# What's the status of time travel? **URL:** \ **Category:** Factual Questions\ **Created:** [March 3, 2003, 1:23pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746 "2003-03-03T13:23:46Z")\ **Posts on this page:** 20\ **Page:** 9
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 6, 2018, 10:33pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/161 "2018-09-06T22:33:27Z")
Nope, you don’t need acceleration to get relativistic effects. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [September 6, 2018, 10:57pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/162 "2018-09-06T22:57:25Z")
> [@Brayne\_Ded](#): > > I accept that time travel is impossible in the sense of going back to the past and changing actual events. IMHO, the only way out of that is if there are parallel universes, so a new timeline opens up in another universe if you went back. The logical problem: which timeline would you be in? Could you be in both at the same time? In which case, what happens to your awareness? > > Just possibly it might be possible to go back into the past - but only as an observer. Could this explain some paranormal phenomena? The question might be whether whether you could get back again to your temporal starting point. If there is a way to do this, then at our present understanding I would guess that it would be done somehow with quantum mechanics. What awareness even is is a question beyond science. If you were to jump into another universe, would that be you, or would it be a person like you spontaneously being created in that universe, and the you here mysteriously disappearing? Would there be any difference in the you that was spontaneously created if the you here was not destroyed? Observing the past is easy. You are doing so right now. If you want to see further back, look further out. If you can find some giant mirror thousands of light years away, you could look back at the earth thousands (times 2) of years ago. If you could “solve” quantum mechanics and get rid of the uncertainty “problem” and the universe becomes both deterministic (which it might be, or it might not be, we don’t know) and determinable(not likely, IMHO), then you could look not just into the past, but into the future. ---
**Author:** ![mandala](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mandala/32/95_2.png) [@mandala](https://boards.straightdope.com/u/mandala)\ **Post date:** [September 7, 2018, 12:12am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/163 "2018-09-07T00:12:12Z")
> [@Exapno\_Mapcase](#): > > Nobody does it none of the time. People only observe the present. > > We do have memories and visual and aural records of the past. Of course those may have just been created by a trickster god. > > I’m betting that no real understanding of time travel will occur before we have a real understanding of time. We’re not even close to that. …and a real understanding of the mind, since only the mind perceives the flow of time, and is supposedly able to experience timelessness (whatever that means) in deep meditative states. We’re not anywhere close to that. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [September 7, 2018, 12:48am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/164 "2018-09-07T00:48:21Z")
When I go out and look at the stars, I’m seeing the past. I’m seeing those stars as they existed years or decades ago. Not millions though, unless you’re using a telescope to observe stars outside our galaxy. ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 7, 2018, 12:54am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/165 "2018-09-07T00:54:24Z")
> [@eburacum45](#): > > But most of the semi-practical methods of time-travel I am aware of involve travelling through a wormhole, with temporally-displaced mouths. This means you turn up at a particular location in time and space, and you know exactly where, and when you are. You can go back and forth through the wormhole at will, since it connects two discrete locations in space/time, and there is no question of suddenly appearing in empty space. Now what I heard is that imagine what you have created the moment you set up that wormhole. The way wormholes are thought to work, mass entering one mouth makes the black hole on that side heavier, while mass exiting the other mouth makes that black hole lighter. (if wormholes can ever be built they probably won’t be transitable by anything but photons or a particle beam - which is just as good as being able to walk through as an intact human if you have sufficiently advanced technology) Anyways by creating a wormhole you create a potential path for photons to go in one mouth, come out the other, travel through space, and interact with their past self. This “short circuit” happens the very instant you move the wormhole mouths enough to allow the tiniest magnitude trip to the past. Anyways in theory this would cause so much transit traffic that one mouth of the wormhole would instantly lose infinite mass. Or, in practice, all of it’s mass. Basically the whole thing would detonate like a mass-energy bomb. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 7, 2018, 1:21am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/166 "2018-09-07T01:21:23Z")
> [@](#): > > Quoth **mandala** : > > …and a real understanding of the mind, since only the mind perceives the flow of time, and is supposedly able to experience timelessness (whatever that means) in deep meditative states. We’re not anywhere close to that. Nah, that part is really, really easy to understand. The explanation is this: It’s meaningless bunk. It’s not even self-consistent bunk: If only the mind were able to perceive time, then _everything but_ minds would be in a state of timelessness. ---
**Author:** ![mandala](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mandala/32/95_2.png) [@mandala](https://boards.straightdope.com/u/mandala)\ **Post date:** [September 7, 2018, 1:45am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/167 "2018-09-07T01:45:57Z")
> [@Chronos](#): > > Nah, that part is really, really easy to understand. The explanation is this: It’s meaningless bunk. It’s not even self-consistent bunk: If only the mind were able to perceive time, then _everything but_ minds would be in a state of timelessness. A bit of a overstatement? I meant to say that while everything is subject to the flow of time, only a self-aware mind is able to \*perceive \*the flow of time. In deep sleep, with the mind quiescent, time flows but with no conscious awareness to perceive it. If a self-aware mind can perceive the flow of time, it may be able to peek into the past and future, a not exactly physical but mental time travel. Gross objects, on the other hand, simply move forward with the arrow of time. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 7, 2018, 2:55am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/168 "2018-09-07T02:55:38Z")
> [@mandala](#): > > If a self-aware mind can perceive the flow of time, it may be able to peek into the past and future, a not exactly physical but mental time travel. Sorry, but I have to agree with Chronos. This is meaningless bunk. ---
