Confused about Special Theory of Relativity?

In summary, the conversation discusses a question about special relativity and the confusion surrounding it. The speaker mentions reading a popular science book by Brian Greene and asks for recommendations on resources to further their understanding of physics. The mention of the four-velocity and its magnitude being the speed of light is also discussed. The conversation also mentions the Feynman lectures as a good resource for learning physics. Recommendations for finding a primary source for learning physics are also given.
  • #1
guitarphysics
241
7
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"?
Thanks
 
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  • #2
guitarphysics said:
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?
 
  • #3
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
 
  • #4
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.
 
  • #5
Simon Bridge said:
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?
 
  • #6
guitarphysics said:
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.
 
  • #7
micromass said:
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)
 
  • #9
Thanks, I will :)
 
  • #10
guitarphysics said:
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.
 
  • #11
guitarphysics said:
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.
 
  • #12
guitarphysics said:
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.
 
  • #13
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.
 
  • #14
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
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
Yeah, you're right. I'll finish up my homework on slants and return those Greene books to the library...
 
  • #17
guitarphysics said:
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:
https://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. :tongue2:
 

1. What is the Special Theory of Relativity?

The Special Theory of Relativity is a scientific theory developed by Albert Einstein in 1905. It describes the relationship between space and time and how they are perceived differently by observers in different frames of reference.

2. How does the Special Theory of Relativity differ from the General Theory of Relativity?

The Special Theory of Relativity deals with objects moving at constant speeds in the absence of gravity, while the General Theory of Relativity includes the effects of gravity on objects in motion.

3. What is the concept of time dilation in the Special Theory of Relativity?

Time dilation refers to the slowing down of time for an object in motion compared to an object at rest. This is due to the fact that time and space are relative and can be affected by the speed and acceleration of an object.

4. How does the Special Theory of Relativity explain the famous equation E=mc²?

The equation E=mc², also known as the mass-energy equivalence, is a fundamental principle of the Special Theory of Relativity. It states that mass and energy are interchangeable and can be converted from one form to another, with the speed of light being the conversion factor.

5. What is the role of the observer in the Special Theory of Relativity?

The Special Theory of Relativity states that the laws of physics are the same for all observers in uniform motion, regardless of their relative velocity. This means that there is no preferred frame of reference and all observers will perceive the same physical laws.

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