# Thread: Is Lightspeed A Constant

1. Newbie
Join Date
Apr 2004
Posts
3
I am wondering if lightspeed is really a constant. It is measured as a constent value most places I read. However, I notice that light is said to slow down in an atmosphere or at some point of resistance.

What then would the slowest possible speed of light before it is no longer light, then what would be the fastest.

If light can travel at variable speeds then how can it be used as a constant for measurement?

2. Join Date
Apr 2004
Posts
8
When light is considered to act as a wave, it can be proven to slow down when it changes mediums (ie: space to atmosphere) the same way regular waves change. This can also be seen with a prism and light.

However, when light is used as a constant, they measure "unaffected" light, or basically light in space. This way they don&#39;t worry about light changing speeds when changing mediums.

As far as light not being light: light as we know it is energy with a certain wavelength (~400-750nm). When it goes smaller then this you have Ultraviolet "light" and above goes to Infarred and up. So if you alter the wavelength (or frequency) you change the colour, and can eventually move light out of the visible spectrum.

I hope that helps.

Chris

3. Another take on the speed of light being a constant would be the fact that the measurement of c (speed of light) in any medium (let&#39;s take space here) remains constant such that c will always be measured to be the same speed (constant) no matter the relative speed or direction of the observer making the measurment. So, if you are moving in a spaceship at half c, relative to earth, and you shine a light beam ahead of you in the same direction you are travelling, you will still measure that light beam to be travelling at c, not c plus half c. Likewise, you will measure a light beam shined directly behind you in the opposite direction as traveling at c, not half c. And, an observer on earth would measure both light beams as c (about 186,000 miles per second). Even two spaceships traveling toward each other, each at half c, would measure beams of light pointed at one another as travelling at c, not 2 times c.

Anything else besides electromagnetic radiation (visible light is only one part of the entire spectrum) does not enjoy this constancy. Throw a baseball at 50 mph (relative to you) off the front of a train moving at 50 mph, in the same direction that the train is moving, and an observer standing still off to the side of the train tracks ahead of the train will measure the ball moving at 100 mph. Not so with light&#33; Now that&#39;s constancy.

Relativity is the key here.

4. Established Member
Join Date
Dec 2003
Posts
488
As far as how slow you can make light go, damn slow is the answer. This link describes a method of increasing the index of refraction of a gas to such a high level that the speed of light in that medium is 90 m/sec. Compare that to the speed of light in a vacuum - 300000000 m/sec.

5. And this link http://science.nasa.gov/headlines/y2002/27...r_stoplight.htm is about how scientists have even stopped light&#33;

6. Newbie
Join Date
Apr 2004
Posts
3
Thanks for those great answers. That was indeed helpful.

However, it creates yet another question.

If you can slow light down to almost nothing, and it still be concidered light, is it possible to accelerate light faster the typical "constant" speed?

I wonder if our constant measurment of light speed is actually slower than its maximum velocity due to intererence from universal forces like gravity or even dark matter/energy etc.

Thanks again.

7. Mr kipper Guest
In answer to your question, no. Once you are talking about the speed of light in "empty" space, that&#39;s the fastest the photon will travel. Anything that might increase the acceleration of matter, like gravity for instance, will increase the frequency of the light. But the speed at which light travels will not increase above the empty space value of 3*10^8 m/s.

Also, gravity doesn&#39;t slow light down, it changes the frequency and possibly the path. If light is heading directly into a gravity well, the frequency increases. If headed out, the frequency decreases. If headed in some manner tangentally, the frequency and the path traveled can both be changed.

Traveling through some sort of matter can and does slow light down. Technically speaking, it is possible that the neutrino flux around every star could effect the speed of light, but as neutrinos don&#39;t react much with anything, the chance of the change being significant is extremely small.

8. John Guest
The speed of light in a perfect vacuum is constant (about 300,000 km/sec), and nothing with any mass can go faster than that speed.

When you are talking about light slowing down you are measuring the time it takes a photon to travel from on point to another in a given medium (air, water, glass, the vacuum of space, etc.). The reason it is said to slow down is that the photon in a medium will collide with the particles in that medium, bounce between them several times, and continue onward. In essence, the speed of the photon is still the same speed, c, but the distance between the two end points has been increased based on the back and forth bouncing the photon has to go through to reach the other side. Technically the reactions with the particles in the medium increases the length of the path the photon takes thus giving the appearance that it has slowed.

The sun is a great example. The photons you see now left the sun&#39;s surface 8 minutes ago, but they were formed in a nuclear reaction in the core of the sun 30,000,000 years ago. The density of the matter in the sun causes the photons to bounce off of the densely packed atoms for millions of years before their random path finally took them to the surface and they finally escape into the relative vacuum of space.

So when scientists claim to have stopped light, they have created a non-opaque medium that causes the photons to become trapped between particles in the medium and continuously bounce back and forth without reaching the other side. This is typically done with exotic compounds that must be modified with lasers to create the trap, and when the lasers are removed the light is freed and can complete its path to the other side.

9. Newbie
Join Date
Apr 2004
Posts
3
Thanks again, very informative and helpful.

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