Anyone here play Portal? Portal + Relativity question

[Ryan_m_b]

If you had these portals, but they had a delay system so that they operated to only allow passage at or below 'c', it would not fall prey to the issue of time travel, right? In short, if you teleported (scifi) halfway around the Earth, but you stayed just below 'c', you would just have a REALLY fast trip, without Relativistic complications, right?

It's the velocity of the wormhole itself, not the velocity of the transition, that gives rise to time travel.

Imagine you are hanging in space in a ship with a wormhole. At rest relative to you is another ship with the corresponding wormhole. When one of you starts speeding up the wormholes fall out of sync so to speak

 

[Misericorde]

It's the velocity of the wormhole itself, not the velocity of the transition, that gives rise to time travel.

Imagine you are hanging in space in a ship with a wormhole. At rest relative to you is another ship with the corresponding wormhole. When one of you starts speeding up the wormholes fall out of sync so to speak

Got yah, thanks!

 

[SeventhSigma]

Actually it's better if the portal allows you to get somewhere FTL (FTL from the point of view of light traveling the "long way", of course you still move slower than any light which travels through the wormhole with you). If the regions of spacetime the portal connects have what's called a "space-like separation", meaning that no signal traveling the regular way could get from one to the other without traveling FTL, then there's actually no danger of time travel here. For example, say there's a star 100 light years away and traveling through the portal on Earth in 2000 would cause you to step out at the location of that star in 1950 (with time being defined relative to the rest frame of the Earth and star). Sure you've gone back in time, but if you send a light signal back towards Earth it won't actually reach them until 2050, and if you step back through the portal in the other direction it takes you 50 years into the future, so you're back at Earth in 2000 (or a little later if you hung out at the star for a while).

On the other hand, suppose there's another star 20 light years away, but stepping through the portal on Earth in 2000 takes you to the star in 2040. In this case the separation is "time-like", meaning that stepping through the portal won't get you to the star faster than a light beam would. It might seem like there's no problem here, but the portal is two-way, meaning if you step back through the portal at the star in 2040, you'll end up at Earth in 2000, and in general if you step through the portal at the star in year Y you'll end up at Earth in Y-40. So now say in 2000 you get in a rocket which flies to the star at 0.8c, covering the 20 light years in 20/0.8 = 25 years. This means you'll arrive at the star in 2025, so if you step through the portal you'll now be on Earth in 2025-40=1985, in your own past!

This assumes the portals already exist though -- as long as I am the person making the portals, then there's no way I can get to any point in my past before the portals were made no matter where I place them or how fast I go. Maybe if there is some kind of tachyon technology involved, but there are plenty of problems with the existence of tachyons to begin with, as far as I know.

 

[JesseM]

This assumes the portals already exist though -- as long as I am the person making the portals, then there's no way I can get to any point in my past before the portals were made no matter where I place them or how fast I go.

Yes, but as soon as the portals do exist then someone can use them to go into their past. For instance, even if the one on Earth didn't exist until 2000 so you couldn't use them to go back to 1985, it'd still be true that if someone left Earth in 2015 at 0.8c, then 25 years later in 2040 they'd arrive at the distant star, allowing them to step through the portal to Earth in 2000, which is in their past so still allows for troublesome time travel scenarios like leaving a message in 2000 for their later selves to read in 2015, telling them everything they are going to experience during their upcoming 25-year journey.

 

[A.T.]

What if we performed the Twin Paradox, but with portals?

You need relativity to create paradoxes with portals?

 

[Misericorde]

How many videogames could possibly engender such an erudite discussion? Oh Valve, I love you.

 

[FtlIsAwesome]

When are they going to make Time Portal? The example of the self-consistency principle on the Wikipedia page I like best is the Star Trek Next Generation one.
But I usually prefer the idea of alternate timelines: traveling into the past makes another timeline with different events, while your original timeline is unchanged.
I really dislike the method of "solving" paradoxes in Back to the Future and Doctor Who.
Good shows though.

As for colliding wormholes, I've thought of that before.
Assuming the wormholes are the same size (so their singularities "match up"), my random guess is that they collapse into a black hole.
I am not sure if they can be connected and different sizes. Someone enlighten me on this?

 

[Ryan_m_b]

As for colliding wormholes, I've thought of that before.
Assuming the wormholes are the same size (so their singularities "match up"), my random guess is that they collapse into a black hole.
I am not sure if they can be connected and different sizes. Someone enlighten me on this?

Assuming you could fit one through the other (I think they would be the same size spheres if they existed though) I guess they would collapse because you are putting the same mass through a wormhole.

Although the freaky thing is if you put something through wormhole Blue (sticking with valve terminology) it would come out through wormhole Orange. If you put wormhole Orange through wormhole blue it should...come out of itself??

 

[JesseM]

As for colliding wormholes, I've thought of that before.
Assuming the wormholes are the same size (so their singularities "match up"), my random guess is that they collapse into a black hole.
I am not sure if they can be connected and different sizes. Someone enlighten me on this?

Traversable wormholes don't have singularities, actually. On the question of what would happen if one mouth fell into the other, I think a clue may be in thinking of diagrams of what a wormhole would look like in a space with only 2 dimensions, like Flatland--in this case the two mouths are circular regions in the plane connected by a tube:

So if you think about what would happen if one opening of the tube was a lot narrower than the other, and the narrower one approached the wide one, perhaps what might happen is the narrow mouth would end up opening onto part of the tube above the wide mouth, creating a shape a bit like a http://www.kleinbottle.com/whats_a_klein_bottle.htm (except with the wide mouth still opening into the larger flat plane, and the narrower mouth just opening onto the surface of the tube rather than going "through" it):

Also look at the bottom part of this complicated diagram of connected wormholes, ignoring all but the central wide mouth...the bottom part twists around and opens up onto its own "throat" in a shape that looks a bit like a French horn:

[PLAIN]http://www.technicianonline.com/polopoly_fs/1.2544686!/image/1587755920.jpg_gen/derivatives/landscape_240/1587755920.jpg

 

[SeventhSigma]

Isn't all this stuff just speculation though? Wormholes and all? Concepts of "negative energy" and "exotic matter" and traversing 3D space to begin with by just jumping through some middle ground? What's the evidence that any of it would be even theoretically possible?

 

[JesseM]

Isn't all this stuff just speculation though?

Yes, it's speculative, but it's not just an arbitrary sci-fi fantasy (like "wouldn't it be cool if there was some way to make a portal connecting different places"), traversable wormholes are valid solutions to the equations of general relativity, and thus of interest to theoretical physicists exploring the consequence of the theory. Whether they are possible in the real world depends on whether the right type of "exotic matter" is possible (certain results from quantum theory like the "Casimir effect" suggest there's a good chance it is, but it's definitely not settled, see here), and also on whether there would be any process that could give rise to one in a region of space where one hadn't already existed since the Big Bang.

 

[SeventhSigma]

What would be an example of a wormhole being a "solution" to a GR equation?

 

[JesseM]

What would be an example of a wormhole being a "solution" to a GR equation?

I don't know what you mean by an "example", are you looking for the equations of a specific spacetime metric involving a wormhole? A solution just means a spacetime with a metric (describing a curved spacetime) and a matter field that satisfies the Einstein field equations (dealing with the relation between matter distribution and spacetime curvature) at every point in the spacetime. Anyway, the detailed equations for one such solution can be found in this paper, for example (the author also has a full textbook on the subject titled Lorentzian Wormholes)