# An upcoming experiment may prove the possibility of time traveling into the past



## Whitestar (Oct 2, 2006)

A friend of mine has sent me the following article about an upcoming experiment on retrocasualty, that is, the possibility of time traveling into the the past. The experiment will be performed by renown theoretical and quantum physicist, John Cramer. The following quote from my friend is in blue, while the following quote from Cramer's daughter, Kathryn Cramer is in orange:

*"I just come a cross an interesting article from New Scientist about variation on Quantum Eraser Experiment where decision in the present can effect the past of a collections of photons:"*

*"Ever wish you could reach back in time and change the past? Maybe you'd like to take back an unfortunate voicemail message, or rephrase what you just said to your boss. Or perhaps you've even dreamed of tweaking the outcome of yesterday's lottery to make yourself the winner.

Common sense tells us that influencing the past is impossible - what's done is done, right? Even if it were possible, think of the mind-bending paradoxes it would create. While tinkering with the past, you might change the circumstances by which your parents met, derailing the key event that led to your birth.

Such are the perils of retrocausality, the idea that the present can affect the past, and the future can affect the present. Strange as it sounds, retrocausality is perfectly permissible within the known laws of nature. It has been debated for decades, mostly in the realm of philosophy and quantum physics. Trouble is, nobody has done the experiment to show it happens in the real world, so the door remains wide open for a demonstration."*

*"It might even happen soon. Researchers are on the verge of experiments that will finally hold retrocausality's feet to the fire by attempting to send a signal to the past. What's more, they need not invoke black holes, wormholes, extra dimensions or other exotic implements of time travel. It should all be doable with the help of a state-of-the-art optics workbench and the bizarre yet familiar tricks of quantum particles. If retrocausality is confirmed - and that is a huge if - it would overturn our most cherished notions about the nature of cause and effect and how the universe works.

Dating back to Newton's laws of motion, the equations of physics are generally 'time symmetric' - they work as well for processes running backwards through time as forwards. The situation got really strange in the early 20th century when Einstein devised his theory of relativity, with its four-dimensional fabric of space-time. In this model, our sense that history is unfolding is an illusion: the past, present and future all exist seamlessly in an unchanging 'block' universe. 'If you have the block universe view, the future and the past are not any different, so there's no reason why you can't have causes from the future just as you have causes from the past,' says David Miller of the Centre for Time at the University of Sydney in Australia.

With the advent of quantum mechanics in the 1920s, the relative timing of particles and events became even less relevant. 'Real temporal order in general, for quantum mechanics, is not important,' says Caslav Brukner, a physicist at the University of Vienna, Austria. By the 1940s, researchers were exploring the possibility of time-reversed phenomena. Richard Feynman lent credibility to the idea by proposing that particles such as positrons, the antimatter equivalent of electrons, are simply normal particles travelling backwards in time. Feynman later expanded this idea with his mentor, John Wheeler of Princeton University. Together they worked out a theory of electrodynamics based on waves travelling forwards and backwards in time. Any proof of reverse causality, however, remained elusive.

Fast forward to 1978, when Wheeler proposed a variation on the classic double-slit experiment of quantum mechanics. Send photons through a barrier with two slits in it, and choose whether to detect the photons as waves or particles. If you put up a screen behind the slits, you will get a pattern of light and dark bands, as if each photon travels through both slits and interferes with itself, like a wave. If, on the other hand, you take a snapshot of the slits themselves, you will find each photon passes through one slit or the other: it is forced to pick a path, like a particle. But, Wheeler asked, 'What if you wait until just after the photon has passed the slits to make your choice?' In theory, you could suddenly raise the screen to expose two cameras behind it, one trained on each slit. It would seem that you can affect where the photon went, and whether it behaved like a wave or particle, after the fact.

In 1986, Carroll Alley at the University of Maryland, College Park, found a way to test this idea using a more practical set-up: an interferometer which lets a photon take either one path or two after passing through a beam splitter. Sure enough, the photon's path depended on a choice made after the photon had to 'make up its mind'. Other groups have confirmed similar results, and at first blush this appears to show the present affecting the past. Most physicists, however, take the view that you can't say which path the photon took before the measurement is made. In other words, still no unambiguous evidence for retrocausality.

