Quantum time travel

Akrasia

Hi Guys

I was reading The Elegant Universe by Brian Greene the other day and I think I have finally grasped some of the principles behind the quantum wierdness,
Can I just run my thoughts through here to see if I am close or completely wrong?

1. Quantum particles can be in many places at once (double slit experiment, where a single photon can create its own interference pattern while travelling through multiple pathways at the same time)

2. As demonstrated through experiments, the act of observing a quantum particle locks it into a definite pathway

3. Experiments have shown that the act of observing a photon can lock it to a pathway even though that photon has already passed through the pathway before the observation was made. (ie, an affect now, can have alter the course the photon made in the past)

4. There is no fundamental law of physics that says time has to travel in one direction (ie, forward, rather than backwards)

5. However, the principle of entropy dictates that everything tends towards disorder. The large number of possible configurations for any combination of matter/energy means that the universe as a whole is travelling in the direction of Low entropy ==> High entropy

6. BUT, on an quantum level, for individual quantum particles, the law of entropy doesn't really apply. The number of configurations of a single particle is the same going forward in time, as it is going backwards in time, so for an individual quantum particle it exists in all times at once. If an observation is made to the quantum particle which causes it to be absorbed in a detector at 9.15am on tuesday morning, this is 'instantly' fed back to 9.07am that morning when that photon was emitted from the surface of the sun and from the instant the particle is absorbed by our detector, that particle now has a locked defined path through which it is 'destined' to travel.

Conclusion
At the quantum level, where the law of entropy is weak (ie, the probability that individual or very small groups of particles might randomly group together to create something that is more ordered than the alternatives is non trivial) time travel is possible backwards as well as forwards and it happens all the time

however, as scale increases, 'reality' is based on the interactions of billions of different particles at the same time, the 'law of entropy' is that time travel is effectively impossible

We don't travel in the direction of time, we travel in the direction of increased entropy.
Time is not really a 'thing' it's just a description of the principle of entropy.

Michael D Not Higgins

Time is a 'thing' as it is measureable and defined. It is possible to decrease the entropy of a closed system. Does this mean it is happening in reverse? No. The fact is that entropy increases over time within our universe but that doesn't mean it has to be like that. It could theoretically be possible to have a universe where entropy decreases over time, e.g. collapsing universe.

Akrasia

Michael D Not Higgins wrote: »

Time is a 'thing' as it is measureable and defined. It is possible to decrease the entropy of a closed system. Does this mean it is happening in reverse? No. The fact is that entropy increases over time within our universe but that doesn't mean it has to be like that. It could theoretically be possible to have a universe where entropy decreases over time, e.g. collapsing universe.

When we measure time, what are we really measuring? It's a vibration or an oscillation that is regular and predictable. The fact that we can measure time just means that the laws of physics are stable and allow for regular motion according to the various forces

Mechanical clocks work by controlling the release of energy from a wound spring using precise mechanisms that take advantage of the properties of the materials used. Quartz clocks use the piezoelectrical properties of the quartz crystal which vibrate at predictable rates depending on their shape, we use these vibrations to measure time.

Our intenal sense of time is much less precise, more subjective and our perception of how quickly time passes seems different depending on our environment and what we are concentrating on

Regarding Entropy, While it may be 'theoretically possible' that the universe could go into a state of decreasing entropy, this is currently not the case, and if entropy did go into reverse in the universe in some kind of 'big crunch' scenario, locally, entropy would apply, ie, eggs would not unsmash themselves so it is possible that 'time' would be meaningful only in local regions of space. If all the expansion of the universe went into reverse and the galaxies started rushing towards each other, in local regions of space presumably, the laws of physics would be the same as they are now
If the laws of physics were fundamentally different in such a scenario, than all bets are off

Justin1982

To really understand quantum mechanics you probably need to do a college type course on it or read a college type book on the subject. Also read "Schroedingers Cat" by John Gribbin. Its very good.

Quantum world is well defined in terms of a mathematical framework called Quatum Mechanics. In this one asks a question about a system of particles and the mathematical framework giving an answer or number of answers with a certain probability of happening.

I think to really understand quantum theory you really need to get rid of all the old classical ideas and prejudices from your mind. Something as simple as speaking about an electron and imagining it as a particle that moves about obeying quantum rules is even wrong.

The most important thing to take from quantum theory in my opinion is this: There is no reality until a measurement is made! There is only question and answer. Before the measurement and answer, there is nothing concrete. After the measurement, things are also merky and mysterious.

