## particles travelling backwards in time

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nIn quantum field theory particles are said to travel backwards in\ntime.\nI assume this is allowable over quantum distance scales.Over what kind\nof distance scale does such particle behaviour stop?\nAnd if it happened when the universe as a whole had a radius equal to\nthe quantum distance scale, would a particle travelling backwards in\ntime still be acceptable to theorists? What was the motivation for\nhaving particles travelling backwards in time in the first place? And\nhow is this sort of time travel compatible with the fact that I always\nsee a clock travelling forwards in time?\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>In quantum field theory particles are said to travel backwards in
time.
I assume this is allowable over quantum distance scales.Over what kind
of distance scale does such particle behaviour stop?
And if it happened when the universe as a whole had a radius equal to
the quantum distance scale, would a particle travelling backwards in
time still be acceptable to theorists? What was the motivation for
having particles travelling backwards in time in the first place? And
how is this sort of time travel compatible with the fact that I always
see a clock travelling forwards in time?
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alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0407201220.b474e1c@po...google.com>... > In quantum field theory particles are said to travel backwards in > time. > I assume this is allowable over quantum distance scales.Over what kind > of distance scale does such particle behaviour stop? No, it is not limited to small time scales. In fact, a particle travelling backwards in time is indistinguishable from its antiparticle travelling forward in time. This is actually the mechanism by which QFT predicts the existence of anti-particles. cheers, Patrick.



alistair@goforit64.fsnet.co.uk (alistair) wrote: > In quantum field theory particles are said to travel backwards in > time. Nothing's moving or travelling. Those are 3-dimensional concepts. Everything's already all there in 4 dimensions. The particles are just worldlines plastered on a Minkowski framework to produce a tapestry. The directionality of the arrow in diagrams is just a graphic representation of the C operator; distinguishing fermions from their anti-fermion states. Time-reveral, the T operator, on the other hand is something entirely different. The underlying theory (that is: the interactions) is invariant under PCT $(P =$ parity), so effecting a T is the same as effecting a PC. But the particles themselves don't have this relation. A particle under T is not the same as the particle acted on by PC. In detail, $P[C[T[\psi]]] != \psi$. So, a positron is not the mirror image of an electron "moving backwards in time" (whatever "moving in time" is supposed to mean), as is commonly stated.

## particles travelling backwards in time

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nnews:&lt;861c1b21.0407201220.b474e1c@p....googl e.com&gt;...\n&gt; In quantum field theory particles are said to travel backwards in\n&gt; time.\n&gt; I assume this is allowable over quantum distance scales.Over what kind\n&gt; of distance scale does such particle behaviour stop?\n\n&gt;No, it is not limited to small time scales. In fact, a particle\n&gt;travelling backwards in time is indistinguishable from its\n&gt;antiparticle travelling forward in time. This is actually the\n&gt;mechanism by which QFT predicts the existence of anti-particles.\n\n&gt;cheers,\n&gt;Patrick.\n\nIt was Dirac\'s combination of special relativity and quantum mechanics\nthat predicted antiparticles.No doubt this made Feynman look out for\nsomething similar.Isn\'t the notion of travelling back in time just a\ncomputational aid in qft - particles don\'t really travel backwards in\ntime, do they?\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>news:<861c1b21.0407201220.b474e1c@p....google.com>...
> In quantum field theory particles are said to travel backwards in
> time.
> I assume this is allowable over quantum distance scales.Over what kind
> of distance scale does such particle behaviour stop?

>No, it is not limited to small time scales. In fact, a particle
>travelling backwards in time is indistinguishable from its
>antiparticle travelling forward in time. This is actually the
>mechanism by which QFT predicts the existence of anti-particles.

>cheers,
>Patrick.

It was Dirac's combination of special relativity and quantum mechanics
that predicted antiparticles.No doubt this made Feynman look out for
something similar.Isn't the notion of travelling back in time just a
computational aid in qft - particles don't really travel backwards in
time, do they?


