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TOPIC: Arrow of Time


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Quantum retrocausality
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If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum "retrocausality."
That's science talk for saying he hopes to find evidence of a photon going backward in time.

"It doesn't seem like it should work, but on the other hand, I can't see what would prevent it from working. If it does work, you could receive the signal 50 microseconds before you send it" - John Cramer.

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RE: Arrow of Time
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Title: Arrow of Time in String Theory
Authors: Brett McInnes

Inflation allows the problem of the Arrow of time to be understood as a question about the structure of spacetime: why was the intrinsic curvature of the earliest spatial sections so much better behaved than it might have been? This is really just the complement of a more familiar problem: what mechanism prevents the extrinsic curvature of space from diverging, as classical General Relativity suggests? We argue that the stringy version of "creation from nothing", sketched by Ooguri, Vafa, and Verlinde, solves both of these problems at once. The argument, while very simple, hinges on some of the deepest theorems in global differential geometry. These results imply that when a spatially toral spacetime is created from nothing, the earliest spatial sections are forced to be (quasi-classically) exactly locally isotropic. This local isotropy, in turn, forces the inflaton into its minimal-entropy state. The theory explains why the Arrow does not reverse in black holes or in a cosmic contraction, if any.

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Feynman-Wheeler theory of time
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The Feynman-Wheeler theory of time describes how every particle emits two opposite carrier waves; a `retarded` wave and an `advanced` wave. The waves travel `through` time; one forward and one backwards. It is the interaction (or absorbsion) with other temporal waves that give the particles a sense of `movement` through time. When a particle absorbs a wave it also emits a retarded and advanced wave. In the original theory the density of the universe plays a critical part; in that, the retarded wave have to be (mostly) all absorbed by matter or else the interference amount drops.
It has to be said that experiments to find advanced waves have found none.
But perhaps the direction/absorbsion is decided by the boundary conditions of the universe; at one end there is the Big-bang (this reflects the wave) and the other end (future) is open (the wave cannot be reflected), and full of virtual particles.

Are these waves present in our normal 3 spatial dimensions?
They don't appear to be.

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RE: Arrow of Time
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Not too long ago, people thought the Earth was flat, which meant they thought that gravity pointed in the same direction everywhere. Today, we think of that as a silly idea, but at the same time, most people today (including most scientists) still think of spacetime as if it were a big box with 3 space dimensions and 1 time dimension. So, like gravity for a flat Earth, the single time dimension for the 'big box universe' points in one direction, from the Big-Bang into the future. A lot of lip service is given to the idea of "curved spacetime", but the simplistic 3+1 'box' remains the dominant concept of what cosmic spacetime is like.

Imagine that 'the arrow of time' in the Universe, like gravity on Earth, is pretty much the same everywhere, yet also different everywhere relative to everywhere else. That means that the 'arrow of time' points in different directions in spacetime depending on where you are, so time has a geometry just like space has a geometry.
The novel idea that there are an infinite number of time dimensions in the Universe revolutionizes gravitational theory and much of modern science with it. A number of outstanding scientific mysteries are definitively solved, including observations that lead to the concepts of 'dark energy' and 'dark matter'. You will know what these are after you read the lectures.


A Correction to the Gravitational Model
Lecture 1 (18 MB signed PDF)

The Many Directions of Time –Solving the Cosmology Puzzle
Lecture 2 (15 MB signed PDF)

Source

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RE: Quantum Theory Looks at Time Travel
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Researchers now speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is "complementary" to the present.
In other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.

Quote:

“The consequences of the basic and most evident consistency requirement-that measured events cannot happen and not happen at the same time-are reviewed. Particular emphasis is given to event forecast and event control.
As a consequence, particular, very general bounds on the forecast and control of events within the known laws of physics result.
These bounds are of a global, statistical nature and need not affect singular events or groups of events. We also present a quantum mechanical model of time travel and discuss chronology protection schemes. Such models impose restrictions upon certain capacities of event control.”


A new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel.
Although the laws of physics seem to permit temporal gymnastics, the concept is laden with uncomfortable contradictions.
The main headache stems from the idea that if you went back in time you could, theoretically, do something to change the present; and that possibility messes up the whole theory of time travel.

"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind" - Professor Dan Greenberger, City University, New York.

According to Einstein, space-time can curve back on itself, theoretically allowing travellers to double back and meet younger versions of themselves.
And now a team of physicists from the US and Austria says this situation can only be the case if there are physical constraints acting to protect the present from changes in the past.
The researchers say these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time.

Quantum behaviour is governed by probabilities. Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated.
So, if you know the present, you cannot change it.
If, for example, you know your father is alive today, the laws of the quantum universe state that there is no possibility of him being killed in the past.

It is as if, in some strange way, the present takes account of all the possible routes back into the past and, because your father is certainly alive, none of the routes back can possibly lead to his death.

“Mach-Zehnder interferometer with feedback-loop”

"Quantum mechanics distinguishes between something that might happen and something that did happen. If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him. But if you know he is alive, there is no chance you can kill him" - Professor Dan Greenberger.

So, even if you take a trip back in time with the specific intention of killing your father, so long as you know he is happily sitting in his chair when you leave him in the present, you can be sure that something will prevent you from murdering him in the past. It is as if it has already happened.

"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind. You wouldn't be able to kill him because the very fact that he is alive today is going to conspire against you so that you'll never end up taking that path leads you to killing him" - Professor Greenberger

Quantum Theory Looks at Time Travel
Authors: Daniel M. Greenberger, Karl Svozil

Abstract:
We introduce a quantum mechanical model of time travel which includes two figurative beam splitters in order to induce feedback to earlier times. This leads to a unique solution to the paradox where one could kill one's grandfather in that once the future has unfolded, it cannot change the past, and so the past becomes deterministic.
On the other hand, looking forwards towards the future is completely probabilistic. This resolves the classical paradox in a philosophically satisfying manner.

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Posts: 131433
Date:
Arrow of Time
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Spontaneous Inflation and the Origin of the Arrow of Time
Microscopic laws of physics are essentially time-reversal invariant, but macroscopic thermodynamics exhibits a profound time-asymmetry; entropy typically increases in closed systems. This intriguing feature of the real world has a cosmological origin: the entropy of the early universe was fantastically small.
After a century of effort, it has been difficult to explain this arrow of time without assuming time-asymmetric boundary conditions.

Jennifer Chen and Sean Carroll have suggested a simple scenario in which increasing entropy is natural, based on the idea that the entropy can increase without bound
(there is no equilibrium state) and that the way entropy increases is by creating universes like our own.
In their picture, any generic state first evolves to an empty de Sitter phase; the small temperature of de Sitter allows for fluctuations into a proto-inflationary configuration, which grows and reheats into a conventional Big-Bang space-time.

The same thing happens in the far past, but with a reversed arrow of time.

On ultra-large scales, therefore, entropy is growing without bound in the asymptotic future and past.
In the absence of inflation, they argue that systems coupled to gravity usually evolve asymptotically to the vacuum, which is the only natural state in a thermodynamic sense. In the presence of a small positive vacuum energy and an appropriate inflation field, the de Sitter vacuum is unstable to the spontaneous onset of inflation at a higher energy scale.
Starting from de Sitter, inflation can increase the total entropy of the universe without bound, creating universes similar to ours in the process.
An important consequence of this picture is that inflation occurs asymptotically both forwards and backwards in time, implying a universe that is (statistically) time-symmetric on ultra-large scales.

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(BTW, this is old news - More on this later...)

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