Slowest recorded, speed of light.

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Discussion Overview

The discussion centers on the slowest recorded speed of light, exploring both theoretical and experimental aspects of light's speed in various media. Participants examine the implications of slowing light, the physics behind it, and related phenomena such as the behavior of light near black holes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant mentions a record of light traveling at just over 40 mph, questioning the scientific methods behind such measurements.
  • Another participant suggests that the slowest speed could be 0 kph, arguing that photons always travel at the speed of light (C), but their effective speed can be reduced in a medium.
  • A participant explains that light can be slowed down in dispersive media, with the phase velocity depending on the refractive index, and emphasizes that this is a wave phenomenon rather than a violation of Einstein's equations.
  • There is a question about whether any special effects can be achieved with light when it is slowed down, indicating curiosity about practical applications.
  • One participant clarifies that while photons do not slow down, their interaction with a medium involves absorption and re-emission processes, particularly in the context of supercooled vapors.
  • A separate inquiry is raised regarding the speed of light near a black hole's event horizon, suggesting a connection to gravitational effects on light propagation.

Areas of Agreement / Disagreement

Participants express differing views on the nature of light's speed in various contexts, particularly regarding whether photons themselves slow down or if their effective speed changes due to medium interactions. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Participants reference various sources and examples, indicating a reliance on specific definitions and contexts, such as the behavior of light in different media and the effects of gravity on light near black holes. There are unresolved assumptions regarding the implications of slowing light and the nature of photons in different environments.

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.
 
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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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