# How do we know time doesn't go backwards? **URL:** \ **Category:** Factual Questions\ **Created:** [May 20, 2004, 4:46am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121 "2004-05-20T04:46:17Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Kel\_Varnsen\_Latex\_Division](https://avatars.discourse-cdn.com/v4/letter/k/aeb1de/32.png) [@Kel\_Varnsen\_Latex\_Division](https://boards.straightdope.com/u/Kel_Varnsen_Latex_Division)\ **Post date:** [May 20, 2004, 4:46am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/1 "2004-05-20T04:46:17Z")
How do we know that time only goes forward? What effects would we see if time started to go backwards and then forward again? How do we know that time is not constantly going forward and backward, but we only can retain memories/knowledge of it going forward? How do know time’s arrow is real? ---
**Author:** ![silenus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/silenus/32/166_2.png) [@silenus](https://boards.straightdope.com/u/silenus)\ **Post date:** [May 20, 2004, 5:33am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/2 "2004-05-20T05:33:35Z")
You asked that tomorrow, remember? 😛 ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [May 20, 2004, 5:38am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/3 "2004-05-20T05:38:42Z")
Stephen Hawking, in one of his books, pointed out that the only difference between the past and the future is which one we remember. This is a borderline philosophy question - is there an objective reality, or is reality defined by how we experience it? ---
**Author:** ![1010011010](https://avatars.discourse-cdn.com/v4/letter/1/ea666f/32.png) [@1010011010](https://boards.straightdope.com/u/1010011010)\ **Post date:** [May 20, 2004, 5:53am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/4 "2004-05-20T05:53:14Z")
Wasn’t there a convention of physicists about a year and a half ago to discuss this sort of thing? As I recall, the popular sentiment was that time has no basis in reality, and is something we, as observers, impose upon experiments. Guess that might explain why certain physical equations work with time flowing either direction. ---
**Author:** ![Thaumaturge](https://avatars.discourse-cdn.com/v4/letter/t/858c86/32.png) [@Thaumaturge](https://boards.straightdope.com/u/Thaumaturge)\ **Post date:** [May 20, 2004, 5:57am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/5 "2004-05-20T05:57:05Z")
“Guess that might explain why certain physical equations work with time flowing either direction.” Actually, as far as I know, pretty much all physics equations don’t specify an arrow of time. I’m sure someone who is less rusty than me will come along and say more, but I was of the opinion that the only equations which deferentiate a direction in time are those which are statistical in nature, such as entropy. ---
**Author:** ![Stringer](https://avatars.discourse-cdn.com/v4/letter/s/eb8c5e/32.png) [@Stringer](https://boards.straightdope.com/u/Stringer)\ **Post date:** [May 20, 2004, 6:03am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/6 "2004-05-20T06:03:35Z")
Not much to contribute to the discussion here, but I remember hearing that until recently there was no evidence that time travels in either direction. However, apparently there is some new evidence that time does indeed travel forward. I of course have no cite, and therefore have nothing to contribute, but maybe this will jog someone’s memory. FWIW, I heard this from my English professor two weeks ago when I we were discussing Martin Amis’ novel, “Time’s Arrow.” He said he discussed the subject of time’s direction with a physicist friend of his. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 20, 2004, 6:12am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/7 "2004-05-20T06:12:07Z")
> [@Thaumaturge](#): > > “Guess that might explain why certain physical equations work with time flowing either direction.” > > Actually, as far as I know, pretty much all physics equations don’t specify an arrow of time. I’m sure someone who is less rusty than me will come along and say more, but I was of the opinion that the only equations which deferentiate a direction in time are those which are statistical in nature, such as entropy. Actually, quantum field theories say that physics is _not_ invariant under time reversal, but that it is if you flip space backwards and swap particles for antiparticles. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [May 20, 2004, 6:12am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/8 "2004-05-20T06:12:29Z")
Actually, I think that **Thaumaturge** got it right but didn’t state it with enough confiction: ;j [sup] **ENTHORPY is what proves that time goes forward and not backward.** [/sup] ---
**Author:** ![Thaumaturge](https://avatars.discourse-cdn.com/v4/letter/t/858c86/32.png) [@Thaumaturge](https://boards.straightdope.com/u/Thaumaturge)\ **Post date:** [May 20, 2004, 6:33am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/9 "2004-05-20T06:33:39Z")
Hmm…I thought that the qauntum feild equation was time-invariant. It was only when you tried to collapse the waveform that you start to get time-invariant equations. But not even all quantum theories say that the waveforms even collapse at all. The many worlds theory says they don’t, though I find it personally repulsive. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [May 20, 2004, 6:40am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/10 "2004-05-20T06:40:13Z")
I got my information from a recent discussion on TV. Methinks you’re just thinking too hard **Thaumaturge**. 😉 ☹ [sup] **Hell I even had trouble spelling ENTROPY.** [/sup] ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 20, 2004, 6:51am UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/11 "2004-05-20T06:51:29Z")
> [@Thaumaturge](#): > > Hmm…I thought that the qauntum feild equation was time-invariant. It was only when you tried to collapse the waveform that you start to get time-invariant equations. But not even all quantum theories say that the waveforms even collapse at all. The many worlds theory says they don’t, though I find it personally repulsive. Okay, now you’re confounding quantum field theory and philosophical interpretations of quantum mechanics. QFT is QM + SR. The CPT theorem proves that physics is invariant under the CPT transform (assuming QFT is correct), but there are experimental results proving violation of CP in weak interactions, thus also T. ---
**Author:** ![vetbridge](https://avatars.discourse-cdn.com/v4/letter/v/b5e925/32.png) [@vetbridge](https://boards.straightdope.com/u/vetbridge)\ **Post date:** [May 20, 2004, 2:03pm UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/12 "2004-05-20T14:03:32Z")
Time flies like an arrow. Fruit flies like a bananna. ---
**Author:** ![Engywook](https://avatars.discourse-cdn.com/v4/letter/e/bc8723/32.png) [@Engywook](https://boards.straightdope.com/u/Engywook)\ **Post date:** [May 20, 2004, 3:17pm UTC](https://boards.straightdope.com/t/how-do-we-know-time-doesnt-go-backwards/246121/13 "2004-05-20T15:17:00Z")
> [@micahjn](#): > > However, apparently there is some new evidence that time does indeed travel forward. I of course have no cite, and therefore have nothing to contribute, but maybe this will jog someone’s memory. How new? IIRC, Brian Green’s **Fabric of the Cosmos** , published in 2004, says that we really haven’t found anything yet to support the idea that time “moves.” > [@](#): > > FWIW, I heard this from my English professor two weeks ago when I we were discussing Martin Amis’ novel, “Time’s Arrow.” He said he discussed the subject of time’s direction with a physicist friend of his. Great book. Required reading for anyone interested in the human perception of time and events.