Alfred Einstead writes >Nothing's moving or travelling. Those are 3-dimensional concepts. >Everything's already all there in 4 dimensions. The particles >are just worldlines plastered on a Minkowski framework to produce >a tapestry. I have a problem with this when one gets to quantum mechanics. Because there is an inherent uncertainty under QM even 'running forwards' will not produce the same result after a period of time even for a system starting in the same state. Perhaps I should add 'if the system is chaotic'. Now that being so (I think) its hard to equate this with a static 4D spacetime under many common situations. Or, put another way, in a *macroscopic* system that is adequately chaotic even if you set a system up to be identical to another, the result will not be the same after a period of time. Or, and this is the key, it is in general possible to distinguish and arrow of time, its basically entropy driven. That is, in teds example using a non-chaotic system (eg a pendulum, but can be much more complex) its not possible to determine the direction of time, but in others (eg a gas expanding into a vacuum) it is. So the question is, IFF (and you dispute this, but I haven't fully grasped your argument) antiparticles travel 'backwards in time' how will their bulk properties differ, if at all? One can argue this both ways. 1) They will see entropy increasing in the -t direction and so think non-antiparticles having a decreasing entropy. 2) They see entropy decreasing in the -t direction and so behave (statistically) like non-antiparticles in the +t direction. Personally I am inclined towards option 2. Anyway my main point is an extension to this. If one assumes option (2) then spacetime isn't rigid and fixed. Its more like a flexible option at short distances/times (one could probably mutter about entanglement and stuff). At longer distances/times, where entropy smoothes everything out, it looks rigid and fixed largely because of the overwhelming abundance of non-antiparticles. Or, put another way, locally spacetime is a 4D flexible structure (although because of this its pretty complicated) but on large scales/long times its a rigid fixed structure. Bearing this in mind that, as I understand it, on small scales antiparticles are rather common (the only way to change a particle?) this would itself suggest a deep schism between GR and QM. But, looked at in this way, a fix doesn't seem impossible. -- Oz This post is worth absolutely nothing and is probably fallacious. BTOPENWORLD address about to cease. DEMON address no longer in use. >>Use oz@farmeroz.port995.com<< ozacoohdb@despammed.com still functions.



whopkins@csd.uwm.edu (Alfred Einstead) wrote in message news:... > The underlying theory (that is: the interactions) is > invariant under PCT $(P =$ parity), so effecting a T is the > same as effecting a PC. But the particles themselves don't > have this relation. A particle under T is not the same as > the particle acted on by PC. In detail, $P[C[T[\psi]]] != \psi$. > So, a positron is not the mirror image of an electron "moving > backwards in time" (whatever "moving in time" is supposed to > mean), as is commonly stated. Well, almost so ! Looking at Peskin and Schroeder, chapter 3, we find that applying P to a fermion field gives $us \gamma_0 \psi(t,-x) ;$ applying T to it gives $us \gamma_1 \gamma_3 \psi(-t,x)$ and finally applying C gives $us -i \gamma_2 \psi_star(t,x)$. So doing the three in a row, C P T, we have: $\psi(x,t) -> -i \gamma_2 (\gamma_0 \gamma_1 \gamma_3 \psi(-t,-x) )*$ Which results in [ 1 ; 1 ; -1 ; $-1] \psi(-t,-x)*$ Now when looking at an individual Fourier mode (a single excitation), we see that $\psi(-t,-x)* = \psi(t,x)$ (for each individual mode). It is only when they are combined with complex coefficients that we have to conjugate those coefficients. So it is true that strictly speaking, there are phase differences between $C P T \psi$ and $\psi$ when considering the different modes, but for one single mode, this does hold. So, you DO can say that for a single electron, applying C P or applying T comes down to the same thing. This is btw verified by crossing symmetry in Feynman diagrams, which comes down to saying that particles incoming from the past can be exchanged (true, sometimes with a minus sign or something like that) with its antiparticle going to the future. cheers, Patrick.



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0407220202.795cfa58@p...google.com>... > news:<861c1b21.0407201220.b474e1c@p....google.com>... > > In quantum field theory particles are said to travel backwards in > > time. > > I assume this is allowable over quantum distance scales.Over what kind > > of distance scale does such particle behaviour stop? > > >No, it is not limited to small time scales. In fact, a particle > >travelling backwards in time is indistinguishable from its > >antiparticle travelling forward in time. This is actually the > >mechanism by which QFT predicts the existence of anti-particles. > > >cheers, > >Patrick. > > It was Dirac's combination of special relativity and quantum mechanics > that predicted antiparticles.No doubt this made Feynman look out for > something similar.Isn't the notion of travelling back in time just a > computational aid in qft - particles don't really travel backwards in > time, do they? Of they don't really travel backwards in time. It's unfortunate that lay people come away with that impression.



Patrick Van Esch wrote: > alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0407201220.b474e1c@po...google.com>... > >>In quantum field theory particles are said to travel backwards in >>time. >>I assume this is allowable over quantum distance scales.Over what kind >>of distance scale does such particle behaviour stop? > > No, it is not limited to small time scales. In fact, a particle > travelling backwards in time is indistinguishable from its > antiparticle travelling forward in time. This is actually the > mechanism by which QFT predicts the existence of anti-particles. No; this is the way the old QM predicted antiparticles. One can go some way with it (as in Volume 1 of Bjorken and Drell), but more sophisticated stuff requires the QFT picture (as in their Volume 2). In QFT there are _only_ particles (and antiparticles) traveling forward in time, corresponding to timelike or lightlike momenta. (Only 'virtual' particles may have unrestricted momenta; but these are unobservable artifacts of perturbation theory.) The need for antiparticles is in QFT instead revealed by the fact that they are necessary to constract operators with causal (anti)commutation relations, in connection with the spin-statistic theorem. See, e.g., Volume 1 of Weinberg's QFT book. Thus talking about particles traveling backward in time is outdated and more misleading than it does good. Arnold Neumaier