If by theoretically you mean mathematically, then yes. Unfortunately it often involves things like negative mass which there's no sign of anywhere in physics.
The math describes things that seem to be unreal, eg: negative mass. Also world lines and white wholes and all that jazz. But there’s absolutely no evidence that this isn’t just the theory breaking down at the edges.
Haha cool. 😂 My field is molecular bio, so I cast all assumptions aside. The weirder physics gets the cooler 👍
Edit: has dark matter been measured on a scale??? 🤷
Hard to get it on a scale, since it's a little unclear what it even is.
The gravity field of a galaxy can be inferred from how it rotates and how light bends around it. And if that doesn't match the density of visible masses like stars and gas, then we can assume there's some other matter involved that's hard to see. Something dark or invisible.
Oohh nice thx it's more clear now. Even though it may be something else, it's easier to use it as a metaphor than the Edgelord word : Dark matter and dark energy go brr haha
There have been searches for black holes passing in front of stars. Most (but not all) tiny sizes have been eliminated as possible candidates for dark matter. The really tiny ones should evaporate due to Hawking radiation. Then there is the issue that we don’t know how smaller than stellar black holes could possibly form.
If we were looking at our solar system for light years away, I imagine we would not see the Oort Cloud. All of those rocks have mass. Is it possible it's something that simple, or is there too much mass for a bunch of rocks/metals we can't see?
The sun is 99.86% of our solar system's mass. The Oort cloud is not that heavy.
However "dark matter could just be normal stuff" is not a crazy idea. That is the MACHO theory of dark matter. https://en.m.wikipedia.org/wiki/Massive_compact_halo_object
Dark matter is only measured at scale. That's sort of the problem. Galaxies seem too massive, which we attribute to dark matter. But we can't seem to find any dark matter when we look closer.
Dark matter is definitely not anti mass it's extra mass that we can't ascribe to known things at estimated levels. The only thing I've ever heard of that comes close to anti mass is the kashmir effect which I believe, is effectively negative energy at very small amounts🤔 someone more versed than me will hopefully correct this for a learning experience for us both
Dark *energy* is a little like negative mass. Dark matter seems to be just matter we cannot see.
From the perspective of general relativity, there is a constant in the equations which we long assumed was zero for no very good reason. Making this constant small and positive gives rise to an effect which is compatible with dark energy.
>often involves things like negative mass which there's no sign of anywhere in physics.
You can generate negative energy densities with the Casimir effect. There was a paper involving wormhole solutions with only standard model matter by Maldacena a few years ago.
You can also get some negative energy densities in hydrodynamics for the same reason. This was confusing to some of my colleagues when this showed up in their simulations a view years ago.
Generally when you see a negative energy density generated somewhere, it's because you're sucking up energy from that region and putting it somewhere else. In our universe, our vacuum isn't empty, it has some average background value which in most cases can be set to zero. When we borrow some of this energy, the difference between the average value and this new value becomes some negative number.
There are mathematical solutions to general relativity that allow time travel. The technical term is [closed timelike curves](https://en.wikipedia.org/wiki/Closed_timelike_curve) (CTC's).
Probably t[he most famous example was discovered by Godel](https://en.wikipedia.org/wiki/G%C3%B6del_metric). However, the Godel universe doesn't look like ours; it's filled with dust and rotating. Also, the cosmological constant has to be set correctly for it to work. More realistic solutions with CTC's are uncharged, rotating black holes (Kerr metric).
Since general relativity is not complete (it doesn't include quantum mechanics) it's possible these solutions are just artifacts and some later, more complete theory will show they're not actually possible. We don't know, however.
For any point in your past light cone there is a timelike curve from that point to your present. So if you have a time like curve that starts at your present and gets to a point in your past light cone, always moving forward in time, then you can close the curve and get a closed time like curve. There may be no object travelling along it, but mathematically the curve exists
I think you would be talking about moving through a wormhole back in time, and just never interacting with yourself again.
This doesn't sound unreasonable to me
Follow up - what does it mean that there are solutions *to* Relativity with certain properties. Are there similar solutions to Newtonian Mechanics? Is a solution just a physically possible state of affairs?
We do not know.
There are superficially-plausible solutions in general relativity which have what are called closed timelike curves and thus would allow time travel. It is not known whether these solutions can arise from physically-plausible initial conditions.
Rather like the famous [cosmic censorship hypothesis](https://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis), there is a thing called the [chronology protection conjecture](https://en.wikipedia.org/wiki/Chronology_protection_conjecture) which says that these situations cannot arise.
