# Time traveling in the future and in the past

• B
Homework Helper
Gold Member
Summary
Time traveling in the future and in the past
As far as i know, current special relativity allows for time travelling but only in the future. In sort what we have to do to travel to the future, according to special relativity, is to make a spaceship that can achieve speed comparable to the speed of light. Then while we are travelling with this spaceship, time will run slower for us than for the people of earth, so we might have that 1hour for us is like 1million hours for people in earth, so we can travel to the future that way. We just don't have the technology yet to make a spaceship that can achieve such speed.

However what about time travelling to the past? Is it possible by special relativity? Are there any theories that allow for time travelling to the past?

benorin and etotheipi

phinds
Gold Member
2021 Award
Summary:: Time traveling in the future and in the past

As far as i know, current special relativity allows for time travelling but only in the future. In sort what we have to do to travel to the future, according to special relativity, is to make a spaceship that can achieve speed comparable to the speed of light. Then while we are travelling with this spaceship, time will run slower for us than for the people of earth, so we might have that 1hour for us is like 1million hours for people in earth, so we can travel to the future that way. We just don't have the technology yet to make a spaceship that can achieve such speed.
No, time will not run slower for you, it will tick away at one second per second, as it will for the stay-at-homes, and this is with the same definition of seconds for each. What WILL happen is that fewer of those seconds will occur for you than for the stay-at-home, provided that you come back. This is called differential aging, which you are confusing with time dilation. It's a very common misconception.
However what about time travelling to the past? Is it possible by special relativity? Are there any theories that allow for time travelling to the past?
no and no

Homework Helper
Gold Member
This is called differential aging, which you are confusing with time dilation. It's a very common misconception.
Can you explain to me what is differential aging and what is time dilation. The way i understand is that differential aging is caused by time dilation.
no and no
Ok i suppose there are no mainstream theories that allow for travelling in the past. But what about non mainstream theories, there must be some books out there that allow for time travel in the past.

phinds
Gold Member
2021 Award
Can you explain to me what is differential aging and what is time dilation. The way i understand is that differential aging is caused by time dilation.
No, time dilation is a observed effect, differential aging is a real effect. Think of this: you and a friend each decide to drive from the same starting point from Washington DC to New York City, each going at 60mph the whole way. You go one route and he goes another. When you meet up, your odometers don't read the same even though your odometers read the same for both trips. This is differential distancing. Continuing the analogy to show how your speed can look different to each other (speed dilation) is doable but tedious and I don't want to bother. The speed dilation will be just something that you see temporarily. The differential distance will be real.

Ok i suppose there are no mainstream theories that allow for travelling in the past. But what about non mainstream theories, there must be some books out there that allow for time travel in the past.
There are. They are all in the Science Fiction section at your local library.

Ibix
Can you explain to me what is differential aging and what is time dilation. The way i understand is that differential aging is caused by time dilation.
In short, if you are moving inertially, "the whole universe, at time ##T##" is usually defined as the plane perpendicular to your path through spacetime that passes through the event where your watch reads ##T##. If you and I are following paths in different directions through spacetime (i.e., we are not at rest with respect to each other) then the time you measure between my ##T## and ##T+1## planes is not 1 because my planes are not perpendicular to your path. That's time dilation.

If you and I meet up, follow different paths through spacetime, and meet up again, the length of our paths through spacetime may be different. The length of a path through spacetime is the time elapsed for someone following that path. That's differential aging.

The two phenomena are related. The distance between your perpendicular planes as measured by me is related to your speed as measured by me, and (in the special case where I remain inertial) so is your path length. But they are not the same thing.

