Anyone here play Portal? Portal + Relativity question

[SeventhSigma]

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

Fire one portal against a wall in a station, another in a spaceship. One twin sits tight, and the other gets on board -- but they can see each other at all times through the portal itself and can walk back and forth between the ship and station.

But now the ship takes off at a high speed, say .99c. What happens?

 

[Ryan_m_b]

I've wondered this before (though not with portals). It's generally accepted that if you had two wormholes and you accelerated one to near lightspeed for a time and then bought it back you would have a tunnel to the past. I've always wondered what would happen if you just walked through whilst the wormholes were moving fast relative to each other

 

[JesseM]

I've wondered this before (though not with portals). It's generally accepted that if you had two wormholes and you accelerated one to near lightspeed for a time and then bought it back you would have a tunnel to the past. I've always wondered what would happen if you just walked through whilst the wormholes were moving fast relative to each other

This is explained nicely in Kip Thorne's book "Black Holes and Time Warps", if you take one mouth of a wormhole on a relativistic journey while the other stays on Earth, then even though the clock on the ship is running slow in the Earth's frame, if you jump through the wormhole the clocks on Earth and the ship are in sync. So for example if you jump through when the Earth clock says it's been 5 years since the ship departed, then when you arrive on the ship the clock there will also say it's been 5 years since the ship departed, even though these readings aren't simultaneous in the frame of the Earth or the frame of the ship. This is actually the key to explaining why a wormhole could be turned into a time machine! According to the usual twin paradox scenario, if the ship departs Earth and returns at relativistic velocity, then its elapsed time is less than Earth's, so for example when the ship is parked on Earth after it returned its clock might show 10 years had passed since it first left Earth, while the clock on Earth right next to where it's parked might show 30 years had passed. But since the times remain synced through the wormhole, if you stepped through the mouth on the ship when its clock showed 10 years had passed since departure, when you stepped out of the other mouth on Earth you would be at the time when the Earth clock showed only 10 years had passed since departure. So in this example, you'd actually have stepped back to a time when the Earth was 20 years younger then before you entered the parked ship!

 

[SeventhSigma]

what about relativity of simultaneity? portals A1 and B1 at both ends of a moving train and portals A2 and B2 on a stationary platform. guy between A1 and B1 holds a light emitter and the guy between A2 and B2 holds two photoplates. if the plates go off at the same time, the guy on the platform dies.

 

[Ryan_m_b]

This is explained nicely in Kip Thorne's book "Black Holes and Time Warps", if you take one mouth of a wormhole on a relativistic journey while the other stays on Earth, then even though the clock on the ship is running slow in the Earth's frame, if you jump through the wormhole the clocks on Earth and the ship are in sync. So for example if you jump through when the Earth clock says it's been 5 years since the ship departed, then when you arrive on the ship the clock there will also say it's been 5 years since the ship departed, even though these readings aren't simultaneous in the frame of the Earth or the frame of the ship. This is actually the key to explaining why a wormhole could be turned into a time machine! According to the usual twin paradox scenario, if the ship departs Earth and returns at relativistic velocity, then its elapsed time is less than Earth's, so for example when the ship is parked on Earth after it returned its clock might show 10 years had passed since it first left Earth, while the clock on Earth right next to where it's parked might show 30 years had passed. But since the times remain synced through the wormhole, if you stepped through the mouth on the ship when its clock showed 10 years had passed since departure, when you stepped out of the other mouth on Earth you would be at the time when the Earth clock showed only 10 years had passed since departure. So in this example, you'd actually have stepped back to a time when the Earth was 20 years younger then before you entered the parked ship!

Interesting. So suppose I stay on Earth next to my wormhole and waved you off on your relativistic rocket (containing the other wormhole) on a circular trip that will take 10 years by your clock but 30 by mine. After 10 years I could look through a telescope and see you still on your journey, or I could step through my wormhole and arrive 20 years in my future!

Time travel always twists my head but would I be right in suggesting that once you get back if I place the two ends of the wormhole in a room facing each other I would have made a time machine that can skip forward and back in 20 year intervals (obviously with the restriction of not jumping to before its creation)? If I step into wormhole P (pastmost) and come out of wormhole F (futuremost) I can just walk straight back into P (now 20+ years) and out of portal F again. If I keep running then every few steps I jump 20 years!