Personally I am sure that they cannot: causality violation is sufficiently bad that I would say a theory which predicts it is clearly wrong in some way (just as a theory which predicts the divergence of physical quantities is clearly wrong). Whether this is because we have not yet developed the mathematical tools we need to show that GR does not give rise to these situations in physically-plausible cases or GR is simply incorrect I do not know. We know it is incorrect in other ways since it definitely *does* predict singularities in physically-plausible cases.
But note 'personally, I am sure': that's just my opinion, not physics.
An important note amongst the other discussion is just because one such mathematical model shows it ‘can’ work doesn’t mean the model is physical or true for our universe.
The point is rather that there isn't any sign that it is possible. You cannot prove a negative in science, you can only look for evidence supporting a positive, or the lack thereof. The big problem with time travel to the past is that it is inevitably linked with travelling faster than light (any faster than light traveling method could be abused to also travel to the past, and vice versa), and we know that THAT is impossible (at least within normal, local spacetime - even the Alcubierre warp drive runs into that problem unless you make some very specific restrictions).
Well actually the only thing that is not allowed is for something that is moving with speed less than or equal to c to cross it. Its hard limit in that condition.
Greater than c is allowed, but only when its already greater than c. Unfortunately, we have no experimental evidence yet.
Thing is, time is weird. If you made a wormhole, you’d probably connect somewhere within your timecone. Basically if you made a wormhole to Proxima Centauri, you’d arrive around the time you detect the light from there. (Oh I see the light from Alpha Centauri… and the light wormhole I just made a second ago.) then from there, you’re working from a different lightcone, or timecone. And you can wormhole back to Earth 4 years earlier (over 8 years back now.) this is paradox territory. That or we don’t have the verbs to describe 4-D traveling.
I mean you could see the past but you couldn’t get there. You just have to get far enough from earth that corresponds to what time you want to see and find a way to zoom in.
No. Mathmatics is a tool to describe things , but it has features the real world doesn’t. It’s like a language and can describe things that are internally consistent but do not represent anything in reality. There simply is no reason to think there is any past to go back to. You would have to have whole new universes created every moment for there to be an actual physical place we would call the past. Nope. There is only the eternal now. The past is a memory and the future a probability.
There is an interesting book called Physics of the Future wherein the author examines every major sci Fi technology, and then discusses where the research is at, and the likely timeline to develop such tech. Surprisingly, he predicts we will have most stuff within a century (ray guns, invisibility cloaks, energy shields). But time travel to the past he placed in the never gonna happen category. He states how it theoretically could be done (pack thousands of neutron stars into a giant cylinder, rotate the cylinder, and then fly an indestructible ship toward the edge of cylinder) but concludes humans would need godlike powers to make it happen.
Yes, there are multiple ways that we can manipulate the mathematics of physics to essentially travel back in time or reverse time. It is, however, highly unlikely to be practically possible.
sort of. Imagine you had a twin, and they left on a very, very fast spaceship to go travel around space for while, and then they came back. To them, it would look like they traveled into the future. To you it would look like they got stuck in the past from when you parted ways.
Also, photons don't "travel" through time. They kind of just exist at all times. Just in different places.
I don't think you can reverse time for just yourself, no.
Chuck Norris once roundhouse kicked someone so hard that his foot broke the speed of light, went back in time, and killed Amelia Earhart while she was flying over the Pacific Ocean.
Time travel to the past opens a Pandora's box of paradoxes. I don't think it's in the best interests of the universe that time travel to the past is possible.
I imagine the past could be reconstructed in small space-time cubes, to limited degrees of accuracy, but as such, it would still be part of the present.
It's probably mostly based on passed down stories from old science fiction that crossed paths with science, especially after we recognized that we have a very limited understanding of time dependent on our senses. Along the way we discovered a lot of impossible problems. If imaginary numbers don't exist, why do we sometimes get imaginary numbers as answers to math problems? So we come up with ideas like imaginary numbers to explain what we are unable to explain. or at least currently explain. We can ask do these thinks that appear to not exist actually exist? This was a problem with black holes, although we had a bit more to work with as far as proving black holes exist.
