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The direction of the gravitational force would not change under time reversal. Your object would feel a force downward, just as it does usually. It might be easier to imagine you had a movie of an object under the influence of gravity. Drop the ball from rest some distance above the floor. You'll see it move downward and speed up. You'd interpret this as a ...

12

Dear Jack, there is no physical phenomenon that could be called the collapse. The collapse of the wave function, as first emphasized by Werner Heisenberg and then many others, is just the event when we learn something about a physical property of a physical system. When we learn that Osama bin Laden is located in a building in Pakistan, his wave function - ...

11

Hannesh, you are correct that the second law of thermodynamics only describes what is most likely to happen in macroscopic systems, rather than what has to happen. It is true that a system may spontaneously decrease its entropy over some time period, with a small but non-zero probability. However, the probability of this happening over and over again tends ...

11

One of the problems you will encounter is causality. Imagine you have a ball resting on the ground. Without already knowing how it behaved in the past you cannot uniquely define the next frame of your game. You cannot tell if the ball should: move upwards vertically. move upwards in any direction. roll on the ground towards any direction. do nothing. ...

8

Loschmidt's paradox is that the laws of thermodynamics are time asymmetric because entropy always increases, but the underlying laws of physics are symmetric under time reversal. It should not therefore be possible to derive the second law of thermodynamics from first principles. Opinions in the scientific community differ as to whether this has been ...

8

There are currently two different accounts that give a larger picture of what happens when a quantum system is measured. One of them is the fact that many random interactions between the system (which might be a 1-body or N-body quantum system) and the environment (which is considered for most purposes a pseudo-classical system with infinite degrees of ...

8

A scalar is defined to be invariant under transformations of the coordinate system. Thus, a vector in one dimension is not a scalar. Time is a "parameter", or a component of a 4-vector in special relativity. In classical mechanics, it is essentially a one-dimensional vector.

7

The laws of physics are time reversible, so a clock could tick backwards as well as forwards. However in our current low entropy universe it is vastly more probable that the clock ticks forwards. In a maximum entropy universe the probablility of a backwards tick would be identical to a forwards tick, so on average the clock time wouldn't change.

7

I think most people would say the paradox is resolved - but, as the answers to this question make clear, they wouldn't necessarily agree about who resolved it or what precisely the resolution is. For my money the paradox was elegantly resolved by Edwin Jaynes in this 1965 paper. In Jaynes' argument, the symmetry is broken by the fact that we, as ...

7

The reasoning in the question is correct. If you have a box with gas particles placed in half of a box but otherwise uniformly random and with random velocities then it is overwhelmingly likely that it entropy will increase with time, but if reverse the velocities, you will still have randomly distributed velocities and the same argument will apply. By time ...

6

The microscopic laws of physics are reversible or, to say the least, CPT-symmetric (processes are invariant if they're run backwards in time, in mirror, and with antiparticles). The CPT symmetry follows from the Lorentz symmetry. Langton's ant as well as pretty much any other Turing machine or cellular automaton fails to be microscopically reversible; ...

5

The summing over final states and the averaging over initial states is a good observation that I always emphasize as the origin of the arrow of time. As soon as one considers mathematical logic, this asymmetry has to arise. Why are we summing over final states? Because "we don't care" about which of them occurs (and no one knows). We're calculating the ...

5

First of all, it's strange how the OP jumps from the Loschmidt "paradox" to dissipation. It makes it very unclear what he or she is actually asking because dissipation has no direct relationship to the Loschmidt "paradox" except that both of them are issues concerned with irreversibility in statistical physics or thermodynamics. The existence of dissipation ...

5

We do all the "cross section business" because we want to predict results of experiments. Let's take for example some particle with two polarizations states: "+" and "-". You know that experimentalists will collide 1 000 000 pairs of particles, with polarisation of initial particles being unknown. Best thing you can do is to hope that in experiment ...

5

You ask: From my studies in quantum mechanics, I don't remember any postulates stating anything like this, but this all makes sense to me. Are there any theories out there that go along these lines? Indeed there is such a theory. It's called decoherence. You mention the comparison with thermodynamics, and this is basically the same way decoherence ...

