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[Edit: I'm not looking for 100% realism. I'm looking for an explanation as realistic as possible, possible with just few exceptions to physics.]

[Edit 2: In my imagination these worlds are different universes and as such not reachable by travelling far. They are similar but not identical to our universe/earth. Parallel indicates that they kind of overlap with our universe, they are parallel to our world, I like to imagine sheets of paper overlapping, and you can just get to another sheet if you rip a hole in your sheet. Or maybe like in 'His dark materials', the universes are here and not here, you'd just need to know how to open a portal.]

I plan to write about several parallel universes, the first being very much like earth including our physics and its restrictions.

Scientists manage to open a portal. My thoughts so far:

  • A huge amount of energy needs to be concentrated on a relatively small area
  • This might be achieved by matter colliding with antimatter
  • There are just the n parallel worlds to which a portal can be opened to, each might require discreet amounts of initial energy to be opened
  • The portal/crack might stay open, get bigger or get smaller and vanish. I think the last option is the most realistic

My questions so far:

  • Would this work as described above?
  • How can it be stablized? My initial idea was that you can put materials through it to stop it from getting smaller, but something like a broomstick would just break or melt and you would need a material that has a high melting point.
  • Does the portal need constant energy to keep from collapsing or would maybe cooling whatever material keeps it in place be enough?
  • Would it be dangerous to be in proximity to it? Would there be radiation?
  • How exactly do the edges of the portal look and behave?
  • Did I miss problems that would appear?
  • Other options or ideas?

In the end it should be possible for humans to walk through the portal without damage.

(Further info: The other universes are about the same, but have one or two additional elementary particles (similar to ours). So the portal would open to the respective place in the selected world.)

I'd really like to make this as realistically/plausible as possible. Thanks in advance!

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  • $\begingroup$ The Sci-Fi show "Dark Matter" used something called a Blink Drive, akin to FTL but nearly instant, to travel instantly from A to B. When the crew installed it, they never properly configured it, which caused them to jump to a parallel universe. The show never explained it in any more detail, however. $\endgroup$ – Kaizerwolf Nov 28 '16 at 20:49
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    $\begingroup$ this is probably best answered with "anyway you want". Once you violate physics or introduce overriding physics, you pretty much are free to declare how it works, including its second order effects. The only advice I have is "be consistent" and "declare how it works at the start of the story so you aren't tempted to make it magical to serve your plot as needed." ;-) $\endgroup$ – SRM - Reinstate Monica Nov 28 '16 at 20:56
  • $\begingroup$ This might help: astronomy.stackexchange.com/questions/3678/… $\endgroup$ – MackTuesday Nov 28 '16 at 21:05
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    $\begingroup$ See my Puzzling SE question on parallel universes - there are several constraints - most notably that if it's parallel or even a copy of the first, you will bonk into your copies when trying to pass through - they'll be trying to pass through too - and killing them means they will kill you simultaneously. It is not possible to enter a parallel universe. $\endgroup$ – Zxyrra Nov 29 '16 at 1:30
  • $\begingroup$ This article on time travel discusses how parallel universes interact. Perhaps somehwere in their research, they discuss how to move from one to the other. $\endgroup$ – Thom Blair III Nov 29 '16 at 5:43
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Short Answer: You can't make this scientifically plausible

Let's be upfront about this.

Stephen Baxter, who has degrees in mathematics and engineering and writes extremely grounded, hard sci-fi, wrote about a device built as an 'escape hatch' to leave our universe.

The device was a ring made of cosmic strings, several million light-years across, spinning at nearly the speed of light.

For a realistic/plausible portal between universes...that's the kind of thing you'd need.

For a portal to be built between something similar to our world and another universe, you're going to need some heavy-duty handwavium. Perhaps you have some substance or material that is able to directly manipulate spacetime, perhaps built into a ring that 'pulls' spacetime apart at the centre.

But whatever you do, I'm afraid you're not going to find a scientifically plausible answer.

Edited to Add:

Home now, so I can expand a bit on my answer.

The problem you're going to run into in creating a 'plausible' portal in the spacetime continuum - we don't actually know what spacetime is.