 

[SeventhSigma]

Wait a second

so let's say I sit in front of the portal on the wall and you sit in your ship, looking back at me. We're literally a few feet apart, talking to each other even though your ship is accelerating off. Let's say we continue to sit and talk (theoretically) for years, being able to see/hear one another. Then the ship returns, and you step out -- such that I can see you stepping out of your ship through my portal in addition to seeing you step out of the ship in real time behind me somewhere. What do I see in terms of the ship directly? What do I see in the portal? What do YOU (the guy in the portal) remember versus the guy now stepping out of the ship?

 

[JesseM]

what about relativity of simultaneity? portal A1 B1 at both ends of a moving train and portals A2 B2 on a stationary platform with a guy in the middle of the train w/light emitter and another guy on the platform with two photoplates. if the plates go off at the same time, the guy on the platform dies.

If you're talking about wormhole portals, it would just depend on how the two ends of either wormhole had been accelerated which would tell you what clocks moving along with each mouth would read at a given moment. For example, if clocks next to A1 and B1 both said 100 seconds simultaneously in the train frame when the light reached them, and clocks next to A2 and B2 both said 100 seconds simultaneously in the platform frame, then the light will exit A2 and B2 simultaneously in the platform frame and the guy will die. On the other hand if A1 said 100 seconds but B1 said 120 seconds simultaneously in the train frame at the moment the light reached both, but A2 and B2 still both said 100 seconds simultaneously in the platform frame, then in the platform frame the light exits B2 20 seconds after the light exits A2 so he'll survive.

 

[Ryan_m_b]

Wait a second

so let's say I sit in front of the portal on the wall and you sit in your ship, looking back at me. We're literally a few feet apart, talking to each other even though your ship is accelerating off. Let's say we continue to sit and talk (theoretically) for years, being able to see/hear one another. Then the ship returns, and you step out -- such that I can see you stepping out of your ship through my portal in addition to seeing you step out of the ship in real time behind me somewhere. What do I see in terms of the ship directly? What do I see in the portal? What do YOU (the guy in the portal) remember versus the guy now stepping out of the ship?

Going by the timescale used previously in the thread the conversation would have gone on for 10 years but he won't get out of the ship for another 20

 

[SeventhSigma]

So I would see the guy in the portal get out of the ship but there would actually be no ship behind me?

 

[JesseM]

Wait a second

so let's say I sit in front of the portal on the wall and you sit in your ship, looking back at me. We're literally a few feet apart, talking to each other even though your ship is accelerating off. Let's say we continue to sit and talk (theoretically) for years, being able to see/hear one another. Then the ship returns, and you step out -- such that I can see you stepping out of your ship through my portal in addition to seeing you step out of the ship in real time behind me somewhere. What do I see in terms of the ship directly? What do I see in the portal? What do YOU (the guy in the portal) remember versus the guy now stepping out of the ship?

Say I left when you and I were both 20 years old, and when the ship returns my age is 30 but your age is 50. That means that when you turn 30, if you're looking through the wormhole you can see me say "well, I've just landed back on Earth, in your backyard" and if you look out your window you don't see a ship in your backyard, but if you look through the wormhole you can see the ship is indeed in your backyard, but it's your backyard as it will look 20 years in the future...you could even see your 50-year-old self come on board the ship to greet me on my return, and you and he could have a conversation! Then when you turn 50, you can actually see the ship return and land in your backyard without having to look through the wormhole, and you can step on the ship to greet me, then look through the other end of the wormhole that was carried on the ship and see your 30-year-old self, and have a conversation with him.

In this type of scenario the 30-year-old version of you could even step through the wormhole on Earth and find himself on the parked ship 20 years in his future, or the 50-year-old version could step through the wormhole on the parked ship and find himself on Earth 20 years in his past. That's why a wormhole could work as a time machine according to general relativity, although there's speculation that quantum effects could actually destroy the wormhole if you tried to do something like this (assuming traversable wormholes are possible in the first place, they are theoretically allowed in general relativity but there's no obvious procedure for creating one if you don't have one already, and they would require "exotic matter" to hold them open).

 

[Ryan_m_b]

So I would see the guy in the portal get out of the ship but there would actually be no ship behind me?

