Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Negative mass

  1. Oct 4, 2013 #1
    Hello, we know that according to special relativity nothing in the universe go faster than speed of light,but space can.Now according to E=mc^2 entity with mass can not reach speed of light and photon is mass less with mass=0.So it can reach speed of light so its meaning that space contain negative mass so it has speed greater than speed of light,and why special relativity does not put any constrain on speed of space.Thanks in advance.
     
  2. jcsd
  3. Oct 4, 2013 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Please provide a source that says this.

    Zz.
     
  4. Oct 4, 2013 #3
    Last edited by a moderator: May 6, 2017
  5. Oct 4, 2013 #4

    BruceW

    User Avatar
    Homework Helper

    In general relativity, two distant objects can move away from each other at greater than the speed of light. (I think that is what you mean).
     
  6. Oct 4, 2013 #5

    BruceW

    User Avatar
    Homework Helper

    In special relativity, (and also general relativity at short distances), objects with mass are always moving at less than the speed of light relative to each other. This is really an experimental fact, as far as I know. (although, you could use it as a postulate of special relativity if you wanted, I guess).
     
  7. Oct 4, 2013 #6
    @BruceW

    Yes space is expanding greater than speed of light currently and after some (Billions) years we can not see speed of light from nearby galaxy clusters.So can we say that space contain negative mass so it posses speed greater than light????
     
  8. Oct 4, 2013 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Then there are a few problems with your conclusion here that space "contain negative mass".

    1. Why should something moving faster than c has negative mass? You can actually solve the SR equation for v>c and get, say, imaginary mass, not negative mass.

    2. We actually know what negative mass do! In solid state physics, negative effective mass occurs often (i.e. in holes). They do not more faster than c.

    Space isn't an object. It is a concept in which we frame location or position.

    Zz.
     
    Last edited by a moderator: May 6, 2017
  9. Oct 4, 2013 #8

    BruceW

    User Avatar
    Homework Helper

    'not really' is my answer.

    edit: I think this is a case of putting two and two together and getting five. don't be disheartened, it's good to keep thinking.
     
    Last edited: Oct 4, 2013
  10. Oct 4, 2013 #9

    phinds

    User Avatar
    Gold Member
    2016 Award

    us40, your problem is that you do not understand the difference between MOVING at some rate and RECEDING at some rate. Look up "metric expansion". Space is not MOVING at > c, objects IN space are receding from each other faster than c, which is not at all the same thing.
     
  11. Oct 4, 2013 #10
    I also do not understand this difference. Please enlighten me.

    Thanks
     
  12. Oct 4, 2013 #11
    It's very simple. Suppose space expanded and doubled in size after 1 billion year. An object that was 1 lightyear away before will be 2 lightyears away afterward, and receded at the rate of 1 lightyear per billion year. Another object that was at 1 billion lightyear away before is 2 billion lightyears away afterwards. That is, it recedes at a rate of 1 billion lightyear per billion year = c!. An object that was more than a billion year away receded faster than the speed of light. How is that possible. That's possible because we are talking about comoving coordinates in a cosmological scale. That is not you garden variety distance between objects.
     
  13. Oct 4, 2013 #12
    So the object at 1 billion ly moves at v=c away from us. An object further away would move faster than c relative to us. That contradicts SR.
     
  14. Oct 4, 2013 #13

    BruceW

    User Avatar
    Homework Helper

    this.
     
  15. Oct 4, 2013 #14
    So SR appears to be not applicable or so.
    I have another question. At what scale of space does the expansion take effect?
    I do not believe a hydrogen atom or the solar sytem are expanding. What about the local cluster of galaxies? Is it expanding or is the expansive motion thermalized by mutual exchange of momentum by gravitation?
     
  16. Oct 4, 2013 #15

    phinds

    User Avatar
    Gold Member
    2016 Award

    us40 and my2cts, Google "metric expansion" for a discussion.
     
  17. Oct 4, 2013 #16

    Drakkith

    User Avatar

    Staff: Mentor

    Any objects bound together through gravity or one of the other fundamental forces of nature do not expand/recede. Gravity finally falls off enough for expansion to occur somewhere around the Supercluster level, which is on the scale of millions of light-years. Clusters and superclusters of galaxies are the largest "structures" that can stay together. Past that, things are receding from each other.
     
  18. Oct 4, 2013 #17
    Following phinds' advise I read on wikipedia, the first google hit:
    "metric expansion is defined by an increase in distance between parts of the universe even without those parts "moving" anywhere"
    The distance between two non-moving objects can increase, even with v>c.
    That is not understandable.
     
  19. Oct 4, 2013 #18
    That makes more sense. But how could Hubble have discovered the expansion if this is true?
     
  20. Oct 4, 2013 #19

    Drakkith

    User Avatar

    Staff: Mentor

    We can measure the redshift of spectral lines in distant supernovas to determine that they are receding from us. It turns out that the further away a supernova was, the more redshifted its spectrum was. Lots of observations and math have led us to believe that this redshit is the result of the expansion of the universe causing objects to recede from us.

    I'd elaborate, but I have to head to class.
     
  21. Oct 4, 2013 #20

    phinds

    User Avatar
    Gold Member
    2016 Award

    I assume you mean YOU do not understand it. Since many of us do understand it, perhaps you should give it some more thought.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Negative mass
  1. Negative mass (Replies: 38)

  2. Negative Mass? (Replies: 39)

Loading...