Anyone here play Portal? Portal + Relativity question

JesseM

In that case, I think when you played the video back you would see this
Something unlikely would have to happen, like some radioactive particles of the same element randomly converging on the location of the lump and replenishing what it lost, or the lump not decaying at the expected rate (and some decay products spontaneously turning back into the nondecayed form) due to an extremely unlikely bit of quantum randomness. But this sort of thing is why I suggested that if you had a theory assigning probabilities to different self-consistent spacetimes, it might just work out that it was extremely unlikely for closed loops consisting of macroscopic objects or information to occur in the first place.

Ryan_m_b

Very interesting yet very confusing stuff lol. I keep finding myself reaching a conclusion before quickly refuting it. Latest thought: If I had two wormholes fixed 10 years apart Future-me could pop out and not only say hello, but stick around. 10 years later Future-me and Present-me could both step into the wormhole so that Past-me meets two people not one. Then all three of us could wait 10 years before making Past(er)-me meet 3 people.

Although self consistency. There would probably only be a small group of us because one of us (the futuremost one) isn't going to make it another 10 years

SeventhSigma

I think the "unlikely event" concept is not satisfying. There's no reason why we should suddenly require unlikely events to "fix" paradoxes when there's no other impetus for those unlikely events to occur.

Ryan_m_b

I think the idea is that they must occur because something had to occur. With the case of the video camcorder, something must have happened to wipe the memory every time it get's passed over because it is an impossibility if not.

It's hard to understand but if impossibilities cannot exist, by that nature something must necessarily happen to prevent them.

SeventhSigma

If it's a self-consistent loop, then by definition it had no real beginning, which means that whatever state the decaying atoms are in means that they were always like that in the past and will always be like that in the future.

I guess I just have issue with "how are these loops brought about to begin with"

Ryan_m_b

Do CTC violate conservation of energy?

It just occurred to me that matter popping in from the future would raise the total amount of energy in the universe. Ten years later the energy will drop again but for that period of time the total energy was greater

Ryan_m_b

Yes me too!

JesseM

Did you read this comment I linked to earlier, outlining a conceptual argument for how a mindless brute-force algorithm could generate entire self-consistent histories? Since the only output would be self-consistent histories, the ones featuring time travel would be guaranteed to have events which prevented contradictions even if some conditions elsewhere in the history (like a simulated time traveler making plans to kill his grandfather) would seem to create the danger of contradictions.

JesseM

No, in the case of wormholes the mass of the wormhole mouth itself changes to compensate for anything entering/exiting it, see this post from pervect.

Ryan_m_b

Again very interesting however in the case which pervect was dealing with there the two wormholes were one above the other. The object would fall until the top Wormhole runs out of mass.

But what if the wormholes were both on the floor side by side? Gravity would pull the object into one which would come out of the other only to have gravity pull it the other way. How is energy conserved here?

michelcolman

And even if it didn't, I fail to see what the big problem would be. Conservation of energy is a "law", not in the sense that everything must obey the law because someone said so, but simply because all our experiments so far seem to confirm this. Whenever an experiment appears to contradict it, research is done and the "law" is adjusted if necessary, for example when it was first discovered that mass is a form of energy. In the case of worm holes, if something is exiting our current universe via a wormhole, this universe is no longer a closed system. That is already sufficient to remove any worries about "oh my God they are violating the law". But, as JesseM said, even in this case it may well be that energy is conserved through a change in mass of the worm hole itself. I don't know, I've never seen a worm hole up close ;-)

JesseM

pervect was answering a question specifically about two wormhole mouths with one above the other, but I don't think he meant for the comment about changing masses to apply only to this situation, I think it's supposed to just be a general property of wormholes that the mouths change mass when things enter or exit them. And neither mouth can ever run out of mass, he also mentioned that the mass of a mouth could eventually become negative.
I don't understand this scenario, why would gravity "pull it the other way"? If you dropped it into one vertically, it would come vertically out the other, and land on the floor before the one it came out of, no? Are you imagining the wormhole mouths themselves are so massive that their own gravitational fields are significantly affecting the object's path?

Ryan_m_b

I was imagining them in the portal sense of being flat circles with the mouth of both pointing up

http://www.youtube.com/watch?v=EZouCf700RU

Misericorde

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?

JesseM

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!

Misericorde

OK, that does make a great deal of sense, and it answers the other question I was going to ask. This forum is fantastic for people trying to write fiction that doesn't utterly trample physics. Thanks JesseM.

I'm curious, what would the radiation field around this kind of portal be like in real life? I'm guessing that an actual "teleport" of this type would have quite an effect on each locale.

Ryan_m_b

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