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

Dispute between Relativity and String Theory?

  1. Apr 9, 2005 #1
    Regarding "The Elegant Universe", page 48-50:

    Relativity says that all objects have a velocity of c. For most objects most of this velocity is in the time dimension. For light, none of this velocity is in the time dimension but one of the other 3 (x,y,z). For an object to give all of this "dimensional energy" to just 1 dimension means the object will have a velocity of c in that dimension.

    Now enter String Theory, which says there are 10 dimensions. A string requires the additional 6 dimensions in order to resonate correctly (to output a photon for example).

    Here's the problem:

    If a string has to vibrate in the additional 6 dimensions, then some of its energy is being expended in those dimensions. Meaning that a photon cannot give all its energy to just 1 dimension (ie, straight forward, the x dimension). That means that a photon cannot travel the speed of light in x, y, or z because of the energy expended in the "other 6" dimensions...

    Where am I going wrong here? Light has to move at c!
  2. jcsd
  3. Apr 9, 2005 #2
    Yes. But that is because photons are small enough to be inside the 10 dimensions of string theory. Because photons are strings, they vibrate in the 9 espacial dimensions. Photons may not be able to go at the speed of light, but becuase they are actually in a 10 dimensional world and not in a 4 dimensional (3+time) they can go at the speed of light at these higher dimension number.
    Last edited: Apr 10, 2005
  4. Apr 9, 2005 #3


    User Avatar
    Science Advisor

    Its not the speed you would need to be worried about, but rather the electromagnetic interaction, since the photon mediates this as a gauge boson. Naively you would expect a suppression factor in the Coulomb term if indeed that mode was allowed to resonate in higher dimensions. Of course it could be that there *is* just such a thing, we'd have to be able to measure the constants down to the planck scale in order to see a deviation. But as far as I know, most models of Stringy physics explicitly prevents such a thing from occuring, only gravity propagates in the compactified dimensions..
  5. Apr 14, 2005 #4


    User Avatar
    Gold Member

    You say 3 space and 1 time, but there's really nno evidence of them being any different, we just percieve them differently. :wink:
  6. Apr 15, 2005 #5

    TRUE, and we probably concieve them diffeently becuase we aren't uosed to travelling in time.
  7. Apr 15, 2005 #6
    You are trying to express a sophisticated point; but instead your statement has no meaning. For example; it makes sense to say that an object is at the same place at two different times while it is impossible for something to be in the same time at two different places.

    The time dimension is very unlike the spatial dimensions, but the distinction has to do with curvature (cone centered on the time axis). In general, dimensions are not divided in to space or time, they just have various curvatures (some of which we interpret as time, or space).
  8. Apr 15, 2005 #7


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Well put, Crosson. Four dimensional vectors can freely move through 3D space without appearing to move [or even exist] from the perspective of any 3D observer.
  9. Apr 17, 2005 #8
    The apparent "same place" is, though, relative to the person who is making the statement (since things continually change). Concerning the impossibility of something to be "in the same time at two different places," doesn't this come up in the Heisenberg uncertainty principle, where things can be in superpositions? This would mean that it is intrinsically possible that an object can be in two places at the same time due to superpositions.
  10. Apr 20, 2005 #9
    It is logically and semantically impossible for one thing to be in two different distinct places at exactly the exact time. Why? Because it would then be TWO things instead of one thing.

    It is possible for one thing to empirically appear to be two places at the same time if it transitions between the two location is fast enough.
    Last edited: Apr 21, 2005
  11. Apr 21, 2005 #10
    Well, if something isn't observered/measured, can it not be in two or more states at the same time? Because of the lack of information of the observer/measurer of an object, can't the object be said to be at various superposition states at the same time, such as Schrodinger's cat? When the observer/measurer makes an observation/measurement of the object, then do such states collapse into a single state.
  12. Apr 21, 2005 #11
    Sure, something can be into two different places at the same time but it than becomes two different things. 1 never equals 2 (unless you make them variables, but then the concept of 1 as a constant and 2 as a constant have no meaning.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?