Is there anything about time-travel......

[peter]

......that flys in the face of the Laws of Physics?

I've heard about the 'arrow of time', but is it a physical Law that matter/information must only travel in one direction; I mean take for example the bubbling cauldron of matter/nonmatter down at the quantum level does this stuff abide by the 'arrow' or does it (Insequant fashion as it were) disregard the direction of time it travels in as it exists for its brief life.

If we consider say the building of a car from scratch there is a continuous 'knowledge chain' from the point where we are living in a cave to the point where we climb into the front seat of the newly built machine and all steps in the development obey the Laws of Physics. If there is no point in the theory where the Laws of Physics are broken then surely the same 'knowledge chain' will be constructed for time machines and we will indeed build them.

[Hashi Lebwohl]

Down at the quantum level there is more flexibility with the thermodynamic arrow we normally refer to as "time". We can create positrons in colliders, measure them, detect them, observe their paths in particle chambers, etc. A positron, of course, in an electron that has a positive charge but the truly amazing thing is that a positron moving forward in time is mathematically equivalent to an electron moving backward in time.
I forget which book I was reading but it listed a valid yet perplexing Feynman diagram. A photon splits into an electron and a positron which subsequently collide and cancel out, resulting in a photon. In reality, there is a time loop--a photon moves along and intersects the positron, resulting in an electron which travels forward in time, emits the photon and reverts to a positron, which travels backwards in time and intersects the original photon. From the electron's point of view, it will be forever emitting a photon, traveling backward in time, intersecting a second photon, sending it moving forward in time where it emits a photon, etc. Crazy stuff.

As matter accumulates and its quantum properties collapse to zero, it loses its ability to have this temporal flexibility so at the macro level traveling through time is simply not possible. Well, I suppose I should say that traveling through time in any direction contrary to the prevailing thermodynamic direction is not possible. Now...since we can create situations where quantum properties exhibit themselves at the macro level, such as the case with superfluids, strange things may occur that he have not experienced before.

As time dilation, according to quantum physics, follows the formula 1/sqrt{1 - v^2/c^2} where v is velocity and c is the speed of light, we can see that the faster an object goes the slower its relative time passes--one year for that object might be 75 years for us, etc. As v -> c the dilation approaches infinity so photons are essentially timeless, which is why they can travel for billions of years without losing energy and we can see stars that are that far away.

Tachyons, the theoretical particles which never travel slower than the speed of light, travel in a timewise manner rather than a spacewise manner but we have no understandable frame of reference to think about what this must be like. If v > c then the dilation is a complex number. Since we have never detected these kinds of particles it is questionable as to whether they really exist outside of a chalkboard in a physics lab.

[peter]

Can I interpret that answer as 'it doesn't quite break the Laws of Physics but it's not really going to happen (with a very small proviso of 'but anythings possible')'.

re the Fennyman diagram - do these diagrams depict what actually happens as opposed to what is theoretically possible that it could happen? Is not an electron/positron couplet massively more....er....massive than a photon and if so where would the energy come from to provide for this increase in mass (if it occurs in the actual rather than just in the theoretical).

[Hashi Lebwohl]

That is a good way to phrase it, yes.

That's just the thing--the photon did not suddenly split into two more massive particles; rather, the photon encountered a positron > the energy change converted the positron into an electron > the electron subsequently emits the photon and becomes a positron > the positron (traveling backward in time) encounters a photon > the energy from the photon converts it into an electron > ad infinitum. Typically, the diagrams describe things that actually happen but they can also describe things that are possible but have not yet been seen.

[Vraith]

Most of the things...like those diagrams...where we find "the math doesn't care which way it is going" only show the difference between kinds of real.
Though a positron is mathematically equivalent to an electron doing back-time doesn't mean that an actual positron is, in any real way, an electron moving back-time. It is a positron going forward.

I mean, I can take Pythagorean and change all the terms to negative [so the sides of the triangle all have "negative length"]...the math doesn't care. But the thing can't be real [materially].

Lots of math is, I think, much like "Jabberwocky."
There's some fun and coolness and use/value...but most of it ain't real.

Unfortunately, it isn't always obvious which parts are real/useful and which aren't.

[Hashi Lebwohl]

It is significantly more fun to view positrons as electrons moving backward through time, though.

[Avatar]

--A

[peter]

But take Hashi's Fennyman Diagram. If the guys at CERN oneday look at one of their results and see a positron-electron pair appear out of thin air, then the math will be in place to account for it. Maybe it's not going to happen (like Vraith's -ve Pythagorus Theorem [still don't understand why you shouldn't have an (right-twisted) anti-universe where the -ve Theorem would be the norm]), but maybe it has and we just didn't see it yet.

(And the state of affairs is getting stranger by the day - take Vraiths post re the discovery that temperatures of less than -273 C. are possible - Temp is a measure of atomic movement and yet you can get temps below the point where it stops altogether. It seems to me that we are so little of the way into our pursuit of knowledge that all the things we now deem 'impossible' will one day make our descendents laugh at us in our simplicity.)

[Hashi Lebwohl]

It seems to me that we are so little of the way into our pursuit of knowledge that all the things we now deem 'impossible' will one day make our descendents laugh at us in our simplicity.)

That is the inherent curse of time and a limited lifespan--our descendants will always laugh at us regardless of whether we were born in 2013 BC or 2013 AD. I suspect my grandchildren will wonder how I could have lived with an Internet speed of only 1 Gbps, they will gasp when I tell them I had to carry actual physical books--printed on paper, no less--to school, and they will wrinkle their foreheads trying to figure out why we didn't use toroids of room-temperature superconductors inside an intense magnetic field to induce an acceleration vector and generate non-Newtonian thrust.

[Vraith]

It is significantly more fun to view positrons as electrons moving backward through time, though.

Heh...I agree with that for sure...it's why I find the "Father-Theory" with two Time factors instead of a boring one more fun even if nearly fatally flawed [maybe someone will come up with a Step-Father-Theory that works better... ]

And yea, peter, I'm dead certain things will be possible that we think impossible. Yet there's a difference between thing "impossible" due to mistakes/limits in current understanding and things literally not possible, and things conceptually possible but not materially possible.

The idea of "negative length" or "anti-length" is the last, I think. Your "right twisted" universe is probably extant in some kinds of math. Might even be useful [even if only on process/functional terms]. But in the material it can't be, in part by definition of what length is.
If I'm in NY, I COULD say London is about 3500 miles away, and Los Angeles is about 3000 anti-miles...but it really isn't a different kind of length/distance. It's just a difference in my perspective/purpose.

Still, it's fun to try and comprehend/visualize. I've been known to spend time, almost a meditational time, trying to visualize what a real 4-d object would look like [4 physical d's]. I've heard that some people can do it. Long ago there was a magazine called "Omni." In one issue they published methods to try and do just that...but I wan't interested then, didn't keep it, have never found the things online.

[peter]

Yeah! read Omni myself for a year or two (couldn't begin to understand most of it!). I remember two things about it. I was forewarned by about ten years that a total eclipse of the sun would happen where I live at the turn of the millenium so when it became public knowledge (in the broad sense) I could say "told you so!"; second I read about this weird impossible thing invented by a polish (?) mathmetician - a cube where the smaller cubes of which it was comprised could be moved independantly of each other. Why the f*** didn't I buy the import rights to the thing on that very day - It's the reason I'll always be the poor shcmuck I am. I never see a Rubik's cube without thinking "It should have been meee!"

[peter]

Just an update. Saw a program 'Stephen Hawhing's Universe' last night where the great man considered this subject. His feeling seemed to be that travel back in time would be an impossibility because a) the Universe would not allow the 'paradoxes' it would create [eg the grandfather paradox etc] and b) the worm-holes needed would be of such a size that they would instantly self-destruct due to 'radiation-feedback' (even if they could be created in the first-place which he felt was debateable). He was more optimistic about forward in time time travel on the basis that it could be effected by utilising the property of time to run at different speeds in different gravitational fields and at different speeds of motion. The technical problems were, he felt, massive, but not necessarily insumountable and, to return to the initial question posed in this thread, he seemed to see no actual stumbling block in the Laws themselves that would stand in the way of the feat being achieved.

[I'm Murrin]

I'm inclined to think backward time travel is impossible, but I don't see how paradoxes would be a compelling argument against. I'd be of the opinion that continuity would be fixed - you would arrive in the past, then you'd leave in the future, rather than a past where you didn't arrive happening and then a new past being created when you travelled back. You'd be moving in spacetime, not rewriting it.

And while the grandfather paradox is problematic (DNA with no ancestor behind it) I don't see it as fundamentally violating causality, because the substances making up that genetic material aren't appearing from nowhere: each member of that short, recursive family line grows their own copy from new materials - the sequence is out of order, but there's not necessarily something appearing from nothing.

