turning time backwards.

[michel123456]

This is a question I could not get a clear answer yet.

When you theoretically turn time backwards, the attractive force of gravitation becomes repulsive. (I hope it is evident)

What happen to the 2 other interactions? (the electroweak & the strong).

[Moontanman]

I have no clue but it's a pretty good question. Would all attractive forces become repulsive? Positive repel negative? Interesting.

[ajb]

I have no clue but it's a pretty good question. Would all attractive forces become repulsive? Positive repel negative? Interesting.

 

No, not quite.

 

The Lorentz force you can show is in fact symmetric in time reversion. (The magnetic field also picks up a minus sign).

 

As for the weak force this is related to CP violation.

 

In quantum field theory it is the combination of Charge Parity and Time (CPT) that needs to be a symmetry.

[Sisyphus]

When you theoretically turn time backwards, the attractive force of gravitation becomes repulsive. (I hope it is evident)

 

That's not evident to me. In fact, I don't think it's true, now that I think about it. An orbit in reverse is still an orbit.

[ajb]

Symmetry usually refers to the dynamics and not the system itself. So, Sisyphus is correct.

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We can easily show that Newton's second law is T-symmetric.

 

[math]m \frac{d^{2}}{dt^{2}}x(t) = F(x(t))[/math]

 

Define [math]T: t \rightarrow -t [/math]

 

and time reversed motion as [math]x^{T}(t) = x(-t) [/math], then clearly the time reversed motion is also a solution to Newton's second law.

 

This would not be true if the velocity i.e. single derivatives wrt time were part of the equations.

Edited December 5, 2009 by ajb
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[michel123456]

Symmetry usually refers to the dynamics and not the system itself. So, Sisyphus is correct.

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We can easily show that Newton's second law is T-symmetric.

 

[math]m \frac{d^{2}}{dt^{2}}x(t) = F(x(t))[/math]

 

Define [math]T: t \rightarrow -t [/math]

 

and time reversed motion as [math]x^{T}(t) = x(-t) [/math], then clearly the time reversed motion is also a solution to Newton's second law.

 

This would not be true if the velocity i.e. single derivatives wrt time were part of the equations.

 

The reversal of Newton's second law shows that intensity is the same. It says nothing about direction i.e. attractive or repulsive.

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At time 1, the apple is on the tree.

At time 2, it falls down to earth (and not upon Isaac's head)

Attraction between the apple and the Earth.

 

reverse:

 

At time 1, the apple is on the ground.

at time 2, the apple is ejected upon the tree.

Repulsion between the apple and the Earth.

 

Now, I never thought about associating a repulsive force and an orbit. Quite interesting comment. Have to think about it.

[Sisyphus]

At time 1, the apple is on the tree.

At time 2, it falls down to earth (and not upon Isaac's head)

Attraction between the apple and the Earth.

 

reverse:

 

At time 1, the apple is on the ground.

at time 2, the apple is ejected upon the tree.

Repulsion between the apple and the Earth.

 

Now, I never thought about associating a repulsive force and an orbit. Quite interesting comment. Have to think about it.

 

That wouldn't be repulsion between the apple and the Earth. You're leaving out forces. Also, think about "time 3," after the apple hits the ground, and is just sitting there. When an apple hits the ground, its kinetic energy is transferred to the ground in a waves that dissipate outwards. Looking at the same event in reverse, you have an an apple sitting on the ground (still held there by gravity), but then shock waves converge and toss it into the air. Not a very likely event in forward time, but that's entropy for you.

 

I think you'll find that any situation put in reverse still shows gravitational attraction working the same way.

[Mr Skeptic]

As far as I know, reversing time results in matter looking like antimatter.

 

As for gravity, it will remain attractive under reverse time. Pass a rock to your friend, and it accelerates downwards until he catches it. In the reverse, your friend passes the rock to you, and it accelerates downwards until it hits your hand. It won't fly into space.

[Mr Black]

is it theoretically possible to make time go backwards in the first place?