Note that all of the above relies on Minkowski geometry, not Euclidean geometry.
Ok i suppose there are no mainstream theories that allow for travelling in the past. But what about non mainstream theories, there must be some books out there that allow for time travel in the past.
General relativity allows for spacetimes that include closed timelike curves (CTC). These are closed paths that an object could follow so that its time repeats. Paths "near" a CTC would be a form of time travel. Spacetimes containing wormholes with an entrance in the other one's causal past would be an example. I don't think that any such spacetime is regarded as physically plausible, however.

sysprog, Delta2 and etotheipi
Ibix
Last edited:
Homework Helper
Gold Member
I am having major trouble comprehending the last two posts, probably because my background in SR and GR is really poor. I ll try to read them carefully and come back with any good questions i might have.

For the time being:
@phinds what is the difference of an observed effect and a real effect. From a layman's view they seem to be the same thing
@Ibix Are there any books or papers on CTCs? From a raw search in google i read the wikipedia entry :" If CTCs exist, their existence would seem to imply at least the theoretical possibility of time travel backwards in time, raising the spectre of the grandfather paradox, although the Novikov self-consistency principle seems to show that such paradoxes could be avoided. "

Homework Helper
Gold Member
@Ibix the skeptical is with the literal meaning of word in greek (from skepsis=thinking) , you put me into serious thinking with the last two posts because i am not able to comprehend them though i think and i think over and over again :D.

PeroK
Homework Helper
Gold Member
2021 Award
Can you explain to me what is differential aging and what is time dilation. The way i understand is that differential aging is caused by time dilation.
Velocity-based time dilation is symmetrical and reciprocal between inertial frames. There is no physical difference between "a moving frame" and "a stationary frame". There is nothing that corresponds to a state of absolute motion.

Differential ageing corresponds to the different proper time associated with different paths through spacetime. The Lagrangian principle applies in relativity, whereby the natural paths are those that maximise (not minimise) the proper time between spacetime points. Travelling inertially maximises this proper time. If you are travelling inertially and your path through spacetime intersects twice with another path, then that path must have less proper time between the two intersections.

This is the basis of the twin paradox.

sysprog, etotheipi and Delta2
Homework Helper
Gold Member
There are. They are all in the Science Fiction section at your local library.
I keep my mind open to all the possibilities (well not ALL but at least some the seem sort of reasonable). Some things that are science fiction for today, might be reality in a few hundred years. Think of Jules Vern that was writing a book about a trip to the moon. It was science fiction at 1850s but it is reality today.

weirdoguy and PeroK
etotheipi
There are. They are all in the Science Fiction section at your local library.

This might be slightly off topic, but this paper co-authored by Kip Thorne explains some of the Physics behind the wormholes in the film interstellar, which I thought was pretty cool.

Ibix and Delta2
PeroK
Homework Helper
Gold Member
2021 Award
I keep my mind open to all the possibilities (well not ALL but at least some the seem sort of reasonable). Some things that are science fiction for today, might be reality in a few hundred years. Think of Jules Vern that was writing a book about a trip to the moon. It was science fiction at 1850s but it is reality today.
You might be interested in this:

Ibix and etotheipi
Ibix
For the time being:
@phinds what is the difference of an observed effect and a real effect.
I'm not sure the terminology is quite right here. Time dilation is not a direct observable, but differential aging is. What this means is that, if I watch you passing me, I'll see your watch tick fast or slow because of the Doppler effect. I'll only find time dilation if I correct for the changing distance - which means adopting a particular inertial frame (probably my rest frame, but it's not obligatory) and its notion of "space". Hence time dilation is an interpretation of my observations, not an observation itself. Differential aging, on the other hand, can be directly observed - we zero our clocks, you go off and come back, and we compare our clocks side-by-side. No interpretation necessary.
Are there any books or papers on CTCs?
Probably. I must say I haven't really looked into them, so I can't make any good recommendations. I know they arise in the interior of Kerr black holes (but we don't think that's an accurate model of a realistic black hole), and in the Gödel spacetime (but that's not remotely physically plausible).

