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Relativity questions

  1. Mar 25, 2012 #1
    Blame it on Woodstock. Can I ask a stupid question? Actually two of them?
    First off, I'm not a physicists. I write php code for a living and that is the boring reality my life is. Just so you know why my questions are stupid. :)

    I was reading about gravitational time dilation - (as an extension of how GPS satellites work) and it said time slows down the lower the gravitational potential. (Which, I took to mean the closer you are to a body of mass.)

    I know time is relative. But, would time be affected significantly by either of these two scenarios?:
    A. In an area of space that has absolutely no mass around at all?
    B. In a black hole?

    As far as gravity goes - aren't those two extremes? Would time change greatly or just a teeny bit?

    My second question is (and this crazy book I bought doesn't even mention it.) this: Let's say I had a big telescope and I was looking at a ray of light traveling through a part of the galaxy where time is different than our own. If I could measure how fast the light was traveling - would c be based on my reference of time or would it tell me how time is different in that area of the galaxy? Does that make sense? Cause I think I just confused myself... :)
  2. jcsd
  3. Mar 25, 2012 #2
    Re: Special Relativity Clocks

    Hello drymetal:

    Better to post such questions separately on your own..but I will give some short ideas.

    Two things affect the apparent, that is relative, passage of time: relative velocity [from special relativity] and additionally gravitational potential [from general relativity] . so...yes to A and B. But we cannot 'observe' time with in a black hole..... nor anything else....only as one approaches the event horizon which surrounds it.

    yes they are 'extremes'. Time appears to slow to a stop for a distant observer watching something else approach a black hole....the distant observer would see dramatic redshift of electromagnetic radiation [light] from that source approaching near the black hole.....the illusion is that the object never reaches the horizon; locally near the BH time appears 'normal' to a free falling observer.

    What you observe is always a local phenomen...light always travels at 'c' locally .....and you 'see' [detect] light as it enters your eye or instrument NOT as it was orginally emitted perhaps from many light years away. You will observe redshift or blue shift depending on circumstances....the dark spectral lines shift one way or another.....For light passing in a strong gravitational field, like past the sun, spacetime curves and the light appears to curve.....it changes direction....

    Another similar effect is due to the accelerating expansion of the universe.....that's more complicated.

    A complete answer is not possible here,,,,for a start check this out:


  4. Mar 25, 2012 #3
    Re: Special Relativity Clocks

    Thank you Naty1. I'll start reading those. I posted here because often on forums - they'll redirect you if there is a post even remotely similar to what you want to know. So I'll create new topics if I have any questions from what you wrote and what I'm about to read. Thanks!
  5. Mar 25, 2012 #4

    Doc Al

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    [Moderator's Note: Posts moved to new thread.]
  6. Mar 25, 2012 #5
    What if two planets were like 20 feet apart and I was floating equally in-between them. Would I not see any shifting?

    Do I have to memorize and learn these equations on these pages to learn how Gravitational redshift works?

    So, to make sure I'm following this. Redshift and Blueshift are essentially when the frequency of light changes?

    I'm probably going to use the worst analogy in the history of this topic, but oh well. If a person is in a swimming pool and yells - someone underwater will hear them just fine - but anyone outside the pool won't because the sound waves would become weaker as soon as they left the water. And this would be like light leaving a stronger gravitational field and that would be called redshift?

    And if the person outside the pool is yelling, the sound waves will hit the water and increase in frequency (I'm guessing because the water molecules are closer together than in the air.) which would be like light going into a stronger gravitational field which is blueshift?
  7. Mar 25, 2012 #6
    I should have also mentioned cosmic microwave background radiation......CMBR

    you can look it up in Wikipedia for a start.

    This is the 'oldest light in the universe'...coming past us now, from all directions, from about 380,000 years after the start of the universe!!..... get ready for a likely long initial and confusing read....there is a lot to it....and another great source to follow on from there that I was introduced to in these forums: [Lineweaver and Davis, MIT]

    the introductory level:

    http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf [Broken]

    and the professional level:

    Last edited by a moderator: May 5, 2017
  8. Mar 25, 2012 #7
    Chronos justed posted that in another discussion.....When we understand exactly what it means we'll be well on our way to understanding light/cosmic expansion/redshift...etc.

    It's a snazzy way to express 'oldest light in the universe' in my post #6...
  9. Mar 26, 2012 #8
    look here:http://en.wikipedia.org/wiki/Redshift#Redshift_formulae
    I am sure not going to 'memorize' all those....

    it helps to 'learn' concepts, rather than memorize, but getting expert interpretation is also very helpsful: Experts here sometimes have different views on 'what the math means.' But one thing you CAN NOT usually do is skim quickly and then assume you understand stuff....not unless you are a lot smarter than most. If you want to work numerical problems you must have the math.

    A difficulty is that in relativity and cosmology [based on relativity] things are not rigid and fixed like, a local ruler measurement or time interval [ticking of a clock] measured locally here on earth. Most other observers [in different gravitational potentials, or accelerating and/or moving at different velcoity] will measure those differently than you.

    that's a good place to start. Now you are forcing me to study some specifics...here goes....
    and I think you really mean GRAVITATIONAL redshift for this discussion....But what does it mean:

    try reading here, too:


    I'd summarize things this way: Redshift due to relative motion between an observer and a source [like radar] is attributable to Doppler shift [and explains cosmological redshift, the observed expansion of the universe]; gravitational redshifts are observed in electromagnetic radiation moving out of gravitational fields.

