Reddit mentions of Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time

Read this in high school it touches on ethics in a future society that uses these matter recreaters to teleport people. For example teleporting from an ambulance stretcher to a hospital bed instantly and how a group of people develops that thinks the soul or ghost gets left behind.

> The only time travel that is possible is perception based

No, it's really something more fundamental than just perception. For example, in the classic twin-paradox setup, one twin stays on earth and observes his brother moving quickly. So, he sees his brother aging slowly. If his brother then slows down and comes back to Earth, he is actually younger than his twin. (Note that this situation is asymmetric due to the acceleration of the traveling twin - that's a whole other story if you want to get into it).

So I'd argue that it's not just a perceptual thing, time travel actually occurred. The now-younger twin has "traveled to the future" since he has aged slower than his Earthly twin. If you're interested in thinking about relativity, as it seems that you are, I highly highly recommend this book: Time Travel in Einstein's Universe. It discusses how relatively allows time travel to both the future and the past, and is written for people without a science background. (The travel to the past part is somewhat controversial - Gott (the author of that book) and Hawking argue about this)

J. R. Gott

Time Travel in Einstein's Universe.

> They are the least problematic because essentially magic is involved, its hand waving and saying "Yeah its there because its always been there"

No, they're least problematic because there are no causality issues at all. All this forth and back through wormholes and black holes creates a closed time like curve where the actions of an object traveling in time and interacting with itself leads back into exactly this path.

Also there's not really a bootstrapping problem. To our minds this looks like violating common sense, because we're so used to the arrow of time. However common sense never works very well with this kind of physics.

But on a quantum level such closed time like curves are formed from nothing all the time: virtual particle / anti-particle pairs forming and destroying themself, if you look at the equations the anti-particle is actually a "normal" particle moving backwards through time.

There's also an excellent pop-sci book, written by a theoretical physicist, that deals with all things time-travel, focusing on how this works in the Einstein view (general relativity) of the universe. I highly recommend reading it. The case of closed loop bootstrapping is covered exhaustively and even discussed as a possibility for how the universe may have come to be in the first place: Amazon link: http://www.amazon.com/Time-Travel-Einsteins-Universe-Possibilities/dp/0618257357

>Okay, so I wouldn't bother going back in time. If there is no reason for something to happen, and you just said that I would have to go back in time because, as you admit, if I didn't have to, then it doesn't make any sense, then you accept that time travel, as described by just about every movie, is simply a silly notion.

Yeah, no. I didn't say that there wasn't a reason for the thing to happen, I said that I didn't know what the reason was.

> If my going back in time to get a car is dependent entirely on whether or not I have that car in my garage (which I think would be the criteria for any fiscally responsible person in existence)

Yeah, see, this is your mistake. Sorry, but all your fancy pants attempt at using formal logic did was make you look even more confused. You never actually explained why any of my scenarios were logically inconsistent (all three directly addressed your false assumption), you basically just moaned that you didn't like them. Probably because you don't like causal loops. Which is a failing of your mental faculties, not a fault with time travel.

If you need convincing that causal loops are not "silly", you should read up on closed timelike curves, which are what physicists call time travel when it happens within the general theory of relativity. There is a book called Time Travel in Einstein's Universe that might be of use to you. It discusses scenarios like this. They can be modelled mathematically, and so assuming ZFC is consistent, they do not lead to a paradox.

But of course, I'm sure to you the combined efforts of some of physics' finest minds are just some "lackluster explanation". Which I gather from context means "thing I refuse to understand".

It is a fairly simple concept that you can easily look up and read up in more detail about (I suggest http://www.amazon.com/Time-Travel-Einsteins-Universe-Possibilities/dp/0618257357 which includes a very good explanation).

It is simply a statement of probability. If you are a random human (and guess what -- you are), it is most probable that you will come into existence when there are more humans than when there are less humans (assuming you are not in some way "special"). If you don't understand this bit, don't waste your time reading further, as that is fundamental.

Gott expresses the principle in terms of confidence levels (as a percentage). e.g. We can be 95% sure we are in the middle 95% of the span of human existence, or we can say we are 50% sure were in the middle 50%. So confidence in the prediction drops as the prediction becomes more narrow.

It makes total sense, and I can't help you if you do not understand the concept (or are unwilling to read one of the many sources that describe it).

The current well-documented rise of human population is completely irrelevant to what we are describing (and is likely to be constrained by resource constraints and disease, anyway). You would have to be pretty nuts to think human population can grow geometrically forever, whilst it has a finite resource base.

Edit: You may also want to try and find this article: http://www.nature.com/nature/journal/v363/n6427/abs/363315a0.html

Not really a physicist in the sense that I have a degree in anything but I am current an undergraduate doing research so I think I somewhat count.

The main reason why I became intrested in physics because, much to the annoyance of my friends, I never was satisfied to just accept that things work, I usually had to know why it worked. While it started off really being just interested in science in general, I became much more interested in physics when my parents got me this book when I was like 10. I was not able to really read most of it until I was older but even at that young age I could appreicate how amazing physics is. I mean if you tell a little kid that if he runs fast enough he is actually slowing down time how can't that kid just be amazed, especially before he understand how fast he would have to run to get that effect lol? Then throughout high school I always would just go on wikipedia and read through all of the astronomy and physics articles that I could understand. My high school education itself though was pretty shacky when it came to physics. I did not like much calc or physics and only a little chem so I became extremly scared to go to uni to major in physics. I went my entire first year of college undeclared since I was so scared that I would not be able to make it. I would have went longer undelcared but I needed to sign up to registar for the second year physics classes. Even though I was not offically a physics major I was enjoying college so much. Being surrounded by people who care about the same subject as I did was so amazing to me. I no longer felt weird and out of place for enjoying learning about the world. Even though I felt like i fit in better at college, even into most of my second year I was scared I would not be able to make it. Now though I feel so much better since I have started taking higher level classes and getting 3.5s or higher in them and also starting to do research. To me one of the best things about being a physics major at college(well probably any major really) is thinking about to over a year ago and how little was able to do compared to now. One year ago I had next to no understand of quantum mechanics, minimal understand of Special relativity, and only basic calculus knowledge. Now I feel so much more knowledgable about everything and I am amazed about how much math I ended up learning this last year. My research and physics classes have also helped me so much this last year with some personal issues I was dealing with this last 6 or so months, as it gave me something to focus on. Granted being in college also has caused a lot of stress. An "easy" class would only require 10 hours of home work a week to me right now. Due to this then I have gotten into the habit of getting minimal sleep and barely eatting during the school year since I just dont have time to do research, go to classes, do homework, and try to maintain an healthy lifestyle. Even though it makes me sound masochistic, I have in a way enjoyed being in that kind of life style as it makes me feel that I am actually earning what I am doing and I am doing this not because it is easy but because I want to go into physics.

Ninja Edit: just posted this and saw how long it was lol. Sorry if it seems a little ranty(dont know if that woud be the right word to use) but like I mentioned I have had some issues the past few months and think this was a nice little refection thing that I could do on myself about the past.