Quantum Field Theory Question


Maybe this should go in future man’s thread - but it’s too late for me to work out if it fits there, so I hope its ok if I post it here.

Anyhow, on the off chance anyone knows the answer to this…

In a lecture whilst I was at uni many moons ago (ok well, about 15 moons now, I reckon) I was told that an antiparticle moving backwards in time is the same as a normal particle moving forwards in time (hence their depiction on Feynman diagrams with the arrows pointing the wrong way) I had the feeling at the time this was a cop-out on the part of my lecturer - but I’m assuming he was basing it on something proper.

So, yeah? Can anyone explain the mystery? Why do we draw antiparticles with the arrows going backwards in time on Feynman diagrams - and if antiparticles moving backwards in time are the same as particles moving forwards in time - and yet we always draw antiparticles as going backwards in time - why do we bother with them at all?

I’m confused… please help! :slight_smile:



Well…that’s a good question for all of the studies mankind has developed. But anywho, it comes down to one basic logical answer: we think it’s nice to know, whether we need to or not.

As for your antiparticle/particle-arrow question (and I’ve only been briefly introduced to Quantum physics), I would say that it is a basically an ‘‘opposite’’ kinda thing. This particle is anti therefore it goes this way…this particle is not anti…so it goes the opposite way.

Just a lil basic logical thinking. Nothing to be harped on about.

This requires more explanation but I believe what you are asking has to with the universe having the same amount of energy in it.

Energy can neither be created nor destroyed, only rearranged. (1st Law of Thermodynamics)

Anti matter is the hole, so to speak, where matter is drawn from. You cannot create matter from nothing; there must be a deficit from somewhere and the deficit is anti matter.

Together matter and anti matter keep the energy balance intact. Whatever you do with one, you must do the opposite with the other. Matter and anti matter are mirror images of each other.

Re time: time is only the measurement of change so don’t get hung up on time, as we know it, or lost in “time travel” ideas etc.

Re the Feynman diagram: don’t think of anti matter (anti particles) as literally going backward according to the diagram but as a deficit now incurred in the total amount of energy through the creation of a particle. You cant store energy or make changes outside the universe so to create a particle, you have to account for a loss of energy in the universe. The loss in creating the particle, is the anti particle.

Note: Anti matter isn’t just a concept or emptiness; anti matter is real and has effects.

I’m sure there’s heaps of stuff on the net giving more detailed explanations.

Chers, guys, that helps. Sort of. :slight_smile:

this is way too complicated.

the positive end of a magnet is regular matter, the negative end is anti matter. (the advanced explanation is that the negative end contains a larger amount of electrons, and the positive end contains a higher proton:electron ratio. protons have been arbitrarily designated as positive/pro, and electrons have been arbitrarily designated as anti/negative)

so lets say you have a big ball of positive matter 10 feet away from where you are standing. you hold in your hand a ball of positive matter. you let go of that ball, and it flies in the opposite direction of that big ball ten feet away from you.

you record in your notebook that “positive/pro matter goes away from the big ball”

then you hold in your hand a little ball of pure electrons, negative/anti matter. it sucks right into that big ball ten feet away from you.

you write into your notebook “negative/anti matter goes TOwards the big ball”

what is a really cool, unintuitive way to describe what has just happened?

ANTI MATTER (/negatively charged matter) IS JUST LIKE REGULAR MATTER GOING BACKWARDS IN TIME!!! wooooahhhh man that shit is crazy! cough pass that shit!

i hold a super ball in my hand. i drop it. gravity pulls it downward, when it hits the ground, It Goes Back In Time and goes upwards. then, when the momentum runs out, It Goes Forwards again. the physics of the universe dictates which direction in time the piece of matter goes. or maybe the physics of the universe relies on rules besides the passage of time. or maybe they rely on the passage of BleeBloo! the 57th dimension!

but hey, maybe the conservation of matter (and the existence of matter, seemingly ex nihilo) really can be reconciled by the idea that negatively charged matter counteracts all the positive matter.
the question is then: are there equal amounts of negatively charged matter and positively charged matter? because, surely, that would be a prerequisite for that theory. is the zpf entirely negative? because atoms are very overwhelmingly positive, and from what i know, there is a very small amount of electron mass and a very large amount of positively charged nuclear mass. mmhuy glayvin.

i could totally be REALLY wrong here. dont let my hilariously cocky attitude fool you.

