<p>This is a tricky topic and my answer will be clearly speculative. I will say that this is also largely a matter about metaphysical allowances and involves personal identity implications. Assuming that time travel is physically possible, we need to make assumptions around how time is interpreted within the time travel journey.</p>

<p><strong>Scenario 1: Time travel based on external time and fatalism</strong></p>

<p>Lewis (1976) distinguishes between two interpretations of time, i.e. <em>personal</em> and <em>external</em> time:</p>

<blockquote>
  <p>Instead I reply by distinguishing time itself, external time as I
  shall also call it, from the personal time of a particular time
  traveler: roughly, that which is measured by his wristwatch. His
  journey takes an hour of his personal time, let us say; his wristwatch
  reads an hour later at arrival than at departure. But the arrival is
  more than an hour after the departure in external time, if he travels
  toward the future; or the arrival is before the departure in external
  time (or less than an hour after), if he travels toward the past.</p>
</blockquote>

<p>Should Bob decide to travel back to 12 PM relative to <em>external time</em>, we can speculate implications on his continuity of personal identity. Le Poidevin (2005) elaborates on discontinuous movement through time:</p>

<blockquote>
  <p>... it seems that discontinuous movement is possible: one can move
  from A to B without occupying places in between.</p>
</blockquote>

<p>If so, it seems probable that Bob could lose his memory of playing tennis at 2 PM if discontinuous movement is allowed. Based on external time, he is in fact moving back 2 hours, however, implied is the suggestion that he won't be psychologically continuous and thus, won't remember that experience. Whether he will treat 12 PM as an ordinary experience or a completely new experience depends on how we interpret causal relations between past and present.</p>

<p>Fatalism suggests that we are unable to deliberate about the future or change it (Taylor, 1962). It is logically and physically impossible to change any events preceding the time travel point. From a fatalist viewpoint, Bob would experience all those events from A to B as they had occurred.</p>

<p><strong>Scenario 2: Time travel based on personal time</strong> </p>

<p>The other scenario presupposes that time travel is continuous from the time traveller's <em>personal time</em> in which the person is psychologically continuous - there is no disconnect between the stages of the psychological and physical continuum when travelling time. If so, it may be possible for Bob to travel back to 12 PM, re-experience eating the pizza while his memory of playing tennis remain intact. Although time travel isn't physically possible, we can still project ourselves backwards in time and recall events (Suddendorf &amp; Corballis, 2007). </p>

<p>Since this scenario doesn't sever the continuity of Bob's personal identity, it seems probable that he would still remember the experience of playing tennis at 2 PM while able to freely experience new events (given that fatalism is rejected) thereafter and the mere possibilities that follow.</p>

<p><strong>References</strong></p>

<ul>
<li>Lewis, D. (1976). The paradoxes of time travel. American
Philosophical Quarterly, 13(2), 145-152</li>
<li>Le Poidevin, R. (2005). The Cheshire Cat problem and other spatial obstacles to backwards time travel. The Monist, 88(3), 336-352</li>
<li>Suddendorf, T. &amp; Corballis, M.C. (2007). The evolution of foresight: What is
mental time travel, and is it unique to
humans? Behavioral and Brain Sciences, 30, 229-351</li>
</ul>

CC BY-SA 3.0
2013-10-03T01:27:21.710Z
I'm mindful this answer relates more so to metaphysics but there are implications for psychological continuity, i.e. how does consciousness, memory, cognitive processes move with temporal changes. While speculative, it still may address the cognitive aspects of time travel.
coeus
CC BY-SA 3.0
2013-10-03T00:38:12.197Z
2013-10-03T00:38:12.197Z
coeus
<p>The answer presumably depends on how you conceptualise the experience of the time traveller relative to the time travel. In most movies the world and most other people in the world are just restored to the original time, but the time traveller is somehow immune to such effects. </p>

<p>For this version of time travel to be true and consistent with neuroscientific theories of the <a href="http://en.wikipedia.org/wiki/Mind%E2%80%93body_problem" rel="nofollow noreferrer">mind-body relationship</a>, then presumably the brain of the time traveller is not altered by time travel, or at least is not altered in such a way to restore the brain to the state it was in at the time of return. Thus, <strong>memories of the future would be retained when going back in time</strong>.</p>

<p>In fact, if the mind of the time traveller was restored to the returned time, then this could result in non-existence if you travelled back further than when you were born. It would also seem inconsistent with the possibility of going back in time but being in the same location that you initiated the time travel.</p>

