Well, it likely depends on what time machine you have in mind.

You could certainly argue that rudimentary time machines already exist - that's in a sense why delayed choice experiments work, for example. Also, the "Transactional Interpretation of Quantum Mechanics", for one, embodies a rudimentary time machine.

Now if by time machine you are thinking of human bodies travelling backwards or forward in Time and then reverting to where they come from, current calculations show that attempting to pass the slightest amount of mass within a wormhole - towards the future or the past - would lead to immediate collapse of the wormhole walls and hence this wouldn't work.

That does not necessarily obviate further developments, but at our current state of the art, building a time machine in the HG Wells sense seems rather unlikely. At the very least, to make it work you'd most likely have to not use wormholes. Alternatively your time machine could be made to work only in higher D universes under certain scenarios - such as a block universe - but definitely wouldn't work with run-of-the-mill 3D, flesh & bones mortals .....

Besides the various paradoxes, I think that a serious obstacle to time travels, at least backwards (i.e. to a time when the time machine was not invented yet...), is the fact that an incredibly high spatial precision would be needed to get to the space-time target point.

Indeed, imagine you want to go to yesterday in time in your office in space. The problem is that your office wasn't in the same position where it is today, because the Earth moves around the Sun, the Sun around the Galactic center, the Galaxy towards the Andromeda galaxy , etc... Although speaking about the "absolute spatial position" of your office makes no sense, you should certainly know what direction, and how far, to go to get to the past position of your office from the present one. But it seems to me hopeless to know or determine such a relative position (given the above-mentioned motions, your office's displacement could have been some 10^7 km per day, and just a very small uncertainty could bring you to a completely wrong place...). The only solution I see to this paradox is that the time machine should already exist at the time and the place we want to go and it should be capable, someway, to send some sort of signal, acting like a "radio beacon" to guide the travel to the space-time target point.

It is possible to construct a non-deterministic (or uncontrollable) time machine that can send a qubit of light backwards in time (quant-ph/0506141). Indeed one has been made based on the quantum scissors device. The machine can be used to study time-travel paradoxes in the laboratory.

There are more basic issues. When speaking of time travel one assumes that we live in a 4 (at least) dimensional universe, and travelling through time means going from one coordinated point to another. Relativity says that it cannot be done backwards, but after all it can be wrong. The other fundamentl issue is what one could find at this past location ? All science fiction stories about time travel assume that the "past" as we have known is still there. So it implies that we (and evrything) do exist at any time : we leave behind us copies of ourselves, frozen at its location. This is a theory which has been advocated by some scientists and philosophers but then the issue is What is changing when the time goes, what makes that we feel that the present is different from the past ?

So beyond the most simplist views of time travel there are still deep interrogations about the meaning of time.

To construct a ``time machine'' it is necessary that the spacetime admit ``closed time-like curves'', that's what ``time machine'' *means*. A nice presentation is given in this paper,http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf

by Kip Thorne. (Relativity does *not* forbid time travel. Anti-de Sitter spacetime has closed time-like curves for example. The absence of a maximal velocity, *allows* for time travel.)

Causality violation?

No, there isn't any causality violation, iff the space-time admits closed time-like curves.

That's what the term *means*.

Causality has been defined by Aristotle. The issues is "is there an effect without a cause (or the reverse)". So, as I said above, if when you go in the "past" you find nothing, there is no risk to meet your grand father and the most obvious paradoxes do not hold. However one can argue that the fields that are transported with you (such as the gravitational field) will propagate, so to be consistent one shall assume that these fields follow the same path as you, and you must be on a closed curve. The issue would therefore be to exit this closed curve...

A time travel like in the science-fiction?. i.e., like at the 12 monkeys movie?. It is impossible. If in any time in the future the time travel will be possible, then, the history and the present should be fully "infected" by "intruders" or "sendings" from the future.

Then, a lot of paradoxes will occur, at first, the principles of conservation will be ruled out, also the principle of casuality.

The whole physic, all the stablished laws and principles, and verified by means of the experimental practice prohibits the time travel, also the existence of spacial dimensions extra.

Our full knowing is done for a Minkowski flat space with three spacial dimentions and one temporal one-directional.

Everything else are unverified conjectures.

Philosophical discussions should be tempered by physics. The physics of time travel is described by closed time-like curves, cf. Thorne's article. While ``philosophically'' there may be different opinions, it's curious that the *physics*, as presented in the paper-or elsewhere-isn't addressed.

