Can the speed of sound or light be slowed down?

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glassynails
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Can the speed of sound or light be slowed down?

Post by glassynails » Wed Jul 13, 2016 10:22 pm

Was wondering if the speed of sound or light could be slowed down.

Why does light travel? Sound?
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slidika
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Re: Can the speed of sound or light be slowed down?

Post by slidika » Thu Jul 14, 2016 11:31 am

I think a black hole would do that.
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Re: Can the speed of sound or light be slowed down?

Post by bear » Thu Jul 14, 2016 12:17 pm

When I was an undergrad I majored (among many other things) in physics. Of course, the world was flat and Helios drove his chariot across the sky.
I came across the article that you might find interesting - http://news.harvard.edu/gazette/1999/02.18/light.html
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Re: Can the speed of sound or light be slowed down?

Post by pogmoor » Thu Jul 14, 2016 4:44 pm

The speed of sound varies with several properties of the medium it is passing through (eg temperature and density). The speed of light in a vacuum is a constant such that the speed it arrives remains the same even if you are moving away from the source. However the speed of light through transparent solids, liquids and gases is less than its speed in a vacuum.
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Re: Can the speed of sound or light be slowed down?

Post by Pat Dodson » Thu Jul 14, 2016 5:11 pm

Here's a rather fun experiment to determine the speed of light that can be done in your kitchen.

http://eufisica.blogspot.co.uk/2012/02/ ... e.html?m=1

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Andrew Fryer
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Re: Can the speed of sound or light be slowed down?

Post by Andrew Fryer » Thu Jul 14, 2016 5:39 pm

Both very easily. Glass slows down light. Thin air slows down sound.
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Re: Can the speed of sound or light be slowed down?

Post by guitarrista » Thu Jul 14, 2016 5:56 pm

pogmoor wrote:The speed of light in a vacuum is a constant such that the speed it arrives remains the same even if you are moving away from the source. However the speed of light through transparent solids, liquids and gases is less than its speed in a vacuum.
Just to add here that it is the apparent speed of light through a medium which is less than that in vacuum. However, it is not true that light slows down - rather there are interactions with the medium it travels through. Although this is too simplistic, it is as if it bounces around in many directions so that it takes longer to progress in the original direction of travel (modified by the refraction angle). In reality there are absorptions and re-emissions going on. But in between interactions, light does still travel at the speed c in vacuum, within any medium.
Last edited by guitarrista on Thu Jul 14, 2016 6:49 pm, edited 1 time in total.
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Re: Can the speed of sound or light be slowed down?

Post by gitgeezer » Thu Jul 14, 2016 6:25 pm

Andrew Fryer wrote:Both very easily. Glass slows down light. Thin air slows down sound.
Sound travels at different speeds through different mediums, but its speed through air is influenced only by temperature, not by density. Sound travels more slowly through colder air because molecules move more slowly as their temperature drops. Sound travels more slowly higher in the atmosphere because the air is colder, not because it is thinner.

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Andrew Fryer
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Re: Can the speed of sound or light be slowed down?

Post by Andrew Fryer » Thu Jul 14, 2016 6:36 pm

Yeah, I knew I wasn't au fait with sound transmission media, but I didn't want to waste a lot of time on my reply.
Temperature is easiest to achieve, as you say. The speed of sound in water is high. Are you sure there's not a converse to that?
You think sound would travel no faster in Krypton than in air?
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Re: Can the speed of sound or light be slowed down?

Post by Andrew Pohlman » Thu Jul 14, 2016 6:55 pm

What's more interesting to me than "if the speeds change" is the effects of a changing wave speed. That's what Doppler Effect and Red Shift are all about. The frequency stays the same, but the observed frequency changes.

The classic train and trumpet experiment explains this. A trumpeter on a moving train plays a single note (a single frequency). The observer on the ground hears a higher frequency as the train is approaching, and a lower frequency as the train moves away. The source frequency is consistent. But the wave speed change - faster as the train approaches and slower as the train moves away.

Ditto for starlight. In an expanding universe, most light sources are moving away from one another, hence we always must correct for red shift. The perceived frequency is lower for objects moving apart.

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Re: Can the speed of sound or light be slowed down?

Post by dtoh » Fri Jul 15, 2016 9:26 am

An electrical signal travels faster through an electrical cable, than an optical signal travels through a fiber optic cable.

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Re: Can the speed of sound or light be slowed down?

Post by Granary Guitars » Fri Jul 15, 2016 1:10 pm

pogmoor wrote:The speed of light in a vacuum is a constant.
I tested your theory with my Dyson but it was too dark and dusty inside for any meaningful results, although I did find a pound coin.

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Re: Can the speed of sound or light be slowed down?

Post by pogmoor » Fri Jul 15, 2016 2:05 pm

Granary Guitars wrote:I tested your theory with my Dyson but it was too dark and dusty inside for any meaningful results, although I did find a pound coin.
:lol:
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Re: Can the speed of sound or light be slowed down?

Post by Michele Franceschini » Tue Jul 19, 2016 1:29 am

Wikipedia is awesome as usual: https://en.wikipedia.org/wiki/Speed_of_sound

In practice for a gaseous medium speed of sound is proportional to the square root of T (in Kelvin) and inversely proportional to the square root of the molecular mass of the gas (also depends on the geometry of the molecule). If you introduce a light weight gas in the air like hydrogen or helium you'll get an increase in the speed of sound.

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Re: Can the speed of sound or light be slowed down?

Post by John Ross » Tue Jul 19, 2016 4:11 pm

gitgeezer wrote:its speed through air is influenced only by temperature, not by density.
Not so, pressure counts, because air is not an ideal gas. So sound would travel more slowly in higher levels of the atmosphere even if there were no temperature difference.

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