# Need help with understanding time travel

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• Physics_is_beautiful
Physics_is_beautiful
TL;DR Summary
My theory on how time travel could work.
need help to find out its pros and cons, and possibilities.
Ridiculous!
I know.

But as a 9th grader, there are a few things that have been bugging me.
I had a conceptualisation of how 'time travel' could work.

However, it's based on the presumption ( a very large one at that) that the many-worlds interpretation of quantum mechanics is correct.

Ridiculous!
hear me out.

I was wondering about the origins of our universe, specifically how it was before the Big Bang.
A deep state of matter-less nothing. In principal, that's how EVERY universe would start off. From completely nothing. Sure, it would vary from the get go, like if and how the Big Bang occurred, but this state remains same for all universes, from the simple reason that for something to be created, there has to be nothing. It's not possible for any universe to just, exist instantaneously. There has to be some change in the status quo. Let's say this is point 'A'.

Now, after some point, our universe is said to return to this state, when there exists nothing. After some time in every universe, the same thing has to happen. Point 'B'.

Now, imagine an infinite set of lines between point 'a' and point 'b' , the ones closer to each other entangled.

That is how our 'infinite set' of universes could exist.
(see image 1 for more clarity)

and the 'points' at which they entangle could represent which events are the same for the universes. Since we're talking about worlds with only the minutest differences, this would be possible.

(EG : in a world where I used a blue coloured pencil instead of red to demonstrate that digram, most events would remain same.)

But how does time travel fit in?

What I thought was Time travel is nothing but travel from 1 universe to another. Let's say I go back to 1972, all I have done is gone to a universe where 1972 is the present day. ( see image 2 for more clarity)

That would solve the two major constituent problems with time travel.

A) If it was there, it would have already been done - it would be, we just wouldn't know because our 'world line' and the ones near to us don't have a device capable of perform travel through other universes. Moreover, whatever time frame the traveller would go to, that would be the present. Ergo, all the actions performed by the time traveller wouldn't 'alter' anything, as the future in itself is not decided.

B) Paradoxes - same logic. We can't change anything if a future isn't even decided. Sure, we could take a guess from the universes around it, but we can never be sure.

[This is how the timeline of it would look.]

The problem that came up to me first was that it's not very 'testable' per se. Even if we assume it's right, we can't prove it.

PS - I might have completely violated the laws of physics with this, I am only in 9th, thus my understanding is limited. still would just like to have another opinion on this.

thanks.

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Of course, you realize this is all personal speculation on your part on how time travel might work but to really understand the physics of it you will need to go back and learn a lot of applied math and the physics to go with it as that is the only way that time travel might be accomplished or declared impossible to do.

Pop sci books are fun to read but they often lead the reader astray with their conceptualizations and one can't develop a meaningful theory of time from them alone. You really need to go back to the math and physics to understand what you don't yet know.

Here's one of my favorite videos from NOVA entitled The Great Math Mystery

Here's another NOVA show on time travel:

Keep in mind that pop sci videos suffer the same problem as books the concepts and words may mislead the lay person and the only way to clarify it is to go back to the math.

Physics_is_beautiful said:
I was wondering about the origins of our universe, specifically how it was before the Big Bang.
A deep state of matter-less nothing. In principal, that's how EVERY universe would start off. From completely nothing.
But would they? We have no idea what the universe was like prior to some point in time. Let's call this point in time t=0. At any time after t=0, we have matter, energy, and the universe works according to known laws. But at t=0 and before we get infinities in our math. Specifically, if we start at, say, t=1, and run the clock backwards towards t=0 the density runs off to infinity. This probably means we don't understand physics at the extremely high energy and density scales of the early universe, but in no way says that before t=0 there was nothing. Note that labeling t=0 is an arbitrary choice. I could have labeled it t=100. Or t=-1,000.