    What is the source of gravitational redshfit? [from Jonathon Scott of these forums]

    Ah ha!!!! I just loved it when I found that in these forums. He says a photon starting from a lower potential energy has less overall energy...a classical viewpoint, I guess

    Another piece of the redshift puzzle:

    Then there is gravitational 'time dilation':

    And I'll post a question of my own:

    In general relativity, what is the source of gravitational redshift? Do we say it is gravitational time dilation or do we say it is gravitational potential?

    PS: sound waves require a medium, electromagnetic and gravitational waves don't they have some very different behaviors....I'd not draw the analogies you did but others might make a connection....
    Last edited: Mar 26, 2012
  10. Mar 26, 2012 #9
    It's too bad I can't just sign up for physics classes with no intention of a degree.

    Well, apparently I have a whole lot to read. I wonder if they have any Dummies books for physics. : ) I'll start reading all that. -and apparently relearning a lot of math. Cause most of that looks like Klingon to me. Well not actually - cause I can make sense of Klingon. But still...

    You know what the problem with physics is? I'm going to have to learn 50 things just to understand one. lol

    Oh well. Thanks for the links. Reading now...

    Oh and the sound wave thing makes sense. Sound waves are the movement of air or water whereas (I think) electromagnetic waves are the movement of protons? Gravitational waves...I've no idea. Gravitons? :) lol I'll add all that to my list of reading too. And all the links on all these articles - and all the links on those articles....
  11. Mar 26, 2012 #10
  12. Mar 27, 2012 #11
    These forums are rather low cost.....and you can 'search' most topics because most stuff has been discussed repeatedly...also look in frequently asked questions [FAQ] some are good, a few great, others not so hot....

    the big disadvantage here is that people post errors [me included and I cringe when I see some of my early posts] and posts, unlike textbooks and general books are not edited, so people may not say what they mean. Other times, like this discussion, you get stuck with one dummy...... like me....

    The big advantage is that often you get six or eight or more different explanations of a phenomena and experts pick apart each others descriptions.....that can be really useful....and there are experts here in many fields [except global warming, it seems.] Another advantage: you find out almost nobody understands it alll!!! partly because science is incomplete.

    very true. But that is only early on; getting started is sometimes the most difficult. Then you get old and don't remember quite as much!!!! THAT becomes a REAL problem!

    you can start with something that interests you and expand from there....About eight or ten years ago I renewed my interest in physics by buying used books at Amazon and taking them with me summers I spent on my boat. I'd read maybe a chapter most days....when I got back home to a computer, I check ??? I noted against Wikipedia to see if that helped....maybe search here, copied explanations that made sense to me and if I could not find what I wanted, asked questions.

    Look for, maybe, non mathematical books for the general public by Lee Smolin,Michio Kaku, Brian Greene, Stephen Hawking, Richard Feynman and after a few of those from Leonard Susskind, Lisa Randall, others....Penrose may have some,too...
    For relativity, look online: 'Einstein Online' for one.... http://www.einstein-online.info/

    and try 'Elementary' Einstein there.....see if it interests you....and how it compares with the material you just read....

    When you get really good at physics maybe you can 'dazzle the chicks'...like the guys on BIG BANG THEORY [an American TV show]. If you succeed, let me know how that works. Or found a 'physics bar and grill'.....
    Last edited: Mar 27, 2012
  13. Mar 27, 2012 #12
    I never used any of the 'dummy books'....never looked at them....do a search in these forums somebody have have written comments .....

    you are probably thinking of 'photons' not protons; photons are the quanta [local particle] of an electromagnetic wave and it is thought a 'graviton' is the quanta of the gravitational wave...photons have been detected but not gravitons; gravitons are still purely theoretical...

    with sound waves, energy moves particles in a medium....like water or air; electromagetic [including light] and gravitational waves required NO medium....they go through 'empty' space.....that's why we can see the heavens, for example, but not so long ago, the best scientists in the world thought electromagnetic waves must utilize 'something' and they dreamed up 'ether'.....
    so look at it this way: already you are on a course with the finest physics minds in the world just 90 years ago!!!! quite an accomplishment....but Einstein was smarter than all of them and dreamed up relativity...somehow he figured out that space and time curve and just about everything we observe is relative....not fixed as it appears....
  14. Mar 27, 2012 #13
    I think that this hasn't been mentioned, but the speed of the light between the star and your telescope doesn't really matter, as long as the average speed doesn't change while you are looking and as long as the star remains at the same distance (no Doppler effect).
    That is easy to understand: suppose that it takes exactly 1 year for the light to reach you. How will that affect the colour that you see? And if light from a similar star, twice as far, takes 2 years? Or if that light went twice as fast? As long as the time delay between emission and reception is constant, you will see the same oscillation frequency - independent of the speed of signal transmission. Else oscillations could get lost in space, or accumulate in space. :wink:
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