Michael D Not Higgins

Akrasia wrote: »

Quartz clocks use the piezoelectrical properties of the quartz crystal which vibrate at predictable rates depending on their shape, we use these vibrations to measure time.

We use the vibration rate, not the vibration to measure time.

My point about the entropy was that entropy can both increase and decrease in forward time. It cannot be a basis for time. In fact time is a basis for entropy, it's even one of the variables that define it.

Also, the fact that entropy can increase or decrease and at different rates in different places makes it a terrible substitute for measuring the distance between events (time).

Akrasia

Entropy never decreases really. While under certain circumstances we can cause what appears to be a decrease in entropy, in reality, the overall level of entropy is increasing.

In order to sort anything from disordered to an ordered state, we need to release energy, thereby increasing overall entropy.

Eg, we can 'decrease entropy' in a messy room by going around, cleaning and picking things up, but the amount of energy expended in this activity will always increase the overall entropy in the room

In a closed system, entropy always increases, entropy can appear to 'decrease' if there is an energy source coming from outside, but the generation of this energy source creates will always increase entropy by more than the decrease in the local state.

The probability of a single broken egg spontaniously unsmashing itself is so low that it is reasonable to work under the assumption that in practical terms it will never happen during the entire span of the universe

In terms of human consciousness, our brain works by generating energy and sending signals to receptors which sets off extremely complex chain reactions resulting in the nervous system coordinating with the brain to cause movement and awareness in the human body.

The odds of single instance of all the signals and components of a single thought travelling along the exact channels and setting off the exact chain reactions required to send the energy back to the mitochondria of a brain cell and reverse the process of generating the energy that would have caused that thought are so low that we can safely say that it will never happen.


The human mind is not capable of 'rewinding' it can not reverse the process of thinking. (remembering is not the same as rewinding to the initial experience, the process in memory are completely different to the processes in experiencing something for the first time)

Quantum theory dictates that once a particle is 'observed' or 'measured' then it is no longer capable of existing in all possible states, it becomes locked down.

The concept of 'observed' seems to imply that it has to be observed by a person, but this is wrong, 'observed' is merely an interaction between the particle and another particle/group of particles

For example a photon travels in the direction that it is sent until it is absorbed somewhere. For that individual photon, there is no such thing as entropy, it is not 'more ordered' if it is in one place rather than another, all states of existence for this photon have equal entropy. Entropy only applies when you talk about arrangements of particles, the more particles included in the arrangement, the more possible configurations there are and the less likely the particle is to be in any one of these specific configurations

If entropy doesn't apply to a particle, and Increasing entropy is the arrow of time, than this explains why particles on a quantum scale are able to exhibit behaviours that violate our perception of cause versus effect.

Lbeard

Justin1982 wrote: »

I think to really understand quantum theory you really need to get rid of all the old classical ideas and prejudices from your mind. Something as simple as speaking about an electron and imagining it as a particle that moves about obeying quantum rules is even wrong.

It's puzzling. In fact very puzzling.

And in regards to time travel, I'm very puzzled by Feynman diagrams.




This diagram seem to show an electron travelling forward in time, and a positron travelling back in time, a photon moving forward in time, and then one of the quarks is travelling forward in time and the other backward in time. I'm not sure how to interpret it.

Is the positron really going backwards in time?

citrus burst

Lbeard wrote: »

It's puzzling. In fact very puzzling.

And in regards to time travel, I'm very puzzled by Feynman diagrams.




This diagram seem to show an electron travelling forward in time, and a positron travelling back in time, a photon moving forward in time, and then one of the quarks is travelling forward in time and the other backward in time. I'm not sure how to interpret it.

Is the positron really going backwards in time?

An electron going backwards in time can be viewed as a positron going forward in time. This is pretty much true for every particle's anti particle. All this diagram says is that an electron and anti electron collide and produce a photon, which decays into a quark-anti quark pair and then a gluon. Feynman diagrams are only used to describe the interactions of particles.

It should be interpreted as a positron and electron moving forward in time. However in Feynman diagrams the convection is that anti particles move backwards in time. Odd but it works, means you can represent different things by just the direction of the arrow.

Lbeard

citrus burst wrote: »

An electron going backwards in time can be viewed as a positron going forward in time. This is pretty much true for every particle's anti particle.

I'm not really at a stage where I can fully understand the maths involved. Is the wave function of an anti-particle simple reversing it's time evolution, by making anti-particle time negative? .....(BTW I'm still very confused)

All this diagram says is that an electron and anti electron collide and produce a photon, which decays into a quark-anti quark pair and then a gluon. Feynman diagrams are only used to describe the interactions of particles.