[npts2020]

is it theoretically possible to make time go backwards in the first place?

Yes, but for humans today only in theory.

[michel123456]

I think you'll find that any situation put in reverse still shows gravitational attraction working the same way.

 

This comment of yours goes against what I thought I knew. I am not sure you are right. I am not sure I am right either. Have to think about.

Do you have any more information, any reference?

[padren]

If gravity became repulsive, the apple would accelerate away from the Earth. However, it's fastest velocity is as it leaves the ground due to "shock waves convergence" as Sisyphus put it, and has the lowest velocity by the time it reaches the tree.

 

This is because the impulse is being slowed by the attraction of gravity - even in reverse.

[swansont]

This comment of yours goes against what I thought I knew. I am not sure you are right. I am not sure I am right either. Have to think about.

Do you have any more information, any reference?

 

Reversing time is like running a movie backwards. The description that Sisyphus gave of a rock jumping off the ground sums it up nicely. It does so because of other forces, not because gravity became repulsive.

[michel123456]

Reversing time is like running a movie backwards.

 

Basically, that was the question in the first place.

Running a movie backwards do not change the nature of matter into anti-matter, as Mr Skeptic coined. The only thing that changes is the sequence of events ( the order in which events happen). I am not sure that it is a good representation of the full "backwards in time" situation.

Simple intuition is not enough here.

 

For example, when time turns back, do the charge of electron / proton change?

[ajb]

Reversing time is like running a movie backwards.

 

 

Motion reversal would probably be a more accurate name than time reversal.

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The reversal of Newton's second law shows that intensity is the same. It says nothing about direction i.e. attractive or repulsive.

 

Consider Newton's force law

 

[math]F(x(t)) = m a(t)[/math] (assume no explicit time dependence).

 

.

 

Under time reversal we have

 

[math]x^{T}(t) = x(-t)[/math] and [math]a^{T}(t) =a(-t)[/math] as acceleration is defined by the second derivative.

 

Then [math]F^{T}(x(t))= F(x^{T}(t))[/math].

 

It all seems invariant to me.

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For example, when time turns back, do the charge of electron / proton change?

 

The usual assumption is that the electric charge is unchanged by [math]t\rightarrow -t[/math], so that the electric charge density in Maxwell's equations is also unchanged. This them means that the current, which depends on the velocity does change sign.

 

In short

 

[math]\rho^{T}(t) = \rho(-t)[/math] and [math]j^{T}(t) = - j(-t)[/math].

 

If we want these to be sources in Maxwell's equations then we need

 

[math]E^{T}(t) = E(-t)[/math] and [math]B^{T}(t)= - B(-t)[/math].

 

 

(I have suppressed any spacial dependence)

 

As an aside, you could also deduce this from insisting that the motion of a charged particle in an electromagnetic field be T-symmetric.

Edited December 6, 2009 by ajb
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[swansont]

Basically, that was the question in the first place.

Running a movie backwards do not change the nature of matter into anti-matter, as Mr Skeptic coined. The only thing that changes is the sequence of events ( the order in which events happen). I am not sure that it is a good representation of the full "backwards in time" situation.

Simple intuition is not enough here.

 

For example, when time turns back, do the charge of electron / proton change?

 

Mr Skeptic did not actually coin it, he merely repeated it. If you violate T symmetry, you need a corresponding violation of CP symmetry to leave CPT unchanged. So this all depends on the conditions of the discussion: are you asking about T symmetry reversal that complies with nature, or reversal that does not? Usually we discuss things that do not violate physical law, since once you've decided to do so, many answers are possible.

[michel123456]

Mr Skeptic did not actually coin it, he merely repeated it. If you violate T symmetry, you need a corresponding violation of CP symmetry to leave CPT unchanged. So this all depends on the conditions of the discussion: are you asking about T symmetry reversal that complies with nature, or reversal that does not? Usually we discuss things that do not violate physical law, since once you've decided to do so, many answers are possible.