But still, I doubt it's possible.

Edit: I was going to say something about time travel allowing perpetual motion through the idea of the grandfather paradox applied to a self-sustaining chemical interaction, but then I realised the same principle debunks that: it'd still lose energy and would need energy put in to replace it. And you'd only actually get one yield, because while the cycle may seem infinite, from the perspective of the timeline it occurs between the arrival and departure points only once. Again, a violation of sequentiality but not thermodynamics.

[Vraith]

Yea, my impression has always been that travel forward in time was less problematic than backwards.
There is actually on fairly easy [heh...easy!] way, I think...travel at, or very near, the speed of light. [if you flew round trip to nearest star and back at 99.9 percent light speed [8 light years] it would take just over 8 years Earth time, but roughly 120 days Ship time...
I know those numbers because....

HERE IS A NIFTY RELATIVITY CALCULATOR ON LINE [the dilation factor is 24 at 99.91% light speed]:

www.1728.org/reltivty.htm?b0=99.99

A machine like HG's machine is more a problem...cuz it not only has to move through time, it has to move through space [or somehow be bound to the same space as the "surface" it is "launched" from, so it is dragged along]...or you pop up in the future...but who knows where you will be? You could pop up in the middle of nothing, [most likely] or an asteroid or star that's passing by. You're "base" will have moved, though.
[If you leapt instantly only 1 second into the future, you would be about 20 miles behind and a quarter mile East of your starting point...and that is only counting EARTH's motion. Since the whole System is moving, and moving much faster, you'd be much farther away than that...closer to 150 miles...though still only 1/4 mile East, if I did it right.]

[Fist and Faith]

Haven't read much of this thread yet, so don't know if it's been mentioned. Brian Greene said the laws of physics absolutely do not require time to move in. the direction we're used to, and he's not sure why out doesn't go the other way at least as often as not. I assume there's principles we simply don't know about yet. After all, if we CAN go backwards in time, we should have visitors from the future. Either there aren't any, or they have a way of covering their existence that's on par with God.

[Vraith]

. Brian Greene said the laws of physics absolutely do not require time to move in.

The rough conclusion I've drawn from the stuff he's done [the TV stuff is really cool, Fabric of the Cosmos ... Great.] is that either:
1) Time is really nothing more than a by-product of the "real" laws...it is an effect, not a cause.
2) We really have no damn idea what Time IS. [though much the same can be said of gravity...and the two are connected/interactive].
3) Some combination of 1 and 2.

[Hashi Lebwohl]

Time travel, especially backward in time, is impossible for macro objects because they cannot express themselves in those smaller dimensions through which quantum objects may travel. The doorways are too small for us to enter. Time travel going forward, as far as moving through time at different rates, is definitely possible is we can go fast enough; Vraith pointed to a dilation calculator available online.

I adhere to the "thermodynamic arrow" definition for time. An object falls from a shelf to the floor; without outside influence it will never jump back up to the shelf again, water will never "un-erode" rocks, etc. Even if we managed to go backwards in time and did something that "disrupted" our knowledge of events that wouldn't mean anything--the new disrupted timeline is now your native timeline while the one you recall no longer exists. Depending upon what you do many things may not change but you might run a risk betting on sporting events--those games will most likely turn out differently than you remember them.

[Fist and Faith]

Even if we managed to go backwards in time and did something that "disrupted" our knowledge of events that wouldn't mean anything--the new disrupted timeline is now your native timeline while the one you recall no longer exists. Depending upon what you do many things may not change but you might run a risk betting on sporting events--those games will most likely turn out differently than you remember them.

It seems that you're stating facts. But all this is entirely speculation. We don't have any idea what is "most likely", or how we would perceive any changes to our "native timeline". Or if backward time travel is possible under any circumstances.

[I'm Murrin]

As I said above, I reject the whole idea that travelling in time would create some new timeline that supplanted the old one. Time travel, to be possible, would be within and consistent with the rules of our universe, and I think that means causality stays in place, it's just that the "cause" is able to move non-linearly in time, but any "effect" we perceive as in the past, is just that - in the past, already happened. There would be no alternate timeline to change.

[Hashi Lebwohl]

It seems that you're stating facts. But all this is entirely speculation.

Are you sure that I am speculating?

Yes, I am speculating. That being said, I suspect my speculations are true and they may be disproven only when someone manages to travel through time.

Please prove me wrong and then show me how to travel through time so I may experience this for myself.