Time Dilation and the Jump Drive

[Tim Soholt]

The problem isn't time dilation, nor is it simultaneity per se. Time dilation only comes into play if the ship is accelerating in normal space, and the real issue is *causality*. The problem comes if you accept Einstein's theory of General Relativity, which as I understand it has stood up to some fairly rigorous experimental testing to date, and accept that things can't happen before their causes happen.

Since the speed of light in a vacuum *as you perceive it* is *always* 300,000 kilometers per second, regardless of how fast you're moving relative to someone else, FTL plays hell with causality.

Depending on how the locations involved were moving relative to each other, you could in potentially jump somewhere based on the receipt of a lightspeed message, Jump there, and tell the locals to send you the message that caused you to show up in the first place.

Personally, I like causality, so Einstein was wrong IMTU.

 

[Vargas]

The problem comes if you accept Einstein's theory of General Relativity, which as I understand it has stood up to some fairly rigorous experimental testing to date, and accept that things can't happen before their causes happen.

Just to play devil's advocate, keep in mind that Newton's 'laws' had been rigorously tested as well before Einstein bent the heck out of them.

Depending on how the locations involved were moving relative to each other, you could in potentially jump somewhere based on the receipt of a lightspeed message, Jump there, and tell the locals to send you the message that caused you to show up in the first place.

Could you give me a concrete example of how this would be done? (I need the help because this causality is begining to damage my calm). I don't understand how you can Jump into the past, (other than theoretically by pointing to a spot on a light cone diagram).

 

[Plankowner]

Vargas is right, you can't jump into the past.

Take the following example:

I send a message at light speed to a station 3 light-weeks away (the message will take 2 weeks to get there).

As soon as the message is sent, I use a Jump Ship to travel to that same station. I will arrive 2 weeks before the message sent through normal space, but a week will have still have passed.

Now say that I jump back immediately to the sending location. I will arrive 2 weeks after the message was sent, I get updated information and immediately jump back again.

I will arrive with updated information at the same time as the original message arrives, but at no time did I jump into the past.

 

[Ptah]

But you can get information about the past, that is look into the past, but you can't change it.

Say raiders attack a merchant ship but leave few or no clues to their identity. Theoretically you can jump ahead of the light from that event and observe it given powerful enough sensors to ID the attacker.

The theoretical possiblity of travelling back in time when travelling faster than the speed of light (demonstrated on a diagram) assumes when you make that diagram that c is the maximum speed of motion through space. Hence anything greater than c necessarily goes into a negative time part of the diagram. It also assumes you travel through space, I'm not sure that is even a safe assumption with Traveller jump drive.

Relativity has survived some rigourous tests but it can't explain everything. Quantum mechanics has also passed some rigorous tests and shows that somethings (I'm hesitant to call it information) do act at a distance/travel faster than the speed of light.

Nevertheless physical observation of the universe shows there is something off and/or incomplete with our theories. Dark matter and dark energy are the examples, two things invoked to fill the gaps. They point to theroies that are off and/or the existance of "matter" and "energy" of a kind we can't explain except by its observed effects.

Finally, who says jump drive move the ship actually faster than light? If there is another physical dimension and regular space is folded about this dimension, jump drive may just allow you to access this dimension. You enter it, travel at less than the local speed of light there, then remerge into the more familiar spatial dimensions. Who knows if particles don't naturally do this already in the Traveller universe, but absent a jump field any information content they have is loss in the transit, they just emerge as a broad microwave radiation background with a little structure here and there. Heck this extra-dimension may interact with ours only by gravity, all the mass in it being the true source of the "dark matter" we evoke to explain why our galaxy sticks together.

 

[atpollard]

But you can get information about the past, that is look into the past, but you can't change it.

Say raiders attack a merchant ship but leave few or no clues to their identity. Theoretically you can jump ahead of the light from that event and observe it given powerful enough sensors to ID the attacker.

THAT is an excellent implication of Traveller FTL Technology. Is it possible to "see" a ship in another system or would it require the Ringworld Engineers to build the telescope?

Imagine an observation outpost 2 light weeks away from a system that can be reached in one week by jump but is watching everything that happened two weeks ago. No time travel/causality issues, just a glimpse into the past.

A ship disappears with no witnesses, but the government has video of the event three weeks later (observation lag plus jump time).

