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Confused about Special Theory of Relativity?

by guitarphysics
Tags: light, special relativity, speed of light
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guitarphysics
#1
Nov10-12, 12:09 PM
P: 180
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 four-velocity "c"?
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Simon Bridge
#2
Nov10-12, 03:32 PM
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Quote Quote by guitarphysics View Post
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.
Well, since that is a popular science book, it is probably a mixture of all of the above ;)
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.
... time dilation and length contraction happen to keep the speed of light constant. Other things are going slower - so you see times and lengths, associated with them, adjusted by a different amount. The amount of the adjustment depends on the relative speed.
On a sort of unrelated note, why is our four-velocity "c"?
It isn't. It is the magnitude of the 4-velocity which is a constant. We usually choose the constant to be 1 and measure distances in terms of the speed of light ... so if time is in seconds, then distance is light-seconds, and |V|=1.
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?
guitarphysics
#3
Nov10-12, 04:55 PM
P: 180
Oh, thanks a ton! That really clears things up. And I should have been more clear with the part about our four-velocity; I did mean magnitude. Why is our four-velocity MAGNITUDE the speed of light?
And no, the book has no math in it :S

Simon Bridge
#4
Nov10-12, 06:41 PM
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Confused about Special Theory of Relativity?

Neato. Note: JIC...
Why is our four-velocity MAGNITUDE the speed of light?
That happens because time dilation and length contraction are complimentary terms. It just a way of keeping track of them.
And no, the book has no math in it
It is really hard to describe this stuff without math.
guitarphysics
#5
Nov10-12, 08:38 PM
P: 180
Quote Quote by Simon Bridge View Post
It is really hard to describe this stuff without math.
Yeah, I know, but Greene usually does a decent job of it. Do you know how I can move forward faster in physics? I'm 15 years old and in my first year of physics (not with calculus) in high school. We're studying Newton's Laws right now, and before that studied motion. I read a ton of popular science books, and I'm teaching myself calculus. Are there any books, resources you can recommend? Are the Feynman lectures on physics any good?
micromass
#6
Nov10-12, 08:49 PM
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Quote Quote by guitarphysics View Post
Are the Feynman lectures on physics any good?
Any good?? They're practically the best!! But don't use them as a primary textbook though, they're not made for that. Use them as a secondary resource.
guitarphysics
#7
Nov10-12, 08:50 PM
P: 180
Quote Quote by micromass View Post
Any good?? They're practically the best!! But don't use them as a primary textbook though, they're not made for that. Use them as a secondary resource.
What should I use as a primary source? (considering I want to go a lot faster than my class, which is going painfully slowly)
micromass
#8
Nov10-12, 08:52 PM
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Quote Quote by guitarphysics View Post
What should I use as a primary source? (considering I want to go a lot faster than my class, which is going painfully slowly)
Perhaps you should ask this in a new thread in the science book forum: http://www.physicsforums.com/forumdisplay.php?f=21
guitarphysics
#9
Nov10-12, 08:54 PM
P: 180
Thanks, I will :)
EskWIRED
#10
Nov11-12, 02:07 PM
P: 97
Quote Quote by guitarphysics View Post
Yeah, I know, but Greene usually does a decent job of it. Do you know how I can move forward faster in physics? I'm 15 years old and in my first year of physics (not with calculus) in high school. We're studying Newton's Laws right now, and before that studied motion. I read a ton of popular science books, and I'm teaching myself calculus. Are there any books, resources you can recommend? Are the Feynman lectures on physics any good?
I am decades older than you, and my methods of educating myself are exactly what you already use. Keep it up. You'll find that school is a great place to get knowledge in a systematic, step by step manner, and that when you supplement it with self taught subjects, you'll learn at a prodigious rate.

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.
EskWIRED
#11
Nov11-12, 02:11 PM
P: 97
Quote Quote by guitarphysics View Post
What should I use as a primary source? (considering I want to go a lot faster than my class, which is going painfully slowly)
Check out the curriculum at some good universities. They often have required reading lists which include their main textbook on the subject you are interested in. If/when you find one book is used commonly, that is a decent indication that it is a good textbook.

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.
nitsuj
#12
Nov12-12, 11:30 AM
P: 1,098
Quote Quote by guitarphysics View Post
Yeah, I know, but Greene usually does a decent job of it. Do you know how I can move forward faster in physics? I'm 15 years old and in my first year of physics (not with calculus) in high school. We're studying Newton's Laws right now, and before that studied motion. I read a ton of popular science books, and I'm teaching myself calculus. Are there any books, resources you can recommend? Are the Feynman lectures on physics any good?
just an FYI, if you have more than a "layman" interest in this stuff, I suggest not reading much, or at least not thinking much about popular physics literature.

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.
Simon Bridge
#13
Nov12-12, 10:02 PM
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There are more than a few questions here arising from Greene's pop-sci 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 back-fill the gaps. It would take a bit of determination though.

Something like cosmology 101 materials should be fine.
guitarphysics
#14
Nov14-12, 04:04 PM
P: 180
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?
Simon Bridge
#15
Nov14-12, 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.
guitarphysics
#16
Nov14-12, 08:03 PM
P: 180
Yeah, you're right. I'll finish up my homework on slants and return those Greene books to the library...
harrylin
#17
Nov15-12, 03:36 AM
P: 3,187
Quote Quote by guitarphysics View Post
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?
As you summarized it, he seems to have omitted the very important issue of synchronizing distant clocks. See the following discussion:
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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