We know that spacetime exists. We know that it has properties. We know that it can be curved and twisted by sufficient mass. We believe, to a high degree of certainty, that if you get enough mass in one place, you can create what is essentially a discontinuity in the universe - a place where two parts of the universe are causally disconnected from one another (a black hole). But we still don't have a really good description of what spacetime is.

To an extent, we're in the same sort of position regarding spacetime as chemistry was before the discovery of the atom. We had a map of what would happen in certain circumstances, but we don't have a clear idea of why.

The only thing we know to be able to manipulate spacetime is mass. All mass causes spacetime to curve, and a spinning mass causes a kind of twisting motion in space as well (called Frame Dragging). If we're going with what is known to exist in the universe, the only way to create a tear in spacetime is with enough mass, spinning fast enough. The amount of mass would be truly immense - far greater than a single galaxy - and certainly not anything that a handful of scientists could knock together in a lab.

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  • $\begingroup$ Can you give a bit more to back up the statement "For a realistic/plausible portal between universes...that's the kind of thing you'd need."? Are you saying that there aren't any other ways, or that you'd need a very large, massive object to travel to another universe? $\endgroup$ – HDE 226868 Nov 28 '16 at 21:27
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    $\begingroup$ With the caveat that we don't really know what spacetime actually is, only what it does - the closest we can come to a place where our universe 'stops' is the event horizon of a black hole. Science based, that's what a rip in space-time looks like. To create one that you can travel through, you need enormous mass pulling from all directions at once, enough that you overcome the pull of the singularity itself. $\endgroup$ – Werrf Nov 28 '16 at 21:42
  • $\begingroup$ ... which requires negative mass, iirc. And a lot of it. Also, negative mass is weird. $\endgroup$ – Draco18s Oct 22 '18 at 20:42
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In order to have a bridge to just walk through, the spacetime needs to be grafted together. It’s the same as any wormhole.

Consider the case first where physics is the same, it’s just history that is different.

If the universe is infinite, then eventually things will repeat. Somewhere, vastly far away, is another copy of Earth and everything in it. Far far away is another identical copy of our entire Hubble volume. In fact, there are an infinite number of repeats.

Suppose a “normal” FTL wormhole creation actually cross-connects different copies. How would you ever know? The wormhole maker breaks through the destination point from a kind of hyperspace, which has no concept of distance (which is the whole point), so how does it find the spot to breqk through to? It has an affinity for the pattern of matter and energy surrounding it. But that is ambiguous because the universe is infinite.

The probability of cross-connecting a universe is dependent on how close of a match it is. So you may open a wormhole to another universe that isn’t some random bizzare thing, but is very nearly identical to ours, exactly as with the sci-fi trope.

Once they figure that out, they can try finding different universes on purpose by setting up the navigation pattern to what they want to find, rather than the actual destination.

As for different laws of physics, what if spacetime had infinite space but the specific laws vary on a scale that's larger than our Hubble Bubble. After all, our observable universe inflated from a tiny patch so will be uniform.

So the different universes are actually different locations in one continuous space. You can’t actually get there by moving normally though because of inflation.

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I'd say one would have to take something dealing with cosmic strings, revisiting Stephen Baxter, or use a wormhole, both of which are not completely proven. I'd say find a way to compress matter into a sizable black hole, find a way to stretch it open, which requires adding dark matter, mind you, then keep it open long enough to send something through.

Far in the future indeed, but tests would need to be done first to make sure this device wouldn't rip apart the earth (what if the universe we first contact is mainly antimatter?), find a way to send something through quickly enough in order to save energy (opening the portal up directly below something or at a surface it is travelling quickly towards). First, we'll have to prove multiverses even exist. I too love the many worlds theory and want to devote my future career to proving it.

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I'm not a fan of many-worlds. That said, we understand almost nothing about dark matter. The first thing we shouldn't assume is that it's all the same. Perhaps it's a zoo of different particles, different physics. Heck, maybe it exists in some dynamic equilibrium, traveling between (or existing in both) two or three or who knows how many "parallel" universes. So, what properties does it have.

