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Last I knew, light is the fastest thing in the universe but I could be wrong.

Anyways, if the speed of light is 299,792,458 m/s (correct me if that is inaccurate) then is it possible to slow it down?

Scenarios

Will an invisible but super dense gas "slow" it down?

Is the speed of light microseconds slower on Earth than in the vacuum of space?

If we could reach absolute zero then would it affect the light's speed?

I am aware that black holes bend and "suck in" light but does the speed stay constant even though it's been rerouted?

MonkeyZeus
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  • https://en.wikipedia.org/wiki/Speed_of_light for your basic question; I would further suggest you to consider what you really want to ask and not ask too many questions at once. – Sanya Jul 05 '16 at 20:02
  • @Sanya I've edited the title; does that help to clarify my question? – MonkeyZeus Jul 05 '16 at 20:05
  • did the linked article answer your question? – Sanya Jul 05 '16 at 20:06
  • Things that slow down light are called 'lenses'. –  Jul 05 '16 at 20:06
  • http://www.dailymail.co.uk/news/article-2380028/Scientists-stop-light-completely-record-breaking-MINUTE-trapping-inside-crystal.html –  Jul 05 '16 at 20:08
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    @Sanya My apologies if this sounds like a completely noobie question but I am not a physicist so I am having a hard time simplifying the article. I'm sorry if this is difficult for you as well but hopefully someone on this site is kind enough to simplify this for the masses :-/ – MonkeyZeus Jul 05 '16 at 20:15
  • "The speed of light in vacuum [...] is 299792458 metres per second [...] The speed at which light propagates through transparent materials [...] is less [...] For example [...] light in glass travels at [...] ≈ 200000 km/s." I am just bewildered, to be completely honest, but I'll leave it at that. – Sanya Jul 05 '16 at 20:22
  • Of interest: http://physics.stackexchange.com/q/466/ and http://physics.stackexchange.com/q/11820/. Also the wikipedia entry for the refractive index of a material is useful. – Wouter Jul 05 '16 at 20:36
  • @Wouter Thank you for those. I am genuinely curious about this subject and will make sure to read those links when I get a chance. – MonkeyZeus Jul 05 '16 at 20:38
  • By "reaching absolute zero", are you talking about temperature? If yes, of what? – Sentry Jul 05 '16 at 20:39
  • @Sentry It's possible that my scenarios are completely misguided. The basic premise behind that scenario is "Does light travel slower through a given medium if the temperature is lower?". e.g. light traveling 100 miles from point A to point B in the North Pole vs Mexico. – MonkeyZeus Jul 05 '16 at 20:42
  • Light only travels at the speed of light, as far we know. One can couple electromagnetic waves to matter and this quasi-particle excitation has a lower velocity. – CuriousOne Jul 05 '16 at 20:52
  • @CuriousOne So the photons are guaranteed to always physically move at 299,792,458 m/s? Essentially the photon's path is altered which results in an apparent "slowdown'? – MonkeyZeus Jul 05 '16 at 21:10
  • Yes. In classical electromagnetism we are talking about the free fields in the vacuum and about the polarization of matter caused by them. The speed of light refers to the vacuum wave velocity. The polarization will slow the waves down, which is expressed by a "normal" index of refraction n>1. When we talk about the quantum mechanical description the quanta of the vacuum waves are called photons and when they interact with solids, we call the result polaritons: https://en.wikipedia.org/wiki/Polariton. Depending on the strength of the interactions, these excitations can be very slow. – CuriousOne Jul 05 '16 at 21:34
  • In this case yes, temperature affects the (effective) speed of light in a medium. In general the refractive index of a material increases slightly with decreasing temperature, so the (effective) speed of light decreases too. You do not have to go all the way down to absolute zero to obtain an effect. – Sentry Jul 06 '16 at 00:14

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The speed of light in vacuum is 299,792,458 m/s - that is an unalterable quantity. However, light doesn't always travel in vacuum. The concept of a refractive index describes the relationship between speed of light in vacuum vs a particular medium, with the value for glass around 1.3 - meaning that the speed of light in glass is about 1.3x slower than in vacuum.

This is why lenses and prisms can work; it is also why the sun seems to change shape just before it sets over the sea; it is what causes the shimmering of the road ahead on a hot day - and many other optical phenomena.

