The Sword and Laser discussion

I must admit that I am a bit of a sci-fi nerd and some of the terminology used in this book is a bit baffling. Can anyone explain a time-debt?

Basically, since they're travelling at FTL speeds, time for them moves at a different rate than it does from someone on Earth(or anywhere else, really).

So if you left a friend on Earth who was the same age as you are, and took a flight out to Moons of Timbuktu that took 3 years of in-flight time, your friend will have aged say 10 years or so.

*note: numbers pulled outta my ass, but you get the gist.

Because the ships are moving at around light speed time on the ship is moving slower than on the target destination. So while 3 weeks of time passes on the ship, 3 years or whatever passes on the destination. So the journey is said to take 3 weeks with a "time debt" of 3 years. I too had to re read a bit to make sure it wasn't referring to time as a mode of currency.

Oh. I don't know why I never thought of this. Or why more sci-fi concerning FTL travel never addresses it. Thanks!

I hear the moons of Timbuktu are nice this time of year. And check out The Forever War, it's (an extremely well done) book that is basically all about this concept, one man fighting in the first and last battle of a war that spanned more than a thousand years.

I'm still having trouble grasping this concept. To me, just because the ship is carrying the person near the speed of light doesn't mean "ship time" would be independent of "planet time".

Take for example a smaller scale of Ala and David's examples: suppose you were on a train that could travel across the U.S. in 30 mins and the train leaves at noon. Because the train takes 30 mins it will arrive on the other side of the country at 12:30. The people on the train were being carried at a very fast rate as apposed to a person standing still on the earth. But in both cases the train still arrives at 12:30 so there would be no time differential.

So in theory, if a person was being carried at the speed of light and it took them 3 "ship weeks" to get to their destination, only 3 "planet weeks" would have elapsed, hence they would arrive without a time differential. The only difference between these two examples is the increased speed and distance traveled. Just because they are moving very fast in a ship doesn't mean their bodies would suddenly age at a different rate than what they did before they got on the ship. Again, we are talking about traveling an incredible distance at an incredible speed. These are independent of time. Just because you are moving really fast in a ship doesn't mean that time will suddenly change for you even though your perception of it might.

I just finished with the second chapter were it deals with 3 of the characters moving at a greatly increased rate of "speed" as opposed to the enemy army. From the characters perspective, the army is moving at a very very slow rate. So 1 hour relative to the characters would only seem like a second (just an example) to the army.

If anything, this example would make it seem as if the faster you are moving, the faster you would age relative to those that are "slower" not vice versa.

If this is the case, your body would be aging at a faster rate than those that are "slower" than you, which would imply that your body would have to be physically augmented to allow this to occur. To me, this is a different scenario than the person traveling in the ship.

I would love to hear your thoughts on this!

Andrew wrote: "I'm still having trouble grasping this concept. To me, just because the ship is carrying the person near the speed of light doesn't mean "ship time" would be independent of "planet time".

Take for..."

According to Einstein's "General Theory of Relativity" (I think) it does. I know, it makes my head hurt too - but there is a solution brought to us care of "Doctor Who" - anything that doesn't make sense or is illogical as far as "time" goes you just call a "paradox" and whallah! Problem solved and very scientific sounding to boot.

Anyway, according to Einstein, time is linked to space and light blah blah blah and moves at different rates relative to the observer as you approach light speed. Even Gravity effects time - ie Time in an airbus in flight moves at a different rate to time on the ground - the difference is negligible but still measurable using atomic clocks - now my head really hurts because I used scientific jargon like blah blah blah.

Andrew wrote: "If this is the case, your body would be aging at a faster rate than those that are "slower" than you, which would imply that your body would have to be physically augmented to allow this to occur. To me, this is a different scenario than the person traveling in the ship. "


This is a different scenario. When travelling in a ship the time dilation/difference is a direct result of travelling at speed and is the "normal" scientific
(according to our best theoretical knowledge to date) way time works. While the example you mention here is something else. The time difference in the battle scene is a result of time itself being manipulated by the Shrike - ie its not a result of them moving at speed but is a result of some mystical(or quasi scientific) power from the shrike or time tombs which results in them moving fast relative to the enemy - but are not in fact moving faster than normal - or are they - I give up, where's my paracetamol!

This may help... http://www.youtube.com/watch?v=Uy7rrr...