Physics_is_beautiful said:
Sure, it would vary from the get go, like if and how the Big Bang occurred, but this state remains same for all universes, from the simple reason that for something to be created, there has to be nothing. It's not possible for any universe to just, exist instantaneously. There has to be some change in the status quo.
That's an assumption. More than one actually, since you are also assuming the universe didn't exist at some point. Which is sort of a problematic statement. How can the universe ever have not existed if time requires the existence of the universe? Non-existence of a universe would seem to imply that time exists independently of the universe.

These issues are why we usually don't worry too terribly much about things we can't observe.

Physics_is_beautiful said:
The problem that came up to me first was that it's not very 'testable' per se. Even if we assume it's right, we can't prove it.

I'd say you have been talking about something else other than time travel. What you've described is more like travel between alternate universes (which aren't known to exist and have absolutely no evidence supporting their existence). This falls more into sci-fi and not science, as there is no evidence for the existence of a multiverse, no known or even theoretical way to travel between them, and not even a theoretical framework to support their existence.

jedishrfu
If time travel was possible, how come we don't have visitors from the past, or from the future with the instruction book?

Will we start to get visitors from the future, once time travel is solved?
If so, why are they not coming here now?

Will we have to build a station "now" to send and receive travellers from the future?

jedishrfu
Drakkith said:
But would they? We have no idea what the universe was like prior to some point in time. Let's call this point in time t=0. At any time after t=0, we have matter, energy, and the universe works according to known laws. But at t=0 and before we get infinities in our math. Specifically, if we start at, say, t=1, and run the clock backwards towards t=0 the density runs off to infinity. This probably means we don't understand physics at the extremely high energy and density scales of the early universe, but in no way says that before t=0 there was nothing. Note that labeling t=0 is an arbitrary choice. I could have labeled it t=100. Or t=-1,000.That's an assumption. More than one actually, since you are also assuming the universe didn't exist at some point. Which is sort of a problematic statement. How can the universe ever have not existed if time requires the existence of the universe? Non-existence of a universe would seem to imply that time exists independently of the universe.

These issues are why we usually don't worry too terribly much about things we can't observe.I'd say you have been talking about something else other than time travel. What you've described is more like travel between alternate universes (which aren't known to exist and have absolutely no evidence supporting their existence). This falls more into sci-fi and not science, as there is no evidence for the existence of a multiverse, no known or even theoretical way to travel between them, and not even a theoretical framework to support their existence.
alright. Poor choice of words from my part. How about we imagine at said t = 0, all matter is compressed to a point which doesn't exist (yet), thus an infinite density. That point is not too prevalent on this idea. We just need to know that every universe starts off in that fashion, and then continues it journey.

Plus, I already did mention that I presume that MWI is 'proven correct' for this logic to apply.

However, that using an interpretation which is unproven, and then basing an idea on it isn't scientifically accurate to say the least. However, it does give us something to think about.

PeroK
jedishrfu said:
Of course, you realize this is all personal speculation on your part on how time travel might work but to really understand the physics of it you will need to go back and learn a lot of applied math and the physics to go with it as that is the only way that time travel might be accomplished or declared impossible to do.

Pop sci books are fun to read but they often lead the reader astray with their conceptualizations and one can't develop a meaningful theory of time from them alone. You really need to go back to the math and physics to understand what you don't yet know.

Here's one of my favorite videos from NOVA entitled The Great Math Mystery

Here's another NOVA show on time travel:

Keep in mind that pop sci videos suffer the same problem as books the concepts and words may mislead the lay person and the only way to clarify it is to go back to the math.

That's what I actually want to know, is there any math that has tried to approach this way of thinking, or has the entire notion of time travel just been consensually been removed by the scientific community for discussion as it's too ambiguous or vague?

Physics_is_beautiful said:
How about we imagine at said t = 0, all matter is compressed to a point which doesn't exist (yet), thus an infinite density.
For clarity, at t=0 the density of the universe goes to infinity everywhere, not just at one point. All of the infinite universe has infinite density everywhere. Which is bad and our theory probably doesn't make the right predictions for this point in time.

