Talk:Speed of light

Measurement of the Speed of Light - Ole Rømer

Rømer observed that Io revolved around Jupiter once every 42.5 hours when Earth was closest to Jupiter. He also observed that, as Earth and Jupiter were moving apart, Io's revolution seemed to take longer.

This statement is ambiguous because it implies that the movement of the Earth away from Jupiter is what makes Io's observed orbital period longer. The Speed of Light in a vacuum is constant and not affected by the movement of the observer or the subject.

Rømer made his observations by taking a previously measured time for Io's orbit about Jupiter, and making a schedule so he could observe the eclipses. He noted that Io was eclipsed earlier than the schedule predicted when the Earth was close to Jupiter, but was late when it was far away. Therefore, whether the Earth was moving toward or away from Jupiter was largely irrelevant, it was their distance apart and its affect on his schedule that he observed and measured.

Good point, we should clear up that ambiguity. Please feel free to adjust the text accordingly if you have not already done so. Regards & happy editing, Wile E. Heresiarch 14:11, 12 Apr 2005 (UTC)

Not entirely. In the paper (which was translated into english from the orginal french), what matters is whether we are on the approaching or the receding segment of our orbit. It says explicitly that it is the motion, relative to Jupiter that matters, and this agrees with standard physics today (Doppler effect in the non-relativistic limit for ligth). Ulcph 17:59, 29 August 2006 (UTC)

Roemer time-lag not similar to Doppler effect

From the article (about Roemer's detection of a variable time-lag in the occultations of Io): "These observations are akin to what today is known as the Doppler effect."

This statement is not correct. It is not the relative velocity of the two systems (Earth and Jupiter/Io) that is causing the time offset, but instead simply the distance between them, combined with the finite speed of light.

Reference to a Doppler effect is undesirable in this astronomical situation because of the potential for confusion with red shift (which is precisely a Doppler effect) or with the aberration effect that Bradley used for this purpose (which could reasonably be said to be akin to the Doppler effect, since both are velocity-based).

The effect is more like echolocation, or perhaps a closer analogy would be the determination of the speed of sound by measuring the differences in thunder delays between lightning flashes to points at known distances. But I do not see an analogy close enough that I think it enhances the article. Hunter 23:10, 17 Apr 2005 (UTC)

The statement "These observations are akin to what today is known as the Doppler effect." is correct. The statement "red shift (which is precisely a Doppler effect)" is wrong. So is much else in the above comment. If it is any consolation, you will find obviously non-sense statements even in well-known textbooks, to the effect that Rømer measured occultations at the closest and farthest points in Earth's orbit. And we wonder how he may have observed Io in broad daylight, with the Sun in between! With all due respect, this misconception seems to have been propagated by Newton himself, in Opticks. Not that we may assume that it was his intention. If in doubt, look up Rømer's original article and check his graphic! - Prof. Guest, Aug. 19, 2006

Now hear this! It is generally recognized that what Rømer observed is the optical (non-relativistic limit) Doppler effect. Indeed, what else could it be? It is a helpful insight for the readers. It would be desirable if people would refrain from editing this out, apparently based on incorrect notions, like the ones expressed in the discussion above. There is, for some reason, an incredible amount of misunderstandings about this particular piece of physics. See the compilation in A. Wroblewski, "de Mora Luminis: A spectacle in two acts with a prologue and an epilogue", Am. J. Phys. 53, 620 (1985), and then check your favorite textbook. Just so that you can appreciate how bad it actually is, here are, taken from this paper, the dates ascribed in textbooks to the discovery: 1656 (1 instant), 1666 (3), 1673 (1), 1675 (14), 1676 (24 correct), 1678 (1), 1876 (1). Related with all due respect. Ulcph 21:28, 31 August 2006 (UTC)

Vehicle

If you were travelling in a vehicle going at the speed of light and you turned your headlights on, would it do anything? Yofoxyman 00:11, 25 April 2006 (UTC)

First of all, it's impossible for a massive object to travel at the speed of light. Second, even if you could travel at the speed of light, in your own reference frame, the very same instant you achieve that speed you would simultaneously slam into your destination, no matter how far away it is, so you couldn't actually spend any time cruising at the speed of light and doing experiments.

