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Theory of einstein

  1. Aug 28, 2013 #1
    Hey guys. I am 17 years old and I love physic. Recently I started to study about special/general theory of relativity. That what I love is the future of light. As eveyone knows light has a speed of "299.000 km/s",and his speed can not cumulated with other speed,so can not become bigger.

    My question is: For example we have a train that runs with 100.000 km/s and a machine that gives to the human speed of 200.000 km/s. So we tell to that human that if the train reach its biggest speed to click the button and machine gives him +200.000, so theoritically will have 300.000 km/s speed.
    But that is banned from the theory. I want to learn, what will happened to that human?? Time will freeze for him??
    (All them are in imagination,not at true.)
  2. jcsd
  3. Aug 28, 2013 #2
    Keep reading your book, and you will how velocities are added in special relativity. That will answer your question.
  4. Aug 28, 2013 #3
    I have not book,I just learn some things from random videos. I do not want a ready answer,so if you have any good book I would really appreciate it.
  5. Aug 28, 2013 #4


    Staff: Mentor

    Perhaps Benjamin Crowell's book on Conceptual Physics will help you with your questions:

    http://www.lightandmatter.com/html_books/7cp/ch04/ch04.html [Broken]

    Chapter 4 covers Special Relativity where they show how velocities are combined and will never exceed light speed.

    With respect to your question on time for the travelling human, he/she would not feel anything in his cabin is running faster or slower. If he happened to pass by an outside clock, he might note that it was running slower. People watching him pass by would notice that his clocks are running slower. However since you can't travel at light speed then your clocks will always keep running.
    Last edited by a moderator: May 6, 2017
  6. Aug 28, 2013 #5
    In 1916, Albert Einstein wrote a book for general public who wanted to understand relativity. That book was "Über die spezielle und die allgemeine Relativitätstheorie, gemeinverständlich". The English translation is "Relativity; the special and general theory". It has also been translated to many other languages. Many of them are now freely available because their copyright has expired. The original: http://archive.org/details/berdiespezielle00unkngoog The English translation: http://archive.org/details/cu31924011804774
  7. Aug 28, 2013 #6

    Doc Al

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    Nothing quite so dramatic happens. But you are correct that even for slow speeds (slow compared to the speed of light), velocities no longer add so simply. (Although it is a very good approximation.)

    See: How Do You Add Velocities in Special Relativity?
  8. Aug 28, 2013 #7
    Thank you all of you. This forum is fantastic,i think that i will learn many things ffrom you guys.
    I will check all the links that you send me,and i hope to find the solution,else maybe i write again,:D.
  9. Aug 28, 2013 #8
    As I read for small speeds we follow newton saying that V=V1+V2(Supposed the same direction)
    But for big speeds we follow einstein saying that V=V1+V2 / {1+ (V1*V1/c^2)}.
    From these things I have some questions.
    1)How einstein proved this law?
    2)In this law speed of light is 300.000 or 299.000 km/h.(which number I will enter?)?
    3)When we saying big speeds what we exactly mean?? Speeds near to light OR only light speeds and more?

    Thank you !!!
  10. Aug 28, 2013 #9


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    That's not quite correct. The second equation applies for all speeds whether the speeds are "big" or "small". It just that if the speed is low compared to c, the difference between the two is not noticeable so we use the first formula as it is easier.

    From experimental evidence, he deduced that the speed of light is a constant. From that a number of surprising results follow including this.

    Neither. According to Wikipedia, the speed of light is 299,792.458 kilometers per second, not per hour. 299,000 and 300,000 km per second are approximations. Most people prefer 300,000 km per second.

    Whatever v is large enough that [itex]v^2/c^2[/itex] is "noticeable". That depends upon how accurately you are calculating.

  11. Aug 28, 2013 #10
    Thank you. "Yea i wanted to write per second. Sorry for that. :D"
  12. Aug 28, 2013 #11
    How Einstein derived the velocity addition law is covered in the sources given in posts 4 and 5 already. It's is not complicated math, high school algebra is plenty.
  13. Aug 28, 2013 #12
    Where I can found the way that he proved that??
  14. Aug 28, 2013 #13
    Read post #5 and click on the link to Einstein's book, its online, its free....

    I found one source in the past with version in several languages...
  15. Aug 28, 2013 #14
    There is the book in Greek? Thank you!!!
  16. Aug 28, 2013 #15


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    Did you do a search for and ebook version in Greek?
    There will almost certainly be a hard copy version in Greek. They must read it in Greek Universities!.
  17. Aug 28, 2013 #16


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    I don't quite agree with this. In the 1905 derivation you need to be able to solve a 1st order PDE. In the 1905 paper there is also algebra missing in the derivation of the PDE.
  18. Aug 28, 2013 #17
    I wonder whether Landau and Lifschitz who just posit matter-of-factly that it is a mere rotation in spacetime is any simpler. No PDE there for sure, though.
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