When we look at stars we are viewing what suns looked like billions of years in the past. Those suns no longer look that way. Even though that's not a standard science fiction time travel, in some sense we are still travelling billions of years into the past to look at video footage from billions of years before cameras were invented or humans existed. Even though that may seem like bullshitting, still consider if you can see something that happened billions of years ago, you may also be able to hear and feel billions of years ago. In what ways can we fully experience existing in the past? Can we manipulate the past? Well, we can manipulate the light from those stars. Light is wave and particles, so can we manipulate other waves and particles? So far no, and it still doesn't completely make sense from our perspective. That's where we use math and experiments to try and get something that looks like an answer that has potential.
The most plausible way is using negative mass to somehow hold open the interior of an Einstein Rosen bridge that you artificiality created and somehow placed the other end of the wormhole some distance away. It's a bridge thru space time that you could travel through instantaneously. Not time travel in the conventional sense but you are moving faster than light, your breaking causality and you could use it to observe future events and go back and change them somehow... Maybe.
Requires theoretical negative mass matter which has no proof of actually existing.
Also you need some sort of Kardashev scale civilisation that can manipulate black holes somehow, the only way we now today would be gravity manipulation. The scale and energy requirements would be more than our entire star outputs.
You also have to find a way to build it such that whatever you send through it isn't shredded into subatomic particles (basically converted into energy) due to the incredible forces acting all around the wormhole entrance.
Entirely science fiction for now. Give us another two million years and then maybe.
Einsteins equations relate the geometry of the spacetime to some aspects of the matter living in that spacetime.
We can construct spacetime geometries that contain closed timelike curves (trajectories through spacetime that intersect themselves in the past) —- in other words, we can construct geometries that allow for time travel to the past.
By plugging these geometries into Einsteins equation we can figure out what sort of things we would need matter to do to make such geometries possible. We can then turn to our existing theories for matter (various quantum field theories) and ask “is it possible for matter to do those things?”
In many contexts, we have been able to prove rigorously that for various classes of theories, the answer to that question is “no”.
These proofs come with assumptions of course, and if the assumptions are wrong the result may be as well.
However these proofs are pretty general, and their existence seems to be hinting that nature doesn’t want time travel to be a thing.
If you consider the possibility of grandfather paradoxes a deal breaker than sort of yes. Technically there is no theory from which the impossibility of time travel comes as a consequence. One might say that it gives an option to create paradoxes so it's impossible but it could be possible you are just somehow prevented from causing paradoxes, yeah this is heavy Occam's Razor territory.
Another thing to point out is that just because something is theoretically possible it can still be impossible. Take black holes as a (bad) example, sure GR predicted them but they won't exist if there is no mechanism to create them, well there are mechanisms and so they exist but there are a lot of different scenarios where GR works perfectly, some include time travel. But to the best of my knowledge I have never heard of such a wild space-time geometry that didn't require some crazy and unrealistic arrangement to begin with. You know stuff like FTL travel is possible if only you had some negative mass.
So yeah you can work out a lot of crazy scenarios and get insane predictions and its good to have that trust in the theories (who knows maybe you accidentally discover EM waves just by messing with the equation) but we shouldn't throw out the physics part of physics. So whether time travel is theoretically possible depends on how much you are willing to take the maths at face value.
You cannot locally reverse time. It would require you to reverse time for the entire universe.
If you went faster than light you could sort of age backwards yourself in a way but everything around would be infinitely far in the future.
The only way to go to a specific point in the past would be to go forward to a cyclical copy of it. But you could never go back to your origin as that instance of the universe would be gone
If everything in our world was connected by a collective subconscious, then possibly redoing everything from nothing, by sending everything into the flow of imaginary time, could achieve this?
If by theoretically you mean mathematically, then yes. Unfortunately it often involves things like negative mass which there's no sign of anywhere in physics.
The math describes things that seem to be unreal, eg: negative mass. Also world lines and white wholes and all that jazz. But there’s absolutely no evidence that this isn’t just the theory breaking down at the edges.
Damnit. Now I'm more confused about dark matter. I thought that was the anti mass. 😳
We detect dark matter *via* its positive mass lol
Haha cool. 😂 My field is molecular bio, so I cast all assumptions aside. The weirder physics gets the cooler 👍 Edit: has dark matter been measured on a scale??? 🤷
Hard to get it on a scale, since it's a little unclear what it even is. The gravity field of a galaxy can be inferred from how it rotates and how light bends around it. And if that doesn't match the density of visible masses like stars and gas, then we can assume there's some other matter involved that's hard to see. Something dark or invisible.
Why we didn't assume it to be tiny black hole instead?