 

The supposition in my question is that physical laws are maintained under time reversal. Otherwise the dicussion is out of meaning.

[swansont]

The supposition in my question is that physical laws are maintained under time reversal. Otherwise the dicussion is out of meaning.

 

Then an antiparticle moving forward in time looks like the particle moving backward in time.

 

 

One intriguing feature of Feynman diagrams is that antiparticles are represented as ordinary matter particles moving backward in time

 

http://www.britannica.com/EBchecked/topic/205708/Feynman-diagram

[ajb]

The supposition in my question is that physical laws are maintained under time reversal. Otherwise the dicussion is out of meaning.

 

Have a look into CP violation in weak interactions.

 

A local, Lorentz invariant quantum field theory must obey the CPT theorem. So as swansont has pointed out, in the context of a local, Lorentz invariant QFT the violation of T-symmetry has everything to do with the combined CP-symmetry.

[michel123456]

What about strong interaction?

[ajb]

This is a bit more complicated.

 

I'll try to outline as best I can.

 

In Yang-Mills theories, such as QCD there is an infinite number of vacua labelled by arbitrary number. (There are fields connecting these degenerate vacua, see instantons. The number is refers to the homotopy class of vacua)

 

The true vacua is thus a sum of all these vacua and is labelled by an arbitrary parameter.

 

[math] | vac\rangle_{\theta} =\sum_{n = 0 \infty}^{\infty} e^{i \pi \theta}|vac \rangle_{n}[/math]

 

 

(The factor ensures that we have gauge invariance up to a phase factor).

 

These vacua are known as [math]\theta[/math]-vacua.

 

If [math]\theta \neq 0[/math] then the vacuum state is complex and time reversal invariance is violated.

 

We also do not have parity invariance.

 

Via experiment, we know that [math]\theta < 10^{-5}[/math].

 

This is an open question. How can [math]\theta[/math] be so small?

 

You should also note that the presence of the [math]\theta[/math]-vacua can be absorbed into a topological term in the action. So, even if we initially set this term to zero the instantons will generate it!

 

So in short, no.

[michel123456]

I would be proud if I could understand 10% of your last post.

 

You wrote "If [math] \theta \neq 0 [/math] then the vacuum state is complex and time reversal invariance is violated" and you also wrote that [math] \theta [/math] is never zero.

 

So, in two words, what happens to the Strong Interaction under time reversal ?

[ajb]

Experimentally we see that it is certainly almost time symmetric, [math]\theta[/math] is small, maybe even zero. But as I said, theoretically this is very unclear.

 

[math]\theta[/math] small has no or little theoretical justification. There seems no real convincing argument that I know of to explain this. The value zero seems even more unnatural. Quantum effects would "push" it away from zero.

Edited December 7, 2009 by ajb

[Zolar V]

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?

 

you may wonder just why would someone come to the past and how would know.

but if someone from the future did come to the past why would they speak our language or look like us, in fact i dont think they would, so they would be very identifiable persons wondering around. basing my opinion on this paradox and logic i conclude that it is not possible to travel back in time.

also on another note, i dont personally believe time is even a force that has anything to do with physics. ie, its not a force but a philosophical ideology created by humans to organize events.

[michel123456]

This thread was not intended to speak about time travel.

It was more intended to clarify the influence of time upon physical events.

 

As I wrote above, I think that "turning time backwards" is more than "turning the reversal sequence" (looking at the reverse movie).

 

For example: your cup of tea falls down from the table and breaks. The reversal movie will show the broken pieces stick together and jump from the floor to the table. That looks completely weird, and you can find in the litterature a lot of discussions on similar themes, involving entropy etc.

 

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

In "real" physics (can we say that?), the broken pieces and the jumping cup of tea are made of anti-matter.

This anti-matter is subjected to attractive gravitation (following the comments on this thread), something that I have not personnally clarified yet (the jumping from the ground still bothers me).

Inside this anti-matter cup of tea, the strong (attractive) force that binds quarks into hadrons becomes ...?