Well, you need to read up on it. Mostly, it doesn't interact much with itself or anything else. If that can't be changed, then I'm wasting time, so let's assume there IS some way to manipulate it, and that by doing something with it (creating a sheet with it, a sphere, a current (flow), concentrating it, dispersing it, the list is limited only by your imagination, but if we could figure out some way to manipulate it, and if that manipulation moved it between universes and if it produced, say, a wake, pulling other things with it, then...there you go.

The problem I have with most fantasy disguised as sci-fi which assumes interdimensional doors, time travel, FTL, etc. is (among other issues) you need to justify why in the world such doors would be pinned to the Earth. Dr. Who had it right long ago; the TARDIS moves in both space and time. I forget how far the Earth travels in a year, but it's a significant distance in terms of light-seconds. It would be really difficult to target a place on the surface of a different planet from ours to put the other end of any hole we make. And keeping that hole fixed there would be another several of orders of magnitude more difficult.

So, there's two ideas I have to manipulate dark matter:

  1. Massive spinning objects in some sort of pattern create a resonance
  2. Neutrino oscillations.

That's all I got. I like the idea of (Farscape) a spaceship going through the "door", having it open up on or even near the surface of a Earth-like planet is not credible (without a huge technology and history behind it)

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I would guess in an incredibly simplified sense, if you wanted to crack open space time, you'd need to find a point in space that would be susceptible to such cracking, where two realities overlap or cross. This is sort of theoretical, but the closest thing we have to this would be the CMB Cold Spot, an area in space about 70-140 μk(micro kelvin) colder than average, that in 2013 was proven to not be a glitch or artifact of the probe observing it(Wilkinson Microwave Anisotropy Probe, which discovered it in 2004, and was observed by the Planck Satellite in 2013), and in 2017 was deemed likely to not be a supervoid(https://phys.org/news/2017-04-survey-hints-exotic-cold.html for proof). One of the most persistent theories to date since its discovery in 2004 has involved Everettian quantum mechanics, or the multiverse theory in Layman's terms. In this theory, the Cold Spot is an imprint of sorts left by another universe. This could technically mean enough spacetime bending properties(gravity, exceptionally high energies or speeds) could possibly(and quite literally depending on the legitimacy of string theory regarding quantum foam) "rip" through the universe and into another. However, the imprint theory falls short on terms of prediction basis, as the theory requires a similar imprint to be on the opposite hemisphere of the Celestial Sphere(paraphrased from the Wikipedia page, I'm no expert on this particular part), and to date, little evidence has been found for it.

Secondly, you need to make some sort of artificial star or dense matter that would use its gargantuan mass to warp, and later rip through, the fabric of spacetime. Another problem is the singularity of the black hole that would inevitably form. You have to figure out how to open it up, so that whatever gets near it at high enough speeds would travel through it to another side, rather than be ruthlessly destroyed and ripped to subatomic bits by the immense and inescapable tidal forces. Again, the vast majority of this answer is incredibly theory-heavy. I say even with constant or even speeding acceleration of our technological advancement, at least some of these problems may be solved in anywhere from the next 100 to 1000 years. Hope this helped.

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I am surprised nobody suggested "borrowing" the approach that the movies/ TV Shows Stargate used.

Essentially, Have a rare material of some kind, Perhaps a mineral that has been found from a meteorite strike - one that came from outside the solar system - machine it or manufacture the material in some fashion. Apply some energy to it and have it generate a stable gateway of some kind.

There are MANY well written science based alternate universe stories that use this particular trope. One by the Grand Master, Heinlein, didn't even use significant energy, just an unusual application of physics. You are possibly familiar with gyroscopes. When one has "spun up" and you push on the axis - it resists the push and therefore moves in a direction perpendicular to the push. - SO he posited - what happens if you push in 3 directions at once? Turns out - in his story - that the Gyro disappears. That book was called the Number Of The Beast, By Robert A Heinlein.

Generally something like this in a story is discovered by accident, while the slightly mad genius was trying to do something else. Then the plucky hero, or his assistant, get's sucked into a vortex of action which allows him (or her) to demonstrate all kinds of resourcefulness and brilliance denied to us lessor mortals.

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the closest you could get to this is transport within our dimension by manipulating gravity to fold the fabric of the universe in a way were two points meet. interdimensional travel isn't something we can describe with modern science that well. but cross-universal travel is

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