In the atmosphere of Earth, light is indeed slower than in vacuum - the refractive index of standard air is approximately 1.00028, meaning light is about 0.03% slower in the atmosphere than in vacuum (but the exact value depends on wavelength, density, composition...)

Floris
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  • Oh ok, that makes more sense now. So the "speed of light" is more of a reference point for how fast light travels through a vacuum rather than being the only speed at which light travels? Is the refractive index still applicable once the photons pass through the medium? For context; do the photons actually travel fewer m/s within the medium or are they just bounced around within the medium so they end up traveling a longer distance? – MonkeyZeus Jul 05 '16 at 20:34
  • What is traveling in optically active matter is not light, though, but quasi-particle states that results from the coupling of the photons and the matter. – CuriousOne Jul 05 '16 at 20:50
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    @MonkeyZeus - actually there is a complex interaction between the EM wave of the light and the electrons in the medium, which results in a delay between when a packet of light enters the medium and when it leaves it - when compared to the same packet traveling the same distance in vacuum. The electrons move under the influence of the electromagnetic wave - and their acceleration causes the emission of more EM waves, which interfere with the incoming wave and result in the apparent slow-down – Floris Jul 05 '16 at 20:54
  • @Floris I am trying very hard to digest your recent comment because I am certain my answer is in there somewhere but I cannot seem to finish cracking the code, yet. For simplicity's sake, would it be correct to say the light's path is being lengthened rather than it's velocity being reduced? Would it be completely wrong to say that the light particle is making more pit stops when traveling through a medium? Also, once the light has passed through the medium then does it go "faster" or has the light particle's speed been permanently reduced? – MonkeyZeus Jul 05 '16 at 21:25
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    It is more like "pit stops" than "longer path". Imagine it's like a Mexican Wave, but the people are a bit overweight, and getting up slowly once the guy next to them gets up. Compared to the fitter side of the stadium, the wave will go more slowly. But as soon as the wave leaves the medium, it resumes its original speed. – Floris Jul 05 '16 at 21:27
  • monkey, the constant speed of light in vacuo really is the speed of any instantaneous interaction. it's not just EM. – robert bristow-johnson Jul 05 '16 at 22:04
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No, the light was not slowed down. They made it interact and travel more. The slowdown in a refractive index material is as Floris and CuriousOne described. It is similar in for instance a light fiber, in essence a waveguide, where it slows down because of reflections against the Fiber edges or walls (i.e., material, so longer distance and also whatever time the interaction lasts). It's been well known for both refraction and waveguides. The rest about light slowing down in vacuum or different speeds for blue and red are nonsense.

As for the Science article if it was not Science I'd ignore it, and the fact they misused the title is disappointing. I looked up another summary, the mask makes it take a longer path. See http://physicsworld.com/cws/article/news/2015/jan/22/structured-photons-slow-down-in-a-vacuum.

The only thing interesting was they were able to slow down through the longer path single photons. This breaks no laws, and they used a dishonest title. Robert in a comment is also right. Photons can not travel at any speed except c. Everything else you doubt it.

Bob Bee
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actually light moves at different speeds not only dependenig on the density of the environment but also the kind of light i.e red light is faster than blue, thats why stars seem to twinkle or change colour its because the defferent coulors rech your eyes at diferent times because of moving at different speeds

arq
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    That's slightly misleading. In a vacuum all wavelengths travel at the same speed. In a medium this is in general not the case and it is indeed possible to observe dispersion due to a difference in speed between wavelengths. Another factor could be the direction in which the light travels or its polarization (birefringence). The wikipedia page on the refractive index is a good place to start reading. – Wouter Jul 05 '16 at 20:32
  • i don't believe for a second that the speed of light, in vacuo, is variable. in a vacuum, it is a constant function of frequency. – robert bristow-johnson Jul 05 '16 at 22:02
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Yes the speed of light is not constant - it can be slowed down IN VACUUM. The discovery was published in 2015 in Science (but is unpopular, for obvious reasons):

http://www.upi.com/Science_News/2015/01/23/Scientists-slow-down-light-particles/1191422035480 "The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently."

http://science.sciencemag.org/content/347/6224/857 "Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860

Pentcho Valev
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