# Is it possible to slow down light?

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?

• 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 5 '16 at 20:02
• @Sanya I've edited the title; does that help to clarify my question? – MonkeyZeus Jul 5 '16 at 20:05
• Things that slow down light are called 'lenses'. – tfb Jul 5 '16 at 20:06
• @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 5 '16 at 20:15

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...)

• 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 5 '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 5 '16 at 20:50
• @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 5 '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 5 '16 at 21:25
• 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 5 '16 at 21:27

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.

Yes, it is possible to slow down light in many ways. They've been mentioned above: send it through some media, like water, glass or air, etc.

If you read Einstein's 1905 paper, you will see that he ONLY mentions the speed of light being a "universal constant" in the "IMAGINARY" universe he created by having all clocks in his IMAGINARY universe be synchronous. There's a highlighted and annotated copy of the paper at this link: http://www.ed-lake.com/Einstein_1905_relativity-annotated.pdf The key passages are on page 3.

The speed of light only has an UPPER limit. As I see it, that upper limit is 299,792.458 kilometers per second in a STATIONARY spot in our universe. But, due to time dilation and the fact that the Earth is FAR from being stationary (it's rotating on its axis, while orbiting the sun as the sun orbits around the center of the Milky Way Galaxy), it can be argued that the speed of light is slower here because the length of a SECOND is longer here.

We also measure the speed of light on earth as being 299,792.458 kilometers per second, but that is only because our seconds are longer.

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

• 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 5 '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 5 '16 at 22:02

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

• That's a really bad case of "science by press release". That the speed of light is a concept that strictly only applies to plane waves was long known. – CuriousOne Jul 5 '16 at 21:36
• i don't believe for a second that the speed of light, in vacuo, is variable. – robert bristow-johnson Jul 5 '16 at 22:01

## protected by Qmechanic♦Jul 24 '16 at 17:38

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