Confused about Special Theory of Relativity?by guitarphysics Tags: light, special relativity, speed of light 

#1
Nov1012, 12:09 PM

P: 149

Hi, I have a question (or maybe more) about special relativity. I'm reading The Fabric of the Cosmos by Brian Greene right now, and what I got from one of the sections is confusing. I might be understanding it wrong, or maybe what he's saying is wrong, or maybe it's just right. So this is what I'm confused about:
Greene says that if we run away from light, in relation to us, it will stay at the same velocity. This is because velocity is just distance/time, and if we measure time slower while we're going faster, we will measure light as going faster. Everything will even out, and we will measure light as going at the same speed. Well, if this is true, then wouldn't this be true for all things coming towards us? Wouldn't everyday objects maintain a constant speed (in relation to you) as well? Sorry if I just got all of this completely wrong, I'm relatively new to physics. On a sort of unrelated note, why is our fourvelocity "c"? Thanks 



#2
Nov1012, 03:32 PM

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That happens because time dilation and length contraction are complimentary terms. I take it the book does not have a lot of math in it? 



#3
Nov1012, 04:55 PM

P: 149

Oh, thanks a ton! That really clears things up. And I should have been more clear with the part about our fourvelocity; I did mean magnitude. Why is our fourvelocity MAGNITUDE the speed of light?
And no, the book has no math in it :S 



#4
Nov1012, 06:41 PM

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Confused about Special Theory of Relativity?
Neato. Note: JIC...




#5
Nov1012, 08:38 PM

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#6
Nov1012, 08:49 PM

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#7
Nov1012, 08:50 PM

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#8
Nov1012, 08:52 PM

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#9
Nov1012, 08:54 PM

P: 149

Thanks, I will :)




#10
Nov1112, 02:07 PM

P: 97

BTW, the Feynman lectures are excellent. Additionally, many notable colleges and universities have video lectures and other resources. Check out MIT and Harvard. I've used their resources, and they are truly excellent. 



#11
Nov1112, 02:11 PM

P: 97

Buy it used on eBay for short money, or go to your local college bookstore and pay a little more for a used copy. And if you can't decide which of a couple alternatives is better, ask here. This place has lots of good people who are happy to help. 



#12
Nov1212, 11:30 AM

P: 1,098

Terminology is important, and at 15 you still have remarkable abilities to "absorb" new concepts / terminology. May as well start off on the right foot. To word this differently, there is literature out there that is more respectful of physics terminology & concepts & is not biased to any particular "school of thought". imo Greene does not meet those expectations of mine. So yea, always be conscientious of who the author is. 



#13
Nov1212, 10:02 PM

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There are more than a few questions here arising from Greene's popsci books ... thinking about it, basic college texts should be accessible to a motivated 15yo. You've almost got the tools at that age and, with the internet, you can backfill the gaps. It would take a bit of determination though.
Something like cosmology 101 materials should be fine. 



#14
Nov1412, 04:04 PM

P: 149

Sorry to bother you again, but I just thought of this; how does Greene's logic (which I explained in my original post) make sense if you're moving TOWARDS light? Wouldn't it be a lot faster than if you were standing still?




#15
Nov1412, 07:39 PM

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Your question actually does not make sense in terms of relativity:
for instance: "standing still" has no meaning. With respect to what? If you rephrased your question so it made sense, then the answer would be apparent. We already know that Greene's logic is flawed  there is no point exploring it further than that. As written, the argument given for an object moving away from the observer does not apply to one moving towards the observer. Move on to real physics. 



#16
Nov1412, 08:03 PM

P: 149

Yeah, you're right. I'll finish up my homework on slants and return those Greene books to the library...




#17
Nov1512, 03:36 AM

P: 3,178

http://www.physicsforums.com/showthread.php?t=641102 In particular posts #3, #10, #12+#15+#38, #18, #46. Then, when you understand the main points (if you are not a genius then that should take some time), you could next try to follow my calculation example in post #50. 


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