Delta2
Ibix
@Ibix the skeptical is with the literal meaning of word in greek (from skepsis=thinking) , you put me into serious thinking with the last two posts because i am not able to comprehend them though i think and i think over and over again :D.
Fair enough - grumpiness retracted.

benorin, etotheipi and Delta2
Homework Helper
Gold Member
You might be interested in this:
Don't worry about me @PeroK, i can understand that speaking about warp drives today is like speaking about smartphones in 1750, but maybe warp drives can become a reality in a few hundred years.

PeroK
Homework Helper
Gold Member
2021 Award
what is the difference of an observed effect and a real effect. From a layman's view they seem to be the same thing

Relative inertial velocity between frames can be seen as a "rotation" of spacetime. Time dilation, therefore, is simply a relationship between coordinates. This is analagous to a rotation in 3D space that changes the spatial coordinates of two points. In that case: the difference in the x-coordinates between two events is not a physical difference. To take an example, if an object is aligned with your x-axis and my y-axis, that is not a physical difference.

SR unfies space and time into Minkowski spacetime, which allows different inertial frames to measure different time and spatial differences between events. This includes time dilation and length contraction. But, these do not represent physical differences, only a difference in coordinates.

Differential ageing is independent of coordinates. Time dilation, in short, is an effect that depends on your choice of coordinates.

Better to learn this stuff properly, though:

https://scholar.harvard.edu/files/david-morin/files/relativity_chap_1.pdf

Delta2
Ibix
Don't worry about me @PeroK, i can understand that speaking about warp drives today is like speaking about smartphones in 1750, but maybe warp drives can become a reality in a few hundred years.
Alcubierre's warp drive is another spacetime that falls under "not physically plausible", requiring negative energy densities. I've seen it stated that it implies the possibility of time travel (via a tachyonic anti-telephone kind of trick) but not a complete mathematical description of a spacetime containing two warp drives, which would be necessary for such time travel.

PeroK
Homework Helper
Gold Member
2021 Award
Don't worry about me @PeroK, i can understand that speaking about warp drives today is like speaking about smartphones in 1750, but maybe warp drives can become a reality in a few hundred years.
There is a difference, however, in being confident in engineering progress and being confident that humans will eventually circumvent the laws of physics. Take nanotechnology for example: are you confident that eventually we will have technologies that use components smaller than those provided by nature? Or, are we ultimately limited to use nothing smaller than elementary particles?

That's a fundamental difference, in my view, to assume that we can go beyond harnessing the physical universe to changing it in ways that nature itself cannot achieve.

phinds and weirdoguy
Homework Helper
Gold Member
There is a difference, however, in being confident in engineering progress and being confident that humans will eventually circumvent the laws of physics. Take nanotechnology for example: are you confident that eventually we will have technologies that use components smaller than those provided by nature? Or, are we ultimately limited to use nothing smaller than elementary particles?

That's a fundamental difference, in my view, to assume that we can go beyond harnessing the physical universe to changing it in ways that nature itself cannot achieve.
Yes I also believe that we cant circumvent the laws of physics, but the problem is that we cant be sure what exactly are the laws of physics. We have some theories today that tell us that warp drive is against the laws of physics (because it requires negative energy densities for example as @Ibix says) but who tell us that the theories we are based today will be changed in the future and make negative densities and warp drives possible.

But what about non mainstream theories

Well, that's simple:

Physics Forums Global Guidelines said:
We wish to discuss mainstream science. That means only topics that can be found in textbooks or that have been published in reputable journals.

but who tell us that the theories we are based today will be changed in the future and make negative densities and warp drives possible.

Maybe they will, maybe they won't. For now we don't have any basis to talk about non-existent theories. But you've been told that not so long ago in a different thread:

We can only discuss theories that actually exist, based on textbooks or peer-reviewed papers that describe what the theory says.

phinds
Gold Member
2021 Award
@phinds what is the difference of an observed effect and a real effect. From a layman's view they seem to be the same thing
Well, if you don't like my analogy in post #4, I think Perok explained it very well in post #16.