Bingo,

The paradoxical thing would be if whilst you are talking through the portal you shoot out a message by radio (at light speed) towards the ship. The guy in the ship will get the before the guy in the portal sees you send it. Time travel, trippy stuff

 

[SeventhSigma]

If you're talking about wormhole portals, it would just depend on how the two ends of either wormhole had been accelerated which would tell you what clocks moving along with each mouth would read at a given moment. For example, if clocks next to A1 and B1 both said 100 seconds simultaneously in the train frame when the light reached them, and clocks next to A2 and B2 both said 100 seconds simultaneously in the platform frame, then the light will exit A2 and B2 simultaneously in the platform frame and the guy will die. On the other hand if A1 said 100 seconds but B1 said 120 seconds simultaneously in the train frame at the moment the light reached both, but A2 and B2 still both said 100 seconds simultaneously in the platform frame, then in the platform frame the light exits B2 20 seconds after the light exits A2 so he'll survive.

what I mean is that on the train, the light fired at A1 and B1 go at speed c in both directions, so the guy on the train sees the light hitting each photoplate at the same time + the guy dying

But according to the guy on the platform, since the train is moving at some fraction of c, he can look ahead and see that when the beams are fired, the one fired back at the end of the train enters that portal before the beam of light fired at the front -- meaning that they don't wind up hitting him at the same time?

 

[JesseM]

what I mean is that on the train, the light fired at A1 and B1 go at speed c in both directions, so the guy on the train sees the light hitting each photoplate at the same time + the guy dying

But according to the guy on the platform, since the train is moving at some fraction of c, he can look ahead and see that when the beams are fired, the one fired back at the end of the train enters that portal before the beam of light fired at the front -- meaning that they don't wind up hitting him at the same time?

No, again it just depends on what the clocks next to each mouth read, which depend on how they were accelerated in the past. Assume the clocks moving along with A1 and A2 were originally synchronized when the two mouths were next to each other with no time differential between them, and likewise for clock moving along with B1 and B2. Then we move A1 and B1 onto the train, and move A2 and B2 onto the platform, and get the train moving relative to the platform. What will all four clocks read at the same moment the guy in the middle of the train sets off the flash? (using the platform frame's definition of "at the same moment", say) The answer will depend on how each clock was moved around beforehand, but if you give some specific answer to that question, it will lead to a specific answer to whether the light hits both photoplates at the same time, an answer that all frames will agree on. If you don't specify these details the problem isn't well-defined.

 

[SeventhSigma]

Say I left when you and I were both 20 years old, and when the ship returns my age is 30 but your age is 50. That means that when you turn 30, if you're looking through the wormhole you can see me say "well, I've just landed back on Earth, in your backyard" and if you look out your window you don't see a ship in your backyard, but if you look through the wormhole you can see the ship is indeed in your backyard, but it's your backyard as it will look 20 years in the future...you could even see your 50-year-old self come on board the ship to greet me on my return, and you and he could have a conversation! Then when you turn 50, you can actually see the ship return and land in your backyard without having to look through the wormhole, and you can step on the ship to greet me, then look through the other end of the wormhole that was carried on the ship and see your 30-year-old self, and have a conversation with him.

In this type of scenario the 30-year-old version of you could even step through the wormhole on Earth and find himself on the parked ship 20 years in his future, or the 50-year-old version could step through the wormhole on the parked ship and find himself on Earth 20 years in his past. That's why a wormhole could work as a time machine according to general relativity, although there's speculation that quantum effects could actually destroy the wormhole if you tried to do something like this (assuming traversable wormholes are possible in the first place, they are theoretically allowed in general relativity but there's no obvious procedure for creating one if you don't have one already, and they would require "exotic matter" to hold them open).

So say I look through the portal after the ship arrives in your frame, and I see my 50 year old self come up to me and he says "Hey! Whatever number I say now, take the binary inverse. 1."

Then 20 years in the future, the ship lands and I decide to enter it -- but I've decided to say "0" this time instead of 1 to my old self. Paradox?

 

[JesseM]

So say I look through the portal after the ship arrives in your frame, and I see my 50 year old self come up to me and he says "Hey! Whatever number I say now, take the binary inverse. 1."

Then 20 years in the future, the ship lands and I decide to enter it -- but I've decided to say "0" this time instead of 1 to my old self. Paradox?

