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

General relativity concept

  1. Dec 3, 2007 #1
    Can anyone help me to know that if General relativity explains the perihelion of mercury or the CMBR...

    If so,how?
  2. jcsd
  3. Dec 3, 2007 #2

    Chris Hillman

    User Avatar
    Science Advisor

    Suggest a good textbook

    Hi, neelakash,

    GR explains the "extra-Newtonian term" in the precession of the perihelia of Mercury (and Venus, various asteroids, and various binary pulsars); in the case of Mercury, most of the observed precession is due to the perturbing influence of Jupiter, but after subtracting away everything they could explain, nineteenth century astronmers were left with an unexplained remainder which agreed perfectly with Einstein's rough computation using GR in 1915.

    The CMBR is not primarily a gravitational phenomenon, so much of the explanation involves physics other than GR, but certainly GR is involved in the explanation of some of the characteristics of this radiation.

    Try D'Inverno, Understanding Einstein's Relativity, Oxford University Press, 1992, for a good discussion (at the advanced undergraduate level) of these phenomena.
    Last edited: Dec 3, 2007
  4. Dec 9, 2007 #3
    [Can anyone help me to know that if General relativity explains the perihelion of mercury or the CMBR...

    If so,how?]

    Newton's theory of gravitation significantly erred when it was applied to calculate the precession of Mercury's orbit. No one knew why. Then Einstein invented General Relativity to explain relativity of accelerated observers in a similar fashion as his Special Relativity had done with inertial observers. To adequately treat acceleration Einstein had to invent stronger equations with more terms that when simplified reduced to Newton's equation for gravitation. Lo and behold the extra terms invented to explain earthly phenomena also explained heavenly phenomena like Merc's weird precession.

    It was the same thing with the CMBR. These guys P & W stuck out a horn looking for stray cat calls and bumped into heavenly hissing. The hissing had a microwave frequency. The CMBR. What did it mean? It came from every direction in the sky so it couldn't be explained by a single source. If you do a calculation something like radiocarbon dating, you get that the temperature of the CMBR implies this radiation has been around as long as the universe, a universe that many believe has evolved from a titanic explosion, a "big bang". Put together these two clues and you have something that apparently comes from everywhere in the universe and is as old as the universe, so it must have come from the beginning of the universe. Voila, it came from the Big Bang.

    So who cooked up the Big Bang? Friedmann, Lemaitre, Hubble, et al, but it was Einstein's GR that explained it, and Einstein missed it when his equations called for it. He couldn't believe it. Something wrong with the simple formulation. Have to add a compensating factor to keep the universe Newtonian! So Einstein invented the Cosmological Constant to keep the universe from flying apart. Along comes Hubble who discovers it IS flying apart. Man, was Einstein pissed. Biggest blunder he ever made. However, solutions to Einstein's equation with or without the cosmological constant indicate the Big Bang and thereby, the source of the CMBR. There are other explanations for the CMBR, other than the Big Bang model, but they're not as good.

    I hope you get the point here. These guys accidentally discovered these things about the universe. They were trying to solve certain problems and then they made a connection to solve seemingly unrelated problems. One can't calculate these "connections". They just come when you spend a lot of time sweating over boring equations that seem to be going nowhere. So that answers your question "how".

    Here's a question for you: What does it mean when someone says a planet's orbit precesses? More formally stated, what is perihelion or aphelion orbital precession?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?