B If gravity was a force wouldn't going back in time cause us to float?

[lolsurround]

TL;DR Summary

Would time travel cause everything to float if we would assume that gravity is a force?

This might sound as a dumb and silly question but if you think about it, it makes sense. If we wrongly assume that gravity is a force just like any other, and given the fact that time is closely related to gravity and that gravitational time dilation is a thing, wouldn't reverse time travel cause everything to float? In order for events to occur, the universe's physical forces has to take place i.e. time must tick. Wouldn't reversing time case forces to act the opposite way? In order for the keyboard on which I'm typing on right now to send a signal, a button has to be pressed. In order for that button to be pressed, a force has to act upon it. My meaning is that reversing that force would cause it to go up, hence a reversed downwards force.

Now if we go back to out wrongly assumption that gravity is a force, wouldn't time traveling cause the gravitational force to be reversed?

 

[Ibix]

It's difficult to say what wrong guesses lead to.

However, time reversed gravity is still attractive. What we describe as a cannon ball shot upwards causing a crater on landing is described as a weird kind of focussed earthquake that un-forms a crater and launches the ball up in the air. It goes up and down just like it does normally, and its landing is cushioned by an un-explosion of gunpowder. The weirdness in this is all about the spontaneous reversal of entropy, but gravity and all physical laws work the same in reverse.

 

[Dale]

Wouldn't reversing time case forces to act the opposite way?

No. Forces are unchanged under time reversal. You can see that in a “handwaving” manner from the fact that the dimensions of force are $F=M\ L\ T^{-2}$ and if you substitute $T \to -T$ it does not change $F$

 

[kuruman]

Wouldn't reversing time case forces to act the opposite way?

No. Consider the description of a force as we understand it using Newton's second law. We write $$\mathbf{F}_{\text{net}}=m\frac{d^2\mathbf{r}}{dt^2}.$$If you reverse time, you must replace $t$ with $(-t)$ in the above equation. Then $$\mathbf{F'}_{\text{net}}=m\frac{d}{d(-t)}\left[\frac{d\mathbf{r}}{d(-t)}\right]=-m\frac{d}{d(-t)}\left[\frac{d\mathbf{r}}{dt}\right]=+m\frac{d}{dt}\left[\frac{d\mathbf{r}}{dt}\right]=m\frac{d^2\mathbf{r}}{dt^2}.$$Thus, $~\mathbf{F}_{\text{net}}=\mathbf{F'}_{\text{net}}~$ which means that time reversal does not result in force reversal.

 

[PeroK]

If you saw a film of the solar system run in reverse, you wouldn't be able to tell that time was running backwards. Each planet would simply be orbiting in the opposite direction.

 

[256bits]

If you saw a film of the solar system run in reverse, you wouldn't be able to tell that time was running backwards. Each planet would simply be orbiting in the opposite direction.

This, and the example by @Ibix of the cannon ball, are examples of 'reversals' of the second law of thermodynamics, which leads to all kinds of absurdities. As far as I know it, the second law of thermodynamics is still a law.

If we are switching the direction of time of the events, then we should also be switching the watching of the events from 't; time to '-t' time - ie reversed film of the revolving earth in '-t' time, not t time. Same for the shooting cannon ball. Oherwise, there would be such a thing as the outside observer of the universe.

 

[vanhees71]

The 2nd law of thermodynamics is derived by using statistical physics and appropriate "coarse graining" of the microscopic details. This leads to the H-theorem, according to which the entropy of a closed system doesn't decrease. Closer inspection of this derivation from first principles shows that it does not follow from time-reversal invariance of the microscopic laws but from the unitarity of the S-matrix (or the unitary time evolution of quantum theory). The corresponding emergent thermodynamical arrow of time coincides with the fundamental causal arrow of time, which is a fundamental postulate.