On the other hand, if you're traveling in a vehicle going at just under the speed of light and you turn your headlights on, they will emit light which, in your own reference frame, travels away from you at the speed of light. A stationary observer will observe the light separating away from you at a much slower speed; the apparent discrepancy between these viewpoints is explained partially by time dilation. Melchoir 00:30, 25 April 2006 (UTC)

I would have to argue against your first point. It implies that the universe is finite and that one could theoretically reach any point instantaneously by travelling at the speed of light. I was thinking that since you are travelling at the speed of light and as are your headlights, the distance that they seem to travel in front of your vehicle would remain constant. Also, at that incredibly high speed, you would most likely be "over-driving your headlights." So because of that I think that if one were travelling at light speed, headlights wouldn't be much use. Yofoxyman 02:46, 25 April 2006 (UTC)

Yes, if you travel at the speed of light, you reach your destination instantaneously. Melchoir 03:09, 25 April 2006 (UTC)

Don't you reach your destination at the speed of light, 300 million meters per second? If you travel to the nearest star instantaneously, surely you would be a few years ahead of the light that follows the same path, and travels, as I understand it, at the same speed? Richard001 06:49, 2 August 2006 (UTC)

I meant "instantaneously in your own frame of reference". Melchoir 07:02, 2 August 2006 (UTC)

I may not understand this thoery of relavtivty but how is light speed infinte and then stay again a constant what ever speed that is?

question

suppose one could travel at the speed of light, and the place you wanted to go was 40 lightyears away, would it take 40 years to get there? --Revolución hablar ver 05:59, 29 April 2006 (UTC)

To a "stationary" observer, it would appear so. But in your own frame of reference, it would take no time at all. Melchoir 06:10, 29 April 2006 (UTC)

Hun? so light by itself has infinte speed?

You can think of it that way. As you accelerate towards the speed of light time slows down relative to a stationary observer, as you reach the speed of light, time stops! So you can travel an infinite distance without observing any passage of time. However it is not this simple as distance also contracts. So the observer at the speed of light sees no distance, taking obviuosly no time, and not requiring an infinite speed. In the moving frame it takes no time as they travel no distance. The idea of infinite speed comes from a stationary observer, using time from the moving frame but distance in the stationary frame, as long as you work in the same frame, everything remains consistent. I think its best not to worry about these things too much its just maths. Obviously it is impossible for an observer to actually travel at the speed of light, so this is all speculation. The slowing down of time has been observed though, for example fast moving particles take longer to decay than slower ones, i.e. time for them is slower.Jameskeates 10:42, 17 August 2006 (UTC)

Question.

If one particle is travelling at the speed of light in one direction and another particle is travelling at the speed of light in the opposite direction and they collide then is the combined speed at point of impact 2x speed of light ?

Um. No. If you and your friend is running against eachother you'll sooner or later hit. The force will be absorbed by the other and you'll gain the exact same force per kg of mass in the other direction. Please correct me if I'm wrong (14years, no uber physican). Cybesystem 00:11, 2 August 2006 (UTC)

nha if a car hits a brick wall at 70 mph is not the same if 2 car are going at eachother at 70 mph tthier force adds up to 140 mph of one car if both cars have similar mass or the mass is doubled with the ligth the momentum is 2X i belive unless there is some werid relativity law i dont understand

No. The speed of impact in the frame of reference of the particles will be less than or equal to the speed of light. i.e. the colliding particles will not experience the collision at twice the speed of light, due to the effects of relativity.Jameskeates 10:39, 17 August 2006 (UTC)

Citation for Subluminal studies?