That's where John Cramer comes in. In the mid-1980s, working at the University of Washington, Seattle, he proposed the 'transactional interpretation' of quantum mechanics, one of many attempts to relate the mathematics of quantum theory to the real world (New Scientist, 24 July 2004, p 30). It says particles interact by sending and receiving physical waves that travel forwards and backwards through time. This June, at a conference of the American Association for the Advancement of Science, Cramer proposed an experiment that can at last test for this sort of retrocausal influence. It combines the wave-particle effects of double slits with other mysterious quantum properties in an all-out effort to send signals to the past.

The experiment builds on work done in the late 1990s in Anton Zeilinger's lab, when he was at the University of Innsbruck, Austria. Researcher Birgit Dopfer found that photons that were 'entangled', or linked by their properties such as momentum, showed the same wave-or-particle behaviour as one another. Using a crystal, Dopfer converted one laser beam into two so that photons in one beam were entangled with those in the other, and each pair was matched up by a circuit known as a coincidence detector. One beam passed through a double slit to a photon detector, while the other passed through a lens to a movable detector which could sense a photon in two different positions.

The movable detector is key, because in one position it effectively images the slits and measures each photon as a particle, while in the other it captures only a wave-like interference pattern. Dopfer showed that measuring a photon as a wave or a particle forced its twin in the other beam to be measured in the same way.

To use this set-up to send a signal, it needs to work without a coincidence circuit. Inspired by Raymond Jensen at Notre Dame University in Indiana, Cramer then proposed passing each beam through a double slit, not only to give the experimenter the choice of measuring photons as waves or particles, but also to help track photon pairs. The double slits should filter out most unentangled photons and either block or let pass both members of an entangled pair, at least in theory. So a photon arriving at one detector should have its twin appear at the other. As before, the way you measure one should affect the other. Jensen suggested that such a set-up might let you send a signal from one detector to another instantaneously - a highly controversial claim, since it would seem to demonstrate faster-than-light travel.

If you can do that, says Cramer, why not push it to be better-than-instantaneous, and try to make the signal arrive before it was sent? His extra twist is to run the photons you choose how to measure through several kilometres of coiled-up fibre-optic cable, thereby delaying them by microseconds (see Diagram). This delay means that the other beam will arrive at its detector before you make your choice. However, since the rules of quantum mechanics are indifferent to the timing of measurements, the state of the other beam should correspond to how you choose to measure the delayed beam. The effect of your choice can be seen, in principle, before you have even made it.

That's the idea anyway. What will the experimenters actually see? Cramer says they could control the movable detector so that it alternates between measuring wave-like and particle-like behaviour over time. They could compare that to the pattern from the beam that wasn't delayed and was recorded on a sensor from a digital camera. If this consistently shifts between an interference pattern and a smooth single-particle pattern a few microseconds before the respective choice is made on the delayed photons, that would support the concept of retrocausality. If not, it would be back to the drawing board.

Cramer says the plan is to do the instantaneous signalling experiment first, to iron out technical glitches from noise or errors in photon tracking, which would wreck the retrocausality experiment. Only after performing that would they add in the delay cables. 'This experiment, if successful, would bring retrocausality into the macroscopic realm,' says Cramer.

Other experts are supportive of the idea but sceptical of what it might mean. 'It would be important to perform such an experiment just because of curiosity about interpretations,' says Brukner. 'If you accept the transactional interpretation, then this experiment would show a retrocausal influence.' Cramer agrees it is speculative, but says the experiment is our best shot at seeing retrocausality in action. Because of the implications he is cautious, but still positive. 'I don't see any show stopper yet,' he says.

If the experiment does show evidence for retrocausation, it would open the door to some troubling paradoxes. If you could see the effects of your choice before you make it, could you then make the opposite choice and subvert the laws of nature? Some researchers have suggested retrocausality can only occur in limited circumstances in which not enough information is available for you to contradict the results of an experiment.

Another way to resolve this is to say that even if the present can influence the past, it cannot change it. The fact that your hair is shorter today has as much influence on your going to the barber yesterday as the other way around, yet you can't change that decision. 'You wouldn't be able to talk about altering, but you could talk about causing or affecting,' says Phil Dowe, an expert on causation at the University of Queensland in Australia. While it would mean we cannot change the past, it also implies that we cannot change the future.

If all that gives you a headache, then consider this: if retrocausality does exist, it says something profound about how the universe works. 'It has the potential to solve what is one of the biggest problems in modern physics,' says Huw Price, head of Sydney's Centre for Time. It goes back to quantum entanglement and 'nonlocality' - one particle instantaneously affecting another, even from the other side of the galaxy. That doesn't sit well with relativity, which states that nothing can travel faster than light. Still, the latest experiments confirm that one particle can indeed instantaneously affect the other (New Scientist, 18 June 2005, p 32). Physicists argue that no information is transmitted this way: whether the spin of a particle is up or down, for instance, is random and can't be controlled, and thus relativity is not violated.

Retrocausality offers an alternative explanation. Measuring one entangled particle could send a wave backwards through time to the moment at which the pair was created. The signal would not need to move faster than light; it could simply retrace the first particle's path through space-time, arriving back at the spot where the two particles were emitted. There, the wave can interact with the second particle without violating relativity. 'Retrocausation is a nice, simple, classical explanation for all this,' Dowe says.

While the jury is out awaiting the results of Cramer's experiment, some researchers expect reverse causality will play an increasingly important role in our understanding of the universe. 'I'm going with my gut here,' says Avshalom Elitzur, a physicist and philosopher at Bar-Ilan University in Israel, 'but I believe that when we finally find the theory we're all looking for, a theory that unifies quantum mechanics and relativity, it will involve retrocausality.' If it also involves winning yesterday's lottery, Cramer won't be telling.

Why we are here
If retrocausality is real, it might even explain why life exists in the universe - exactly why the universe is so 'finely tuned' for human habitation. Some physicists search for deeper laws to explain this fine-tuning, while others say there are millions of universes, each with different laws, so one universe could quite easily have the right laws by chance and, of course, that's the one we're in.

Paul Davies, a theoretical physicist at the Australian Centre for Astrobiology at Macquarie University in Sydney, suggests another possibility: the universe might actually be able to fine-tune itself. If you assume the laws of physics do not reside outside the physical universe, but rather are part of it, they can only be as precise as can be calculated from the total information content of the universe. The universe's information content is limited by its size, so just after the big bang, while the universe was still infinitesimally small, there may have been wiggle room, or imprecision, in the laws of nature.

And room for retrocausality. If it exists, the presence of conscious observers later in history could exert an influence on those first moments, shaping the laws of physics to be favourable for life. This may seem circular: life exists to make the universe suitable for life. If causality works both forwards and backwards, however, consistency between the past and the future is all that matters. 'It offends our common-sense view of the world, but there's nothing to prevent causal influences from going both ways in time,' Davies says. 'If the conditions necessary for life are somehow written into the universe at the big bang, there must be some sort of two-way link.'"
*
picture:

http://www.newscientist.com/data/images/archive/2571/25710901.jpg 

More information, check out the following links:

http://www.kathryncramer.com/

http://realityshifters.com/pages/articles/retrocausalrs.html

Scary huh?

Whitestar


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## Dave (Oct 3, 2006)

I'm not sure I followed that correctly, so correct me if I'm wrong, but technically I don't think that experiment sent particles back in time. By loosing one set of particles in coils of fibre-optic cable you are simply slowing them down relative to the other set. The analogy would be to the confidence trick known as "The Wire", which you will know about if you ever saw the film "The Sting", or the BBC TV series "Hustle". In that scenario, announcements of race results are held back after the finish, so that bets can be placed on the winner before the race has appeared to have begun. But in the real world, the race finished a long time before.


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## Paige Turner (Oct 3, 2006)

I remember when that happened.


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## chrispenycate (Oct 6, 2006)

It doesn't appear to be particles that move into the past (or exist , not simultaneously exactly, but at all times), but information, which is massless and energy free, about the future state of a wavicle. Assuming that a wavicle does change state, rather than both the "wave" and "particle" explanations being inadequate descriptions of another, apparently inimaginable state that is "reality" for a photon. 
Even so, the experiment has no "useful" offshoots.(apart from the depressing certainty of predestination) Suppose we could lay our hands on ten light-minutes of fibre-optic cable. Now, you say, we can wait to see if a particular horse wins the grand national, and code our individual photons to inform us.(only a "yes/no" question for the time being; still, computers are good at that) Now, if there is anyway to measure the state of the photons coming out, then, by measuring it we collapse the state vector (by finding out the answer we define what that answer is - straight Heisenberg), so render the information useless. Thus we have demonstrated that all choices that can be made are being made, will be made and have been made, all in the same static four dimensional matrix, without even having the possibility to gain a few bob on the side.


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## Whitestar (Oct 13, 2006)

chrispenycate said:
			
		

> Even so, the experiment has no "useful" offshoots.(apart from the depressing certainty of predestination) Suppose we could lay our hands on ten light-minutes of fibre-optic cable. Now, you say, we can wait to see if a particular horse wins the grand national, and code our individual photons to inform us.(only a "yes/no" question for the time being; still, computers are good at that) Now, if there is anyway to measure the state of the photons coming out, then, by measuring it we collapse the state vector (by finding out the answer we define what that answer is - straight Heisenberg), so render the information useless. Thus we have demonstrated that all choices that can be made are being made, will be made and have been made, all in the same static four dimensional matrix, without even having the possibility to gain a few bob on the side.


 
Its not a useless experiment. First of all, it tells us about the nature of physical reality. From the beginning of quantum mechanics there has been no explanation of why particles behave in this manner, and while we can use the formula of quantum mechanics to predict a wide range of behavior, the "why" part has eluded physicists. 

If Cramer is correct, that is, particles do send signals into the past and can alter events due to action in the present, it would solve one of the greatest questions of quantum mechanics, the mechanism behind the collapse of the quantum wave function. 

This would be the starting point of a new era of physics. Present arguments against faster than light travel would be dispelled due to the fact that the so-called weak energy condition, (dealing with time travel and paradoxes due to altering the past) which is used as an major argument would be proven false. 

What benefits this can bring to mankind, I cannot predict, but no one was able to predict the changes of the world the quantums mechanics has brought to our world. The rise of the PC and internet is directly due to semiconductors, which was the result of solid state physics and is one of the many childfields of quantum mechanics.

As for military uses, well that is a sad side effect. No scientific discovery has ever been completely devoid of miltary application, remember those same chemicals were used to encourage the growth of crops are used in making explosives and nerve gas.

For those who think this experiment is useless on a practical standpoint, Iet me remind you that these are energies propelling us to a better objective, that is, none better than revealing the ultimate nature of reality!!!  


Whitestar


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## carrie221 (Oct 13, 2006)

It would be cool if it was true that they could do that


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## Whitestar (Oct 13, 2006)

carrie221 said:
			
		

> It would be cool if it was true that they could do that


 
Well, if you're referring to experimenting the technology on humans, you can forget about it. Just like with quantum teleportation, the process involves quantum entanglement and that requires reducing a human being to a state of coherence, which would kill you. Hence, there will be no human testing anytime soon, if ever.


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## carrie221 (Oct 13, 2006)

Whitestar said:
			
		

> Well, if you're referring to experimenting the technology on humans, you can forget about it. Just like with quantum teleportation, the process involves quantum entanglement and that requires reducting a human being to a state of coherence, which would kill you. Hence, there will be no human testing anytime soon, if ever.


 
I still want to travel back in time and get to see a lot of events and places like the Library of Alexiandria before it was destroyed, find out who built stonehedge, if Atlantis existed, Rome during the high point,...


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## Whitestar (Oct 13, 2006)

carrie221 said:
			
		

> I still want to travel back in time and get to see a lot of events and places like the Library of Alexiandria before it was destroyed, find out who built stonehedge, if Atlantis existed, Rome during the high point,...


 
Well, there might still be a way: a ship that travels faster than light (FTL). Time travel is a natural byproduct of FTL, some modes of FTL travel tend to avoid this, but any potential FTL stardrive can be turn into a time machine with a little work. But if you had such a technology, you have to consider the possible paradoxes that may occur if you time travel into the past. For instance, let's say you time travel back into the past to prevent your parents from meeting and succeed. Your "past" self would never have been born and your "present" self would cease to exist. Your atoms would transform into something else, but they would not literally disappear like in the Back To The Future movies or the Star Trek tv shows because the laws of energy conservation strictly forbids this, which also rules out the silly, yet fascinating notion of parallel universes/many-worlds interpretation, even though they've been proven to experimentally exist at the quantum level. And while they may exist at the microscopic world, they do not, however, subscribed to the macroscopic world in which we live in. 

I also have reason to theorized that if you time travel back into the past to prevent your parents from meeting, your "past" would never be born, but your "present" self will not cease to exist either. Your "present" self would continue unharmed and unaltered, but you would essentially be the woman with no past. However, until we have a theory on quantum gravity, the final resolution of time travel will remain unsolved and paradoxes will continue to haunt us. Once we do find the solution and come up with a theory of quantum gravity, we'll see that time is the same in the quantum world as it is for you and me. Its just that quantum particles have an easier time when it come to time travel, but to accomplish this enormous feat we would need to harness vasts amounts of energy, consisting of several black holes, or the power of a galaxy.

But here's the real interesting part about this experiment on retrocausalty. If you wanted to change history, there's no need to time travel into the past, that is, you can change history just by staying in the present!


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## carrie221 (Oct 13, 2006)

Whitestar said:
			
		

> Well, there might still be a way: a ship that travels faster than light (FTL). Time travel is a natural byproduct of FTL, some modes of FTL travel tend to avoid this, but any potential FTL stardrive can be turn into a time machine with a little work. But if you had such a technology, you have to consider the possible paradoxes that may occur if you time travel into the past. For instance, let's say you time travel back into the past to prevent your parents from meeting and succeed. Your "past" self would never have been born and your "present" self would cease to exist. Your atoms would transform into something else, but they would not literally disappear like in the Back To The Future movies or the Star Trek tv shows because the laws of energy conservation strictly forbids this, which also rules out the silly, yet fascinating notion of parallel universes/many-worlds interpretation, even though they've been proven to experimentally exist at the quantum level. And while they may exist at the microscopic world, they do not, however, subscribed to the macroscopic world in which we live in.
> 
> I also have reason to theorized that if you time travel back into the past to prevent your parents from meeting, your "past" would never be born, but your "present" self will not cease to exist either. Your "present" self would continue unharmed and unaltered, but you would essentially be the woman with no past. However, until we have a theory on quantum gravity, the final resolution of time travel will remain unsolved and paradoxes will continue to haunt us. Once we do find the solution and come up with a theory of quantum gravity, we'll see that time is the same in the quantum world as it is for you and me. Its just that quantum particles have an easier time when it come to time travel, but to accomplish this enormous feat we would need to harness vasts amounts of energy, consisting of several black holes, or the power of a galaxy.
> 
> But here's the real interesting part about this experiment on retrocausalty. If you wanted to change history, there's no need to time travel into the past, that is, you can change history just by staying in the present!


 
 Wow... I like the idea of even if I affected my past so much that I wouldn't have been born that I would still be there, without a past but still there


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## Whitestar (Oct 16, 2006)

carrie221 said:
			
		

> Wow... I like the idea of even if I affected my past so much that I wouldn't have been born that I would still be there, without a past but still there


 
I have been in contact with Dr. J. Richard Gott III, a physicist professor of astrophysics at Princeton University, and he is one of the top experts of time/time travel and in his book entitled, "Time travel in the Einstein's Universe", he states in laymen terms on how time travel into the past might be possible. In his view, if you time travel back into the past to prevent your parents from meeting, you will not succeed because they did meet and you were born. No matter how hard you try, you can't change the past. Now if a time traveler travels into the past, its because they were always a PART of it and always will be. This is exactly what happened in Babylon 5 with the character of Commander Jeffrey Sinclair when he traveled back 1,000 years to become Valen. If a time traveler is a part of that history, then he or she will behave the way they did according to history books. If we follow this mode of logic, then this means that if you were never part of that history, then you can never time travel back into the past before you were born. 

Here is an interview with Dr. J. Richard Gott III on time travel:

Did you know that we have already traveled in time? Do you know the longest time travel so far?

We usually enjoy fantasies about time travel in science fiction movies and books. We are not even aware that the distinction between reality and fantasy is often quite vague. 

Meet the physicist who makes it even more blurred. J.Richard Gott, III, a professor of astrophysics at Princeton University is one of the leading cosmologists of our time and a person whose mind wonders about problems like the creation of the Universe and organizing high school science fairs. We spoke with him about the possibilities of time travel, how the Universe is its own mother, the significance of science fiction genre, why would somebody believe his theories in the first place, and why all of us should know more about science. 


*About time travel*

*Q: Time travel has been described in many books and movies. We usually call them science fiction. While they deal with the fictional part, you are dealing with the science part. Therefore, here comes the basic question: is it possible to travel through time?*

Einstein showed in 1905 that the time travel to the future is possible within his theory of special relativity. According to the theory, clocks tick slowly and this has actually been observed. We observe that if we take atomic clocks on an airplane trip around the world toward the East, where the velocity of the plane adds to the rotation velocity of the Earth, these clocks come back about 59 nanoseconds slow. In fact, we observe that particles that are moving with speeds nearly the speed of light decay more slowly. So, if you wanted to visit the Earth in the year 3000, all you have to do is to get on a rocket ship going at 99.995% of the speed of light, go to a star 500 light years away, turn around and come back at that speed. When you get back, the Earth would be 1000 years older, but you would only aged 10 years. The biggest time traveler so far is an astronaut named Sergey Evdeyev, who was on the Mir Space Station for a total of 748 days on three space flights. When he returned to earth he had aged about a 1/50 of a second less that if he stayed home. That is, when he returned to earth he found the Earth a 1/50 of a second to the future than what he expected it to be.

Time travel to the past is more difficult but it is a theoretical possibility that seems to be allowed by Einstein's theory of general relativity. There, space and time are curved, and you can have solutions to those equations that are sufficiently twisted that allow you to circle back and visit an event in your own past. This is in the same way that Magellan's crew left from Europe, went steadily west, and circled the globe. There are solutions like this to Einstein's equations of general relativity: wormhole solutions, some that I found involve cosmic strings, and so forth. However, the question is whether these possibilities can be realized in our Universe or not. To understand that, we may need to understand the laws of quantum gravity. That is why gravity is particularly interesting project. We take solutions to Einstein's equations quite seriously because they have been tested. For example, light bending around the Sun and so forth. Therefore, the time travel to the past is a topic under current investigation. 

*About the Universe *
 
*Q: There are several universal questions that humankind has asked itself since the beginning of its time. One of them is about the beginning of everything, beginning of the Universe. You have an answer to this question, too.*

_*We believe today that the Universe started after a state of very rapid expansion called inflation, back at the Big Bang. The Universe inflated very rapidly during this phase. This has been confirmed by recent observations of the cosmic microwave background, as the theory of inflation is made definite predictions of what we should have seen and they have been confirmed quite nicely. Professor Linde at Stanford has shown that if you have an inflating Universe like this that the quantum fluctuations cause it to create baby universes, like branches growing out of a tree. Each branch grows up to be as big as the trunk and it sprouts branches of its own. Thus, you have an infinite fractal tree of branching universes in this theory of inflation. You still might ask yourself the question, though, where did the trunk come from. Li-Xin Li and I proposed that simply one of the branches circles back around and grows up to become the trunk. This is a model where the Universe is its own mother. It is a model with a little time loop at the very beginning. This is a model that has quite interesting properties. *_

_*Q: Nevertheless, is it possible to test your theory of self-creating universe?*_

When we made this model we found that it had one interesting very testable prediction. It predicted that there would be an arrow of time. One of things that we notice in the Universe is that if I shake charges, electrons, here, light waves would go out with the speed of light and they would get to Alpha Centauri, which is 4 light years away, 4 years later. However, Maxwell's equations of electrodynamics equally allow light beams to go toward the past and intersect Alpha Centauri 4 years ago. We never see that, we never see light beams going to the past, even though Maxwell's equations are time symmetric, they make no difference between the future and past. We see that the light beams are always emitted toward the future. This explains the cause and effect that we usually see in the Universe today. Since it is not the laws of electrodynamics that are causing this, we would suppose that this must something to do with the initial conditions of the Universe.

In our model, there is a very simple explanation. The only way to have a self-consistent model in our case is if the light beams are only allowed to go toward the future. Any light beam going toward the future would go out one of these branches and just continue run out, no problem. But, a light beam going to the past go back down that branch, back down the branches, back into the trunk, and then it would circle this time loop an infinite number of times which would cause it to blow up and destroy it. This would not be the geometry you started with. In this time travel solution, you have to have a self-consistent solution where you are not killing your grandmother. The only way how to get a self-consistent solution is to have an arrow of time pointing away from the time loop at the beginning. 

That was a very interesting and testable prediction that it had. In the future, one will have to see how this picture of our fits in whatever future superstring theory of everything. We think that it already fits in it very well conceptually because these branches originally had circumfereces that were very small and the superstring theory tells us that the big dimensions of space that we see used to be very tiny and curled up. And there are some curled up microscopic extra dimensions today that we don't see. The macroscopic dimensions that we see today are dimensions that used to be tiny as well. Thus, all of the dimensions in our Universe, all of the spatial dimensions were curled up in time. In our model, the time dimension is curled up in time as well. This is a little time loop at the beginning. We think it fits well into the superstring theory, but we will have to wait and see how that superstring theory close out and whether it allows solutions of this kind. We will have to see other predictions of the superstring theory that would convince you that it is the right theory.


*About science fiction*

*Q: You often use examples from the science fiction literature to explain complicated physics and mathematics behind your theories. Since many often scoff this genre, what do you think about science fiction?*

I think that science fiction is very interesting because a lot of times ideas are explored first in science fiction and then later by scientists as well. For example, H.G.Wells wrote the book "The Time Machine" in 1895. This is 10 years before the special relativity. At that time, with the Newton's theory, time travel looked impossible. But with the Einstein's theory of special relativity moving clocks tick slowly, and later, with the general theory of relativity, space and time are bendable. That is how a possibility of traveling in time came up. The same thing is true with the Carl Sagan's book "Contact" about wormholes. He asked Kip Thorne to examine whether his wormhole physics make sense. Then Kip Thorne looked into the wormhole physics seriously and found that circumstances and solutions like this might be possible. If you had wormholes like this, you might use them to make a time machine to visit the past. Thus, I think science fiction has often sparked interesting science investigations.

*Q: What is your favorite science fiction book or movie?*

I would have to say it is H.G. Wells' "The Time Machine". It is an amazing book and, as I said, 10 years or so ahead of its time. 

*About understanding science *

*Q: However, even the most of physicists cannot grasp the complexity behind your work. What can then an ordinary person expect?*

I wrote a book "Time travel in the Einstein's Universe" about this. In the book, I am explaining my theory to the lay audience, too. Everyone should be able to understand this. Einstein's theory is about curved space-time, and this is very visual. You can understand the concept that space and time might be bent. You can visualize bending of a piece of paper. You can visualize a curved surface because you see the surface of the Earth. Many concepts of this can be explained rather simply. However, working out the equations for general relativity is difficult. Nonetheless, many of the solutions in general relativity that solved those equations are rather simple. The black hole solutions, for example, are beautiful. I think that the average person can understand what the implications of this theory are. I also go to some trouble to try to describe how Einstein came to some of these ideas.

*Q: While everybody can appreciate and admire your work, most of humankind is struggling with very basic problems of daily survival. From their point of view, what would you say it is the importance of your work, except satisfying the human curiosity?*

The first of all, the human curiosity is very important, because, as a species, we have been around for 200 thousand years. That is not remarkably long. Tyrannosaurs Rex existed for two and half million years, for example. However, in that very short time, one of the amazing things that we have accomplished is we really understood where we are in the Universe, we understood something about the laws of physics. That is what everybody should be proud of. Also, I think it is important to say that you never can tell quite what the implications of understanding science at a deep level would be. Einstein was trying to understand the basic laws of physics and the Universe. Well, one of the things that came out of that is E=mc2, which is nuclear energy. One cannot often foresee that understanding science may have profound applications for the future of the human race. And I think one other thing that we should be doing as a human race is going to other planets, colonizing every world. This would be a benefit to our survival. This is something that you need science and technology to do. If we stay on the Earth we are subject to extinction events as other species are, and our survival chances to spread out into the Universe would improve.

_*About teaching science*_ 

_*Q: You served as the chairman of the judges of the National Westinghouse and Intel Science Talent Search, the premier science competition for high school students. How do you describe this complex theoretical physics to high school students?* _

The important thing that is stressed in that kind of competition is that high school students can actually do research. They do research projects, and they enter these research projects. We think that one of the best ways to find science talent is to actually encourage high school students to do their own research. Students in high school can actually do many frontier research projects. They may work for the scientists at a university, but many projects people can do in their own basement. A lot of science is learning about it by doing it. I teach a course in general relativity for undergraduates, and I think the Einstein's theory of general relativity, should be something that is on the undergraduate curriculum for physic students. It shouldn't necessarily be put off to graduate school. I think high school students should certainly be exposed to the ideas of special relativity and general relativity because TIME magazine picked Einstein as the person of the century. Thus, if you want to be an educated person, you should understand what Einstein did, why he is there.

*Q: Why do kids need these scary science classes if just a small number of them will become scientists?*

First of all, I would describe science classes as fun, not scary. I think that exposure of high school students to science is important. Science is a part of our modern world. It is one of the things that human beings have discovered. Just as in high school, you ought to read some Shakespeare because he is one of the important people who have written wonderful plays. That is why I think you should know something about science. You should know something about science to understand our modern world. If you are going to be a politician, you may have to decide scientific questions. If you are going to run a business, scientific considerations may come into some decisions that you have to make on the future of your company, whether to use some technology or not and why. Therefore, I think science education is a part of our educational system. That is something that people would find very worthwhile.