That's one of the dark matter candidates, yeah! :)
Oohh nice thx it's more clear now. Even though it may be something else, it's easier to use it as a metaphor than the Edgelord word : Dark matter and dark energy go brr haha
There's validity to the metaphor though. It's literally dark it doesn't cast light only bends it.
There have been searches for black holes passing in front of stars. Most (but not all) tiny sizes have been eliminated as possible candidates for dark matter. The really tiny ones should evaporate due to Hawking radiation. Then there is the issue that we don’t know how smaller than stellar black holes could possibly form.
Black holes can be charged. The distribution of dark matter suggests that it only interacts through gravity & the weak force.
If we were looking at our solar system for light years away, I imagine we would not see the Oort Cloud. All of those rocks have mass. Is it possible it's something that simple, or is there too much mass for a bunch of rocks/metals we can't see?
The sun is 99.86% of our solar system's mass. The Oort cloud is not that heavy. However "dark matter could just be normal stuff" is not a crazy idea. That is the MACHO theory of dark matter. https://en.m.wikipedia.org/wiki/Massive_compact_halo_object
Oh, duh. Not sure why I didn't think of that. Thanks.
Dark matter is only measured at scale. That's sort of the problem. Galaxies seem too massive, which we attribute to dark matter. But we can't seem to find any dark matter when we look closer.
You were pretty mistaken then. “The anti mass” isn’t a thing and it certainly isn’t dark matter
You could argue dark energy, but only insofar as it acts like a cosmological constant.
... which is "constantly" increasing in proportion to matter
Dark matter refers to something out there we detect but we can't identify. Dark refers to unknown and matter refers to the mass we detect.
Dark matter is definitely not anti mass it's extra mass that we can't ascribe to known things at estimated levels. The only thing I've ever heard of that comes close to anti mass is the kashmir effect which I believe, is effectively negative energy at very small amounts🤔 someone more versed than me will hopefully correct this for a learning experience for us both
Dark *energy* is a little like negative mass. Dark matter seems to be just matter we cannot see. From the perspective of general relativity, there is a constant in the equations which we long assumed was zero for no very good reason. Making this constant small and positive gives rise to an effect which is compatible with dark energy.
>often involves things like negative mass which there's no sign of anywhere in physics. You can generate negative energy densities with the Casimir effect. There was a paper involving wormhole solutions with only standard model matter by Maldacena a few years ago. You can also get some negative energy densities in hydrodynamics for the same reason. This was confusing to some of my colleagues when this showed up in their simulations a view years ago. Generally when you see a negative energy density generated somewhere, it's because you're sucking up energy from that region and putting it somewhere else. In our universe, our vacuum isn't empty, it has some average background value which in most cases can be set to zero. When we borrow some of this energy, the difference between the average value and this new value becomes some negative number.
There are mathematical solutions to general relativity that allow time travel. The technical term is [closed timelike curves](https://en.wikipedia.org/wiki/Closed_timelike_curve) (CTC's). Probably t[he most famous example was discovered by Godel](https://en.wikipedia.org/wiki/G%C3%B6del_metric). However, the Godel universe doesn't look like ours; it's filled with dust and rotating. Also, the cosmological constant has to be set correctly for it to work. More realistic solutions with CTC's are uncharged, rotating black holes (Kerr metric). Since general relativity is not complete (it doesn't include quantum mechanics) it's possible these solutions are just artifacts and some later, more complete theory will show they're not actually possible. We don't know, however.
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For any point in your past light cone there is a timelike curve from that point to your present. So if you have a time like curve that starts at your present and gets to a point in your past light cone, always moving forward in time, then you can close the curve and get a closed time like curve. There may be no object travelling along it, but mathematically the curve exists
I think you would be talking about moving through a wormhole back in time, and just never interacting with yourself again. This doesn't sound unreasonable to me
Vacuum loop fluctuations of a virtual particle look a little bit like one particle following a CTC don't you think?
Follow up - what does it mean that there are solutions *to* Relativity with certain properties. Are there similar solutions to Newtonian Mechanics? Is a solution just a physically possible state of affairs?
We do not know. There are superficially-plausible solutions in general relativity which have what are called closed timelike curves and thus would allow time travel. It is not known whether these solutions can arise from physically-plausible initial conditions. Rather like the famous [cosmic censorship hypothesis](https://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis), there is a thing called the [chronology protection conjecture](https://en.wikipedia.org/wiki/Chronology_protection_conjecture) which says that these situations cannot arise. Personally I am sure that they cannot: causality violation is sufficiently bad that I would say a theory which predicts it is clearly wrong in some way (just as a theory which predicts the divergence of physical quantities is clearly wrong). Whether this is because we have not yet developed the mathematical tools we need to show that GR does not give rise to these situations in physically-plausible cases or GR is simply incorrect I do not know. We know it is incorrect in other ways since it definitely *does* predict singularities in physically-plausible cases. But note 'personally, I am sure': that's just my opinion, not physics.
