Is Time Travel (going backwards) Possible?

[KarinsDad]

As I noted upthread, this is not possible. Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.

Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.

Precisely.

So, if the laws of thermodynamics are limited to "3 dimensions" (i.e. not time if one considers time a dimension) then backwards time travel in any sense is not possible.

 

[freyar]

<snip...>

Right, that's what I mentioned (way) upthread. There's not a proof of it at the moment (that I'm aware of), but most physicists believe that CTCs, even if they can exist, can't allow time travel. The examples we know tend to be unstable, meaning that sending matter along them makes that part of spacetime pinch off.

 

[Umbran]

So, if the laws of thermodynamics are limited to "3 dimensions" (i.e. not time if one considers time a dimension) then backwards time travel in any sense is not possible.

I don't believe that follows, and wasn't the point I was trying to make.

Most folks, when considering time travel, have one thing that bakes their noodle - causality. Being able to change the past means you can have paradox. The typical defense is, "we can *observe* the past, but not interact with it." I'm merely saying that, in and of itself, isn't possible. If I observe the past, I must at least take energy out of the past, thus changing it.

If you don't mind changing the past, that's not an issue.

Robert L Forward wrote a nice novel considering time travel* - his basic posit was that the Universe didn't care if you traveled in time, but the end result would be self-consistent - that was the one law. You could violate whatever other laws you wanted, but there would be *no* logical paradox. If you went back in time to kill your own grandfather before your father was born, then, well your genes would get together in some other manner.

The end result was that the time traveler had his free will removed - once he'd changed the past, he also became the vehicle for enforcing self-consistency. It wasn't like the universe started falling apart if he didn't act, or anything. He just acts - whatever he thought his choices might have been, in the end, they were choices that enforced the self-consistency.


*Timemaster - his science, as always, was good, but his characterization leaves a lot to be desired.

 

[KarinsDad]

I don't believe that follows, and wasn't the point I was trying to make.

Most folks, when considering time travel, have one thing that bakes their noodle - causality. Being able to change the past means you can have paradox. The typical defense is, "we can *observe* the past, but not interact with it." I'm merely saying that, in and of itself, isn't possible. If I observe the past, I must at least take energy out of the past, thus changing it.

If you don't mind changing the past, that's not an issue.

If one considers the "Butterfly Effect", then removing any energy from the past violates causality as well as violating the first law of thermodynamics.

That was the point that I was trying to make.

 

[Nytmare]

Robert L Forward wrote a nice novel considering time travel* - his basic posit was that the Universe didn't care if you traveled in time, but the end result would be self-consistent - that was the one law.

It's been a while since I read it, but if memory serves, one of Mark Hodder's 'Burton and Swinburne' novels, [ame="http://www.amazon.com/Strange-Affair-Spring-Heeled-Swinburne/dp/1616142405"]"The Strange Affair of Spring Heeled Jack"[/ame] centers around the follies of a time traveler who keeps jumping further and further back trying to right some wrong that he himself unwittingly created.

 

[Umbran]

If one considers the "Butterfly Effect", then removing any energy from the past violates causality as well as violating the first law of thermodynamics.

Actually, in my example, the laws of thermodynamics hold - it is their holding that leads to causality violation.

The laws of thermodynamics say you can't move the information without moving energy. So, fine, you move the energy (the laws of thermodynamics hold). But now, whatever that energy did in the past doesn't happen. Even that doesn't automatically violate causality - causality is only violated if your future is not consistent with that energy not reaching its original destination.

 

[KarinsDad]

I want to come back to this again briefly. From studying the CMB, we know that there must be some kind of non-normal dark matter in essentially the right amount to explain the speeds of galaxies in clusters, rotation of stars in galaxies, etc.

Well, if this is true, then there would be an experiment that could (more or less) prove it once and for all.

The planets revolve around the sun in ellipses. These ellipses are distorted ever so slightly by the gravity of other planets (and to some degree, other objects in our solar system).

There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.

If DM exists, then the distortions should match those predicted by DM theories.

If DM does not exist, then the distortions should match those predicted by a total lack of DM.

And, of course, it could be somewhere in between.

It would be a complex analysis (having to take into account the speed of light and many other factors to determine the exact position of the planets over an extended period of time), but it should be doable.


This type of experiment (done properly and with the proper precision) would seem to be a long pole in the tent to decide the matter one way or another.

 

[tomBitonti]

There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.

Wouldn't that distortion only measure the mass within the radius of the sun's orbit through the milky way? The field strength is determined by the total mass within that radius, not by the distribution of the mass.

Probably, the ongoing surveys will help to provide more accurate measurements of stellar velocities, so to tell the mass distribution.

E.g.:

http://www.latimes.com/news/science/sciencenow/la-sci-sn-milky-way-star-map-20121025,0,4431104.story

http://www.astronomynow.com/news/n1201/09APOGEE/

This image from the ESA is simply fantastic:

http://www.eso.org/public/images/eso1242a/zoomable/

If you play with that image, when you zoom give it time to fill in the details!

Thx!

TomB

 

[KarinsDad]

Wouldn't that distortion only measure the mass within the radius of the sun's orbit through the milky way?

Not the sun's radius, the planet's radius. When planets are closer to the galactic center, the radius of where the planet is located around the galaxy is smaller than if they are away from the galactic center. If DM is a halo around the entire galaxy with or without a higher density towards the center, then a smaller radius of the planet on one side of the sun should have a more distorted ellipse. The velocity when heading towards the galactic center should be greater (by a slightly greater amount) than if DM did not exist, and the velocity away from the galactic center should be less. The amount this is influenced would require extreme precision to measure.

Thinking about it some more, this could disprove DM (or minimally create observational problems for the theory), but it couldn't prove it. If there were other reasons for attraction (like electromagnetic fields), those attractions might still exist.

 

[Nagol]

Our instruments aren't sensitive enough for us to detect the minute variation in something as small as a planetary orbit.

I saw a report of a similar experiment using stellar motions of nearby stars. http://www.eso.org/public/news/eso1217/