5

It is a reasonable question at the elementary particle physics level , since the mathematical formulae of all the models we have are reversible as to time. It is in the thermodynamic manifestation of the laws that an arrow of time appears, and in special relativity which separates observations in timelike and spacelike regions. So it is one of those ...

4

By increasing the precision of the measuring instrument AND making the measurement, the observer CAN decrease the entropy of the observed system (by redefining what is macro-state). But the total entropy of the observed system plus the observer will increase. This is because every act of measuring increases entropy of the entire system. In other words, it ...

4

First of all, physics does not ever talk about the question of existence, but about useful descriptions and predictions of observations. No physicist will ever prove to you he is not just a figment of your imagination but he can prove to you that Newton's law works pretty well for what you see. In the scientific method, a theory is indeed used until it ...

4

I suppose that what you are thinking about is the principle of causality. We have two events: a cause and an effect, where the second event is a consequence of the first. That is how we perceive all events around us and what we intuitively accept as true. In physics, however, we sometimes obtain two different solutions: first with the cause before the ...

4

Just a few pointers for you to explore more on this. Check out Aharonov's paper the time symmetric formulation of quantum mechanics: http://arxiv.org/abs/quant-ph/9501011 Tony Leggett talks about this: http://www.youtube.com/watch?v=IGim9uzcumk It's a nice video and quite simple to understand.

4

A vector in a $1D$ space is not a scalar. But if we choose a basis (which in this case consists only in one vector, say $E$), any other vector is of the form $vE$, with $v$ a scalar. So we can identify the $1D$ vector space with $\mathbb R$, but the identification depends on the choice of $E$. In the case of the time, things are similar. For the Minkowski ...

4

A scalar with a unit is a 1-dimensional (axial) vector; changing the basis corresponds to changing the unit. A number (without a unit) is not a 1-dimensional vector in the terminology used by physicists. However, it is a 1-dimensional vector in the terminology used in linear algebra.

4

The clock cannot move it's hands forward rather than backward in a maximum entropy universe--- the two processes would be symmetric. This is a simple point--- any computational process requires increasing entropy as a side effect, so if you have an internal conceptual notion of time defined by the relation of physical systems, there must be a background of ...

4

Time seems to "pass" because it is not symmetric -- it is T symmetric. This is often called the "arrow of time." The arrow of time points in the direction of increasing entropy. More: http://en.wikipedia.org/wiki/Arrow_of_time The real question you are asking is why our minds perceive this direction...

4

I'm not sure if there's a definitive answer because I've seen it discussed recently at high level. I do think there's some broad agreement that entropy is important because it has an irreversible property: closed systems progress from low entropy states to higher entropy states. So we can define the passage of time more precisely by talking about increasing ...

4

Contrary to the general tone of the answers I have read so far, I feel there are indeed real difficulties in quantum mechanics which fall under the general category of "collapse of the wave function". I think part of the problem is the tendency of people to talk in generalities instead of dealing with specific issues. I am therefore going to list a few of ...

4

Imagine a flowerpot sitting on a ledge. A breeze blows the pot off of the ledge, and it falls to the ground. When it hits the ground it shatters into a bunch of pieces, it kicks up dust, it makes a sound, it vibrates the ground, and the shards come to rest. The time-reversal of this is that some pieces of flowerpot are sitting on the ground. Suddenly a ...

3

To make such a game and have it make sense, you would have to use a much simpler physical system than anything you would reasonably encounter on Earth's atmosphere. For heat-dissipating systems, the laws of thermodynamics give time a direction: the direction of increasing entropy. Thus an object that falls to earth dissipates its energy, mostly as heat. You ...

3

The statements of the age of the universe timescale are related to the cosmic time, a timescale derived from the expansion of the universe in general relativity of a roughly homogenous universe (the Friedmann-Lemaitre universe/metric). Different homogenous densities of the universe define different cosmic times. The assumption is a homogenous ...

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