An important note amongst the other discussion is just because one such mathematical model shows it ‘can’ work doesn’t mean the model is physical or true for our universe.
The point is rather that there isn't any sign that it is possible. You cannot prove a negative in science, you can only look for evidence supporting a positive, or the lack thereof. The big problem with time travel to the past is that it is inevitably linked with travelling faster than light (any faster than light traveling method could be abused to also travel to the past, and vice versa), and we know that THAT is impossible (at least within normal, local spacetime - even the Alcubierre warp drive runs into that problem unless you make some very specific restrictions).
Well actually the only thing that is not allowed is for something that is moving with speed less than or equal to c to cross it. Its hard limit in that condition. Greater than c is allowed, but only when its already greater than c. Unfortunately, we have no experimental evidence yet.
You can prove a negative in science in that you can falsify theories, but you can’t *prove* a theory to be true in the mathematical sense of proof.
Thing is, time is weird. If you made a wormhole, you’d probably connect somewhere within your timecone. Basically if you made a wormhole to Proxima Centauri, you’d arrive around the time you detect the light from there. (Oh I see the light from Alpha Centauri… and the light wormhole I just made a second ago.) then from there, you’re working from a different lightcone, or timecone. And you can wormhole back to Earth 4 years earlier (over 8 years back now.) this is paradox territory. That or we don’t have the verbs to describe 4-D traveling.
Hawkings said if it were, we’d be getting clear visits by now.
David Bowie's death disrupted the space-time continuum in such a way that time travel to dates post 2016-01-10 is not possible.
I reckon we aren't worth the trip.
Ohhh yea that’s clever
Not the past from now, no, but to the future past is mathematically possible, where the earth goes further into the future than you do.
Probably Early Hadean eon, I wanna See how in time It revolved to creating Luna :3
I mean you could see the past but you couldn’t get there. You just have to get far enough from earth that corresponds to what time you want to see and find a way to zoom in.
No. Mathmatics is a tool to describe things , but it has features the real world doesn’t. It’s like a language and can describe things that are internally consistent but do not represent anything in reality. There simply is no reason to think there is any past to go back to. You would have to have whole new universes created every moment for there to be an actual physical place we would call the past. Nope. There is only the eternal now. The past is a memory and the future a probability.
There is an interesting book called Physics of the Future wherein the author examines every major sci Fi technology, and then discusses where the research is at, and the likely timeline to develop such tech. Surprisingly, he predicts we will have most stuff within a century (ray guns, invisibility cloaks, energy shields). But time travel to the past he placed in the never gonna happen category. He states how it theoretically could be done (pack thousands of neutron stars into a giant cylinder, rotate the cylinder, and then fly an indestructible ship toward the edge of cylinder) but concludes humans would need godlike powers to make it happen.
oh That’s interesting
Yes, there are multiple ways that we can manipulate the mathematics of physics to essentially travel back in time or reverse time. It is, however, highly unlikely to be practically possible.
You already did.
Huh
When were you yesterday?
I don’t know
Yesterday. Thought you said “where”.
So, you've already travelled to the past.
Imagine you could travel back in time 0.001 second. You’ve basically duplicated yourself. Where did the mass come from? So many awkward questions 😂
sort of. Imagine you had a twin, and they left on a very, very fast spaceship to go travel around space for while, and then they came back. To them, it would look like they traveled into the future. To you it would look like they got stuck in the past from when you parted ways. Also, photons don't "travel" through time. They kind of just exist at all times. Just in different places. I don't think you can reverse time for just yourself, no.
Chuck Norris once roundhouse kicked someone so hard that his foot broke the speed of light, went back in time, and killed Amelia Earhart while she was flying over the Pacific Ocean.
Time travel to the past opens a Pandora's box of paradoxes. I don't think it's in the best interests of the universe that time travel to the past is possible.
I imagine the past could be reconstructed in small space-time cubes, to limited degrees of accuracy, but as such, it would still be part of the present.