I think with as many references to studies as there are in the subluminal section, there should be some citation... --HantaVirus 14:05, 21 July 2006 (UTC)

Since this is a two-year-old Featured Article, it doesn't have the inline citations one would expect. I can look for a couple for that section... Melchoir 23:58, 21 July 2006 (UTC)

Question

Hi. I am having a hard time understanding how the speed of light is measured (and not "defined"). I hope someone will clarify this process for me. I've seen the diagrams that show lightbeams reflecting off of mirrors, but I can't make sense of what these images mean?

For instance, in the real world, there is what I call a "source" i.e a source of heat, light or sound. There is also a "receiver"; like your ears, eyes, or skin. From the diagrams given, I see a "source" sending out a beam of light, that is then redirected using mirrors, but what is the instrument that "receives" this signal? and how does it work? 2c me 04:23, 29 July 2006 (UTC)2c me

Don't light have a charge? If so, can you measure when the charge is changed in the reciver? I'm also confuzed Cybesystem 00:08, 2 August 2006 (UTC)

2c me: could you specify which diagrams or setups you find confusing, and whether any of them in Wikipedia need improvement?

Cybesystem: light doesn't carry charge, although it can certainly move around the charges already present in whatever it strikes. (If light did carry charge, then it would be able to support longitudinal polarization modes... but it doesn't.) Melchoir 00:18, 2 August 2006 (UTC)

In the 1850s, I assume that a human eye was the detection equipment used with the Fizeau-Foucault apparatus. In the 1890s, I also assume that the human eye would have been used with early versions of the Michelson-Morley experiment. These days, a light-sensitive semiconductor device would be used. Do those articles explain the diagrams and the experiments in sufficient detail for you to understand what is going on? -- ALoan (Talk) 10:01, 2 August 2006 (UTC)

Going at the speed of light and turn on the lights

Dumb question, but I do really want to know (can also be included in the A.)

Scenario:

You are traveling at the speed of light (though it might be impossible, but imagine), in your "spacecar". You then turn the front lights on. What happends?

Ok, I'm no physican but heres ny therory, please correct me:

Since Mass is another form of energy, energy is another form of mass. Therefore light have mass. We also know that the speed of light is constant, does not change if the car moves. And we are moving at the speed of light. Since the light can't go faster than we are, the light will be "trapped" inside the lightball. The light will continue to come, and get "stuck". As the amount of light increases, so does the mass. When the lightball is as full as it can be the light will create a preassure inside the lamp. When the preassure exeeds the force the lightball can take it will explode.

So, what do you think of a 14yr olds therories? Cybesystem 00:22, 2 August 2006 (UTC)

This doesn't seem like such a dumb question; it's actually very popular. My own answer is above at #Vehicle. For a longer answer, try this link. Melchoir 00:40, 2 August 2006 (UTC)

according to the thoery it is impossible to be going at speed of light casue it says if you apporach the speed of light ur mass increases infintyly and the engery requied would also be infite w/e lets says ur going .99 of the speed of light according to the theory the speed of light is the smae to all obververs so to ur it is going at the speed of light but lets says there is some guy doing how fast light is going experiment lets says as soon as you turn on the head lights the observer only see the light going the 1% outa the 99% ur going you you in the car see light go 100% its werid but idk even if the driver was apporaching a wall that was senstive to light and saw the light hit it from far away while the second obsever was near the wall apparantly she wouldn't see it unless he was getting extremly close to the wall as the light was going the 1% of the speed that she was observing. if what i said is wrong plz tell me what is right according to the thoery?

If you go the speed of light, you cannot turn on your lamp because no time is passing for you — you are frozen at that instant. At any speed, even if only a snail's pace slower than light, you will see the illumination of your light travelling at the speed of light. Interior cabin lights would act perfectly normal to you. An outside observer would see the light from the headlights go at the speed of light, which is only fractionally faster than the car. She would also see it blue (if approaching) or red (after passing her). —Długosz

so the light would hit the wall at the same time for both observers?