Here are the basics for relativistic travel (travel near the speed of light).

Imagine a clock that is composed of 2 mirrors (A and B) facing each other with a separation of length L. A single photon of light is bouncing between them, and each time the photon hits A, the clock ticks once.

Then we know that the time between ticks is (where c is the speed of light, a constant). With me so far?

Now imagine that you're moving parallel to this clock at speed v, such that you are seeing the light trace a different path, as so:

Now, and this is important, from your perspective (i.e. frame of reference), the light is traveling a distance 2D rather than 2L, making the period of the clock

See the right triangle? The Pythagorean Theorem () tells us that .

Do a little math (substitute D into the 2nd equation and solve for delta t'), and you reach the following:


But look! That part on top is just our earlier equation for the observed time when you're at rest, therefore:
.

What does this mean? As v approaches c, the denominator of the equation approaches zero, which means that the observed time of a person moving in a very very fast frame of reference will grow very very large the closer they come to the speed of light.

I doubt anyone's actually read this far, but I swiped all the images from http://en.wikipedia.org/wiki/Time_dil..., where you can go for a more thorough explanation.

Just wanted to say this is one of the reasons I love Sci-Fi and discussing it with people, because we get to have rad conversations like this one.

Also, I really like that Simmons brings up the "time debt" when mentioning space travel at light speed. Too many books ignore this for the sake of convenience. Although I don't get too bogged down in "facts" if the story is good. : )
Plus, even if it wasn't scientifically sound it's still a cool concept anyway.

Others have explained it and are correct in that its all about the theory of relativity.

The concept of time debt will be important in several of the tales, most notably the Poets, Scholars, and especially the Consuls.

Sky wrote: "What does this mean? As v approaches c, the denominator of the equation approaches zero..."

After speaking with my wife, I think an example might help. Say you're not moving. Then v = 0 and t' = t, so time moves normally, as expected.

Now say that v = one half the speed of light = c/2. Then t' = t/(1-1/4)^(1/2) = 2t/3^(1/2). So for every 2 units of time (seconds, days, years...) that take place at rest, only 3^(1/2) take place while moving at half the speed of light. Therefore the faster you move, the slower time gets.

Which may make you wonder, what happens if you somehow exceed the speed of light? Maybe you should ask the Shrike and his imaginary (see what I did there?) Time Tombs that are moving the wrong way in time...

Sky,

Fantastic time equations there.

The only problem I have with them and time relativity generally speaking is that it doesn't explain how "observed time" is more than optical illusion as a consequence of the light having a time delay between source and destination. You still have to assume that light space and time are somehow "physically" linked to get to time being relative to the point that you will age "slower" on a ship relative to someone at your destination.
And I do believe your earlier equation (the last one) demonstrates why travel at or faster than the speed of light is theoretically impossible - ie if v=c you have a denominator of 0 ie mathematically unsound. But to me that just means its impossible to "observe" time/speed if travelling at the speed of light - it does not necessarily mean above light speeds are actually impossible unless again you assume light and time are "physically" linked.

Now let me take a couple more paracetamol before throwing this out - Nobody ever sees the present - we are always looking at the past because there is a delay between every event that occurs and when we actually see the event. The delay may be negligible but theoretically measurable. Does that mean we are always living in the past? And if you're blind and don't see any light, does that mean you are in a different "time" to those around you? More paracetamol. And seeing that light itself is travelling at the speed of light - what happens if we assume for argument sake that light was intelligent - how would time be perceived by the light photon in the equations you posted?

Now I need something a little stronger for my headache

Thanks Sky. Now I feel bad pulling numbers outta my ass like that.

:P

(Note for Americans who aren't pharma nerds, paracetamol = acetomeniphen, i.e. Tylenol)

@Ala, I think you meant to say, because they're not traveling at FTL speeds. FTL space travel is the usual way SF authors solve the time dilation problem - have the ships travel at non-relativistic speeds when they're near planets, then FTL to the next destination. Ta-da, everyone ages at the same rate.

Hyperion is interesting because it includes both the farcaster network and ships traveling at relativistic speeds, implying that FTL travel the way it normally exists in SF is technologically impossible in this universe. (Except of course for the Shrike and the Time Tombs, who don't seem to answer to the laws of physics.)

I don't even know what I was trying to say anymore. I've slept since that post.

