The Relativity of Simultaneity

[Robin]

I'd like to see someone who knows what they're talking about provide a response to this, if possible:



I know that there were recent clickbaity headlines saying that time might not exist, but they were all about a new book explaining Loop Quantum Gravity (which I might have to get hold of a copy of), one aspect of which is that time is an emergent property rather than an inherent characteristic of reality.

I would be interested in the answer. I suspect it might be, no one knows at this point in time.

I would just point out that we don't need to assume that time is an actual thing to be independent of or unified with anything for the purposes of this thread.

So that stone wall isn't a stone wall.

I am guessing that none of us assume that time is an actual thing to be independent of or unified with anything.

We are merely open to the possibility that science will show us what time is.

 

[arthwollipot]

I've heard that entropy defines the arrow of time.

Let's see if my explanation of entropy passes muster.

There is only one way that you can stand a pencil on its point. You have to arrange the pencil so its centre of balance is precisely over its point. You can rotate the pencil around its axis, but the number of states that the pencil can be balanced is limited to those in which its centre of balance is directly above its point. But there are a lot of ways in which a pencil can be not on its point. It can be on its side, pointing in any direction, it can be angled on a slope, it can even be on its flat end. The pencil on its point is in a state of low entropy, and its natural inclination is for it to fall over, into a state of higher entropy. This tendency for a system to change to higher entropy states is what defines the arrow of time.

This explanation is almost certainly inadequate, and I look forward to finding out why.

 

[shemp]

Suppose that Bob and Alice, instead of clapping, each emit a photon.

Bob and Carol are standing near each over, each motionless with respect to the other.

Alice flies close by at considerable speed. Each Alice and Bob emit a photon.

Carol is standing midway between Bob and Alice at the time Bob emits the photon.

Carol registers photons from Bob and Alice at the same time. This implies that Bob and Alice emitted the photons simultaneously.

Now transform the frame where Alice is motionless and Bob and Carol zip by her at considerable speed.

In this frame Carol still registers the photons at the same time.

But in the transformed coordinates Carol was not midway between Bob and Alice at the time Bob emitted the photon.

So in this frame they did not emit the photons simultaneously.

It wouldn't take any more than high school maths to establish that this really does follow from the basic postulates and the acceptance of a Lorentz Transform as the correct way to transform coordinates.

But what is Ted doing while all this is happening?

 

[Robin]

But what is Ted doing while all this is happening?

Time travelling with Bill

 

[Robin]

I've heard that entropy defines the arrow of time.

Let's see if my explanation of entropy passes muster.

There is only one way that you can stand a pencil on its point. You have to arrange the pencil so its centre of balance is precisely over its point. You can rotate the pencil around its axis, but the number of states that the pencil can be balanced is limited to those in which its centre of balance is directly above its point. But there are a lot of ways in which a pencil can be not on its point. It can be on its side, pointing in any direction, it can be angled on a slope, it can even be on its flat end. The pencil on its point is in a state of low entropy, and its natural inclination is for it to fall over, into a state of higher entropy. This tendency for a system to change to higher entropy states is what defines the arrow of time.

This explanation is almost certainly inadequate, and I look forward to finding out why.

No, actually it is a pretty good explanation.

(PS I kind of assumed you meant you hadn't explained it well.

If you meant that then I disagree.

If you meant that entropy was not necessarily a good explanation for the arrow of time then I might agree.)

 

[arthwollipot]

I meant both so it's nice to hear that I succeeded at one of them, at least. I find that one good way to test my understanding of a subject is to try to explain it to someone else.

So why is entropy not a good explanation for the arrow of time?

 

[Robin]

I meant both so it's nice to hear that I succeeded at one of them, at least. I find that one good way to test my understanding of a subject is to try to explain it to someone else.

So why is entropy not a good explanation for the arrow of time?

Basically there is the "why does it go that way?" Question. If the laws of physics are reversible then why can't entropy increase in the other direction, timewise?

If you take the basic laws of Newtonian Physics and change the delta t to a minus delta t you can also see entropy increase.

If you model a heat engine step by step and about half way to equilibrium you multiply the.delta t by -1 it just continues to run down to equilibrium.

So that is the part which goes against the idea of time as a dimension.

 

[arthwollipot]

Basically there is the "why does it go that way?" Question. If the laws of physics are reversible then why can't entropy increase in the other direction, timewise?

If you take the basic laws of Newtonian Physics and change the delta t to a minus delta t you can also see entropy increase.

If you model a heat engine step by step and about half way to equilibrium you multiply the.delta t by -1 it just continues to run down to equilibrium.

So that is the part which goes against the idea of time as a dimension.

I was under the impression that entropy wasn't time-reversible. If you take a jar full of ball bearings (low entropy) and pour it out on the floor so that they go everywhere (high entropy), it won't look at all the same if you run that film in the projector backwards, unlike, say, an electromagnetic interaction which is time-reversible.

 

[psionl0]

Suppose that Bob and Alice, instead of clapping, each emit a photon. ...

The way that I heard of this thought experiment is that Tom is standing next to a railway track and he is exactly half way between two light sources A and B which are also next to the railway track. Dick is on a train that is moving from light source A to light source B.

At the instant when Dick is directly adjacent to Tom, both light sources flash simultaneously (in Tom's reference frame). Since Tom is exactly the same distance from both light sources, he will see the light from both sources at the same instant and will conclude that both light sources flashed simultaneously.

However, because Dick is moving towards light source B, he will see B's flash before he see A flash. Since the speed of light is the same in both reference frames, Dick's (equally valid) conclusion is that light source B flashed before light source A.

I don't know what Harry was doing all this time.

 

[arthwollipot]

Brian Greene describes a peace treaty between the presidents of Forwardland and Backwardland. They only agree to a treaty if they sign the document simultaneously - neither wants to be the first to sign. They sit at either end of a table with a lamp placed directly in the middle. When the lamp goes on, they sign the document.

Now put the table, the presidents, and the lamp on a train, moving smoothly and steadily and undergoing no acceleration. By Einstein's principle of equivalence, no experiment done inside the train will produce any result that indicates that the train is moving (inertial reference frames are equivalent), so both presidents still see the light go on at the same time, and from their point of view the documents are signed simultaneously.

But from the point of view of a spectator on the platform as the train whizzes past, the president of Forwardland signs the document first, since she is moving towards the photons that are from her point of view coming towards her. She travels less distance before seeing the light than does the president of Backwardland, who is rushing away from the photons that the lamp emits. The citizens of Forwardland are outraged at being tricked and the war between the two countries continues.