<p><em>You might also wish to see what answers you get on <a href="https://scifi.stackexchange.com/">https://scifi.stackexchange.com/</a></em></p>

CC BY-SA 3.0
2013-10-03T00:20:25.573Z
2013-10-03T00:32:16.807Z
2017-04-13T12:43:40.813Z
Jeromy Anglim
<p>Suppose it is possible to travel back in time. If Bob traveled back in time, wouldn't he lose the memory of what happened since his neurons are also traveling back in time? In other words, traveling back in time would feel like the present moment. Consider this thought experiment:</p>

<p>Bob eats pizza at 12 PM.</p>

<p>Bob plays tennis at 2 PM.</p>

<p>Bob travels back in time to 12 PM.</p>

<p>Would Bob lose the memory of him playing tennis at 2 PM and essentially treat 12 PM as an ordinary experience?</p>

CC BY-SA 3.0
2013-10-02T22:58:50.167Z
I think this is too speculative and unanswerable at present.
Chuck Sherrington
CC BY-SA 3.0
2013-10-03T00:28:21.183Z
My first instinct was to vote to close this question, but part of me finds it quite interesting to ponder the implications of changing one aspect of reality but holding psychological theory broadly constant. Anyway, I [posted a meta question to discuss these types of questions](http://meta.cogsci.stackexchange.com/questions/713/are-questions-involving-improbable-human-conditions-on-topic) and whether they are on topic. I also think the scientific nature of the answers supports keeping the question open.
Jeromy Anglim
CC BY-SA 3.0
2013-10-03T16:23:20.133Z
@ThinksALot there is definitely a basis to discuss this sort of thing on a physics forum in the context of [closed timelike curves](http://en.wikipedia.org/wiki/Closed_timelike_curve), and there is non-trivial things that can be proved about the [computational consequences of CTCs](http://www.scottaaronson.com/papers/ctc.pdf). However, the person asking the question is obviously not aware of this sort of work, and so the question ask asked does not belong here since it is not framed by any actual physical or psychological theory.
Artem Kaznatcheev
CC BY-SA 3.0
2013-10-03T16:24:44.170Z
Voting to close because the question shows no initial research, no basis in existing physical or psychological theory. Even if it was properly grounded in physical theory, it would belong on physics.SE, not here. As a generally 'fluffy' question, it belongs on scifi.SE
Artem Kaznatcheev
CC BY-SA 3.0
2013-10-03T22:07:33.027Z
There are plenty of discussion on this on the [physics](http://physics.stackexchange.com/questions/2166/) forum. I reckon the question should move there.
Izhaki
<time>
If time travel were possible, would travelling back in time result in memory loss?
guweoiwje
# If time travel were possible, would travelling back in time result in memory loss?

(Asked by: username_0 on 2013-10-02)

Suppose it is possible to travel back in time. If Bob traveled back in time, wouldn't he lose the memory of what happened since his neurons are also traveling back in time? In other words, traveling back in time would feel like the present moment. Consider this thought experiment:

Bob eats pizza at 12 PM.

Bob plays tennis at 2 PM.

Bob travels back in time to 12 PM.

Would Bob lose the memory of him playing tennis at 2 PM and essentially treat 12 PM as an ordinary experience?

**Tags:** time

> **Comments:**
> **username_1**: I think this is too speculative and unanswerable at present.  
> **username_2**: My first instinct was to vote to close this question, but part of me finds it quite interesting to ponder the implications of changing one aspect of reality but holding psychological theory broadly constant. Anyway, I [posted a meta question to discuss these types of questions](http://meta.cogsci.stackexchange.com/questions/713/are-questions-involving-improbable-human-conditions-on-topic) and whether they are on topic. I also think the scientific nature of the answers supports keeping the question open.  
> **username_3**: @ThinksALot there is definitely a basis to discuss this sort of thing on a physics forum in the context of [closed timelike curves](http://en.wikipedia.org/wiki/Closed_timelike_curve), and there is non-trivial things that can be proved about the [computational consequences of CTCs](http://www.scottaaronson.com/papers/ctc.pdf). However, the person asking the question is obviously not aware of this sort of work, and so the question ask asked does not belong here since it is not framed by any actual physical or psychological theory.  
> **username_3**: Voting to close because the question shows no initial research, no basis in existing physical or psychological theory. Even if it was properly grounded in physical theory, it would belong on physics.SE, not here. As a generally 'fluffy' question, it belongs on scifi.SE  
> **username_4**: There are plenty of discussion on this on the [physics](http://physics.stackexchange.com/questions/2166/) forum. I reckon the question should move there.  

