Expert: James Gort - 4/24/2007

Question
QUESTION: in special relativity speed od of light is constantn & works as a linkage of connecting space & time & SR has basic postulate that (c) is invarient, but GR rejects this and says that speed of light is variable . doesn't this contradict basic postulate of SR ?
ANSWER: Hi Mohit,

You're probably referring to Einstein's statement that "according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity."  While GR does not require constant c, neither does it prohibit constant c.  In fact, recent experiments have strongly indicate that gravity travels at c, thereby confirming the constancy of c in GR.  

Steve Carlip, a professor of UC Davis and postdoc at the Institute for Advanced Study, Princeton (http://www.physics.ucdavis.edu/Text/Carlip.html) has a good summary article at:  http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.htm


However, quantum electrodynamics (QED) may seem to contradict SR.  In QED, a photon can (and usually does) travel at speeds less than c or more than c, and NOT in straight lines.  But when you vectorally add all the probabilities for each photon's path, the average speed (in vacuum) will be exactly c.  

Hope that helps explain a difficult concept.

Prof. James Gort

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QUESTION: SIR
with recent obvervation it is shown that universe is expanding at an accelerating rate. but dark matter & dark energy were created as other matter was not accounting enough to halt expantion of universe now we know that it is not going to stop ever then what's the need of dark matter ?

Answer
Hi Mohit,

The concept of datk matter was actually developed to explain the rotational dynamics of the Milky Way.  Beyond a few kiloparsecs, the Galaxy rotates as a rigid body, in spite of what the observed distribution of mass is.  Based on the laws of rotational dynamics, dark matter was postualted to account for this "strange" behavior.  Similar rotations have been observed in other galaxies.  Since then, dark matter has successfully accounted for gravitational lensing in distant objects.  However, as you note, dark energy is more than enough to compensate for its effects and cause the apparent acceleration.

Prof. James Gort