@Jean claude:

you mentioned closed space-time curves and the actual meaning of what we *feel* as the flow of time. It's intriguing to me the fact that, classically speaking, if every particle in the body (brain included) of the time traveler describes an exactly close space-time path, then that traveler won't presumably recognize to have done the time travel itself, because the whole "state" of his/her brain would have just done a space-time "loop", coming back to the "initial" state...

Of course, things would be different in the Quantum Mechanics context (in which particles "trajectories" have no real sense).

In the 1940s Godel found solutions to the equations of General Relativity that described universes that allowed closed time-like curves that would permit time travel to the past. This implied hat the past must still exist. This led him to conclude, and later to prove, that past, present and future all exist and that the flow of time is an illusion (see thread "is the flow of time an illusion?) In fact the inability of any two observers to agree on the interval between two events, which is an inescapable conclusion of the ToR, already implies a block time rather than a flow.. In this case we live all our lives all the time but every instant feels like "now". Time travel is impossible. Travel takes time and there would in any case have to be another time dimension through which we would travel in order to move through the first dimension (just as travel in space takes time)........

To Paolo,

Your remark is good. But travelling on a close curve does not necessarily imply a reversible process (where you change the sign of t). And QM does not matter, because I do believe that trajectories have a meaning, just that the traditional interpretation of QM is neither necessary nor useful.But this is another story.

But the key point stays that time travel would not be very exciting if in going in a region of the past one finds nothing. The issue is then if we exist at any time, and if is so what makes that we feel each instant different.

Sorry, but the "theoretical discovery" of Godel and of Thorne's articles are pure speculations that starts from several confusions at the interpretation on GR. That is, the space curvature as a form of modelling the gravity, is just a model, a representation of the action of gravity on the relative movements of the bodies and its reciprocal interactions. This model have a range of conditions of validity, as a newtonian model. But, we don't must to confuse the model with the reality. Of course, Thorne's conjectures about wormholes or similar things are "unphysical". Today, is verified that the whole space, is flat.

Besides, the time is not an ilusion, like is not ilusion that we grow old with the flow of time.

Let us suppose, one can go to past; but at which space point? The position of space point is relative and not absolute. If some one goes into past (14 Mar 1879) to see birth of Einstein in Ulm (Germany). Now the question is: Where is that place in space, because relativity does not allow that point to be located uniquely. It seems relativity does not allow time-travel.

No time travel is physically real because:

1- Flow of time.

2- Time arrow is unidirectional.

3- Spacetime: changing the time direction will affect the space too.

4- No time traveller from the future or from the past was reported.

5- At least, our universe is based on causality principle and no violation was reported since the big bang.

6- Any conjecture is possible in other universes or multiple dimensions, not in our universe.

However, time is relative and time dilation is a physical fact, along its unidirectional arrow.

All our experience is an illusion We do not experience the electrical signals generated by our senses but only the images and impressions created by our brains to make them comprehensible. If we could see at small enough wavelengths we would see nothing because atoms are transparent.

We define 'now' in terms of our memories, hopes and expectations but it would be hopelessly confusing to experience them all at once. Our brains therefore give us our lives as a panorama which we interpret as a flow of time. Of course our belief in a flow of time is so ingrained that we take it for granted but assertions that we grow old with the passage of time does not constitute a proof since it is no different from an assertion that we are different at different times, irrespective of whether time flows or not.

But, this is a philosophic position, that starts at a notion of reality, as a creation of our brains, or a seemed concept, of we can't know the reality. This is, "I think, after, I exist". This philosophic conception was born with Platón and, in fact, negate the scientific method as a tool for the knowing of nature. This means that for each of us, all rest of the people are just an "illusion". i.e. Parviz Parvin is just an illusion for every except for himself!. This conception is called "idealism"

On the other hand, it is demostrated that the brain works as a recorder that registers and processes the reality. A reality that exist at a independent mode of us, as its laws. Our scientifics theories and concepts are reflexes, aproximations, incorrect or correct of the reality, this can be verified by mean of experience.

This conception is called "materialism" or "realism".

You can choose one point of view.

The purpose of science is to provide an explanation of the physical world, clear, that can be largely understood and shared, and which is efficient to act on this world.

It does not pretend to tell what the world is really. But of course it assumes that this world exist, and does not depend of our minds. So philosophical skepticism is excluded.

We have theories of time and space, notably in GR, which are powerful enough, so far. They can be revised, but only if we find concepts and theories which are more efficient. It does not seem that they are sufficient motive to give them up. The job of scientists is to solve mysteries, not to invent new ones, even if this is fashionable.

I am a scientist, not a philosopher. The absence of a flow of time is not a new mystery but a logical and inescapable consequence of the theories of relativity. So long as these are not shown to be deficient in respect of what they tell us about the nature of time, I must accept their consequences, including the impossibility of time travel.

Quantum theory warns us to be careful of our perceptions of reality, not that there is no reality. A rose is as real to me as my family or a paving block and I do not lose my appreciation of its colour, perfume and texture in knowing that these are sensations created by my own brain.

We do not need to know anything about quantum theory to observe the effects of the accumulation of matter in stars and galaxies but it helps us as we gradually develop our understanding of our Universe, which is the object of scientific research. Science progresses by observation, hypothesis, testing, and rejection or revision until we have a theory that accounts accurately for one or more classes of observation. Great advances come when we find a way to enlarge the scope of a theory by merging or by assimilation because this leads to new insights. The greatest advances come from resolving conflict, inconsistency or incompatibility between theories or observations.

I think that "the Crisis in Physics" (Sciam May 2014) arising from the failure of the LHC to detect any of the 250 GeV mass particles foreseen by supersymmetry may well be the moment to see what happens to M theory in a universe in which time does not flow.

The uncertainty in atomic transitions:

Delta t * Delat E ~ h/2*pi

level liftime ~ 1/ enegy wtdth of transition

It means that the energy is inversely correlated to time in quantum trasitions of atoms and molecules. It is possible to excite the transition frequently and deactivate it based on spontaneous emission while level lifetime remains an invariant parameter. Since delta E = h *delta f, then lietime is inversely propotional to frequency width of emitted photon. Energy and frequency are physical facts, therfore the lifetime is not an illusion. The atomic clock is designed based on this principle.

Parviz - How would you distinguish between a universe in which time flows and one in which the particles have identical quantum states at the same time intervals but where time does not flow and the future already exits?

Stephen

I do not know the physical laws in another universe but I may know some fundamental rules in our universe where there is no absolute quantity and all parameters are relative. Atomic time scale is innate and we do not have any access to change it because the lifetime of a certain atomic state always invariant and repeatable, therefore it is a fact. I mean time duration is a fact which arises from enegy width. On the other hand, time and energy are correlated together. We can measure the energy flow, consequently the time flow may exist.

Parviz - The fact that two quantities have different values at different points in time does not imply or prove the existence of a flow of time. The most we can say is that it is compatible with a flow of time.

Time is strictly periodic in nature. You can only compare one period of time with another. Linear time flowing in one direction is a delusion of human mind. In this sense time machine is an absurd concept.

To “construct a time machine” is necessary to understand – what is “time”? (And, of course – what is space, since any material object, including, hope, a “time machine”, operates in the 4D spacetime.) Mainstream physics doesn’t answer on this question, in spite of some declarations that the “fundamental properties of space and time are explained in the relativity theory”. Including the assertion that real spacetime in Matter is Minkowski pseudoeuclidian continuum, where space and time depend, in certain sense, from each other. All that is evidently incorrect – nobody observed till now, e.g., imaginary space or time, as well as didn’t observe imaginary distance between a couple 4D spacetime points, etc.

In reality the Space and the Time are some independent logical Rules that control (govern by) the informational structures: Space establishes that different fixed informational patterns must be separated by something what in Matter a human calls “space (spatial) interval”; Time Space establishes that different states of changing informational patterns must be separated by something what in Matter a human calls “time (temporal) interval”. Both Rules are universal in the Set “Information”, when all/ everything what exist, including material objects, is/are some informational structures; Matter is some subset of the Set.

So for any changing in some structure “time” – or more correct “time interval” – is always additive (positive by convention); there principally doesn’t exist of any logical step that make the “changing of the time” negative. “Time machine” cannot exist principally.

Besides, Matter in our Universe consists of uninterruptedly operating automata; when their algorithms are, as a rule, reversible. That is, in spite of automata states are uninterruptedly changing “in same time direction”, the results of running, for example two of algorithms with relatively reverse command orders, are reverse, it seems as one automate “runs in positive time direction”, when the other one runs “in negative time direction”. Such organization of the informational patterns (first of all – elementary particles) is critically necessary for existence of our Matter. So in Matter besides “true time (above) Rule” acts other “time Rule” that allows (controls, govern by) reversible processes.

Next – it is clear from above that the Rules themselves are universal and act in whole Set, they don’t determine totally how they should be realized in concrete cases. In our Matter, as it seems, the Rules act as rigid 4D (3D “space” and 1D “time”; but this “time” isn’t true time, that is “coordinate time” ) dense lattice of “material” (Rules, since they are rules, aren’t, of course ,“material”) fundamental logical elements (aether), when any elementary particle and any system of particles is/ are some disturbances of the aether. At that, for example, antiparticles move

in negative “time direction”, when particles – in the positive one, by convention. But the movement in the negative temporal dimension 4D Matter’s spacetime by no means is a “travel into the past”, for any step in the spacetime is necessary to spend [positive] true time interval. So, for example, if somebody meets some “scientific” new that other somebody found a method – often such a methods appear as some sequence of general relativity (wormholes, etc.), then with a great probability that is a next fantasy…

More – see “Space and Time” http://arxiv.org/abs/1110.0003

Cheers

Manuel:

The answer is yes. I already have a device that is opening micro wormholes so that photons can pass through which means that information can be sent through both in the RF and IR range. Look up Verdrehung Fan. 

As for time travel in general, a cutting edge authoritative book that answers all of the questions that usually remain unresolved on the topic can be found here http://www.blurb.com/b/5622324-paradox-lost-the-public-edition.

 We must recognize the world dominated also "principle of equal a priori probabilities", This is demanded by  Uncertainty principle of Heisenberg. This means ,for example, behavior of collision of atoms is unpredictable. Normal explanation is too many. But I think not only too many but also tiny of particle and  uncertainty  of collision exist.In the field of  kinetic theory of gases、particles get omnidiectional dispersion. By this reason, normal distribution function is derived and solution of low dimension diffusion equation is similar to normal distribution function. From this reason, I think that we must be careful to adopt other function of deviation density model.

 In this background, we must calculate dependency of entropy. This is simple. With considering diffusion phenomenon of kinetics ink spreading from fixed point in perfectt stasis water, this model indicate that entropy becomes negative value in the case of going back past time. In thermodynamics, state of negative value of entropy can not exist. So time machine for past, we can not make, unfortunately.

We must recognize the world dominated also "principle of equal a priori probabilities", This is demanded by Uncertainty principle of Heisenberg.

it seems to be acceptable providing tools for doing that. Based on the quantum mechanics and its principles, it is possible because sub-Atom particles have created anything even our body, So, I think that it is feasible to travel in time in next decades for human. But we do not must forgot that the travel in time means to move our Creator particles based on Quantum Principles, and it is very exciting.

With Best

Amazing phenomena as quantum tunneling by electron movements around the Atom kernel and its key features as its presence in all directions in real-time, All of them are reasons for feasibility, because the presence of electron in all directions in our mind is a phenomena which should be investigated by the time dimension.

If science can solve this problem, that is the proofing some complex quantum actions, we will enable to provide Novel composed sciences for doing the Time travel and inventing some tools for its practicality.  At results , it seems that this problem is still located at first phases of progression, because we do not already have a Comprehensive quantum Computer can solve all quantum operations. IBM, Google and some Technology Companies have already developed quantum Computers by only doing a few operations, not all problems.

@Nicolis:

"No, there isn't any causality violation, iff the space-time admits closed time-like curves.

That's what the term *means*."

I doubt that. Traveling around a closed time-like curve once means that you will meet your former self. (You reach the same point in spacetime from which you started.) If you live for an infinite amount of time, you will meet your former self infinitely often.

That's why many physicists believe that solutions such as the Gödel universe would be forbidden by some as yet unknown mechanism. These solutions might be generically unstable.

My answer to the original question would be. Nobody knows whether "true" time machines are possible or not. The logical paradoxes to which their existence seems to lead must be avoided somehow. For example, if there was no free will or rather, free action, most of the logical paradoxes due to negative-feedback causal loops would disappear.

Others must be handled differenty.

For example, suppose you send the plans of the time machine to the past, your earlier self picks the sheets up, builds the time machine and, being your current self now, sends the plans back to the past to allow your earlier self to build the time machine. This would be a self-reinforcing causal loop, not a logical paradox. But the difficulties arise as soon as you ask yourself the question what is the age of the paper on which the plans were drafted.

first one must know what time really is!? what/how is the definition of time?