turning time backwards.

[ajb]

But I think that in real physics you cannot simply assume that "reverse the movie" = "reverse time".

 

In "real physics" we have to take into account large ensembles and the rules of probability. This I believe is why T-symmetry is lost.

[Sisyphus]

http://en.wikipedia.org/wiki/Arrow_of_time

(Should have brought this up earlier.)

 

What's all this with antimatter? What is the distinction you're making between "reverse time" and "reverse movie?" It seems to me that if you "reverse time" and end up with a sequence of events that is something other than the "forward" events in reverse order, that just shows you've done something wrong. So yes, the ceramic shards get knocked together into a teacup and tossed up onto the table.

[RobbieG]

You know, if it was theoretically possible to travel back in time. Then my paradox is if you can in the future why haven't you come to the past to do anything or tell anyone?

 

To my understanding (which isn't very much mind you), people could only travel back in time if:

 

1) They were travelling faster than the speed of light

 

OR

 

2) They invented some sort of mechanical device to grant them time travel.

 

But in #2, they would only be able to go back in time, as far back as when the device was created and no further.

[ajb]

You know' date=' if it was theoretically possible to travel back in time.

[/quote']

 

To my understanding (which isn't very much mind you)' date=' people could only travel back in time if:

[/quote']

 

As already noted, this thread is not really discussing time travel. We are discussing T-symmetry which is the replacement of [math]t[/math] with [math]-t[/math] in physical theories. It should be viewed (as pointed out by several posters now) that we are really discussing reversing motion in physics and not really thinking about time-travel as such.

 

Time travel is a fascinating subject, but outside of our discussion here.

[michel123456]

http://en.wikipedia.org/wiki/Arrow_of_time

(Should have brought this up earlier.)

 

What's all this with antimatter? What is the distinction you're making between "reverse time" and "reverse movie?" It seems to me that if you "reverse time" and end up with a sequence of events that is something other than the "forward" events in reverse order, that just shows you've done something wrong. So yes, the ceramic shards get knocked together into a teacup and tossed up onto the table.

 

You have got all the meaning of this thread, in some sense.

Yes, I agree when you write " (...) if you "reverse time" and end up with a sequence of events that is something other than the "forward" events in reverse order, that just shows you've done something wrong."

That is the reason I believe that the "reverse movie" is not enough. When you reverse the movie, the world becomes weird. Broken objects distributed at random on the floor stick together and become a cup. The milk separates spontanously from coffee and jumps into the bottle, and so on. Endless descriptions.

I think that those descriptions are weird because they are incomplete.

That was the reason of my first question: (in other words) when time goes backwards, what are the following physical consequences.

From the discussion, it appears that turning back time may also involve a transformation of the charge. Antimatter is described as matter of reversed charge (electron/positron, wrong?), so I believe that we could accept antimatter as regular matter travelling backwards in time, which is quite the same as giving the definition of antimatter as matter travelling backwards in time.

I know that is a little bit too much of an interpretation, but I believe that it complies with the laws of physics. Of course I have not the knowledge to confirm anything of it. Otherwise, i wouldn't be here.

 

And still, still, I cannot get what is happening to the strong force. As much as I know, the Strong force is always attractive. Do this force become repulsive through time reversal?

[Sisyphus]

That is the reason I believe that the "reverse movie" is not enough. When you reverse the movie, the world becomes weird. Broken objects distributed at random on the floor stick together and become a cup. The milk separates spontanously from coffee and jumps into the bottle, and so on. Endless descriptions.

I think that those descriptions are weird because they are incomplete.

 

They are weird because they are astronomically unlikely in the ordinary sense, because they are examples of negative entropy on the macroscopic scale. However, such events do not actually violate any physical laws on the microscopic scale, and are "possible" (though extremely, extremely unlikely) without changing the behaviors of any forces.

 

This is why I think "time reversal" should focus on what entropy means, and why it increases. Which I think is closely related to the question of why entropy was so low in the "initial conditions" of the universe. But I'm rather out of my depth.

 

And still, still, I cannot get what is happening to the strong force. As much as I know, the Strong force is always attractive. Do this force become repulsive through time reversal?

 

Well surely common sense can be employed, as with gravity. If the strong force suddenly and permanently became repulsive throughout the universe, what events would occur? Have similar events in reverse order just occured in the immediate past? I think not.

