Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of Light/Relativity

  1. Sep 11, 2006 #1
    My biology professor keeps talking about how the speed of light is no longer considered a constant. Anyway, he went on to quote how scientists have stopped a beam of light using magnetic fields. I've read a few popular science articles in the magazine New Scientists but found nothing that would indicate a violation of relativity or even one using magnetic fields. He also stated that the speed of light is changing over time. He used an example regarding how the sun will eventually run out of energy & between that time the speed of light will slow down as a result.

    Can anyone point me in the right direction of information on the stopping light with magnetic fields? (if there really were/are any)

    Does anyone know of any articles regarding the slowing of the speed of light?

    Overall, I understand a biology professor is not a physics professor but he keeps using various examples as if they were facts. Often stating "we once though the speed of light was a constant."
  2. jcsd
  3. Sep 11, 2006 #2


    User Avatar

    Staff: Mentor

    When we refer to the speed of light being a constant, we're usually referring to the speed of light in a vacuum. Any slowing of the speed of light involves its transmission through a medium of some sort. For example, the speed of light in water is about 3/4 the speed of light in vacuum. This is the basis of refraction in lenses etc.

    I understand there are theories which incorporate or predict a variable speed of light (in vacuum), but as far as I know they are speculative, and not widely accepted.
    Last edited: Sep 11, 2006
  4. Sep 11, 2006 #3
    i'd like to see the peer-reviewed article for that one. I highly doubt its as he represented it.

    He said what? Slow down as a result of what? The sun running out of hydrogen fuel?

    I'm pretty sure he's mistating and misquoting whatever he's read (probably not intentionally).

    What jtbell said pretty much covers it.
  5. Sep 12, 2006 #4

    Meir Achuz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "a biology professor is not a physics professor."
  6. Sep 12, 2006 #5


    User Avatar

    This is a nice paper on "slowing light down": http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000095000025253601000001&idtype=cvips&gifs=yes [Broken]
    Last edited by a moderator: May 2, 2017
  7. Sep 12, 2006 #6


    User Avatar
    Science Advisor

    The speed of light in vacuum is constant!
    Your biology professor should stick to his field of knowledge.
  8. Sep 12, 2006 #7


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The professor is perhaps referring to hypotheses that c may have varied over the course of the universe. He should not be treating these mere hypotheses ("theories in search of confirmation") as facts or draw social conclusions from them.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook