https://journals.aps.org/prd/abstract/10.1103/PhysRevD.46.603

It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past. This paper examines this possibility in the case that the causality violations appear in a finite region of spacetime without curvature singularities. There will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon. This shows that one cannot create closed timelike curves with finite lengths of cosmic string. Even if violations of the weak energy condition are allowed by quantum theory, the expectation value of the energy-momentum tensor would get very large if timelike curves become almost closed. It seems the back reaction would prevent closed timelike curves from appearing. These results strongly support the chronology protection conjecture: The laws of physics do not allow the appearance of closed timelike curves.

My advice is to not work on Time Machines.

 

Years ago I saw a show about a man who claimed that rather than travel back in time, he was working on communicating with the past. Sadly his motivation was to send information back to save his father's life. The claim was that communication was possible but only as far back as the required receiver was constructed. That meant that for him to save his father he needed to find an alien civilization that had constructed the device and get them to help him.

I'd guess communicating with the past suffers the same limitations as outright travel. You could send a message back to your mother telling her not to marry your father, but she would necessarily ignore the message. Except she would have kept the letter and showed it to you as you were growing up and you'd have known she ignored the message so you wouldn't bother to send it. lol.

 

well,the most known physics experiment may beat casuality:

 

well,the most known physics experiment may beat casuality:

“Time flies like an arrow. Fruit flies like a banana.

The delayed-choice quantum eraser is not a refutation of the principle of causality. It only shows how quantum mechanics violates our intuitions.

https://arxiv.org/abs/1908.03920

Demystifying the Delayed-Choice Quantum Eraser
Tabish Qureshi

The delayed-choice quantum eraser has long been a subject of controversy, and has been looked at as being incomprehensible to having retro-causal effect in time. Here the delayed-choice quantum eraser is theoretically analyzed using standard quantum mechanics. Employing Mach-Zehnder interferometer, instead of a conventional two-slit interference, brings in surprising clarity. Some common mistakes in interpreting the experiment are pointed out. It is demonstrated that in the delayed mode there is no which-way information present after the particle is registered on the screen or the final detectors, contrary to popular belief. However, it is shown that another kind of path information is present even after the particle is registered in the final detectors. The registered particle can be used to predict the results of certain yet to be made measurements on the which-way detector. This novel correlation can be tested in a careful experiment. It is consequently argued that there is no big mystery in the experiment, and no retro-causal effect whatsoever.

 

Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit. And then I procrastinated on filming the retraction! Thanks for laying it out more clearly than I could have. I have some thoughts to share that might add one more nail to the coffin ... coming to a video real soon!

https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g

 

It seems nonsensical to me to consider going back in time.
Time is a sequence of events, so how can you go back to an event that has already happened and finished?
That event is not just sitting back there, waiting for someone to return to it, unless you believe in the infinite universes theory.

 

From a casual thinker about light speed perks.

If one launches a rocket that increases speed to above the speed of light, and after a billion or so miles, then begins to slow down as it's camera is focused somewhere specific on earth, then it should capture images of previous earth light particles catching up to it. It could view it's own actual launch or some crime scene from Detroit, as long as it happened outdoors on a clear sky day.

If it's then traveling at a half-speed of light, the image it records could be a slow motion, half-speed capture.

And back to my chicken dinner...indoors now.

 

A little more info on the same subject:

Physicist Discovers 'Paradox-Free' Time Travel Is Theoretically Possible

​

Stephen Hawking certainly thought it was possible – and if we do then this new research suggests we would be free to do whatever we wanted to the world in the past: it would readjust itself accordingly.

 

A little more info on the same subject:

Physicist Discovers 'Paradox-Free' Time Travel Is Theoretically Possible
View attachment 310316​

More mathematical quackery. Our physical universe makes it impossible.

 

Time travel is an interesting idea. Only problem is, there doesn't seem to much (if any) evidence to support it. In fact one might even go as far as to argue that it fundamentally violates just about every law of physics. (Not to mention logic itself.) Just consider the Grandfather Paradox. Sure you could introduce a loophole here, a loophole there. Maybe then you'd get a somewhat reasonable resolution to these kinds of questions. But otherwise I think Occam's Razor pretty much sums it up. The more ways that you have to suspend your disbelief (ie. without any evidence whatsoever) to accept a theory, the less likely it is to be true.

 

Hello there,

Yes, there has been some recent interest in this idea of time travel again.
If it is true I think it is safe to say that humans from Earth have not discovered how to do this yet, if it is possible that is. That's because nobody has come back from the future and nobody went back in time yet, that we know of.

There are so many problems with this.
For one, if someone went back in time then the time machine would not have been invented yet so they would be stuck there and probably deemed "crazy" by the local population there. That's of course true unless they could take their time machine with them.

The grandfather paradox is interesting though. If you went back in time and killed your grandfather, your parents, as they are now, would never have existed, and if so then you would not have been born, and so how did your grandfather die.
I don't see any way out of that unless there are some other drastic changes that occur along with the time travel, such as your grandfather was killed by someone else even though it was you. In other words, it becomes a control theory problem where the feedback alters the future but in a more complete way. However, this kind of thinking seems to indicate that it just isn't possible because this would involve a huge number of changes at the instant right after death. Not only those that involve the original timeline, but also possibly deep into the past before the murder took place. That gets harder to believe. Of course we have no proof of any of this either way so after all is said and done, we really still have not figured anything out for sure.

What we have done is created a problem and have no way to prove it true or false.

BTW, it has been proven that time travel is possible, but only for very small particles and extremely short times. Now if that is really true then maybe we can project that idea onto larger objects like we did with quantum entanglement, but there could still be a controlling factor that prevents it. What we forget I think is that some of these things may require some infinite variable and we don't consider that to be realistic in most cases, except in theory, and real life isn't really theory.
Remember an exponential time factor never decreases to a perfect zero, except in the infinite limit, and we can't get to an infinitely large time in the future.

Could things about this change in the future? I wish I could predict that but that's even harder to guess.

 

If it is possible for a human to travel backwards in time, then it certainly will never become common, because we would have already seen travelers from the future. Perhaps we have but did mot recognize them. Perhaps every one of them was disciplined and careful enough to avoid revealing themselves. But I find that highly unlikely.

 

It's makes for good SCI-FI but that's all it is, writers or mathematical fictions.

 

It's makes for good SCI-FI but that's all it is, writers or mathematical fictions.

Near the end of that video, there is an explosion and a round, approximately 6 ft diameter Chippendale style table goes flying through the air. It now sits in my dining room!

The film was partially shot in my home town, Worcester, MA, and the props were sold in the Habitat for Humanity store about a mile from my home.

 

Near the end of that video, there is an explosion and a round, approximately 6 ft diameter Chippendale style table goes flying through the air. It now sits in my dining room!

The film was partially shot in my home town, Worcester, MA, and the props were sold in the Habitat for Humanity store about a mile from my home.

That's really cool. People will abuse 'Time Travel' like any other powerful tool. The instant it's ever possible IMO that part of the universe collapses into a 'real' singularity were time doesn't exist or the 'Time Cops' kill the inventors before they invent it so that doesn't happen.

This guy is a true believer.

 

If it is possible for a human to travel backwards in time, then it certainly will never become common, because we would have already seen travelers from the future. Perhaps we have but did mot recognize them. Perhaps every one of them was disciplined and careful enough to avoid revealing themselves. But I find that highly unlikely.

It could be that they not only learned about time travel, but that going back to the past would wreak havoc in the world and so it was outlawed, and everyone obeyed that because the ramifications would be too severe.

 

It could be that they not only learned about time travel, but that going back to the past would wreak havoc in the world and so it was outlawed, and everyone obeyed that because the ramifications would be too severe.

Not likely. If something is exploitable, someone will exploit it despite any consequences to others. That is human nature.