What is a Light Year?

@BillRM,

Sorry but that is incorrect. Nothing changes the speed of light. When travelling through matter, light bends, as in the sky, giving it a blue tint. When traveling past a massive object like the sun, light also bends.

If you travelled just under the speed of light, light would pass you AT THE SPEED OF LIGHT. These are the facts of the universe that made Einstein famous.

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@Ionus,

Nope, Bill is correct. Light does propagate at different speeds through different media. In fact-- this is why light bends (it is called refraction and happens when light passes from one media, where it goes one speed, to another).

http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/refr.html

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@ebrown p,

Quote:

. . . Sound travels much much slower than light in any medium . . .

This is why some people seem bright until you hear them speak.

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@George,

That depends on the observer, George.

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light doesn't travel, light is the absence of dark, what is a dark year? I can turn on my bedroom light and be in bed before it gets dark. i travel light, lady diane travels with 4 steamer trunks and 5 carry-ons.

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@Ionus,

The sky is "blue" because of the scattering of light in what is for us the visible bands as a result of the atmospheric composition. Do a web search for Rayleigh scattering some time.

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@BillRM,

Half? Labs around the country have brought down the speed of light to around 30mph using Bose-Einstein condensates. Here's one:

Quote:

But a few physicists are intent on getting as close as possible to that theoretical limit, and it was to get a better view of that most rarefied of competitions that I visited Wolfgang Ketterle's lab at the Massachusetts Institute of Technology in Cambridge. It currently holds the record"at least according to Guinness World Records 2008"for lowest temperature: 810 trillionths of a degree F above absolute zero. Ketterle and his colleagues accomplished that feat in 2003 while working with a cloud"about a thousandth of an inch across"of sodium molecules trapped in place by magnets.

Read more: http://www.smithsonianmag.com/science-nature/phenom-200801.html#ixzz0PJPrx1SW

and, from the same article, another lab says it can stop light completely:

Quote:

The speed of light, as we've all heard, is a constant: 186,171 miles per second in a vacuum. But it is different in the real world, outside a vacuum; for instance, light not only bends but also slows ever so slightly when it passes through glass or water. Still, that's nothing compared with what happens when Hau shines a laser beam of light into a BEC: it's like hurling a baseball into a pillow. "First, we got the speed down to that of a bicycle," Hau says. "Now it's at a crawl, and we can actually stop it..."

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@High Seas,

Scarcely credible. And after all that, is it any use any more? Is there enough to read by, for instance?

Thought not. So what good is that?

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are you all trying to confuse me???

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@Dorothy Parker,

Dorothy Parker wrote:

are you all trying to confuse me???

The original answers are still correct, especially as relates to your daughter's original question.

If your daughter takes a science or physics class and starts asking about atomic propagation delay, quantum fluctuations and Bose-Einstein condensates, then review the later posts. Until then, a Light Year is the distance Light travels in a year. Smile

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. . . in the near vacuum of space.

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Not to talk about the fat years..

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@Setanta,

We've got some lights and a vacuum in our house. Are you saying one affects the other?
Should I switch out the light when using the vacuum, for example?
Then I won't be able to see the dirt.
Damn.

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@McTag,

Are you saying that you ordinarily see it?

You need to use that vacuum more frequently.

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@Setanta,

Quote:

Are you saying that you ordinarily see it?

What are you, Jewish? That a question with another question you would answer?

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@McTag,

McTag - aren't you a Brit? Don't you say "a hoover" instead of "a vacuum"? I suspect you of speaking of the interstellar vacuum and using "light" to indicate a laser beam, though how you managed to build a multiplexed pulsed beacon in your attic without Mrs McTag noticing is a mystery to me.

Optical scattering is of course caused by interstellar dust grains - Setanta has a point, you can see those after a few kiloparsecs.

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@McTag,

So what, are you the Grand Inquisitor?

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@ebrown p,

Thanks for the correction and the ref. Is this scattering due to the higher levels of electrons plus the angle of light ? And the following posts on stopping light..what is the big deal ? I have a brick wall that does that.. why is that experiment so groundbreaking ?

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@Ionus,

Don't you see? Once we control the speed of light we can build purely optical computers:

Quote:

...ultra-cold atoms known as Bose-Einstein condensates (BECs) can be used to perform "controlled coherent processing" with light. In ordinary matter, the amplitude and phase of a light pulse would be smeared out, and any information content would be destroyed. Hau's work on slow light, however, has proved experimentally that these attributes can be preserved in a BEC. Such a device might one day become the CPU of an optical computer.

http://www.physorg.com/news3679.html

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@High Seas,

Thinking aloud it could also be used as data storage. The stopped light could be identified as one and the escaped light or passed thru light as zero. There, you have the base two system.

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What is a Light Year?

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