Time travel physics explaination needed...

[sphenisc]

That's kind of right, but not exactly. What physics really tells us is that time is a dimension, much like the three of space. Our lives can be thought of as lines, with a thickness of a meter or so in the space direction and a length of 70 years or so in the time direction. And while there's no fundamental law of physics that distinguishes future from past, we can still attach an arrow to the line (pointing from birth to death, say).

In that view there is really no passage of or traveling through time - different positions along the line correspond to the different times, but that's all. Your comment about time dilation and speed simply means that two such lines can be at an angle (that's relative motion), and then it's obvious then from the geometry that one ages slowly relative to the other.

"Time travel" would be a loop in the line, where it doubles back on itself. As I said above, such things are believed to be impossible, although they do exist in certain solutions to Einstein's equations.

I suppose that raises the question of why all our arrows point in the same direction? And why does there seem to be privileged position on the arrow we call now?

 

[RobDegraves]

The idea of time travel is a popular one, though widely misunderstood.

1. No one actually knows whether it's possible or not. Arguments are common even amongst accomplished physicists as to the nature of time and the answers are not definite at this time. To state that time travel is possible or not is simply impossible.

2. The biggest argument against time travel is based on causality, ie, that cause precedes effect. In our normal universe this is axiomatic. However, there have been some interesting suggestions that naked singularities are anathema to causality and some solutions have been proposed (see Roger Penrose's weak cosmic censorship hypothesis). The many worlds hypothesis also serves to solve the causality based arguments against time travel.

If causality is not an obstacle (a big if, granted), then time travel does not have any logical obstacles... only technical ones.

3. Most ideas for time travel require the use of materials that are unknown to exist at this time. A good example would be negative energy or negative mass. Though such have never been shown to exist, they are not theoretically impossible.


I have what I think is an interesting thought experiment for time travel.

Let's say I have a simple circuit consisting of a battery, a button and a lamp connected in series. The only difference is that the electricity I am using has the unique property of being able to go back in time 5 minutes. From the observer's point of view, the lamp should light up 5 minutes before I press the button. So... the lamp lights up, five minutes pass but I decide not to press the button.

What happened...

Causality is broken. We have an effect without cause. Is this possible.

The answer to that would be the solution as to whether or not time travel is possible.

 

[sphenisc]

...I have what I think is an interesting thought experiment for time travel.

Let's say I have a simple circuit consisting of a battery, a button and a lamp connected in series. The only difference is that the electricity I am using has the unique property of being able to go back in time 5 minutes. From the observer's point of view, the lamp should light up 5 minutes before I press the button. So... the lamp lights up, five minutes pass but I decide not to press the button.

What happened...

Causality is broken. We have an effect without cause. Is this possible.

The answer to that would be the solution as to whether or not time travel is possible.

I refer you to:

Asimov, A. 1948 The Endochronic Properties of Resublimated Thiotimoline

 

[sol invictus]

On the flip-side, if you could cause a copy of some object to exist as anti-matter, an anti-copy as it were, then according to some ideas in Quantum Field Theory, the anti-copy would go backwards in time.

That's not true. Physicists deal with anti-matter all the time, and anti-particles do not go "back in time". You're probably confused by a technical fact about quantum field theory - which is that given any solution to the equations of physics, the time reverse and charge conjugate (which exchanges matter with anti-matter) and parity reverse is always another solution.

I suppose that raises the question of why all our arrows point in the same direction? And why does there seem to be privileged position on the arrow we call now?

The best answer to that question is that the arrow of time is defined by the direction of increase of entropy. That translates the question into why the entropy was low in the past (i.e. why it was lower in some direction, which is then automatically the past) - to which there is no good answer.

2. The biggest argument against time travel is based on causality, ie, that cause precedes effect. In our normal universe this is axiomatic. However, there have been some interesting suggestions that naked singularities are anathema to causality and some solutions have been proposed (see Roger Penrose's weak cosmic censorship hypothesis). The many worlds hypothesis also serves to solve the causality based arguments against time travel.

No - MW doesn't help with that at all. Quite the contrary, actually.

 

[Vorpal]

Closed time-like loops using Goedel's solution, or use a wormhole. Problem with Goedel's solution is no material can withstand the rotational stresses needed to spin a cylinder fast enough (maybe other problems) to allow for time-travel, and wormholes require exotic states of matter that probably do not exist (negative pressure to the rescue).

What cylinder? Are you thinking of Tipler instead?

Let's say I have a simple circuit consisting of a battery, a button and a lamp connected in series. The only difference is that the electricity I am using has the unique property of being able to go back in time 5 minutes. From the observer's point of view, the lamp should light up 5 minutes before I press the button. So... the lamp lights up, five minutes pass but I decide not to press the button.

What happened...

Causality is broken. We have an effect without cause. Is this possible.

I'm not sure I buy that. For an atom with an electron in an excited state, what's the cause of spontaneous emission? One can probably interpret it as emission stimulated by vacuum fluctuations, but the cause of that is still fundamentally probabilistic. So once we're talking about particular outcomes (stated decayed or not, lamp turns on or not), we can have effects without causes.

On the other hand, if spacetime stays flat and one thinks of causality as effects staying within the light cone of a cause, then causality is violated even if I press the button, so my choice is irrelevant in that regard.

Yet one more possibility: if the button is supposed to trigger the formation of a CTC from this hypothetical electricity, then I suspect the resolution is simply that the whatever is the hypo-electricity is made of is the sort of stuff that has a nonzero probability of forming wormholes even if the button stays off.

 

[kittynh]

wow I know I learned a lot. And hopefully my UFO buddy will also learn a lot (one heck of a long follow up answer....). Point is, how cool are all the theories and the science...and the science fiction!!!

Way more interesting than boring old UFO stuff.....