In the article We've seen signs of a mirror-image universe that is touching our own in the popular magazine “New Scientist” (which describes a real scientific theory though, though in a popular form, but I think that's not a problem in this case) one can read that Leah Broussard has dusted off an already old theory (by Lee and Yang), and tried to reanimate it: The Mirror Universe.

It says, for example:

The article you are reading is written back to front.

But how can this be? I can imagine that the clock goes backward (though not time itself). Just look in the mirror and you'll see it's true.

But when I look in the mirror I can't see that someone who's writing a piece of text starts at the end and working his/her way to the beginning. What I do see though is that the one who writes starts at the beginning but writes from the right to the left, like in Arabic (you can check this out again by looking to someone who's writing in a mirror). Now, according to me, someone can't write a book from the back to the cover. Causality is violated here. Can the dead come out their graves alive, growing "older" until they are being pushed back in the uterus, after which they get smaller and smaller until only semen and an egg is all that's left and one can say they are dead.

Now, do I interpret the quote wrong, or are texts really written backward from the end to the beginning?

In the answer by@PM 2Ring (below) one can read:

The t=0 instant of the Big Bang is at the left side of that diagram, and time progresses towards the right side. Make a mirror-image of that diagram, with its t=0 Big Bang point on its right side, so time in that diagram progresses towards the left.

How can the arrow of time have two different directions? I mean, of course, you can mirror an arrow. But time in both Universa proceeds in the same way as in ours (or relative to ours: if 100 000 years have passed in our Universe then also in the Mirror Universe this "amount" of time will also have passed. Also from our point of view time in the MU passes normally and only the direction of time's arrow is different.

Doesn't this mean that time isn't really going backward which is implied by my first quote (surely the clock ticks opposite to ours, just look in a mirror to see this) and the only differences between these two Universes are the fact that we see only particles while in the MU

I know this is not mainstream (at this time anyway), and maybe a link can be laid with the Rishon model which proposes a substructure of quarks and leptons and says that in our Universe there are equal amounts of matter and anti-matter, and maybe another Universe where all the particles are the anti-particles of ours (though the amount of Rishons and anti-Rishons is the same in both Universes, they are differently combined in each), but I'm really fascinated by this theory and that's why I nonetheless ask this question.

  • $\begingroup$ Re Broussard's experiment mentioned in the answer, Forbes mag. was negative about its potential for success. Concerning the relation between time and entropy, which I think's usually expected to increase as a result of passage thru time in either its familiar direction or in an opposite direction that Aguirre & Gratton have (in their 2003 paper "Steady-state eternal inflation") associated with the opposite side of a (rather unmotivated) Cauchy surface, a projected collaboration between Guth and Carroll on the possibility of infinite entropy will be of great interest, if it ever appears. $\endgroup$
    – Edouard
    Oct 20 '20 at 4:03

I haven't looked at the New Scientist article (it's behind a paywall), but in the original Mirror Universe model, the arrow of time in our universe points in the exact opposite direction to the arrow of time in the mirror image universe.

So relative to us, their time is running backwards. But that's not an issue, since the only point the two universes have in common is the exact instant of the Big Bang, so no causality paradoxes can arise.

To illustrate this, let's start with this Big Bang diagram from the top of Wikipedia's Big Bang article.

Big Bang diagram

The $t=0$ instant of the Big Bang is at the left side of that diagram, and time progresses towards the right side.

Make a mirror-image of that diagram, with its $t=0$ Big Bang point on its right side, so time in that diagram progresses towards the left.

Join those 2 diagrams at their bases, the $t=0$ instant of the Big Bang. Now you have a diagram of the mirror universes, according to the original theory.

Leah Broussard is claiming that maybe that picture is misleading, and the topology is curved in a way that allows the space of our universe to be in some kind of contact with the mirror universe.

  • $\begingroup$ But why is their time running backward if things are only mirrored. A car (as in a given example) drives on the wrong side of the road, but they still drive forwards. Likewise, a neutrino moves with an opposite chirality as in ours, but this doesn't imply that time is running backward. And what about people? They don't get out of the grave alive first and then your live runs backward. Our hearts would be on the right side, the two halves of the brain interchanged, instead of writing lefthanded you're writing righthanded, etc. So what's the reason time runs backwards in a mirror Universe $\endgroup$ Jun 21 '19 at 18:01
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    $\begingroup$ @descheleschilder As far as the people in the mirror universe are concerned, time is running forwards, so for them entropy increases over time, etc. But the stuff that they call matter, we'd call antimatter, if we could observe it, but we can never do that. $\endgroup$
    – PM 2Ring
    Jun 21 '19 at 18:21
  • $\begingroup$ So time for them doesn't go backward? The last part of your comment seems to be contradicted by an experiment conducted by Leah Broussard. She claims some interaction between the two Universes is possible and tries to establish this by experiment. $\endgroup$ Jun 21 '19 at 20:04
  • $\begingroup$ Haha!! That's true. I read it in a bookstore while waiting for a train to Amsterdam. Don't you think that when our Big Bang took place, they had that already behind them? $\endgroup$ Jun 21 '19 at 23:19
  • $\begingroup$ So after the common Big Bang, the arrow of time in the mirror Universe pointed in the opposite direction to the direction of time in ours? But If they had the same initial conditions (apart from the particle-antiparticle distinction), how can time have different directions. Doesn't time has always one direction (towards increasing entropy)? $\endgroup$ Jun 23 '19 at 0:59

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