And here is the link:

http://www.sns.ias.edu/~dejan/CCS/work/news/Gott_interview/J.Richard.Gott.III.eng.html

Whitestar


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## carrie221 (Oct 16, 2006)

Wow... that you for all that info


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## Whitestar (Oct 16, 2006)

You're welcome!


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## Starbeast (Jun 15, 2021)

Oops.


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## tinkerdan (Jun 30, 2021)

In my own wild theory:
I would guess that this is a combination of theoretical physics plus MathMagics.
By that I mean that it possibly involves the chaos mathematics in predicting the patterned path behavior of a particle trajectory.

Particle trajectories often are shown using math and it is shown that if particles started in the same place they would follow the same trajectory or path and also that there is a margin of 'error' or some such where a particle could start a bit away from that start point and still be drawn back into the trajectory.

My guess is that in some theoretical experiment they use some method of drawing the particle{or possible quantum entanglement}toward a target(that lies outside of the path)and possibly far enough from the predicted pattern that if they trace back the trajectory using their math that the particle mathematically starts far enough away from the starting point that they have theoretically 'changed' its starting point thus theoretically changing the past.


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## alexvss (Jun 30, 2021)

I think the guy above resurrected this thread by accident   

Anyway, as someone allergic to exact sciences who writes the most soft sci-fi, I can't contribute properly, but Stephen Hawking once threw a party for time travelers. The twist is that the invitations were only printed years after. Although they had the exact coordinates of Cambridge University, nobody should up (at least not in this universe). Hawking saw this as a proof that time traveling isn't possible, but his Memorial acknowledged its possibility years later.

I came across this a couple days ago, and I'm already writing a story about it.


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## Ori Vandewalle (Jun 30, 2021)

Before I saw this was posted in 2006 and thus the experiment is no longer upcoming, I really wanted to reply with, "Well, did it work?"


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## Venusian Broon (Jun 30, 2021)

Ori Vandewalle said:


> Before I saw this was posted in 2006 and thus the experiment is no longer upcoming, I really wanted to reply with, "Well, did it work?"



I found this on Wikipedia in his entry:

_From 2007 to 2014, Cramer investigated the possibility that quantum nonlocality might be used for communication between observers through the use of switchable interference patterns. In the course of this work, he gained new understanding of the "show stopper" within the quantum formalism that prevents such nonlocal signaling: For each interference pattern, nature also provides and superimposes an "anti-interference pattern". These are always combined in a way that "erases" potential nonlocal signals. The two interference patterns complement each other, resulting in no perceptible interference pattern. Measurement changes can dramatically modify the individual interference patterns, but always so that this erasure occurs. In this way, nature is protected from the possibility of retrocausal signaling and its consequences and paradoxes_

I didn't read all of the above in blue, but I think the above suggests that he didn't find any retrocausal signalling! Ah well, back to the drawing board...


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## Ori Vandewalle (Jun 30, 2021)

That strikes me as too... cute an explanation. Like there's some force of the universe sneaking into particular experiments/setups and doing juuuust enough to protect causality. I would bet there's a more parsimonious explanation for the lack of interference.


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## Ori Vandewalle (Jul 1, 2021)

This, from physicist Sean Carroll, isn't a direct response to the experiment described above, I think, but to the general category of delayed choice quantum eraser experiments. He throws cold water on the whole idea of retrocausality, explaining that these experiments are neat but by no means evidence of time travel, and they have a (relatively) simple explanation... if you accept the Many Worlds/Everettian interpretation.






						The Notorious Delayed-Choice Quantum Eraser – Sean Carroll
					






					www.preposterousuniverse.com


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## Harpo (Sep 7, 2021)

Here’s what I think: last year was so ridiculous, everything seeming to be worse with each passing month everywhere, there was a theory that it was all caused by time travelling historians from the future who travelled back in time to study that calamitous year of 2020. And inevitably they caused it all in the first place, and their subsequent attempts to fix things only made it worse every time they tried.
The solution, and what prevented such worsening increasing further, was to go even further back and make sure that such time travel would not be possible in the first place, thus saving the world. 

Ta-daaah.

Of course, this is nonsense. You know what I think about time travel, and this isn’t it.


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