If you look at a movie of an everyday situtation, run backwards, we realize that it's running backward in all situations, where "macroscopic complexity" is involved. E.g., if we film a cup of coffee falling down from my table and fragmenting in zillions of pieces when hitting the floor, we'll immediately realize if the movie is run backwards, because the corresponding "time-reversed motion" is extremely unlikely to occur, but it doesn't contradict any fundamental laws at play here, which are all time-reversal invariant (and in this example for sure we can necglect the weak interaction, which breakes T invariance). Put in a more instrumental way, it's easy to understand: to realize time revearsal means that you take a "final state" of an event and prepare the corresponding time-reversed event as an "initial state". If you can do this in all microscopic details, and only time-reversal invariant dynamics is involved, you could realize indeed the time-reversed motion. No in the example with the falling cup it's extremely difficult to time-reverse all the splinters' state just hitting the floor, and thus it's practically impossible to repair the cup with the coffee in place by "just time-reverse" its state hitting the floor.

In @PeroK's example of filming the motion of our solar system and running the movie backwards is different, because in this case it's sufficient to describe a quite small number of heavenly bodies circling around the Sun, and the time-reversed state indeed doesn't in any way "unnatural", i.e., it's not unlikelier that all the bodies run in time-reversed motion than that they run as observed in our solar system.

 

[256bits]

In @PeroK's example of filming the motion of our solar system and running the movie backwards is different, because in this case it's sufficient to describe a quite small number of heavenly bodies circling around the Sun, and the time-reversed state indeed doesn't in any way "unnatural", i.e., it's not unlikelier that all the bodies run in time-reversed motion than that they run as observed in our solar system.

One could just as well view the solar system from above or below, giving a mirror image, or parity symmetry, if one wants to see the earth revolving around the sun in the opposite direction.

The time reversed films do illustrate that some physical quantities remain the same such as position, energy, acceleration and force, while others negate such as velocity and momentum.
But, If the time reversed film lasts long enough, one will notice some physical deviations from the laws of kinematics and dynamics, as well as electromagnetics.
For the solar system, the tidal bulge becomes retarded, having the effect that the moon's orbital radius is decreasing, along with a decrease in the earth's rotational period. The sun has become a heat sink, while the earth converts high entropy into low entropy radiation.

For illustratation,

T

The event A (A') occurring at time 0, evolves symmetrically in t time, or -t time to state B (B'), with A and A' having the same state conditons, and respectively B and B'.

In the second figure, state A has evolved to state B.
State B, in non-symmetric time reversal ( state B is shown transposed back to the origin of time 0 showing the non-symmetry ). State B could evolve back to state A, OR, most likely to some state C different from A.

 

[phinds]

gravitational time dilation is a thing

Yes, it is but I don't think you understand just what that "thing" IS.

 

[vanhees71]

One could just as well view the solar system from above or below, giving a mirror image, or parity symmetry, if one wants to see the earth revolving around the sun in the opposite direction.

The time reversed films do illustrate that some physical quantities remain the same such as position, energy, acceleration and force, while others negate such as velocity and momentum.
But, If the time reversed film lasts long enough, one will notice some physical deviations from the laws of kinematics and dynamics, as well as electromagnetics.

How so? All interactions except the weak interaction are T-invariant. So if there's any possible process then also the time-reversed process is possible as long as you can neglect the weak interaction.

 

[256bits]

How so? All interactions except the weak interaction are T-invariant. So if there's any possible process then also the time-reversed process is possible as long as you can neglect the weak interaction.

So true.
It's a question of whether information can be forgotten.
for macroscopic processes it does leave the system, is where I am coming from.

 

[mpresic3]

Watch re-runs of the science fiction series by Irwin Allen, "The Time Tunnel". The actors as scientists are floating as they are going back (and forward) in time. I don't think gravity is mentioned as a force though.
I'm not making fun of the question or the questioner, just pointing out the humor in the question

 

[mpresic3]

Would time travelers on closed timelike curves from a rotating blackhole experience free-fall. Maybe there is something more to this question? From what I have read though, this would require negative energy and that has not been observed, and may not be possible.

 

[Ibix]

Would time travelers on closed timelike curves from a rotating blackhole experience free-fall.

I'm not sure without checking if those CTCs are geodesics or not. However, even if they are it's easy to find "nearby" trajectories that are not, so "some CTCs are geodesics" doesn't mean reversed gravity is repulsive.