the animation depicts that 13 billion year trip, so that would seem to be a good place for it, except that the article is already too large. With this in mind, instead of putting the animation itself in the Astronomy section, a link to it could be put in a footnote that is added to the existing Notes section. Here's the subject sentence as it presently appears in the Astronomy section of the article, along with the proposed footnote.

"For example, it has taken 13 billion (13×10⁹) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images."

Notes

1. An animated depiction of the light's 13 billion year trip can be found in the lead section of The Distance Scale of the Universe.

--Bob K31416 (talk) 20:08, 7 December 2011 (UTC)

I don't see what that would add. (Not to mention that the quoted website is not a WP:RS.)TR 22:15, 7 December 2011 (UTC)

Me neither. As for RSness, hell, the light doesn't even redshift in that picture! (Plus, I would have used a grid with closer spacing.) ― A. di M.​  10:59, 8 December 2011 (UTC)

Conversion for kilometre and miles per hour is NOT incorrect on chart in infobox

If the speed of light is 299,792,458 metres per second this works out to 299,792.5 Km/s or 17,987,547.5 km/h. I believe the chart stated this as being 1,080,000 km/h. Similarily, when you convert the km/h to mph you should arrive at 11,176,943.8 mph.

Can someone, verify this calculation and then edit the chart as I am unable. — Preceding unsigned comment added by 207.195.52.1 (talk) 18:28, 15 December 2011 (UTC)

You seem to have converted from seconds to hours with a factor of 60. There are 3600 seconds in an hour. Quondum^talk_contr 18:34, 15 December 2011 (UTC)

Yes it appears that I did thank you kindly sir — Preceding unsigned comment added by 207.195.52.1 (talk) 18:08, 21 December 2011 (UTC)

The listing gives the speed as approximately 1,080 million km/h, i.e., 1,080,000,000. Hertz1888 (talk) 18:48, 15 December 2011 (UTC)

Questions

a very very interesting article.

I have 2 questions and would much appreciate any replies or observations. (nb. i am a historian, not a physicist, so in lay terms please)

A. why is C such an exact value.

B. assuming a source of photons, what distance is estimated between the emission of a photon and its achievement of velocity C and what stimulii create this acceleration.Miletus (talk) 19:49, 14 January 2012 (UTC)

A. It's a constant and because of the way it relates distance and time it's defined to be an exact value. This fixes the metre in terms of the definition of the second

B. They attain that speed instantaneously and always go at that speed. They just do: it's an intrinsic property of light (and other electromagnetic waves) that it goes at this speed.

Sorry, that's maybe not in lay terms but it's difficult to explain properly without using pretty advanced theory. E.g. the answer to A depends on an understanding of special relativity.--JohnBlackburne^words_deeds 20:34, 14 January 2012 (UTC)

Actually, the answer to can be put in very simple terms. The meter is defined is the distance that light travels in a certain fraction of a second. Consequently, the speed of light (the distance travelled divided by the time taken) is exactly this fraction. This is just a consequence of the way the units are defined. Compare this to the easier to understand situation of the speed of light measured in lightyears per year. Since a lightyear (by definition) is the distance traveled by light in a year, the speed of light is exactly 1 lightyear per year. (What requires more knowledge to understand, is why this is a good definition of the meter.)

B. is indeed almost impossible to explain in laymans terms. The thing here is that photons always travel at the speed of light, they are created at their source travelling at that speed. There is no acceleration involved.TR 22:47, 14 January 2012 (UTC)

Miletus, you have two good answers above. I hope they make sense to you. Let me just add that photons are quantum entities. If you start thinking of them as little balls travelling at a specific speed and following a trajectory you have misunderstood what they are. Martin Hogbin (talk) 00:48, 15 January 2012 (UTC)

Isn't this something for the wp:reference desk/science? - DVdm (talk) 10:00, 15 January 2012 (UTC)

Strictly, yes, or at least for user's talk pages. I would be happy to continue to discuss this on my talk page. One thing I agree with Brews on is that statements from readers that they do not understand something in an article can be very useful in improving the article, so there is a logic to at least starting the discussion here. Martin Hogbin (talk) 10:09, 15 January 2012 (UTC)

One further thought on B. The reason why things take time and energy to accelerate is because they have mass and inertia, and it takes time to overcome that inertia. The heavier something is the longer it takes to accelerate it. Conversely the lighter something is the less time it takes. Light has no mass, no inertia, so needs no time to get up to speed or change direction.--JohnBlackburne^words_deeds 11:05, 15 January 2012 (UTC)

B is a tough question. Questions like that are humbling because it's hard to give a satisfying answer. You don't learn them in physics class. By definition, a photon travels at the speed c. If it doesn't, as when accelerating to the speed c, then it's not a photon. All I can add is that the microscopic world of quantum mechanics that describes photons is not accessible through our common sense experience. It's quite a different world that our common sense can't explain and is constructed with theories expressed by mathematics. The only reason we believe it, is that it ultimately predicts results that we can check by doing experiments. --Bob K31416 (talk) 03:52, 20 January 2012 (UTC)

We should be careful of even of statements such as "By definition, a photon travels at the speed c." I think that c is best defined as a well-established physical constant, and would remain unmodified even if we discovered that photons had a rest mass. For instance, the experimentally known upper limit on the rest mass of a photon is rather small, but it is nonzero. And we could perhaps in principle find a new ultrashort-range force that the photon couples to, which might result in a sub-lightspeed propagation near some objects. Photons also "have" a rest mass inside superconductors; with an energy below that they can only penetrate it as a virual particle. And before you write off a superconductor as "not vacuum", vacuum is treated in some standard approaches as a superconductive quark condensate. — Quondum^☏✎ 04:45, 20 January 2012 (UTC)

Parts of your comment seemed to be in the context that anything's possible. Other parts:

Re "Photons also 'have' a rest mass inside superconductors..."— I guess the quotes around "have" mean that it's not true. Was your comment based on the following?