**Author:** ![tampora](https://avatars.discourse-cdn.com/v4/letter/t/9de0a6/32.png) [@tampora](https://boards.straightdope.com/u/tampora)\ **Post date:** [September 7, 2018, 4:36am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/169 "2018-09-07T04:36:17Z")
> [@Alessan](#): > > But isn’t that effectively FTL travel? I mean, that way you can travel to a point in spacetime one second in the future and 100 light years away. No law of physics prevents ‘effective’ FTL travel; only FTL travel in local regions. Consider the furthest galaxies from us are moving away faster than the speed of light. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [September 7, 2018, 9:36am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/170 "2018-09-07T09:36:53Z")
> [@tampora](#): > > No law of physics prevents ‘effective’ FTL travel; only FTL travel in local regions. Consider the furthest galaxies from us are moving away faster than the speed of light. Local regions? Does that mean there’s like a minimum range? ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [September 7, 2018, 9:48am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/171 "2018-09-07T09:48:43Z")
> [@Alessan](#): > > Local regions? Does that mean there’s like a minimum range? Yes, that’s right. You have a personal future light cone; if you open a wormhole to a location within your own light cone, now-you can travel to your own future (and more importantly, future-you can travel back into your present). But if the other wormhole mouth is outside your personal light-cone, you are safe from paradox. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 7, 2018, 10:36am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/172 "2018-09-07T10:36:31Z")
Clocks perceive time a heck of a lot more accurately than human minds do. And of course the human mind can (inaccurately) peek into the past and future: That’s called “memory” and “planning”. Which has nothing to do with time travel, on either count. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [September 7, 2018, 10:52am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/173 "2018-09-07T10:52:41Z")
The subjective or personal experience of time is also a fascinating subject. For instance, we can see at least two million years into the past with the unaided eye, by looking at the Andromeda Galaxy. I’ve seem M81 through a telescope, more than eleven million years into history. Conversely a photographer needs to photograph the future, especially if photographing fast-moving objects; if you want to photograph runners, a car race, a horse race or a bike race, you need to aim the camera where the racers will be when the shutter opens. This is slightly ahead of where your eye sees the racers, depending on how quickly the camera responds. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [September 7, 2018, 12:25pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/174 "2018-09-07T12:25:40Z")
> [@eschereal](#): > > Thing is this: the time dilation effects of SR are based on acceleration. > [@Chronos](#): > > Nope, you don’t need acceleration to get relativistic effects. These two posts seem to be contradictory. I’m confused. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 7, 2018, 12:37pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/175 "2018-09-07T12:37:13Z")
Special Relativity deals with inertial frames of reference. That means no net forces, which means in turn no acceleration. The gamma function is driven by v[sup]2[/sup]/c[sup]2[/sup] not by any a variable. Acceleration shows up in General Relativity. ---
**Author:** ![RaffArundel](https://avatars.discourse-cdn.com/v4/letter/r/87869e/32.png) [@RaffArundel](https://boards.straightdope.com/u/RaffArundel)\ **Post date:** [September 7, 2018, 12:46pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/176 "2018-09-07T12:46:40Z")
It only requires velocity, not acceleration: > **[Lorentz factor](https://en.wikipedia.org/wiki/Lorentz_factor)** > > The Lorentz factor or Lorentz term is a quantity expressing how much the measurements of time, length, and other physical properties change for an object while that object is moving. The expression appears in several equations in special relativity, and it arises in derivations of the Lorentz transformations. The name originates from its earlier appearance in Lorentzian electrodynamics – named after the Dutch physicist Hendrik Lorentz. > It is generally denoted γ (the Greek lowercase letter gamma)... Side note, I recall seeing a video on the Fermilab Youtube channel about how the twin paradox had nothing to do with acceleration (which apparently was a common misunderstanding) and demonstrated that it was only the velocity that mattered. ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [September 7, 2018, 1:57pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/177 "2018-09-07T13:57:26Z")
The Twin Paradox is not actually a paradox, not in the same sense as the Grandfather Paradox. This entire discussion is about violations of causality, where you have events where the effect precedes the cause, or the effect has no cause at all. The Twin Paradox merely describes a situation where two observers in two different IRFs have different observations and there is no answer to the question of which one of them is right. So what? This does not violate causality (unless the two IRFs have a relative velocity greater than c). So, let’s drop the word “paradox”. Sending information of any kind faster than c can create conditions that can violate causality. Sending information of any kind backwards in time can create conditions that can violate causality. Building a wormhole that has the ability to do both of those things can create conditions that can violate causality. Two people passing each other in space ships at different speeds (less than c) does _ **not** _ violate causality. We are always moving forward through time without violating causality. Moving forward at a “slower” or “faster” rate would not violate causality either. Moving backwards through time is a completely different problem. That’s where you can violate causality. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [September 7, 2018, 5:16pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/178 "2018-09-07T17:16:15Z")
> [@Alessan](#): > > But isn’t that effectively FTL travel? I mean, that way you can travel to a point in spacetime one second in the future and 100 light years away. Yes. According to Matt Visser this is one of the few methods of FTL that does not cause temporal paradoxes, so long as you stick to the rules. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [September 7, 2018, 5:40pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/179 "2018-09-07T17:40:49Z")
> [@Grey](#): > > Special Relativity deals with inertial frames of reference. That means no net forces, which means in turn no acceleration. The gamma function is driven by v[sup]2[/sup]/c[sup]2[/sup] not by any a variable. > > Acceleration shows up in General Relativity. Ah, I see. Thanks! Is GR the thing that is corrected for in GPS satellites and such? ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [September 7, 2018, 5:52pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/180 "2018-09-07T17:52:04Z")
> [@eburacum45](#): > > Yes. According to Matt Visser this is one of the few methods of FTL that does not cause temporal paradoxes, so long as you stick to the rules. What are the rules? [Previous page](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746.md?page=8) [Next page](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746.md?page=10)