Homework Helper
Gold Member
Ok well thanks for all the replies, i believe i have learnt something, especially the thing called CTC. So a mainstream theory like general relativity doesn't forbid timetravelling in the past, though CTCs seem to be for general relativity, like plane EM waves are for maxwell's equations. Plane EM waves are theoretical solutions to Maxwell's equations in vacuum, but there is no way of producing plane EM waves through some sort of finite antenna structure. It seems that CTCs are purely theoretical thing in the same manner too.

George Jones
Staff Emeritus
Gold Member
Ok i suppose there are no mainstream theories that allow for travelling in the past. But what about non mainstream theories, there must be some books out there that allow for time travel in the past.

Here are some serious references.

At the popular level, there is the excellent, non-technical reference on time travel, the second edition (make sure that it's the second edition) of "Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction" by Paul Nahin. This is a wonderful book that is written for the educated layperson.

Physicist and Nobel Laueate Kip Thorne wrote a foreword for the second edition of this book, and here's a quote from this foreword: "It now is not only the most complete documentation of time travel in science fiction; it is also the most thorough review of serious scientific literature on the subject - a review that, remarkably, is scientifically accurate and at the same time largely accessible to a broad audience of nonspecialists."

More technical references by serious physicists:

1) "The Quantum Physics of Chronology Protection" by Matt Visser
https://arxiv.org/abs/gr-qc/0204022

2) "Wormholes, Warp Drives, and Energy Conditions", Francisco Lobo, editor

3) "Back-in-Time and Faster-Than-Light Travel in General in General Relativity" by Sergui Kasnikov.

2) and 3) are interesting recent books at the about the level of a beginning grad student, and are based on papaer publised in top research journals like Phy. Rev. D, and the journal of Classical and Quantum Gravity.

etotheipi, Delta2, Ibix and 1 other person
robphy
Homework Helper
Gold Member
Possibly useful technical references on causal structure and on time travel [I haven't read through all of them]:

Penrose's "Techniques of differential topology in relativity"

Geroch & Horowitz's "Global structure of spacetimes" in Hawking&Israel - General Relativity: An Einstein Centenary Survey [look for the books.google.com link]

Geroch's "General Relativity: 1972 Lecture Notes" (see "29. Time-Orientation" and "30. Causality Violation")
typset draft at http://home.uchicago.edu/~geroch/Course Notes
published https://www.amazon.com/dp/0987987178/?tag=pfamazon01-20

Thorne's "Closed Timelike Curves" https://www.its.caltech.edu/~kip/index.html/PubScans/II-121.pdf

In addition to physicists and mathematicians,
I've found that there some useful references from the "philosophy of physics" community.
The following approach the problem using mathematical tools (like light-cones and spacetime diagrams) from relativity

http://faculty.poly.edu/~jbain/philrel/index.html
http://faculty.poly.edu/~jbain/philrel/philrellectures/16.TimeMachines.pdf
http://faculty.poly.edu/~jbain/philrel/philrellectures/17.TimeMachineST.pdf

http://www.lps.uci.edu/~jmanchak/gss.pdf

From Earman's website:
http://www.pitt.edu/~jearman/
http://www.pitt.edu/~jearman/Earman_1986PrimerOnDeterminism.pdf
http://www.pitt.edu/~jearman/Earman_1989WorldEnoughAndSpacetime.pdf
https://www.amazon.com/gp/product/0262050404/?tag=pfamazon01-20
http://www.pitt.edu/~jearman/Earman_1995BangsCrunches.pdf
https://www.amazon.com/gp/product/019509591X/?tag=pfamazon01-20

Smeenk & W¨uthrich "Time Travel and Time Machines"
http://philsci-archive.pitt.edu/4915/1/SmeenkWuthrich2009TimeTravelPittArchive.pdf

Delta2 and Ibix