Relativity does not require events to happen simultaneously for both observers, it is possible for the light to be observed hitting at different times. There are even situations where the order of events can be altered for different observers raising interesting questions about cause and effect.Jameskeates 10:45, 17 August 2006 (UTC)

Gravitational lensing

I just removed the following claim:

In an analogous way, the light speed is also affected by gravity. This gives rise to the phenomenon of gravitational lensing, in which large assemblies of matter can refract light from far away sources, so as to produce multiple images and similar optical distortions. The constant speed of light then belongs to those who may be in free fall, or for other reasons may disregard such effects of gravity on light.

This is not true in any sense that I can make out. There are certainly coordinate systems in which the speed of light appears to be different (just double t, say) but one of the fundamental principles of general relativity is that the speed of light is the same in any inertial frame. Gravitational lensing occurs because spacetime is curved, not because the speed of light changes.

Maybe you can't make it out, but to edit here you are obliged to know your physics. For electromagnetic waves, the gravitation (curvature if you like) has a representation as refractive index, so these are really equivalent viewpoints. These are not inertial frames, of course, this is general, not special relativity. You will find that in standard textbooks. Ulcph 20:13, 1 September 2006 (UTC)

Overview

The opening paragraph seems redundant:

"According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed (or velocity) in a vacuum. This physical constant is commonly known as the speed of light and denoted as c. This speed c is also the speed of the propagation of gravity in the theory of general relativity."

Most of this was said right before, or will be right after. I suggest to delete it, in the interest of readability (but haven't).

The author of the last sentence probably was referring to a linearized approximation, for the propagation of a "small" wave-like disturbance in otherwise gravitation-free space. Of course, in general relativity coordinates are freely chosen, and speeds can come out as nearly anything, merely as a consequence of this. It is not covariant. To obtain the constant, go to a locally inertial frame (free fall), but you only get it locally, and light will still bend globally. This would be rather much to saddle the reader with right up front. Besides, it is not directly testable (so far), and only established indirectly in the binary pulsars - but convincingly, of course. Besides, this is, after all, an article about light. Ulcph 23:41, 8 September 2006 (UTC)

Question.

Although there is much reasoning over this, lets say for now, that light does have a mass. Would it therefore be possible to move something using light, if the 3 factors which make up light were strong enough?

I'm not sure what you mean by "3 factors." But light has momentum, so you can use it to move things anyway, in principle. You'd need very high total energy, but very low individual photon energies to avoid blowing the object apart; but low energy means long wavelength and a tendency to pass through things, so this isn't practical in the slightest.

In the future, can you please ask science questions at Wikipedia:Reference desk/Science? Article talk pages are for discussing changes to the articles. -- SCZenz 15:55, 12 September 2006 (UTC)

You may want to consult the article Radiation Pressure. Ulcph 04:19, 13 September 2006 (UTC)

Photon in Peer Review

Hi all,

The Photon article is now in peer review, in preparation for its Featured Article candidacy. Could you please give us some tips on reaching FA? Your article is already FA (congrats!) and it's very close to ours in its subject. Thanks! Willow 11:40, 15 September 2006 (UTC)

Doppler and GPS?

The article says that Doppler effects have to be taken into account in understanding how global positioning satellites work, but it does not actually give a rationale for that statement. On the other hand it mentions the Doppler effect in the context of the accurate synchronization of the clocks on satellites and on the ground. Doppler effects change the apparent frequency of waves emissions. Such effects might have an impact on the tuning of radios for reception or transmission, but Doppler effects would not have any effect on the times reported over those frequencies. A relativistic effect would, on the other hand, have an inevitable effect. Time dilation is well known to occur when, e.g., two clocks are synchronized, one is sent into orbit and remains there for some time, and then the two clocks are reunited. Less time has passed for the clock that was in orbit.

So is there a misidentification of the time dilation phenomenon as being "Doppler effect"? Or are both factors involved somehow? P0M 06:11, 19 September 2006 (UTC)

Speed of Light

Why is the speed of light 1,079,252,848.8 km/h and not something else?

One answer is that we don't know. Another answer is that the speed of light is the universe's fundamental "conversion factor" between space and time, and we define our unit systems around it. See meter#Timeline of definition. -- SCZenz 02:42, 20 September 2006 (UTC)