beggining of time and tachyons

Factual Questions
21

Ficer part of the problem is that when I say “classical” gravity, I mean general relativity (and hence Einstein). This is to be opposed to some quantum mechanical formulation of gravity, and NOT to Newtonian gravitation. Newtonian gravity is not now and never has been correct, except as an excellent approximation to GR in certain situations.

erl, that’d actually be really helpful of you to post, with respect to CPT. I know people are looking for CPT violation, since it’d be terrifically important if found, but since I’m not really a particle physicist, I don’t really keep up on that sort of thing at all well.

22

Ah, it was my mismemory. The discussion was about the weak force not obeying CP symmetry. Not a thing about CPT which, it is said, is the lynchpin of physics. I am so sorry.

What threw me was a comment that to preserve CPT, if the kaon system wasn’t invariant under CP, it also couldn’t be invariant in T.

23

Gotcha. And so my current understanding of physics isn’t too out of date after all. This is always good to know. :slight_smile:

24

But now I want to know what affects a T variance has, which is mysteriously right back on track (in a way… maybe). I understand it means that if we only reverse time in our experiment, then the result won’t be the same. We’d need time, charge, and parity reversal to run everything the same. But what significance does this have? What does a T variance tell us—can it be tested?

General tachyon info: http://www.macalester.edu/astronomy/people/chrissy/Links/Tachyon.html

This one seems rather interesting, too, though considerably over my head:
http://www1.shore.net/~ewall/
Reference to some scientific journals in that one… very nice.

25

I’m not going to say that the second link is wrong per se, but be advised that it looks, from a relatively casual reading, to be decidedly non-mainstream. In fact, I’ll go far enough to say that, admitting that I’m a rather conservative physicist, it looks kind of crankish to me.

I have no idea how you’d test T invariance; this is partially because I’m not an experimentalist, partially because I’m also not a particle theorist, and partially because T invariance has always been the one that I understand least well. I’ll give it some thought over the weekend and see if I can dig up some decent stuff in my plethora of books, if you’d like.

26

An article about tachyons not being mainstream physics?! :wink:

27

Heh. Touche.

28

Apparently there are more than a few theories (apparently consistent, except for that whole “causality” thing) discussing what tachyons are and how they could be detected… some even going so far as to say that neutrinos could, in fact, be tachyons. Like the paper I linked, even this physicists isn’t holding his breath: Robert Ehrlich. As his anti tardyon-centrism image indicates, he seems to take the whole thing with a grain of salt. But that doesn’t mean: not seriously.

29

G8rguy - I caught that much, concerning the difference between Einstein’s model, and Newton’s. Newton did not have the equipment that Einstein had hence his view was very limited.

Despite that important difference, if Einstein’s model of the universe became dominant within a very short time after the Big Bang, like in planke (sp again) moments, then the mass of the expanding particle of matter would have been very great, enough to prevent light from escaping, and enough to prevent any matter from escaping.

If space is expanding and matter is getting farther away from other matter, there should still be a center point of the universe.

If gravity is strong enough, then the universe will one day collapse.

When the universe collapses, is that the moment that tachyons come into being? Because, it seems that it requires a signifigant event to generate particles that go backwards in time. Have they been detected experimentally?

30

No, they haven’t. Check the link: Robert Ehrlich.

31

Oh, sorry Ficer. I’ll try for a really brief explanation of Einsteinian gravity, to clear up any misconceptions you might have, since part of what you say is right and part is not as right.

As you have grasped, what the Big Bang tells us is that space is expanding. As you have also grasped, if there’s sufficient things gravitating, space will stop expanding, start collapsing, and we’ll have a Big Crunch instead. Current indiciations are that this is not what will happen, but it’s a theoretical possibility.

The part that’s confusing you a bit is that you seem to be thinking of the Big Bang rather like an explosion, and so assigning it a center. This isn’t so true.

There’s an old analogy, which likens the fabric of space to the surface of a balloon. As we blow up the balloon, the surface expands, distance between two dots you’ve drawn on the surface gets bigger. Every point is getting farther away from every other point, simultaneously, and hence, one can’t define a center to the universe.

Matter tends NOT to expand in the same way, because gravity holds it together. So the expansion of space is making the Earth get farther from distant galaxies and the like, but isn’t making the Earth get bigger.

As to why there wasn’t a black hole style singularity to prevent the expansion taking place, there’s one important part of the Big Bang story we have deemphasized somewhat which will perhaps make this a little more palatable to you, and that’s inflation. The idea behind inflation is that the universe got really really tremendously huge really really tremendously fast, before Einsteinian gravity took over.

On an unrelated note, erl, I swear I really will try to come up with something about the breaking of time reversal symmetry, but I’m still at home and most of my references are in the infamous office.

32

No pressure, g8rguy, whenever you get around to it. I’m not going anywhere! :slight_smile:

33

Right. I’ve a few things on tests of time reversal, finally.

One implication of CPT invariance is that the mass and lifetime of a particle and the mass and lifetime of the antiparicle are identical. This would also follow, of course, if C were a perfect symmetry by itself, but since it’s not, we have to use CPT invariance to get these. And of course, maintaining CPT invariance with CP non-invariant requires breaking T also. So one way to test for T breaking is to find a system in which it’s known that CP is broken and then check to see if particles and antiparticles have identical masses and lifetimes.

A second implication is that the electric dipole of an elementary particle has to be zero, for reasons that are a little hard to explain. Basically, the electric dipole has to point along the same direction as the spin because that’s the only direction available, but the direction of spin changes sign under T and dipole doesn’t. So if there’s a non-zero electric dipole, there’s breaking of T invariance.

34

Well, the antiparticle mass and lifetime makes sense, but I don’t follow the dipole business. Interesting, though.

35

Hmm… okay, this is hard to explain, as I said, but I’ll try to make it a bit clearer.

The hardest part for people to swallow is typically that the dipole would have to be in the same direction as the spin. There is a reason for this, though, which is that for a fundamental (i.e. point) particle, the dipole moment would establish some preferred direction. But there’s already a preferred direction: the direction of spin. If we had TWO preferred directions, we’d run into all sorts of funny problems about being able to simultaneously measure two directions of spin, and that sort of thing. Since we know we can’t do this, we have to have the dipole and the spin point in the same direction.

Okay, now think of a spinning top. Obviously, if I were to run a tape of this thing in reverse rather than normally, it would look like it’s spinning in the other direction, right? This means that the direction of spin changes sign under time reversal. Since the dipole moment is supposed to point along the direction of spin, and spin changes direction under T, the dipole moment would have to do the same thing.

But electric dipole moments don’t have anything to do with time, and hence shouldn’t change directions under T at all. So it must simultaneously change directions and not change directions, and give the same answer; it must equal minus itself. And the only vector that’s minus itself is zero, of course.

The upshot, then, is that if a fundamental particle has an electric dipole, it must violate T, because if it doesn’t violate T, the dipole has to both change signs and not change signs. Does that help at all?

36

Ah! Very good. Thanks so much!

37

If something was moveing into the past how would we know??? Take a tachyon for example. If we could aquire a nutrino and study it and to us it seemed very normal but the nutrino was accually a tachyon traveling back in time to it’s origin/begining (or destruction/end) the supernova or whatever we observed it had (from our point of view) came from.