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Is everything relative?

  1. Nov 26, 2013 #1
    Is everything we know relative or is there something absolute in this universe?
  2. jcsd
  3. Nov 26, 2013 #2


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    This is rather vague. Let's start with something clearer.

    Do you know about Special Relativity? Yes? Then what have you concluded from that?

    No? Then maybe we can start you with that.

    Secondly, what do you mean by "everything"? There are covariant/invariant values and expressions in physics that are NOT relative.

  4. Nov 26, 2013 #3
    I just have a basic idea about special theory of relativity.by everything I meant the physical quantities like energy,mass,velocity.
    From special theory of relativity I have concluded that velocity of light is the same for all observers so maybe velocity of light is not relative?
  5. Nov 26, 2013 #4
    The speed of light is absolute. So is mass. The charge of an electron is absolute. Etc
  6. Nov 26, 2013 #5
    Isn't mass relative?
  7. Nov 26, 2013 #6
    Mass is NOT absolute.
    ##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
    Mass, length, time, kinetic energy are all relative.
    Charge, spin, baryon no. etc are not relative
  8. Nov 26, 2013 #7
    I believe it depends on how you learn special relativity. I learned it as mass being absolute, but I only just learned it in a classroom this semester, so I'm not an expert.

    Why is this necessary?
  9. Nov 26, 2013 #8


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    Mass?But I heard that mass increases with Speed(Kinetic Energy)

    EDIT:Look at the Equation given by Enigman.
  10. Nov 26, 2013 #9
    So bottom line velocity of light and charge are the two physical quantities which aren't relative ?
  11. Nov 26, 2013 #10
    I hate arguing anything that I'm not too confident in, but for the sake of education!:

    I learned relativistic kinetic energy as:
    [itex]T=(\gamma_u - 1)mc^2[/itex] where mass is absolute. This is from the book "Modern Physics" second edition by Randy Harris

    Also, total relativistic energy:
    [itex]E=\gamma_u mc^2[/itex]. Where mass is absolute.

    There are most certainly other quantities as well..
  12. Nov 26, 2013 #11
    Isn't charge relative because electric and magnetic feilds are relative?
  13. Nov 26, 2013 #12


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    No charge is a Lorentz invariant. This follows from local charge conservation which is itself a consequence of Maxwell's equations.

    Rest mass (more appropriately called invariant mass) is also a Lorentz invariant.
  14. Nov 26, 2013 #13
    Perhaps you are talking about the rest mass or the invariant mass? This the mass observed in an inertial frame where the object in question is at rest. (I perhaps should have mentioned this before)

    ##M=\frac{M_0}{\sqrt{1- \frac{v^2}{c^2}}}##
    gives the mass as observed from a frame in which object in question is moving with velocity v.
    But often mass and rest mass are used interchangeably
    Derivation here- http://www.scribd.com/doc/98591006/Simple-Derivation-for-Relativistic-Mass.
    (WBN beat me to it...)
  15. Nov 26, 2013 #14


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    The quantity ##M## in that equation is not frame-invariant, but ##M_0## is. It's something of a matter of taste which one you consider to be "mass", and that taste has changed over the years.
  16. Nov 26, 2013 #15
    Yes, I was talking about relative or 'observed mass' as I thought it would be obvious from the context and not rest mass which is by definition frame-invariant.
    This is of course correct, provided m represents relative mass. And as ##m_{rel}=\gamma m##
    The mass-energy equation reduces to ##E=m_{rel} c^2##.
    ##m_{rel}## is the mass that would be observed from a frame in which the object moves with velocity v and m is the mass in the frame in which relative velocity is zero.
    You may want to read-
  17. Nov 26, 2013 #16


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    Y'know, the more things change, the more they remain the same. This thing keeps coming back like an unwanted guest.


    Please note this FACT: when you read the mass values of the various particles in the Particle Data Book, you'll notice that they never cite the corresponding speed. If mass is "relative", then there will not be a unique, unambiguous value.

  18. Nov 26, 2013 #17
    Thanks for this link. This person has some very interesting points :)
  19. Nov 26, 2013 #18


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  20. Nov 26, 2013 #19
    Why would I be referring to Lev Okun? He doesn't even make an appearance in the link I quoted...

    And when you say "Zz." at the end of your post it makes it seem like you are facepalming at the post you quoted lol
  21. Nov 26, 2013 #20


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    Lev Okun's name appears in the link if you scroll down to my post. So I mistakenly thought you were referring to his view on why we shouldn't be using the term "relativistic mass". The OP's view in that link isn't that interesting considering that it was mainly a question that needs clarification.

    In any case, I obviously thought you were referring to something else.

  22. Nov 26, 2013 #21
    This view seems pretty interesting too...
    P.S. Don't listen too him the Zz is one of the best signatures in the forum.
  23. Nov 26, 2013 #22


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    Still, if he wanted to facepalm about an argument on whether mass is relative, which uses the word "invariant", I'd sympathize.
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