Among physicists like Thorne who take the possibility of time travel seriously, probably the most popular resolution for time travel paradoxes Novikov self-consistency principle, according to which any attempt to create this sort of contradiction would fail for one reason or another (for example, maybe your older self would misremember what he had heard his older self say 20 years ago). See my post [post=2914902]here[/post] for why this doesn't require any "intelligence" on the part of nature, how a lawlike rule could generate a history involving which is bound to be consistent, even if it involves intelligent beings who are trying to create mischief.

 

[SeventhSigma]

Would self-consistency imply that even if I wanted to go back in time and kill my grandfather, I shouldn't bother because since I am alive, I must have tried and failed -- but then by not trying in the first place, I'm ensuring that I'm alive, which is consistent as well, etc.

So even if my future self told me that I should say the inverse of binary 1, this means that somehow -- in some way -- in the future I will wind up saying the same thing?

 

[JesseM]

Would self-consistency imply that even if I wanted to go back in time and kill my grandfather, I shouldn't bother because since I am alive, I must have tried and failed -- but then by not trying in the first place, I'm ensuring that I'm alive, which is consistent as well, etc.

Well, it implies you didn't kill him, it doesn't tell you whether that's because you tried and failed or just because you knew about self-consistency and decided not to bother.

So even if my future self told me that I should say the inverse of binary 1, this means that somehow -- in some way -- in the future I will wind up saying the same thing?

Yup.

 

[SeventhSigma]

Well, it implies you didn't kill him, it doesn't tell you whether that's because you tried and failed or just because you knew about self-consistency and decided not to bother.

Yup.

But I mean that by me choosing not to bother, I immediately know why I didn't kill him.aka the way time travel worked in LOST

 

[JesseM]

But I mean that by me choosing not to bother, I immediately know why I didn't kill him.

But how do you know you won't change your mind and give it a try later?

 

[SeventhSigma]

But how do you know you won't change your mind and give it a try later?

Then I know that I'll end up failing -- my point is that whatever action I decide to "stick with" in the end ultimately tells me what will happen. So even if I actively decide to go kill him, I know that it'll be futile in the end.

 

[Ryan_m_b]

Would self-consistency imply that even if I wanted to go back in time and kill my grandfather, I shouldn't bother because since I am alive, I must have tried and failed -- but then by not trying in the first place, I'm ensuring that I'm alive, which is consistent as well, etc.

So even if my future self told me that I should say the inverse of binary 1, this means that somehow -- in some way -- in the future I will wind up saying the same thing?

Oddly enough I was thinking about this today. Yes if you try to kill you're grandfather you fail, perhaps you will shoot your grandma's secret boyfriend by mistake which will guide your grieving grandma into your grandfather's arms

 

[SeventhSigma]

reminds me of that michio kaku time travel story where you've got a one-person family (one person who winds up acting as both husband, wife, child, father, mother, accomplice, and kidnapper)

 

[JesseM]

reminds me of that michio kaku time travel story where you've got a one-person family (one person who winds up acting as both husband, wife, child, father, mother, accomplice, and kidnapper)

Kaku was actually summarizing a Robert Heinlein story there, it's called All You Zombies.

 

[Ryan_m_b]

Kaku was actually summarizing a Robert Heinlein story there, it's called All You Zombies.

Good story but one massive flaw, s/he shouldn't actually exist. The main character (and most others) are acausal

 

[SeventhSigma]

unless it's like an Ouroboros-type of story where the causal chains are circular

 

[JesseM]

Good story but one massive flaw, s/he shouldn't actually exist. The main character (and most others) are acausal

Self-consistent causal loops are logically possible in a universe obeying the Novikov principle, see the bootstrap paradox, also sometimes called the "ontological paradox". Of course it might be that if you had a fully developed theory of time travel that could assign probabilities to different possible histories, such things could turn out to be very unlikely, who knows.

 

[Ryan_m_b]

Self-consistent causal loops are logically possible in a universe obeying the Novikov principle, see the bootstrap paradox, also sometimes called the "ontological paradox".

Fair enough though I still don't understand how something with no beginning can exist. If something has to cause it's own existence so that it can exist surely it would never come into existence

 

[SeventhSigma]

Reminds me of the compass in LOST

A time-traveling Locke appears in the 1950's and gives a compass to Richard Alpert, who then gives the compass to present-day Locke in 2007, who eventually goes back in time to the 1950's, etc.