 

[Tim Soholt]

Just to play devil's advocate, keep in mind that Newton's 'laws' had been rigorously tested as well before Einstein bent the heck out of them.

A fact which I definitely take advantage of.

Could you give me a concrete example of how this would be done? (I need the help because this causality is begining to damage my calm). I don't understand how you can Jump into the past, (other than theoretically by pointing to a spot on a light cone diagram).

Unfortunately, a) I grossly oversimplified and b) I'm sick right now, so my own brain isn't firing on all cylinders. Hopefully there's a physics teacher who can provide a better example. You actually need three points, and I *think* some normal-space acceleration as well. You change positions faster than light, find a place and an acceleration where the event you want to go to is in your present, and go there faster than light. Compared to your original frame of reference, you've traveled back in time.

Quantum mechanics has also passed some rigorous tests and shows that somethings (I'm hesitant to call it information) do act at a distance/travel faster than the speed of light.

Right. The one hope for a world where quantum mechanics and general relativity still exist is the fact that it currently appears impossible to get coherent information to travel faster than light.

Finally, who says jump drive move the ship actually faster than light? If there is another physical dimension and regular space is folded about this dimension, jump drive may just allow you to access this dimension. You enter it, travel at less than the local speed of light there, then remerge into the more familiar spatial dimensions.

That still doesn't get around relativity. It doesn't matter how you get there, just that you get there faster than a photon could under ideal circumstances.

THAT is an excellent implication of Traveller FTL Technology. Is it possible to "see" a ship in another system or would it require the Ringworld Engineers to build the telescope?

That's an excellent implication of *any* FTL technology. At least in Traveller, you have to wait a couple of weeks. I'm not sure about the telescope issue, but I'd guess it's possible under certain circumstances to ID a vessel at a distance of a light-week or more.

A ship disappears with no witnesses, but the government has video of the event three weeks later (observation lag plus jump time).

A ship with good enough sensors would be able to jump out a little more than a light-week, take a reading, and jump back to its home system. Call it 17 days max.

 

[Tobias]

Personally, I like causality, so Einstein was wrong IMTU.

Meh. Causality is overrated. Universe. Causality. Pick one.

 

[Ptah]

Unfortunately, a) I grossly oversimplified and b) I'm sick right now, so my own brain isn't firing on all cylinders. Hopefully there's a physics teacher who can provide a better example. You actually need three points, and I *think* some normal-space acceleration as well. You change positions faster than light, find a place and an acceleration where the event you want to go to is in your present, and go there faster than light. Compared to your original frame of reference, you've traveled back in time.

I think I see where you are going here. Part of it is that we define what is by the reception of information. You can not find a place and know anything about it as it currently exists, the best you can do is know what it was like when the light you are seeing left the place. If FTL was through real space and I was observing a ship coming at me at FTL it would arrive before I saw it leave. Which really screws with our sense of causality.

Right. The one hope for a world where quantum mechanics and general relativity still exist is the fact that it currently appears impossible to get coherent information to travel faster than light.

Well at least macrophysics wouldn't call it information. I'm thinking primarily of the coherence of particles and how measurement of the state of one particle can "force" a distant particle to assume a specific state instantaneously. And before you think the particle always had that state, there a few mind-bending experiments that indicate that is not the case. Brian Greene's book The Fabirc of the Cosmos has a nice discussion of this work as well as the implications of relativity.

That still doesn't get around relativity. It doesn't matter how you get there, just that you get there faster than a photon could under ideal circumstances.

I think it does. Relativity only talks about a speed limit of an object inertially moving through space. It does not set a speed limit on how fast space itself can move, and I think there is astrophysical evidence space once expanded faster than light. The extra dimension idea is very similar to the idea of wormholes, brane theory as well allows for the possibility. My made up game idea is light could also access this other dimension, so the ship is not travelling faster than a photon could under ideal circumstances from A to B since we assume light can enter and emerge from jump space, but in my psuedo game hypothesis it emerges as the background microwave radiation so is not recognized for what it really is at TL8.

I'll move mountains to find a rationalization, that can't be broken by players, for FTL travel, otherwise you don't have Traveller but Metamorphis Alpha, or better yet Brian Aldiss' book Starship circa 1958.

