# Slowest recorded, speed of light.

1. Jan 7, 2012

### MartinJH

I googled for the slowest record for the speed of light and it came up with just over 40mph or 60kph.
I am fairly knowledgeable with Einsteins equations regarding the speed of a massless particle but what were scientists able to do when they were able to do this with light.

Heres where I got the answer;
http://wiki.answers.com/Q/What_is_the_slowest_speed_light_has_ever_been_measured

I know its not the ideal place to get this type of information from but I wanted to try and find an answer before I asked here.

2. Jan 7, 2012

### Ryan_m_b

Staff Emeritus
Actually I think the slowest ever is 0kph
http://news.bbc.co.uk/1/hi/sci/tech/1124540.stm

However as I understand it this is technically incorrect, photons always travel at C but passage through a medium can result in slower transit times
https://www.physicsforums.com/showthread.php?t=511177 [Broken]

Last edited by a moderator: May 5, 2017
3. Jan 7, 2012

### tommw

You can slow down light simply by putting it into a dispersive medium. Light in water is slower than in air.
In a medium with refractive index n, the phase velocity vp is given by vp = c / n
So if you find a transparent medium with a very high refractive index, you can in principle slow down light to whatever speed you want. The problem is thus really related to material science and you don't need to look to much into Einstein's relations because its a simple wave phenomenon.

4. Jan 7, 2012

### MartinJH

Thank you both.
This question is probably asked a lot here but I felt I needed to ask myself.

Also, are we able to do anything 'special' to light/photons when we slow them down.

Last edited: Jan 7, 2012
5. Jan 7, 2012

### alexg

The photons themselves do not slow down. But their passage through a medium involves absorbtion by electrons and re-emission.

In the case of the photons which were 'stopped', they were absorbed in a supercooled vapor, but not re-emitted until a laser stiumlated the emission.

http://cerncourier.com/cws/article/cern/28403

6. Jan 7, 2012

7. Jan 8, 2012

### MartinJH

Ahhh thank you alexg, I couldnt get my head around it. Thats cleared it, thank you.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook