The Ur-Quan Masters Discussion Forum

That is not how MWI works. QM obeys forwards causality, and not retrocausality.

What they're seeing must have an 'also a consequence' relationship to the causes of more notable events. Or QM is actually wrong and the universe is even weirder. I'd put my money on the former.

That is not how MWI works. QM obeys forwards causality, and not retrocausality.

The fundamental laws of physics work in the same way whether time moves forward or backward. Yet, while a glass can fall and scatter shards across the floor, glass shards never gather together and leap back onto the counter to form a complete glass. The source of this temporal asymmetry is one of the deepest mysteries in physics. We tackle this problem by combining two different disciplines, computational and quantum mechanics. Our results illustrate that the asymmetry could emerge from forcing classical causal explanations on observations in a fundamentally quantum world.

Computational mechanics asks the following question: Given a sequence of observations, how many past causes must we postulate to explain future behavior? This quantity is asymmetric when time is reversed. There is an unavoidable memory overhead cost for modeling a process in the “less-natural” temporal direction—one must pay a price to enforce explanations adhering to a less-favored order of events.

We show that quantum models always mitigate this overhead. Not only can we construct quantum models that need less past information than optimal classical counterparts, these models can always be reprogrammed to model the time-reversed process without additional memory cost. This remains true even for observational data where this classical overhead diverges, such that all classical models for the less-natural temporal direction require unbounded memory.

We illustrate scenarios where classical favoritism for particular causal orders vanishes when quantum models are permitted, thus highlighting a new mechanism for the origin of time’s arrow.

Greater proximity to this simple boundary condition is what we mean by 'earlier', and being further from it is what we mean by 'later'.

So you believe that we have memories from the past since the particles in our region of the universe move away from a boundary condition? I assume this would be somewhat similar to a particle moving away from a black hole, or a particle decelerating from light speed.

So in no event do you get memories - or any consequences - from the future.

C) in the retrocausal regime, you'd see an exponential un-decaying as decay products seemingly randomly flew in and combined. It would be exactly like normal events going backwards.

I don't think it would look like that at a macroscopical scale. Only at a microscopical scale

I'd ask why you would think so

except that the rest of the paragraph is word salad. You have left the realm of physics and entered word games that aren't even philosophy.

Use of cognitive capacity was measured during reading of text in six experiments. Content features of the texts were varied. A secondary task technique was used to measure use of capacity. In all experiments, capacity was filled more completely while reading narrative text than while reading expository text. The finding was generalized over two pools of passages, over two modalities of secondary task probes— auditory and tactile—and over two secondary tasks—simple reaction time and choice reaction time. Three hypotheses for the narrativity effect were investigated. Not supported were an hypothesis based on the interest value of the passages and the hypothesis that difficult to comprehend passages induced problem solving cognitive operations that filled capacity. The results were consistent with a comprehensibility hypothesis, which states that when more meaning is produced in the reader's cognitive system while reading a text, more cognitive capacity is filled by reading it. Narrative passages produce more meaning because schemata for narrative texts are more familiar, more frequently encountered, and easier to comprehend than schemata for expository texts.

Antimatter is in every precise meaningful sense matter moving backward in time. The notion of "moving backward in time" is nonsensical in a Hamiltonian formulation, because the whole description can only go forward in time. That's the definition of what the Hamiltonian does--- it takes you forward in time a little bit. So if you formulate quantum mechanics the Hamiltonian way, this idea is difficult to understand (still it can be done--- Stueckelberg discovered this connection before the path integral, when field Hamiltonians were the only tool).

But in Feynman's particle path-integral picture, when you parametrize particles by their worldline proper time, and you renounce a global causal picture in favor of particles splitting and joining, the particle trajectories are consistent with relativity, but only if the trajectories include back-in-time trajectories, where coordinate time ticks in the opposite sense to proper time.

Looked at in the Hamiltonian formalism, the coordinate time is the only notion of time. So those paths where the proper time ticks in the reverse direction look like a different type of particle, and these are the antiparticles.

Sometimes there is an idenification, so that a particle is its own antiparticle.

Precise consequence: CPT theorem

The "C" operator changes all particles to antiparticles, the P operator reflects all spatial directions, and the T operator reflects all motions (and does so by doing complex conjugation). It is important to understand that T is an operator on physical states, it does not abstractly flip time, it concretely flips all momenta and angular momenta (a spinning disk is spinning the other way), so that things are going backwards. The parity operator flips all directions, but not angular momenta.

The CPT theorem says that any process involving matter happens exactly the same when done in reverse motion, in a mirror, to antimatter.

The CPT operator is never the identity, aside from the case of a real scalar field. CPT acting on an electron produces a positron state, for example. CPT acting on a photon produces a photon going in the same direction with opposite polarization (if P is chosen to reflect all spatial coordinate axes, this is a bad convention outside of 3+1 dimensions).

This theorem is proved by noting that a CPT operator corresponds to a rotation by 180 degrees in the Euclidean theory, as described on Wikipedia.

Precise consequence: crossing

Any amplitude involving particles A(k_1,k_2,...,k_n) is analytic in the incoming and outgoing momenta, aside from pole and cut singularities caused by producing intermediate states. In tree-level perturbation theory, these amplitudes are analytic except when creating physical particles, where you find poles. So the scattering amplitudes make sense for any complex value of the momenta, since going around poles is not a problem.

In terms of mandelstam variables for 2-2 scattering, s,t,u (s is the CM energy, t is the momentum transfer and u the other momentum transfer, to the other created particle), the amplitude is an analytic function of s and t. The regions where the particles are on the mass shell are given by mandelstam plot, and there are three different regions, corresponding to A+B goes to C+D , Cbar + B goes to Abar+ D, and A + Dbar goes to C+Bbar. These three regimes are described by the exact same function of s,t,u, in three disconnected regions.

In starker terms, if you start with pure particle scattering, and analyticaly continue the amplitudes with particles with incoming momentum k's (with positive energy) to negative k's, you find the amplitude for the antiparticle process. The antiparticle amplitude is uniquely determined by the analytic contination of the particle amplitude for the energy-momentum reversed.

This corresponds to taking the outgoing particle with positive energy and momentum, and flipping the energy and momentum to negative values, so that it goes out the other way with negative energy. If you identify the lines in Feynman diagrams with particle trajectories, this region of the amplitude gives the contribution of paths that go back in time.

So crossing is the other precise statement of "Antimatter is matter going back in time".

Causal pictures

The notion of going back in time is acausal, meaning it is excluded automatically in a Hamiltonian formulation. For this reason, it took a long time for this approach to be appreciated and accepted. Stueckelberg proposed this interpretation of antiparticles in the late 1930s, but Feynman's presentation made it stick.

In Feynman diagrams, the future is not determined from the past by stepping forward timestep by timestep, it is determined by tracing particle paths proper-time by proper-time. The diagram formalism therefore is philosophically very different from the Hamiltonian field theory formalism, so much so Feynman was somewhat disappointed that they were equivalent.

They are not as easily equivalent when you go to string theory, because string theory is an S-matrix theory formulated entirely in Feynman language, not in Hamiltonian language. The Hamiltonian formulation of strings requires a special slicing of space time, and even then, it is less clear and elegant than the Feynman formulation, which is just as acausal and strange. The strings backtrack in time just like particles do, since they reproduce point particles at infinite tension.

If you philosophically dislike acausal formalisms, you can say (in field theory) that the Hamiltonian formalism is fundamental, and that you believe in crossing and CPT, and then you don't have to talk about going back in time. Since crossing and CPT are the precise manifestations of the statement that antimatter is matter going back in time, you really aren't saying anything different, except philosophically. But the philosophy motivates crossing and CPT.

Calling antimatter 'regular particles going back in time' is cute but only really helpful if you've got a Feynman diagram and you want to spin it around to face a different direction.

Zanthius, which of us has actually taken a graduate school class in quantum field theory? You keep pulling out pop-sci explanation videos. I've derived the time-propagator and proven causality for quantum fields.

Matt O'Dowd is an Australian-born astrophysicist. He is an assistant professor at the Physics and Astronomy Department at the Lehman College of the City University of New York[1] and writer and host of PBS Space Time on YouTube.[2] He is a frequent guest on Science Goes to the Movies on CUNY TV, and StarTalk radio with Neil deGrasse Tyson.[3]

The Hamiltonian in classical mechanics is derived from a Lagrangian, which is a more fundamental quantity relative to special relativity. The Hamiltonian indicates how to march forward in time, but the time is different in different reference frames. The Lagrangian is a Lorentz scalar, while the Hamiltonian is the time component of a four-vector. So the Hamiltonian is different in different frames, and this type of symmetry is not apparent in the original formulation of quantum mechanics.

If the Lagrangian is invariant under a symmetry, then the resulting equations of motion are also invariant under that symmetry. This characteristic is very helpful in showing that theories are consistent with either special relativity or general relativity.

Antimatter is in every precise meaningful sense matter moving backward in time. The notion of "moving backward in time" is nonsensical in a Hamiltonian formulation, because the whole description can only go forward in time. That's the definition of what the Hamiltonian does--- it takes you forward in time a little bit. So if you formulate quantum mechanics the Hamiltonian way, this idea is difficult to understand (still it can be done--- Stueckelberg discovered this connection before the path integral, when field Hamiltonians were the only tool).

You can describe it that way for discussions of individual interactions because it's convenient - I'm not saying those people are wrong - , but when you get to discussions of macro-scale phenomena like thermodynamics, talking of actual propagation backwards in time is a mistake.

The propagation operator itself points straight back to time being pointed away from simple boundary conditions.

New particle links dark matter with missing antimatter

Physicists in the US and Canada have proposed a new particle that could solve two important mysteries of modern physics: what is dark matter and why is there much more matter than antimatter in the universe?

The yet-to-be-discovered “X” particle is expected to decay mostly to normal matter, whereas its antiparticle is expected decay mostly to “hidden” antimatter. The team claims that its existence in the early universe could explain why there is more matter than antimatter in the universe – and that dark matter is in fact hidden antimatter.

And that math says that the two pictures, one with actual backwards time travel and one without, are equivalent.

And diagrams are not fundamental. They are how you organize your knowledge about what the propagation operator is doing.

Sure. I am not arguing that they aren't equivalent. But don't you get twice as many possibilities if you include both the forward possibilities and the backward possibilities? Why do you think the g-factor is around 2 and not around 1?

You need to include those in one way or another; taking the time-travel interpretation of the mathematics is not necessary.



Well, well... the path integral formulation is based on mathematics, not a diagram. But diagrams and mathematical formulations can be equivalent. For example, you can use regression or a machine learning algorithm to find a formula that fits a graph or geometrical shape. You can use the Schrödinger equation to visualize atomic orbitals, but you could probably also use a machine learning algorithm to find the Schrödinger equation from the shapes of the atomic orbitals.

You might think that graphs and geometrical shapes are "dumbed down" mathematics…

They don't need to be, but look at it this way - each diagram corresponds to an integral. Do you know what that integral is? In this sense, it is dumbed down. Important parts were left out. And when you look at those important parts, you find that there is no need to, and indeed, it hurts to, consider the action of antimatter to be traveling backwards in time.

In quantum electrodynamics, the anomalous magnetic moment of a particle is a contribution of effects of quantum mechanics, expressed by Feynman diagrams with loops, to the magnetic moment of that particle.

The QED prediction agrees with the experimentally measured value to more than 10 significant figures, making the magnetic moment of the electron the most accurately verified prediction in the history of physics.

This is getting incredibly frustrating.

Because the math is identically the same whether you have one kind of stuff going forward and backward in time or two kinds of stuff only moving forwards in time.

Ok. What about this?

(https://i.imgur.com/kC5o8yC.png)

A) Bosons don't stack. You're thinking of Fermions.  

B) GR predicts black holes without reference to QM.  

We might not see any stacking of bosons in our 4 dimensional perspective of spacetime. But can't bosons be stacked in another 5th dimension that is hidden to us, and still appear to have the same location in our 4-dimensional perspective?