 

[Vargas]

Am I missing something? The discussion keeps veering into FTL in normal space, however, if we use Marc's Jumpspace article from JTAS as the baseline, the Traveller Jump drive involves transit through a dimension seperate from our own. Given the existence of this entirely fictional Jumpspace, are there causality implications?

 

[pendragonman]

As any mathematician can assure you, there are more dimmensions that the basic four (length, width, heighth, and time). The concept that J-space is a fifth dimmension where things from the four basic dimmensions transit from one point in the first three dimmensions to another in the first three dimmensions is easily seen when the J-space dimmension is concidered to be a dual space of the first three dimmensions. Thus, for J-space to work, we only need to find a function that maps itmes in the first three dimmensions to the J-space dimmension and back to a different point in the first three dimmensions (or to the dual space of the J-space, the dual-dual space of the first three dimmensions).

For our J-space to work, all we need is to find that function. And since this is a fictional space, we can safely hypothesise that this function exists.

 

[Tim Soholt]

I think I see where you are going here. Part of it is that we define what is by the reception of information. You can not find a place and know anything about it as it currently exists, the best you can do is know what it was like when the light you are seeing left the place. If FTL was through real space and I was observing a ship coming at me at FTL it would arrive before I saw it leave. Which really screws with our sense of causality.

Well, it can be more than our *sense* of causality. FTL potentially allows one to literally act on information before that information is generated.

Am I missing something? The discussion keeps veering into FTL in normal space, however, if we use Marc's Jumpspace article from JTAS as the baseline, the Traveller Jump drive involves transit through a dimension seperate from our own. Given the existence of this entirely fictional Jumpspace, are there causality implications?

The causality issue is caused by information arriving somewhere before photons from the same event arrive. It doesn't matter whether you're moving FTL in normal space or removing yourself from normal space at one point and putting yourself back in in another. It's also an issue for ships moving in normal space and transmitting FTL signals to each other.

 

[Vargas]

It doesn't matter whether you're moving FTL in normal space or removing yourself from normal space at one point and putting yourself back in in another.

OK, so you've violated the concept of causality by stepping out of normal space, transiting Jumpspace and returning to normal space at a point far removed from where you started and done so faster than a photon could. What's the practical problem with that? What is inherent to the act that causes reality to want to rise up and smite you?

 

[Ptah]

...The causality issue is caused by information arriving somewhere before photons from the same event arrive. It doesn't matter whether you're moving FTL in normal space or removing yourself from normal space at one point and putting yourself back in in another. It's also an issue for ships moving in normal space and transmitting FTL signals to each other.

That I can see. I'm at planet X which blows up, I jump to system Y one light-year away. But from system Y's perspective nothing has happened at planet X yet because the photons from the event haven't arrived yet. But that doesn't mean we assume nothing has happened to the planet X, we just don't know about it yet. It also doesn't mean an event hasn't happened until we see the light of it.

(So if a tree falls in the woods and no one is there to hear it does it still make a sound?)

Our intuition tells us, which could be wrong, that when we see light that is a light year old the event we are seeing occured a year ago, we just didn't know it. It's not like the event that we are seeing just winked into existance the moment we see it.

So a starship jumps and brings us information about planet X, before we see the light of the event. That doesn't mean the event hasn't happened yet. So you can act on the information before the light of the event arrives, no big deal, you still can't change the event that generated that information. That is you cannot travel to the past of that event and change it.

I'm not sure Relativity really talks about past and future, that is it says nothing about a direction in time. It does talk about time dilation effects where when v is greater than c you get imaginary delta t. In some equations this leads to negative time, but since the relativity equations are symmetric in time past and future have little meaning. Hence all of Stephen Hawkings and other scientists struglling to explain from our theories the direction of time that gives rise to our experience of past and future. Settling on entropy seems like a good explaination.

Let's get to quantum mechanics, I'd have to read the experiments again, but the action at a distance/correlated particle experiments seem to allow transfer of information faster than light. For example, you'd communicate by changing the state of particle A at a certain frequency, the frequency in the change in state would be your information (like morse code) as the correlated particle B would instantaneously change state as well. It doesn't matter what the state is, you are just looking at changes. If this idea falls flat, I beleive their are several other algortihms out there to get information out of a correlated quantum system without disturbing it's state by measurement, odd as that may sound.