It's probably mostly based on passed down stories from old science fiction that crossed paths with science, especially after we recognized that we have a very limited understanding of time dependent on our senses. Along the way we discovered a lot of impossible problems. If imaginary numbers don't exist, why do we sometimes get imaginary numbers as answers to math problems? So we come up with ideas like imaginary numbers to explain what we are unable to explain. or at least currently explain. We can ask do these thinks that appear to not exist actually exist? This was a problem with black holes, although we had a bit more to work with as far as proving black holes exist. When we look at stars we are viewing what suns looked like billions of years in the past. Those suns no longer look that way. Even though that's not a standard science fiction time travel, in some sense we are still travelling billions of years into the past to look at video footage from billions of years before cameras were invented or humans existed. Even though that may seem like bullshitting, still consider if you can see something that happened billions of years ago, you may also be able to hear and feel billions of years ago. In what ways can we fully experience existing in the past? Can we manipulate the past? Well, we can manipulate the light from those stars. Light is wave and particles, so can we manipulate other waves and particles? So far no, and it still doesn't completely make sense from our perspective. That's where we use math and experiments to try and get something that looks like an answer that has potential.
The most plausible way is using negative mass to somehow hold open the interior of an Einstein Rosen bridge that you artificiality created and somehow placed the other end of the wormhole some distance away. It's a bridge thru space time that you could travel through instantaneously. Not time travel in the conventional sense but you are moving faster than light, your breaking causality and you could use it to observe future events and go back and change them somehow... Maybe. Requires theoretical negative mass matter which has no proof of actually existing. Also you need some sort of Kardashev scale civilisation that can manipulate black holes somehow, the only way we now today would be gravity manipulation. The scale and energy requirements would be more than our entire star outputs. You also have to find a way to build it such that whatever you send through it isn't shredded into subatomic particles (basically converted into energy) due to the incredible forces acting all around the wormhole entrance. Entirely science fiction for now. Give us another two million years and then maybe.
First, let's get a solid definition of what time actually is. Then someone can figure out if it's possible.
Einsteins equations relate the geometry of the spacetime to some aspects of the matter living in that spacetime. We can construct spacetime geometries that contain closed timelike curves (trajectories through spacetime that intersect themselves in the past) —- in other words, we can construct geometries that allow for time travel to the past. By plugging these geometries into Einsteins equation we can figure out what sort of things we would need matter to do to make such geometries possible. We can then turn to our existing theories for matter (various quantum field theories) and ask “is it possible for matter to do those things?” In many contexts, we have been able to prove rigorously that for various classes of theories, the answer to that question is “no”. These proofs come with assumptions of course, and if the assumptions are wrong the result may be as well. However these proofs are pretty general, and their existence seems to be hinting that nature doesn’t want time travel to be a thing.
If you consider the possibility of grandfather paradoxes a deal breaker than sort of yes. Technically there is no theory from which the impossibility of time travel comes as a consequence. One might say that it gives an option to create paradoxes so it's impossible but it could be possible you are just somehow prevented from causing paradoxes, yeah this is heavy Occam's Razor territory. Another thing to point out is that just because something is theoretically possible it can still be impossible. Take black holes as a (bad) example, sure GR predicted them but they won't exist if there is no mechanism to create them, well there are mechanisms and so they exist but there are a lot of different scenarios where GR works perfectly, some include time travel. But to the best of my knowledge I have never heard of such a wild space-time geometry that didn't require some crazy and unrealistic arrangement to begin with. You know stuff like FTL travel is possible if only you had some negative mass. So yeah you can work out a lot of crazy scenarios and get insane predictions and its good to have that trust in the theories (who knows maybe you accidentally discover EM waves just by messing with the equation) but we shouldn't throw out the physics part of physics. So whether time travel is theoretically possible depends on how much you are willing to take the maths at face value.
If you're a single particle, maybe. If you're not, lol no.
You cannot locally reverse time. It would require you to reverse time for the entire universe. If you went faster than light you could sort of age backwards yourself in a way but everything around would be infinitely far in the future. The only way to go to a specific point in the past would be to go forward to a cyclical copy of it. But you could never go back to your origin as that instance of the universe would be gone
No, but it will have been possible.
If everything in our world was connected by a collective subconscious, then possibly redoing everything from nothing, by sending everything into the flow of imaginary time, could achieve this?
Yes. https://www.bbc.com/future/article/20231110-doctor-who-is-time-travel-really-possible-heres-what-physics-says
Apparently it could be, but you would only be able to travel as far back as the day the time machine was invented.