Calls into question "Where did the compass actually come from if all it ever experiences is being passed back and forth between Locke and Richard? How do we account for the age of the compass (does it not tarnish over time, etc)?"

 

[Ryan_m_b]

Reminds me of the compass in LOST

A time-traveling Locke appears in the 1950's and gives a compass to Richard Alpert, who then gives the compass to present-day Locke in 2007, who eventually goes back in time to the 1950's, etc.

Calls into question "Where did the compass actually come from if all it ever experiences is being passed back and forth between Locke and Richard? How do we account for the age of the compass (does it not tarnish over time, etc)?"

Exactly. To steal wikipedia's example if tomorrow a portal opens in front of me and an older me steps out and says "Hey young me. Here's a time traveling portal gun. Enjoy!" and after subjective years of time travel I pop into my house on the 26th of April 2011 to hand over the time traveling gun where did it originally come from? It seems to come from nowhere! It just appears yet it's subjectively eternal

 

[JesseM]

Exactly. To steal wikipedia's example if tomorrow a portal opens in front of me and an older me steps out and says "Hey young me. Here's a time traveling portal gun. Enjoy!" and after subjective years of time travel I pop into my house on the 26th of April 2011 to hand over the time traveling gun where did it originally come from? It seems to come from nowhere! It just appears yet it's subjectively eternal

The gun's word line is a good example of a closed timelike curve. It wouldn't be "subjectively eternal" though, if there was a little gnome riding on the gun throughout its existence, it wouldn't remember multiple distinct loops (or at least, its memories at each point on the circular world line would be fixed, it's not like it would make multiple "loops" and remember something different the "first time" it reached a point on the world line than it did the "second time" it reached the same point)

 

[Ryan_m_b]

The gun's word line is a good example of a closed timelike curve. It wouldn't be "subjectively eternal" though, if there was a little gnome riding on the gun throughout its existence, it wouldn't remember multiple distinct loops...

Interesting. I was just thinking that if along with the time traveling portal gun (TTPG) the future me hands over a video camera with the instructions to record, continuously, everything I do. Now video cameras obviously have a finite memory storage and if future me has been recording everything he was doing the camera should be full. However when he got it it was also full and so on and so forth infinitely backwards. So...who filled the memory of the camera?

 

[JesseM]

Interesting. I was just thinking that if along with the time traveling portal gun (TTPG) the future me hands over a video camera with the instructions to record, continuously, everything I do. Now video cameras obviously have a finite memory storage and if future me has been recording everything he was doing the camera should be full. However when he got it it was also full and so on and so forth infinitely backwards. So...who filled the memory of the camera?

Well, what if you watch what's on it, then erase the memory, and then start recording, recording the same information that was on it when it was handed to you?

 

[Ryan_m_b]

Well, what if you watch what's on it, then erase the memory, and then start recording, recording the same information that was on it when it was handed to you?

Well yes that would solve the problem but what if I didn't? Another way to look at it would be what if future me handed me an amount of radioactive material that was decaying. How is it that I could hand over the same lump after another subjective decade without it having decayed away?

It's tempting to answer my own question by saying "I couldn't do that, after 10 years I would have to find an indistinguishable lump to handover and say it is the same one I received." But I'm still drastically confused about where the original lump came from!

 

[SeventhSigma]

That's a lot like the compass example -- it should become tarnished and older-looking over time and yet it's being passed back and forth across two timeframes forever

 

[JesseM]

Well yes that would solve the problem but what if I didn't?

In that case, I think when you played the video back you would see this

Another way to look at it would be what if future me handed me an amount of radioactive material that was decaying. How is it that I could hand over the same lump after another subjective decade without it having decayed away?

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]

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.

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 "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.

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]

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

Yes me too!

 

[JesseM]

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.

Did you read [post=2914902]this comment[/post] 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]

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

No, in the case of wormholes the mass of the wormhole mouth itself changes to compensate for anything entering/exiting it, see [post=819700]this post from pervect[/post].

 

[Ryan_m_b]

No, in the case of wormholes the mass of the wormhole mouth itself changes to compensate for anything entering/exiting it, see [post=819700]this post from pervect[/post].

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]

No, in the case of wormholes the mass of the wormhole mouth itself changes to compensate for anything entering/exiting it, see [post=819700]this post from pervect[/post].

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]

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.

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.

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?

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]

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?

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

 

[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]

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?

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]

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!

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.