I had seen the diagrams before, and thought "so what?". Then reading Feyman's public lectures on QED, it seems there is more to the diagrams. That they represent relativistic interactions - it's not simply an electron and positron collide and release a photon. I don't really fully understand it, but from bits I've gleaned from those lectures Feyman seems to be saying the particles exist virtually before they become real - if they exist virtually before the interaction, when the interaction happens you can go back in time (you're not really going back in time to make something happen - it's already happened virtually).

This stuff is really mind bending.

Justin1982

Lbeard wrote: »

It's puzzling. In fact very puzzling.

And in regards to time travel, I'm very puzzled by Feynman diagrams.




This diagram seem to show an electron travelling forward in time, and a positron travelling back in time, a photon moving forward in time, and then one of the quarks is travelling forward in time and the other backward in time. I'm not sure how to interpret it.

Is the positron really going backwards in time?

Again I think that Feyman diagrams are a bit misleading. That diagram you gave above for example is a very basic Feyman diagram and your considering it in isolation. In reality, for any interaction of an electron with any other particle (like another electron for example), you would represent that one interaction by drawing a lot of different possible Feynman diagrams (infinite numer of diagrams in most cases) and adding them together.
The other thing is that the diagrams are taken too literally. They became popular because they allowed physicists to graphically illustrate the complicated maths of quantum field theory.
The best way to view Feynman diagrams is to imagine each diagram as a mathematical calculation rather than a representation of how the particles are really behaving.
If someone showed you the complicated equation that the diagrams represent then 99% of people would be very confused. But by drawing diagrams it makes it easy for people to understand the math.

Ironically I think that the math is the only thing that should be taken seriously in Quantum Mechanics. Its always right, its logical and once you learn how to use it, well it is easy to understand the quantum world.
But even today if I take the diagrams and try develop a physical picture of the quantum world in my head then it becomes very confusing and contradictory.

Lbeard

Justin1982 wrote: »

In reality, for any interaction of an electron with any other particle (like another electron for example), you would represent that one interaction by drawing a lot of different possible Feynman diagrams (infinite numer of diagrams in most cases) and adding them together.

I'm not 100% sure what they do at particle accelerators. But I'm guessing. Each diagram is a term in the equation. And each diagram is given a probability. Then a flood of particles is accelerated at a target and the results should match the probabilities for different diagrams.

The particles are accelerated to near light speed. Why is that done? In the Feyman lectures I have he represents light speed by a 45 degree angle in the diagram, but he doesn't say why the particle should go at light speed.

The other thing is that the diagrams are taken too literally. They became popular because they allowed physicists to graphically illustrate the complicated maths of quantum field theory.

The best way to view Feynman diagrams is to imagine each diagram as a mathematical calculation rather than a representation of how the particles are really behaving.

I know the diagrams are not a literal representation. At the same time they do show particles.

Justin1982

They accelerate the particles to really high energies so that they smash into each other at high speed and spew out new particles that only have a decent chance of becoming real at specific energies. These specific energies are generally very high. Essentially its a long story.

Yes the diagrams do represent particle processes.
Lets say we have an electron colliding with another electron. This collision is thought of classically like the diagram below where the electrons exchange a virtual photon.
But thats the wrong way to think of it. The real interaction is represented by an infinite number of diagrams like the one below, all different and of all levels of complexity.

Lbeard

Justin1982 wrote: »

Essentially its a long story.

I know it's a long story. I'm trying to learn it by myself, in my spare time. For the last few weeks I can't access most of the books I have. And I haven't really found a book that's comprehensive and comprehendable. I have bunches of physics books and maths books.

They accelerate the particles to really high energies so that they smash into each other at high speed and spew out new particles that only have a decent chance of becoming real at specific energies. These specific energies are generally very high.

I know that. But is there a reason apart from high energy to generate heavier particles, for accelerating particles to relativistic speeds?

Yes the diagrams do represent particle processes.
Lets say we have an electron colliding with another electron. This collision is thought of classically like the diagram below where the electrons exchange a virtual photon.

Yes. I can't access some of my books at the minute. I should start with electron collisions. I know the diagrams are not classical interpretations - but they are interpretations.

They are confusing and understanding a gauge boson in an electron collision, seeing an electron/positron loop in a photon is puzzling.

But thats the wrong way to think of it. The real interaction is represented by an infinite number of diagrams like the one below, all different and of all levels of complexity.

I know. What I'm just trying to do at the minute is just interpret single diagrams, then I can move on the infinite number. And I can't quite do the maths at the moment, but I'm getting there.