Slowest recorded, speed of light.

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SUMMARY

The slowest recorded speed of light is approximately 40 mph (60 kph), but this measurement is contingent upon the medium through which light travels. In a dispersive medium, such as water, light slows down due to the refractive index, where the phase velocity (vp) is calculated as vp = c / n. While photons always travel at the speed of light (C), their effective speed can be reduced by interactions with matter, such as absorption and re-emission in supercooled vapors. This phenomenon is primarily a wave-related occurrence rather than a deviation from Einstein's equations.

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  • Understanding of the refractive index and its impact on light speed
  • Familiarity with Einstein's theory of relativity and massless particles
  • Knowledge of wave phenomena in physics
  • Basic concepts of material science related to optics
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Physicists, optical engineers, and students studying wave phenomena and the behavior of light in different media.

MartinJH
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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.
 
Science news on Phys.org
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
 
Last edited by a moderator:
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.
 
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:
The photons themselves do not slow down. But their passage through a medium involves absorption 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
 
Ahhh thank you alexg, I couldn't get my head around it. Thats cleared it, thank you.
 

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