 

Charge, on the other hand, is rather different. If every positive charge simultaneously became negative and vice versa, would there be any way to detect that? (Non-rhetorical question.)

Edited December 8, 2009 by Sisyphus

[michel123456]

If the strong force suddenly and permanently became repulsive throughout the universe, what events would occur?

 

I have some idea about it, but it is highly speculative, not suitable for this thread.

I supposed that the question could be answered independently from the peculiar consequences. In any case, intuition will be a very bad adviser.

Edited December 8, 2009 by michel123456
Consecutive posts merged.

[Sisyphus]

But you must have some reason for suggesting it. And the suggestion is, rephrased, that the strong force is responsible for some time-asymmetrical process. What process?

[michel123456]

No no, my suggestion is entirely different. I cannot propose anything of it and certainly not here (it should be posted under the "speculations" entity at least, after some introduction).

For the moment I just want to gather information I cannot find on the Web. I am not an expert in any field I try to discuss, and I supposed people on this forum would maybe give some clue, some information or reference.

 

The question of the strong force comes naturally because IMO physical nature is a whole. When you change something, it must obligatory have some influence somewhere else. Call that CPT symmetry, equilibrium, law of conservation,or wathever. For me the meaning is the same.

 

So we talked about graviy, about electromagnetism, and about the weak force (considered as the electro-weak).

I still don't know about the strong force.

[ajb]

The strong force is most definitely symmetric under CPT.

 

But like I said, it is not so obvious if it is symmetric under T, or equivalently CP. Experimentally, it looks CP (or T) symmetric, but theoretically this just seems impossible. Well, at least without adding something else to the theory like axions.

 

The amount of CP violation in the strong force cannot be enough to explain the baryon asymmetry in the universe.

[michel123456]

In the meanwhile I found this:

_"In 1942 he proposed the interpretation of the positron as a negative energy electron traveling backward in time." from the wiki article http://en.wikipedia.org/wiki/Ernst_Stueckelberg

And "By considering the propagation of the negative energy modes of the electron field backward in time, Ernst Stueckelberg reached a pictorial understanding of the fact that the particle and antiparticle have equal mass m and spin J but opposite charges q." under "The Feynman-Stueckelberg interpretation" section in http://en.wikipedia.org/wiki/Antiparticle.

 

Thus nothing new.

 

So, as a resume, under time reversal the once broken cup of tea (that jumps from the floor) is probably made of antimatter, we don' t know for sure wether the particles of the cup are bounded together through the attractive strong force, and Michel still doubts that gravity keeps its attractive properties between the cup and the Earth.

 

Maybe this could be considered more than a simple "movie reversal".

[Mr Skeptic]

But there would be a difference between reversing time and rewinding it. If you are reversing time, statistical effects like increasing entropy would apply equally well. However, if you were rewinding time entropy would be reducing and that would manifest as extremely unusual things happenning, like teacups un-breaking. So long as a movie does not depict increasing entropy you could play it in reverse and it would look the same.

[Sisyphus]

But there would be a difference between reversing time and rewinding it. If you are reversing time, statistical effects like increasing entropy would apply equally well. However, if you were rewinding time entropy would be reducing and that would manifest as extremely unusual things happenning, like teacups un-breaking. So long as a movie does not depict increasing entropy you could play it in reverse and it would look the same.

 

I suppose in the case of non-deterministic events, that would be true. Maybe. In a classical system I'm pretty sure you would be seeing the unbreaking teacups. I have to think about this.

[Mr Skeptic]

Well you might see unbreaking teacups for a while, but eventually non-deterministic effects are going to leak in and mess things up.

[michel123456]

I am glad that the discussion could last so long discussing such a weird thing as negative time, which has never been observed.

Time is a scalar. Negative time mean negative scalar.

 

Using negative time in equations gives strange results from the basics.

For example :Speed

[math]

V = \frac{x}{t}

[/math]

Introducing negative T, you obtain negative speed (which is a scalar too), very weird.

In this little "game", negative scalars are blowing out from everywhere.

 

So, IMO, there are 2 alternatives:

_either you refuse to introduce negative scalars in the first place, assuming it is pure nonsense.

_either accept that negative scalars can be used, and offer the theoretical possibility that ANY scalar can become negative, independently of what intuition drives to reject at first sight.