LoL… so that’s how you spell it

Quantum Mechanics confuses most physics professors too, they are not really sure regarding how it works. My cousin is a nuclear physics instructor, now expanding into nano technology, and he can’t even clearly explain it.

As for me, I am totally lost in the science and math disciplines. Earned A’s in the basics after killing myself, but there is no real connection with the disciplines.

You might enjoy Crichton’s Time Line or Heinlein’s The Cat Who Walked Through Walls for some interesting speculation, and remember Crichton is an MD and Heinlein is not lightweight and goes into physics in some sections of Expanded Universe.

Quantum mechanics offers no ontological explanations. It is a statistical tool used to determine the probabilities of certain things existing [in space and time]. ‘Things’ are instances of categorisations of behaviour, so they require a categoriser (an observer). At the moment, observations almost always require information to be transferred using electromagnetism or gravitation. A particle can only be observed after it has produced such information, thus altering the original behaviour of whatever the particle was, or was doing, prior. Or, alternatively, whatever transfer of information was in progress prior to the emission of said radiation completes and a new transfer of information takes place to the observer, and that transfer is the particle. The particle is the observation, not the observed.
It would be wise not to take Quantum mechanics too literally. I think it is best seen as a mathematical tool used to make good guesses about microcosmic events.

to answer the original poster :

i’m not sure exactly what your understanding of physics is, so i will hope for the best and think you have legitimately gotten to study quantum fields, and not just happened in a class while waiting for some friends.

essentially, exotic particles present two main problems.

firstly, and most importantly, particles such as tachions appear to be moving faster than the speed of light. this, inasfar as we understand these things, is impossible. however, it might be impossible to ACCELERATE something over the speed of light, not simply for something to exist at that speed. maybe. if we apply the math we have to the circumstance a particle moves faster than the speed of light, the result is that it’s subjective time is going backwards. (at the speed of light, it’s subjective time is stitting still). it is not very clear at the moment precisely what is going on there.

secondly, anti matter. anti matter has been proven to exist, and in playing around with it, we have found it perfectly follows (at least in broad lines) everything we would have expected it to do, based on our math for normal particles and the symetry principle.

the reason feynman diagrams show it moving backwards however, is just a limitation of the diagrams themselves. they were conceived (and work pretty well) to describe properties of common matter, which, incidentally, was what they were designed to do. looking at anti matter, we are forced to symetrize them, and the result is we HAVE to think the arrow is pointing backwards.

we are currently not at all sure, or even convinced, that in either case it REALLY means the respective particles actually are moving back in time. many people simply belive its an effect of overstretching our mathematical apparatus. a methodologic artefact, if you will (for instance, if you look at yourself in a concave mirror, you will appear upside down. are you upside down, or is it just a matter of the mirror ? what if you DON’T KNOW the mirror is there ?)

this is highly speculative, but i for one belive such “paradoxes” are nothing more than the result of trying to cram too much “reality” into too narrow a model.

Hum, but don’t cell phones operate using this theory. At least, this is what I have read. As explained earlier, I really do not know.


Interesting, and yes, anti-matter does exist. Wow, good for you as you offered some insights. Thanks. :sunglasses:

Probably, but then architects use geometry. Geometry makes some good predictions, but it does not mean the universe is made of lines and triangles. Similarly QM is a statistical tool. It does not offer any ontological explanations. As far as I know, wave function collapse does not describe a physical process. Although an electron in motion can be represented by a wave function, it does not mean that a) electrons in motion exist b) wave functions exist.

It’s just the way it is.

There’s more if you want some more. Please let me know.

If the two (particle and anti-particle) collide, they annililate leaving massless entity. Hope this helps.

Becuase it is really fun to think about!
I mean it seriously.

Thanks for the article. If I understand it correctly, and I probably am not, this supports the idea of time travel, warping time for space travel, and multiple universes. Time travel is the major theme in Crichton’s Time Line. :smiley:

Thanks for the article. If I understand it correctly, and I probably am not, this supports the idea of time travel, warping time for space travel, and multiple universes. Time travel is the major theme in Crichton’s Time Line. :smiley:

You are very welcome. If you are interested in ''Time treavel", you might want to read Kaku’s ‘Hyperspace’. It is very fun to read.

I don’t think that time travel is possible in this physical world. Only in theory it is possible.