---

## 2 answers

### Answer by username_5 on 2013-10-03. Score: 1

This is a tricky topic and my answer will be clearly speculative. I will say that this is also largely a matter about metaphysical allowances and involves personal identity implications. Assuming that time travel is physically possible, we need to make assumptions around how time is interpreted within the time travel journey.

**Scenario 1: Time travel based on external time and fatalism**

Lewis (1976) distinguishes between two interpretations of time, i.e. *personal* and *external* time:

> Instead I reply by distinguishing time itself, external time as I
> shall also call it, from the personal time of a particular time
> traveler: roughly, that which is measured by his wristwatch. His
> journey takes an hour of his personal time, let us say; his wristwatch
> reads an hour later at arrival than at departure. But the arrival is
> more than an hour after the departure in external time, if he travels
> toward the future; or the arrival is before the departure in external
> time (or less than an hour after), if he travels toward the past.

Should Bob decide to travel back to 12 PM relative to *external time*, we can speculate implications on his continuity of personal identity. Le Poidevin (2005) elaborates on discontinuous movement through time:

> ... it seems that discontinuous movement is possible: one can move
> from A to B without occupying places in between.

If so, it seems probable that Bob could lose his memory of playing tennis at 2 PM if discontinuous movement is allowed. Based on external time, he is in fact moving back 2 hours, however, implied is the suggestion that he won't be psychologically continuous and thus, won't remember that experience. Whether he will treat 12 PM as an ordinary experience or a completely new experience depends on how we interpret causal relations between past and present.

Fatalism suggests that we are unable to deliberate about the future or change it (Taylor, 1962). It is logically and physically impossible to change any events preceding the time travel point. From a fatalist viewpoint, Bob would experience all those events from A to B as they had occurred.

**Scenario 2: Time travel based on personal time**

The other scenario presupposes that time travel is continuous from the time traveller's *personal time* in which the person is psychologically continuous - there is no disconnect between the stages of the psychological and physical continuum when travelling time. If so, it may be possible for Bob to travel back to 12 PM, re-experience eating the pizza while his memory of playing tennis remain intact. Although time travel isn't physically possible, we can still project ourselves backwards in time and recall events (Suddendorf & Corballis, 2007).

Since this scenario doesn't sever the continuity of Bob's personal identity, it seems probable that he would still remember the experience of playing tennis at 2 PM while able to freely experience new events (given that fatalism is rejected) thereafter and the mere possibilities that follow.

**References**

* Lewis, D. (1976). The paradoxes of time travel. American
  Philosophical Quarterly, 13(2), 145-152
* Le Poidevin, R. (2005). The Cheshire Cat problem and other spatial obstacles to backwards time travel. The Monist, 88(3), 336-352
* Suddendorf, T. & Corballis, M.C. (2007). The evolution of foresight: What is
  mental time travel, and is it unique to
  humans? Behavioral and Brain Sciences, 30, 229-351

> **Comments:**
> **username_5**: I'm mindful this answer relates more so to metaphysics but there are implications for psychological continuity, i.e. how does consciousness, memory, cognitive processes move with temporal changes. While speculative, it still may address the cognitive aspects of time travel.  

---

### Answer by username_2 on 2013-10-03. Score: 0

The answer presumably depends on how you conceptualise the experience of the time traveller relative to the time travel. In most movies the world and most other people in the world are just restored to the original time, but the time traveller is somehow immune to such effects.

For this version of time travel to be true and consistent with neuroscientific theories of the [mind-body relationship](http://en.wikipedia.org/wiki/Mind%E2%80%93body_problem), then presumably the brain of the time traveller is not altered by time travel, or at least is not altered in such a way to restore the brain to the state it was in at the time of return. Thus, **memories of the future would be retained when going back in time**.

In fact, if the mind of the time traveller was restored to the returned time, then this could result in non-existence if you travelled back further than when you were born. It would also seem inconsistent with the possibility of going back in time but being in the same location that you initiated the time travel.

*You might also wish to see what answers you get on <https://scifi.stackexchange.com/>*


---

