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

Another confusing thought on relativity!

  1. Feb 11, 2012 #1
    So, as anything approaches the speed of light, times slows, and once an object reaches the speed of light, aka a photon, time stops passing for it. I'm fine and dandy with this concept, until I start thinking about light traveling through space from distant galaxies. If you're looking at a galaxy that is 1.28 million light years away, the light you are observing is 1.28 million years old. My problem with understanding this is that, if time does no pass for a photon, wouldn't the light have arrived on earth 1.28 million years ago instead of now?
  2. jcsd
  3. Feb 12, 2012 #2


    User Avatar
    Gold Member

    Nitpick: Nothing in the universe "approaches and then reaches the speed of light". Things with mass will never reach the speed of light. Massless things always go the sped of light. And ne'er the twain shall meet.

    No. 1.28 million years still passes for the rest of the universe.
  4. Feb 12, 2012 #3
    Good one. :smile: No, according to our measurement that light is 1.28 million years old in the sense that it was emitted that long ago. However its "proper age", if we use that term for a photon, is indeed zero - just as a space traveler going at almost the speed of light will hardly age. But if he leaves (according to our counting) during our life time, he will still arrive after we are long dead.
  5. Apr 16, 2012 #4
    To open a new though, if we have spent time and though based on space time being bent by gravity and its not, are we not in a rut that has trapped us for a long time and will subvert our line of though till we get out of that rut. If Einstein, was wrong and those people at the solar eclipse saw light simply being bent by the suns atmosphere, not time and space being bent by gravity, do we not have a problem starting from a mistaken point and would not out thoughts on relativity be changed.
  6. Apr 16, 2012 #5


    User Avatar
    Science Advisor

    The real waste of time and thought is the inability to accept a theory that was proved 100 years ago. If you think that gravitational lensing is a mistaken effect of the sun's atmosphere, let me remind you of the lensing produced by the galaxy cluster Abell 2218.
  7. Apr 16, 2012 #6
    If we shift this question a little to a space ship, can anyone answer me this one please?
    Twin Paradox. Twin Brian is merrily zooming through space at almost the speed of light heading for the star Castor.It will take him 50 years to cover the 49.8 light years.
    Brian had computers on his space ship, they can measure his speed.
    Wayne is back on Earth. Remotely manning a space telescope orbiting earth, carefully watching his twin brothers spaceship travel through space.
    I understand that Wayne should see the journey take some 50 years.
    Brian ?? Will think that the journey was ?instant? because time is slowed down for him and he does not age.
    But if he thinks that he get there in a matter of minutes, how fast did he think he was going?
  8. Apr 16, 2012 #7


    User Avatar
    Science Advisor
    Gold Member

    Brian and Wayne will both agree on the speed that Brian is traveling at except that Brian will see Wayne as traveling away from him.
  9. Apr 16, 2012 #8


    User Avatar
    Gold Member

    As Brian approaches c, he will see the distance between Earth and Castor shrink. By the time he hits top speed, the distance he measures between them will be much less than a light year - short enough that his journey only takes a fraction of a year.

    At no time will Brian measure his speed as exceeding c.
  10. Apr 16, 2012 #9


    User Avatar
    Science Advisor
    Gold Member

    Brian will age a only a little bit, thinking the trip took <<50 years to complete.

    He will, however, measure the distance to be extremely contracted, and so will measure his speed as ~c.
  11. Apr 16, 2012 #10
    Sorry, but my understanding was that questions were a good thing, (but having been talked out of school by the US. Government, to kill people back in the 70's), my knowledge base is very limited but I still have wide interest and I have found that to many times confusing questions and answers that are more confusing than the questions, often result from a basic mistake from the starting point, as I understand it no original theory, or understanding in any time, or age, has every survived the test of time. My apology for this question as it seems you were offended, so I'll stay off this cite and leave it the educated, Sorry but Facebook and the rest, really sucked, I looked for something more.
  12. Apr 16, 2012 #11
    very different answers there! But thank you Dave - that one looks good to me.
    In a mind bending Einstein was darn clever wasn't he and maybe I still won't sleep tonight even though I now know the answer...
    Paul (knows a little more now)
  13. Apr 16, 2012 #12
    xtrapper: get over it...you need a thick skin here when you disparage proven theories. In fact you'll need a thick skin in general!!!

    I disagree with your entire first post since you post a false premise and consequently arrive at a false conclusion. Maybe next time just ask : Do we have a better theory of gravity?
    and you'll likely get some responses that are useful both to you and others.

    It's not too late to learn: I only avoided 'killing people' in the sixties because I was in graduate school then....but I'm still learning....even more slowly than back then!!

    I would point out to you that most physicsts believe general relativity is NOT the final theory....nor is quantum theory....The reason it is suspected neither is absolutely accurate is that both appear to fail at the center of black holes and at the point of the big bang.....both appear as 'singularities', not likely much of anything actually 'infinite' as the math says, but rather where such 'infinite' results probably imply a breakdown of our theories. Where spacetime curvature becomes reallly, really severe, we have some more theoretical work ahead....quantum gravity attempts that, but is apparently not complete yet.
    A good read is THREE ROADS TO QUANTUM GRAVITY by Lee Smolin....all ideas, no math, and likely still cheap on Amazon books. Then you'll be able to ask some really cool questions...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook