Any arguments for time travel back in time (the past)?

[ThomasT]

Something that hasn't been mentioned is the Everett Many Worlds Interpretation.

http://en.wikipedia.org/wiki/Many-worlds_interpretation

In this system (which has a fairly sound basis) you nulify many time travel paradoxes. We do know that if a closed timelike curve were to exist it's mathmatically sound that travel to the past is possible.

When incorporated with the above theory the past that you would travel to would be unique to the one you left. The mere act of appearing would instantly create a new reality in which any change you made would affect that reality only. You could kill your grandfather and in that world you would never be born. However because this reality is entirely different from the reality you originated in, it would have no causality reprocussions.

This would also explain Hawkins' quote away quite nicely. There are no tourists because the moment they traveled in time they'd create their own personal reality.

While this all seems to be quite the Sci-Fi channel special it has fairly sound physics and mathmatics. Check it out.

The MWI of QM and CTC's of GR don't have a sound physical basis. In fact, they have no physical basis. They're mathematical fictions, not physics. As such, they're part of why QM and GR shouldn't be taken as descriptions of physical reality.

The observational evidence suggests that we live in an expanding universe whose spatial configuration is transitory (ie., what we see via EM propagations no longer exists, in precisely the same state as we've perceived it, outside our perceptual machinery) -- and in such a universe, backward time travel is a physical impossibility.

 

[WaveJumper]

Theres the Butterfly effect and Grandfather paradox which makes logical time travel into the past difficult. Is there any theory or anything which suggests it may be possible?

Here is one very strong argument - Where are the flocks of time travellers from the future? They should be easily recogniseable by holding 10 000 megapixel cameras.

 

[a4mula]

The MWI of QM and CTC's of GR don't have a sound physical basis. In fact, they have no physical basis. They're mathematical fictions, not physics. As such, they're part of why QM and GR shouldn't be taken as descriptions of physical reality.

The observational evidence suggests that we live in an expanding universe whose spatial configuration is transitory (ie., what we see via EM propagations no longer exists, in precisely the same state as we've perceived it, outside our perceptual machinery) -- and in such a universe, backward time travel is a physical impossibility.

QM does an excellent job of describing the reality of submicro and macro systems. GR does an excellect job of describing the gravational fields of large objects. Are these any less descriptive of reality than classical physics, or any physics for that matter? Just the opposite! They provide a more accurate description.

There are plenty of mathematical fictions throughout history that have in turned proved to be very accurate descriptions of reality. As it stands today MWI does the best job of describing the universe without observers. It's one of a handful of interpretations that are taken seriously by mainstream physics. We know wavefunction collapse happens, that is reality. We do not know the mechanism behind it however. MWI provides this mechanism while violating no known laws of physics, eliminates the need of an observer, and solves multiple paradoxes along the way.

As far as closed timelike curves are concerned... I think it's quite premature to state that these are merely mathematical fictions. Are you going to claim that black holes are mathematical fictions as well? CTCs have long been predicted, not by just one or two theories, but by many. They show up in equation after equation and as of yet we've been unable to find anything that seems to dispute their existence.

Are either of these reality? Who knows, I don't but then again you don't either. All we can do is model our reality and then see where the predictions take us. While you seem to have some disdain for the mathematics involved in the process it's a necessity in Scientific Method. We do not live in the world where intuition and perception allows us to define reality. We know reality often times defies both.

 

[ThomasT]

QM does an excellent job of describing the reality of submicro and macro systems. GR does an excellect job of describing the gravational fields of large objects. Are these any less descriptive of reality than classical physics, or any physics for that matter? Just the opposite! They provide a more accurate description.

They provide more accurate predictions of instrumental behavior. But that doesn't necessarily qualify them as descriptions of reality. This is maybe clearer wrt QM than wrt GR.

There are plenty of mathematical fictions throughout history that have in turned proved to be very accurate descriptions of reality.

Mathematical fictions can be good quantitative (predictive) models -- while at the same time being not very good qualitative descriptions of reality.

As it stands today MWI does the best job of describing the universe without observers. It's one of a handful of interpretations that are taken seriously by mainstream physics. We know wavefunction collapse happens, that is reality. We do not know the mechanism behind it however. MWI provides this mechanism while violating no known laws of physics, eliminates the need of an observer, and solves multiple paradoxes along the way.

As a quantitative model, MWI does exactly the same thing as standard uninterpreted QM. The rest is just untestable, and conceptually questionable, fiction. MWI doesn't eliminate observers -- at least not insofar as it deals with filters and detectors and data. And, if no unwarranted assumptions about the physical meaning of wavefunctions (and their 'collapse'}, quantum superpositions, etc., are made, then there aren't any paradoxes in the first place.

As far as closed timelike curves are concerned... I think it's quite premature to state that these are merely mathematical fictions.

GR is on its way out. CTC's will likely be mothballed with it.

Are you going to claim that black holes are mathematical fictions as well?

There's observational evidence of their effects. Also, black holes, like white dwarfs, neutron stars, etc. don't require the GR block universe, spacetime geometric model. Black holes, etc., can exist in an evolving, transitory universe -- where the spatial configurations corresponding to the past really don't exist other than as subjective recollections or objective records of them. But, CTC's can't exist in a transitory universe (which I think is a more accurate description of the real one that we are part of).

CTCs have long been predicted, not by just one or two theories, but by many. They show up in equation after equation and as of yet we've been unable to find anything that seems to dispute their existence.

Or anything to confirm their existence. CTC's are an untestable prediction, afaik. So that leaves us with a mathematical construction that has no physical basis.

We also don't see advanced waves, or broken stuff spontaneously reassembling itself, etc. -- and I think a better approach than taking these things as physical possibilities simply because they're allowed or appear in one model or another is to assume that they're precluded by the physical dynamics which govern our universe.

Quantum experimental phenomena and QM (even though it shouldn't be taken as a description of reality because the extent to which the formalism corresponds to an underlying physical reality is unknown) provide indications that physical reality is fundamentally wavelike. In my view, current physics is a necessary simplification of a fantastically complex system of wave interactions in a fantastically vast hierarchy of particulate media (the particles being bound wave structures) arising from a few, and maybe just one, fundamental wave dynamic(s).

While you seem to have some disdain for the mathematics involved in the process it's a necessity in Scientific Method.

Disdain for the mathematics involved? What gives you that idea? I love mathematical models. But there's usually an important difference between mathematical models and 'descriptions of reality'.

The normal procedure is to develop mathematical models that are consistent with the current standard theoretical structures and also fit the data, and then worry about the physical conceptual (qualitatively descriptive) issues as they arise. This approach is somewhat efficient, but has led to a situation where there isn't a unified conceptual framework.

But, I think that eventually this will happen. And when it does, then I think that backward time travel will be theoretically, not just practically, prohibited.

 

[leonstavros]

Time travel to the past is possible but only as an observer. Spacetime is affected by matter therefore when matter moves through spacetime it leaves a "recording". This recording can be read by a spaceship that has traveled to Earth's wake trail. A laser inteferometer can scan the spacetime recording and detect any spacetime changes. The changes then can be translated to the matter that caused them.

 

[OB 50]

The simplest reason you can't travel back in time is that it would violate the second law of thermodynamics. You cannot create or destroy energy or mass.

Think about all of the atoms in your body. They each have a unique history. Some of those atoms were once part of a ham sandwich (you know they were).

Now, when you time travel, you have to reverse the history not just for yourself, but for all the matter in your body. So, you would either revert back to a pile of ham sandwiches and mountain dew, or you would step through a magical instant time portal and violate the second law by introducing duplicate atoms into the past universe, thus effectively creating matter.

Either way, it doesn't look so good for time travel.

 

[casp07]

I am not sure if this post is still active, i hope it is. anyway, OB 50 makes a great point about the law of conservation of mass. so far in the observable universe, there are NO contradictions to this law. that doesn't mean there won't be a first, but it does give us a pretty good idea that were in no position to make any such arguments. I have come across a theory of time travel, that while a little logically obtuse and hard to comprehend at first glance, is amazingly well adapted to the universe at we know it.
I should note that this theory turned me from a skeptic of time travel, like most everyone else, into a firm believer. Before i begin however, i should make a disclaimer involving the scientific content of this theory, seeing as how this is a physics forum: DISCLAIMER: THIS IS NOT A SCIENTIFIC THEORY, AND SHOULD NOT BE COMPARED TO THE MANY SCIENTIFIC THEORIES OF TIME TRAVEL THAT ARE BACKED BY PEOPLE WITH ACRONYMS IN THEIR NAMES.

I am a firm believer in time travel. Around the turn of the twentieth century there was a growing body of thought concerning meditation induced time travel, that like so many other things, was overshadowed by the magnanimous scientific leaps of the early 20th century, and all but forgotten.
A couple years ago, I became interested in the works of Jack London and the book "The Star Rover" in particular. while it is purely a fictional work, it is based on a supposedly true story about a man who is able to leave his body, and even travel back in time, and live another life in the past, only to be jerked back into the present, sometimes unexpectedly.
Now, if you've read this far, the key point is about to be made: this person is NOT aware of the time travel until s/he returns to the present, therefore cannot make any paradox inducing changes to his world. in other words, this is two people, experiencing one life. When this person then returns to the present, the tree of cause and effect splits, and this one person becomes two people.
The idea of multiple souls, if you will, with different fates and experiences inhabiting one body is more fantastical that the wildest science fiction, but for me, an idea made more believable by personal out of body experiences.
I continued to be interested in this book, and the factual elements of its backstory. I learned that a man named Ed Morrell, had written a book about his own experiences with time travel, while an inmate in San Quentin prison. This book is rather hard to track down, and is expensive to purchase. I managed to get a copy through the Harvard University library system, that had not been checked out in some 60 years. Anyhow, I highly recommend this book to anyone courageous enough to break free from the shackles of empiricism, and take a leap of faith into the unknown. I understand most people will not want to believe this theory, simply because it is beyond the realm of their imagination. Please let me know what you think of this theory, and respond.

 

[Boy@n]

Since there is no time (time is just an illusion in a human brains, arising due to existence changing) there is no time travel.

All there is, is NOW.

Past and future are just human concepts - past doesn't exist, future doesn't exist, thus you cannot travel to that which doesn't exist.

If you had enough power though (aka God), you could simulate time travel, meaning that you would have to change the NOW, this exact existence you are living/experiencing NOW, to the kind of existence you imagine to be the one of past or future.

In a way we all can do it, in our dreams. Try to master your dreams, so they are lucid and under your control (there are techniques for that), and you will achieve 'time travel' ability ;)

 

[leonstavros]

Since there is no time (time is just an illusion in a human brains, arising due to existence changing) there is no time travel.

All there is, is NOW.

Past and future are just human concepts - past doesn't exist, future doesn't exist, thus you cannot travel to that which doesn't exist.

If you had enough power though (aka God), you could simulate time travel, meaning that you would have to change the NOW, this exact existence you are living/experiencing NOW, to the kind of existence you imagine to be the one of past or future.

In a way we all can do it, in our dreams. Try to master your dreams, so they are lucid and under your control (there are techniques for that), and you will achieve 'time travel' ability ;)

If space-time is a fabric that is modulated by matter and this fabric is expanding then the past exists as a "recording". If you could travel back to the spot that Earth was a day ago and you're able to detect the "recording" by some instrument then you could "observe" the happenings of yesterday. By observing only, you would not create any time paradoxes.

 

[casp07]

Boy@n. Your argument seems a bit simplistic. Perhaps time travel is only an idea, but many things are just ideas before they become more widely accepted and acknowledged. One could argue that the entire universe is contained in our minds, and in terms of observable cause and effect, you would be right, for each of us experiences our own version of the universe. Time travel is not just a concept. you should keep an open mind when talking about stuff like this. The theory of relativity shows time dilation. Tachyons theoretically travel in time.
In the end, everything is just a concept. That does not mean it is not worth talking about. Also, God is not relevant to a discussion in a physics forum. You personal beliefs of an all powerful god are not shared by everyone.

On a less scientific note, perhaps you would enjoy the song "all we have is now" by the Flaming Lips. It's namesake is how Boy@n sees time, but in the song, the anonymous narrator meets a future version of hilmself. Perhaps there is room for both of these ideas in our shared multiverse.