 

[Ptah]

OK, so you've violated the concept of causality by stepping out of normal space, transiting Jumpspace and returning to normal space at a point far removed from where you started and done so faster than a photon could. What's the practical problem with that? What is inherent to the act that causes reality to want to rise up and smite you?

The Information Management Transtellar Union (IMTU) Local 505.

 

[Tim Soholt]

OK, so you've violated the concept of causality by stepping out of normal space, transiting Jumpspace and returning to normal space at a point far removed from where you started and done so faster than a photon could. What's the practical problem with that? What is inherent to the act that causes reality to want to rise up and smite you?

Well, nothing, as long as you don't mind a universe where you can react to events that haven't happened yet.

That I can see. I'm at planet X which blows up, I jump to system Y one light-year away. But from system Y's perspective nothing has happened at planet X yet because the photons from the event haven't arrived yet. But that doesn't mean we assume nothing has happened to the planet X, we just don't know about it yet. It also doesn't mean an event hasn't happened until we see the light of it.

No, it doesn't. The problem happens when you change position and it reorders events. Say, for instance, that a signal is sent from system A to system B, causing a ship to jump to system A and send the original signal to system B. Not normally possible, right?

But thanks to relativity, it's entirely possible that because system X is moving in a different direction than system B (and again, I think normal-space acceleration has to take place as well, but I can't remember for sure), from system X's perspective the signal from system A *hasn't been sent yet* (not just hasn't been received yet; if system A is two light-years away, the signal will arrive more than two years from now) when the ship jumps. If the ship jumps to system X before heading to system A, it arrives *before the signal telling it to show up*.

I was really hoping that someone with a more solid background in relativity would step in by now. I'm basing most of this on an AP Physics course I took 18 years ago. I don't even have access to any decent books on relativity right now.

Let's get to quantum mechanics, I'd have to read the experiments again, but the action at a distance/correlated particle experiments seem to allow transfer of information faster than light.

In theory, yes. All the experiments I've heard of, though (and your information may well be more current than mine) indicate that in practice the information is so garbled as to be useless. More refined experiments may change that, but as far as I know there isn't a way to transmit information FTL right now, so the jury can still waffle on the verdict.

 

[Vargas]

Well, nothing, as long as you don't mind a universe where you can react to events that haven't happened yet.

But why would you react to a non-event in the first place? If Star A suddenly explodes, and in the absence of FTL information transfer, the residents of a planet around Star B won't know it until a year later; they're not reacting to an event which they can't know about. If a starship carries news of the exploding Star A, the residents of the Star B system learn about what's happened before the light of the explosion reaches them; they're reacting to an event which has happened, it's just that sluggard light is lagging behind the starship.

Once again, how does the universe go ass over tea kettle?

No, it doesn't. The problem happens when you change position and it reorders events. Say, for instance, that a signal is sent from system A to system B, causing a ship to jump to system A and send the original signal to system B. Not normally possible, right?

The part in bold is where you lose me; the ship Jumping from B to A can't send the original signal because that happened in the past (message travel time in normal space plus one week).

But thanks to relativity, it's entirely possible that because system X is moving in a different direction than system B (and again, I think normal-space acceleration has to take place as well, but I can't remember for sure), from system X's perspective the signal from system A *hasn't been sent yet* (not just hasn't been received yet; if system A is two light-years away, the signal will arrive more than two years from now) when the ship jumps. If the ship jumps to system X before heading to system A, it arrives *before the signal telling it to show up*.

Sorry, I lost the bubble on this one; what are the relative positions and velocities of A, B and X to start?

 

[Tim Soholt]

The part in bold is where you lose me; the ship Jumping from B to A can't send the original signal because that happened in the past (message travel time in normal space plus one week).

Which is why it's a causality paradox.

Sorry, I lost the bubble on this one; what are the relative positions and velocities of A, B and X to start?

That's the part I need to be at the top of my mental game to explain. I know it works, I've done the thought experiment in the past, I know I've seen it worked out in multiple books, but I can't *find* any books with a worked out example and I can't think straight enough to reconstruct the thought experiment on my own.

 

[Vargas]

Which is why it's a causality paradox.

