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)