Physics_is_beautiful said:
That's what I actually want to know, is there any math that has tried to approach this way of thinking, or has the entire notion of time travel just been consensually been removed by the scientific community for discussion as it's too ambiguous or vague?
It's not that it is 'removed' by the scientific community, it is that it has been investigated and so many problems arise from the notion of time travel in the form of paradoxes that it just doesn't make sense. You simply cannot make a working theory if it results in a paradox. Paradoxes generally expose false statements, half truths, and logical problems that render any physical theory dead if it incorporates them. They represent areas where your theory not only doesn't work, but is actual nonsense. They are at least as bad as a singularity in the math of your theory and are something you always, always want to avoid.

And, like I already said, your idea is more like jumping to parallel universes, not true time travel. It avoids paradoxes by avoiding time travel altogether.

Given your age we (the staff) didn't want to just shut your thread down without providing at least some feedback, but unfortunately time travel is not an allowed topic here at PF. I applaud your interest in science and your willingness and ability to come up with an idea and develop it and I encourage you to continue to learn about science, but as a moderator of the website I cannot allow a topic like this to stay open for discussion. I trust you understand.

Please feel free to message myself or another mentor if you have any questions, comments, or concerns.

DeBangis21, berkeman, topsquark and 3 others
In closing and to elaborate more on what @Drakkith said, we have developed site policies to maintain the quality of discussion on PF:

which are listed in our FAQ above.

With respect to time travel and science, physics has approached the subject through study of Einstein's Relativity theory which led to the discovery of black holes and speculatively to wormholes which was brought up in the NOVA episode I posted earlier.

The math needed to understand this more would be differential geometry, tensor calculus and the physics of General Relativity and of course to complete the picture Quantum Mechanics so pretty much all of physics.

The basis for these math topics are Calculus 1,2,3, Differential Equations, and Linear Algebra... and the list goes on usually covered as an undergrad physics major in college.

Member @gmax137 suggested this book by Paul Nahin to kick-off your studies:

### Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction​

https://www.amazon.com/Time-Machine...?asin=0387985719&revisionId=&format=4&depth=1

This book explores the idea of time travel from the first account in English literature to the latest theories of physicists such as Kip Thorne and Igor Novikov. This very readable work covers a variety of topics including: the history of time travel in fiction; the fundamental scientific concepts of time, spacetime, and the fourth dimension; the speculations of Einstein, Richard Feynman, Kurt Goedel, and others; time travel paradoxes, and much more.
Lastly, there are several books that may pique your interest:

- Gravitation by Wheeler, Thorne and Misner

- Science of Interstellar by Thorne

- Theoretical Minimum books by Susskind on Special Relativity, General Relativity and Quantum Mechanics

You find other interesting books in addition to the ones I mentioned abover here on Amazon:

https://www.amazon.com/dp/0805387323/?tag=pfamazon01-20

In 2006, there was a physicist who wrote a book on his quest to investigate time travel Prof Ronald Mallett for very personal reasons having lost his father early in his career. All of us who have lost loved ones often ponder traveling back in time to make things different.

https://en.wikipedia.org/wiki/Ronald_Mallett

The wiki article has some criticism of his approach to the problem. It kind of highlights the problem in physics of going to far astray from the mainstream theories. Physicist don't really speculate in the way laypeople do as they must backup their thinking with established theories. Failure to do so can lead to be ostracized in the physics community.

One such movie approaching time travel in a personal way, About Time had an interesting premise where the protagonist found he could redo events in his life to make things better but found there were limits in his abilities. The movie hints at the philosophical implications of traveling back in time where science is vague on what limits nature has imposed on such travel.

I close this now by saying interest and speculation can be good tools if you realize the weaknesses in your thinking and then proceed to learn more to strengthen your understanding. It's how all great scientists, mathematicians, and engineers have started their careers.

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