See here's my problem: Simply stating that something violates causality, falls outside the light cone or is a paradox doesn't help me understand the means by which the ship is capable of going back in time to send the message. I understand that Einstein's equations state/make it appear that you can go back in time if you exceed c in normal space, but we're not dealing with even getting close to c in normal space; in fact we're side stepping normal space altogether, (with the apparent exception of the dimension of time). I also understand it just might be impossible to answer the question without being able to comprehend some very advanced math but i'm hoping a thought experiemnt will do.

Last, I appreciate your hanging in there and trying to explain this Tim. My comments aren't meant as a criticism of you or what you're saying, I'm just trying to work through this in my head.

 

[Ptah]

.... The problem happens when you change position and it reorders events. Say, for instance, that a signal is sent from system A to system B, causing a ship to jump to system A and send the original signal to system B. Not normally possible, right?

This is interesing, I'm still trying to see if this is really responding to something that hasn't happened yet.

Let's say system A and B are 1 light year apart.

1. A signal is sent from system A to B causing a ship to jump to system A (so this has happened) and the ship jumps into A 1 year + 1 week later from the perspective of system A. Assuming no time dialation between system A and B, which is probably insignificant given the small difference in relatively velocity.

How can the ship send the signal for itself as it won't jump until it gets the signal?

I was really hoping that someone with a more solid background in relativity would step in by now. I'm basing most of this on an AP Physics course I took 18 years ago. I don't even have access to any decent books on relativity right now.

I still have my books, but I'm not sure I'm much better. It's been about 20 years since I had the 2nd year pyhysics course where we went over this. I did get an A and it was at Berkeley so not shabby.

My gut feeling is that relativity does predict some travel into the past for something that travels faster than light at the local speed of light, but do to the delta t making the time experience by the traveler go negative, not so much being able to outrace the light. Hence my earlier comment about how jump space may circumvent this problem. That is the weirdness of relativity is that time progresses at different rates for two observers dependent on your relative velocity.

To add confusion, for a computational mathematics class I once wrote a program to calculate the gravity wave emission from a rotating black hole. What I found on reading up on this is that there is a physically possible trajectory that allows you to go faster than light, i.e, into the imaginary time part of a space-time graph, without hitting the Schwartzchild radius. Of course the gravitation forces so close to the balck hole would rip you apart.

In theory, yes. All the experiments I've heard of, though (and your information may well be more current than mine) indicate that in practice the information is so garbled as to be useless. More refined experiments may change that, but as far as I know there isn't a way to transmit information FTL right now, so the jury can still waffle on the verdict.

It's been about 20 years since I had my graduate quantum mechanics, my most recent information is Brian Green's book The Fabric of the Cosmos.

He doesn't talk about information exchange but to even verify the happening and some other strange events that make it seem that the particle knows what is going to happen before it happens, the very thing you mention where you change an event before it happens!

The problem I see with using this as a communication device is that sure it may work across a lab bench, but the coherence is easily destroyed so it may present practical difficulties to separate the two entangled particles far enough apart to have a useful device. For example, confining fields may readily destroy the entanglement.

Back to relativity and such, wormholes and aspects of m-brane theory predict it is possible to get from A to B with an apparent speed faster than light by short circuiting as it were the normal 3 spatial dimensions, without violating relativity or going back in time. My understanding of course.

Then one could also postulate more bizar physics. Let's call it a quantum freezer, you generate a field that for a brief second you exactly, or close enough, define your momentum, every particle of your ship all at once. (Yes large scale quantum systems like this can happen, e.g. Bose-Einstein condensate but under special conditions) By the uncertainty principle you've just made you position uncertain, make it uncertain enough and you could really be a light year away at the next star. You haven't gone faster than light (or so the explainantion goes I believe) as the wave function that represents your ship (your ship is a wave as well as a particle) always existed a light year away and theoretically exists everywhere, the amplitude was just vanishingly small. By mysteriously defining you momentum so well, all of a sudden your probability of actually being a light year away increased.

 

[Tim Soholt]

Okay. I'm still out of it, but I'm starting to piece together in my head what the missing element is. Give me a week, week and a half and I think I can put together a cogent explanation of the causality problems inherent to FTL in a relativistic universe. It will probably involve at least one point of "There's a complicated equation that tells us X", but I should be able to at least explain what "X" is, which is where I'm stumbling right now.