"A particle of mass M has associated with it a Compton wavelength given by $\hbar /Mc$, where $\hbar$ is Planck’s constant and c is the speed of light. The Compton wavelength tells how fast the field of a particle falls off with distance. For example, the exponentially falling penetration of the magnetic field into a superconductor means that the photon has acquired a mass $M_{\gamma} = \hbar / \lambda c$. (The expert reader will object that this is not a true photon mass, because the condensate is nonrelativistic, and the electric field behaves differently from the magnetic field. But it’s close enough for our purposes.)"[1] (p. 28/6)

Re your last sentence, "And before you write off a superconductor as "not vacuum", vacuum is treated in some standard approaches as a superconductive quark condensate."— Would you happen to have a link to the approaches you are referring to? There's the Affine Higgs mechanism which has a non-zero photon mass, but that came out well before quarks.

--Bob K31416 (talk) 17:29, 20 January 2012 (UTC)

You are correct about the effective source of "photon mass" in a superconductor (I read a different article, though). It is merely an analogy, and is not to be taken literally. The presumed quark condensate would give mass to the photon's sister particles W and Z, but not to the photon itself; sorry about any confusion. I was only suggesting that it is plausible to have some unknown mechanism giving the photon a small rest mass: my point is merely that we cannot define the rest mass of the photon. — Quondum^☏✎ 19:32, 20 January 2012 (UTC)

wave length = rotation

please note there is no wave length as such but the rotation of the electron charge, think of the riffeling effect of a round bullet. the bullet travels forward in a straight line but the charge rotates giving the effect of an up and down motion ie wave length. the faster the rotation the shorter the apparent wave length this solves the wave particle problem. I am interested in this stuff and do not know how to use this forum you can email me at tom@thehomeofmusicandsound.com — Preceding unsigned comment added by 60.229.175.8 (talk) 12:35, 31 January 2012 (UTC)

Note that this is not a forum. Article talk pages on wikipedia are exclusively for discussing improvements to the article. For general discussions on physics topics like this please visit a physics forum such as [physicsforums.com].

Internal Propogation delay of the observer system

Lets imagine a micro processor based intelligent digital system that can detect and measure electro magnetic radiation.

Assume that the system has not been calibrated to any human standard of measurement units.

So the system's unit of time, is dictated by its own clock speed (t).

Assume that the system starts with no memory or knowledge of earlier data to compare its current readings, but it has memory to store readings from the time it is switched on.

Let us assume that the system's scale of measurement is 0 to 65536 counts.

Let us assume an environment in which the electromagnetic radiation is a perfect sine wave of a constant frequency and constant amplitude.

Assume that the sensor that detects the AC radiation produces a average DC signal which when read by the digital system is equal to 30,000 counts.

Let us assume that the intelligent system takes a sample reading once in 10,000 clock ticks and saves it in its memory.

As long as there is no change in the frequency and amplitude of the radiation, the intelligent system will keep recording the same 30,000 counts after every sampling.

What is the speed "c" of the radiation in this environment?

It is irrelevant because, the concept of "speed" is applicable only for the propagation of a "variation" of either frequency or amplitude. Since both are constant, the concept of "speed" does not arise.

Now let us assume, that there is a 10%change in amplitude of the waveform.

If this change occurs and dies down (restores to previous level) within the sampling time of 10,000 clock ticks, the system will never know. However, if the change persists, over multiple sampling periods, then the system will record it as such.

Now let us assume that the sampling interval is reduced to 1 clock tick from 10,000 clock ticks.

Now if an event occurs in the environment, which caused the sensor's signal (and hence the count) to increase from 30,000 to 30,300 the earliest the system will know about it is after "One clock tick".

Assume there is no other means to know, when the event occured, and at what speed the impact of the event propagated in the environment and reached the sensor of the digital system.

So if the system wants to measure the speed of propagation of a "Change" event, it has to induce an event by itself and measure the return signal.

Assume that the system has a means to disturb the raditation in its environment.

However the speed of propagation it will eventually measure will be limited by

a) the "t" of its own clock tick b) the response speed of its "digital to analog to radiation transducer" c) the response speed of its "radiation to analog to digital transducer".

There is no way to know, whether the propagation speed measured by the system is its own internal speed of propagation only, or the propagation speed of the radiation per se in the environment, or a combination of both.

Let us now hypothesise that a "radiation changing impact event propagates instantaneously and simultaneously across the entire environment". Then the speed of propagation measured by the system will be nothing but its own internal propagation speed.

If we replace the intelligent digital system, with the human system, then the speed of light c measured by us, includes

(a) the speed of propagation internal to the human system + (b) the propagation speed of the radiation in the environment. Is there a way to prove or disprove that b = 0 or b <> 0

(----) Ramkumar R S

Note that this is not a forum. Article talk pages on wikipedia are exclusively for discussing improvements to the article. For general discussions on physics topics like this please visit a physics forum such as [physicsforums.com].TR 06:59, 3 February 2012 (UTC)