Thinking past is history and now is present and future is not coming yet, this framework obviously restricted the idea of time travel.

Since this is a topic on Time Travel - The Past, Present, and Future, would like to quote the following:

"The distinction between past, present and future is only an illusion, however persistent" - Albert Einstein

His view on time is not time's arrow or the river of time, where time is not flowing through us and every moment we experience now should not be labeled present.

It is merely an event happened in the time medium itself, just like the event happened in the space (x,y,z).

As such, just like we could "go" north, south, east, west (-x, +x), if condition is right, we could "go" in time medium (-t, +t) as time is relative, just like space is relative depending on the observer state.

One needs to put down the past, present, future model and replace it with the elasticity of time to appreciate the idea of time travel.

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Added on October 13, 2009, 2:26 pm
You are referring to casual loop, where past is causally linked to the present. Changing the past is paradoxical, but affecting the past is logically possible, which means that there is no breaking in logic if one event caused another event to occur.

Example, A rich person now rich due to inherited a wealth from a "mystery person" who befriend with his great grandmother 100 years ago. He financed a time machine project now to go back to discover the source of his wealth. At the same time, he bring today news paper to his young great grandmother. With the news paper information on finance, his great grandmother made a fortune out of it, hence this is the source of the rich person wealth.

The rich person was infact the "mystery person" his great grandmother befriend with. Of course later he go back to the future where he stays.

No paradox issue here as the causal loop is self-consistent.

Another example of killing his own ancestor is a paradox as it breaks the cause and effect loop, hence is non-consistent.


This post has been edited by nice.rider: Oct 13 2009, 02:26 PM

Never says never......When you said time travel is not possible, I believe you referred to going back 20 years before and altered the event at that time which impacts the future outcome.

In classical mechanic era, almost everyone by default shared the same view.
Absolute view:
Drama - Nature
Cast - all the atoms, matters
Stage - Space and time

In this classical view, the physicists were tasked to work out the "plot" of this cosmic drama within the absolute entity of space and time or to analysis the cast who plays on the stage.
The centric or core of this "stage" idea is that there is an unidirectional of the arrow of time that the casts are playing throughout the entire cosmic drama.

It took Einstein to challenge this very conceptual foundation of the absolute entity of space and time.
Relativity view:
Drama - Nature
Cast - all the atoms, matters, space and time

The stage is not like what is defined earlier in classical view, there is "no stage", and space and time are part of the cast. What does this means?

Space and time play a full and active role in the drama. They are not simply there as an unchanging, absolute entity of nature, ie the "stage"; they are physical things, changeable, and subject to physical law.

A physical thing (space and time in this case) which is subjected to physical law, means they are subjected to alteration and modification pending on the external factors that influence them.
Can we go to +t or -t if we were in the right condition? Think again...............

P/S: In this cosmic drama, "by default", one will ask if "an intelligent director" exists who decide how the drama start, ie "the begining" and how he influences the "play" and how he would like the drama end. This became an determinism and free will arguments, and has been pretty much covered under different topics altogether.

Quite a number of interesting discussion here. But I didn't see Second Law Of Thermodynamic being mentioned anywhere here.

At Time1, we dropped an egg. At Time2, the egg hit the floor and broke to pieces. Assume we had invented a time machine, and we are now at Time2, can we "go" back to Time1 where the egg was intact?

Second Law tells us that this is impossible as entropy (order to disorder, wasted heat) in an enclosed system increases and matching towards maximum entropy.

Says the entropy at Time1 is W, the entropy at Time2 is W+n=X, in order to go back to Time1, we build a time machine, this project takes resources and manpower, and the process of building the machine produces more entropy, all digging, manufacturing, the sweat introduces entropy. At Time3, the machine is created and entropy now is X+m=Y. At Time3, if we travel through time to Time1, the entropy of the total system (at the so called "Time1") would be Y+l=Z.

The moral of the story is Z doesn't equal to W at Time1, as the so called new Time1 is a state with more entropy then Time2, even Time3. That leaves us little room to discuss about travel through past isn't it.

If time travel ever possibe, this scenario inevitably forces us to branch out to a new Time1. Although we back to the timeline of Time1 earlier, we are indeed branched out to a new universe, same time line but different universe, hence the idea of multiverses.

Angel, H. G. Wells's Time machine is a science novel. The idea of creating a time machine can not changes the outcome of the past is not entirely accurate. When the scientist travel back through time, the "new's old time" should be a new state, where he should be able to safe his wife in theory. However, he produced the book in the year of 1895, he is no simpleton and definitely ahead of his time.

Back to the topic, time travel inevitably lead us to multiverse. More and more physicists are leaning towards this and bubble universes, pocket universes are postulated which incorporate the idea of quantum physics and relativity.

One major drawback of this multiverse theory is it is not "testable". Our consciousness is always stick to one instant of the multiverse (ie the universe), eventhough we continue to make different decision of condition branching continously. To us, we always perceive it as an "uni"verse as our consciousness flows though a single straight line of continuum out of the infinite continous branching.

We always tends to perceive that we are the only one who make the choice out of the "uni"verse, which is actually an illusion. We could propose to a partner to marry us, and there are two outcomes, he/she accepted and we get married and have kids OR he/she refused and let assume we remain single the rest of our life.

These two outcomes are quite the opposite and the main point here is "we are not the only one who make the choice for our own destiny, decision branching of others impact our life too". Now, considering we interact with N number of people and everyday everyone make a choice which impact themselves and also impacting us, how could it be possible that there is an only a single state of "uni"verse that we live considering there are NxM conditional branching out there? What we know is there is only a single "uni"verse instant that we could observe, it doesn't mean it is the only instant out there.

Even time travel to the past is happening now, it is unlikely that we even "know" that happened.

Going back from a state with higher entropy to a state with lesser entropy violates the 2nd law. No matters what you do, what machine you built, all your efforts introduce more entropy to the system.

A simple analogy is you find a plain surface and dig a hole, there is no way you could make it back to the original plain surface again, all your efforts ended up producing more disorder to the system.

Good luck with your time travel journey. Hope you do it for good.

This post has been edited by nice.rider: Nov 1 2010, 09:35 AM