Jess wrote: "(Note for Americans who aren't pharma nerds, paracetamol = acetomeniphen, i.e. Tylenol)

@Ala, I think you meant to say, because they're not traveling at FTL speeds. FTL space travel is the usual ..."

Actually they do have FTL speed in the Hyperion Universe. Martin the Poet has a line in his story talking about a voyage he once took on a ship that was slower than light. The obvious implication is that most ships travel faster than light.

In that case, ignore everything I said above...clearly I don't understand relativity as well as I thought I did!

I thought I understood time dilation just fine until I read (yes completely) Sky's post... WAAAY too early in the decade to be doing math like that... Where's my coffee? :-P

By the way, if anyone watches Sanctuary, there was a very good Time Dilation example that did not involve Space Travel. It was towards the end of season 3.

Our own astronauts who go into space actually experience time dilation at the speeds they move at. Its only in the fractions of a second but it still happens.

Yeah, I included the derivation because the math major in me is still convinced that everyone else wants to know about how we get to the answer as well...

But that last equation is useful, and kinda cool when dealing with sci-fi. Check this out: in the Consul's story, we see the effects of time dilation in all their bittersweet detail. But maybe you want to know exactly how fast Shipman Merin is traveling. We actually have everything we need to figure that out.

t' = 11 years = 132 months
t = 10 months

Solving for v gives...

v = c(1 - (10/132)^2)^(1/2) = 0.99712627 times the speed of light. This is his average speed going from and to the planet.

Cool, right? Yeah, I know, not very...

As for FTL travel, the farcasters are basically wormholes. The time it takes to get from point A to point B is just the distance divided by the speed of travel. Speed is hard capped at the speed of light, but by connecting two locations with a wormhole you reduce the distance between, and avoid violating any laws of nature.

Hmm, If I ever write a sci-fi book I would totally make up new terms just so that I don't have to see if what I am saying makes sense.... lol

But thanks for the explanation Sky!

Blah, blah, blah, paradox. Ok, got it. Where is my "I Believe" Button?

Sky wrote: "Yeah, I included the derivation because the math major in me is still convinced that everyone else wants to know about how we get to the answer as well...

But that last equation is useful, and kin..."

Thanks for showing the derivation! As a former physics major and current astro grad student, I appreciate your thoroughness :)

Anna wrote: "Thanks for showing the derivation! As a former physics major and current astro grad student, I appreciate your thoroughness :)"

Glad you enjoyed it, it was actually fun to write, especially while I was pretending that it could help people understand a little bit of special relativity. =P

This is why I love this group!

There have been numerous experiments that prove the time dilation theory.
One experiment (Called the Hafele, Keating Experiment) involves taking 2 atomic clocks and synchronizing them. Leave one of them stationary on the ground and fly other around in a fast jet for several hours. When you check them again you will find that the one that was on the plane is now slower than the one that was left on the ground. It's only going to be a very small difference, but it proves the basic relativity rule 'the faster you go, the slower time passes'.
More info here : http://hyperphysics.phy-astr.gsu.edu/...

I agree with the concept of time moving at different rates in FTL and on "planet time", but I am unclear on why they go into cryogenic sleep as well... My understanding of cryogenic would be to preserve the body over long times of travel, if we are FTL doesn't that negate the need for Cryogenics?

It'd still be a long time cooped up in a ship. Plus, with everyone in a cryogenic freeze, they can conserve power and wouldnt need to carry a bunch of foodstuffs or other necessities in a large cargo.

The easiest way to get around the time dilation paradox for an author is the concept of hyperspace. Where an FTL drive removes the ship from normal space-time and travels outside of it leaving normal laws of physics behind. You could also say the FTL drive creates a wormhole through space-time to reach it's destination or even folds it. This way, instead of the ship following the curve of space time, it takes a short-cut through it.

Most Sci-Fi authors use one of these tricks to get around the time issue to keep things tidy. Star Wars, for instance, uses hyperspace as their FTL solution, where as Stargate uses the wormhole method for gate to gate travel.

Normal physics causes headaches for Sci-Fi authors because it makes Faster Than Lightspeed travel virtually impossible based on our current understanding of the Standard Model of physics.

Wolfman-k wrote: My understanding of cryogenic would be to preserve the body over long times of travel, if we are FTL doesn't that negate the need for Cryogenics?"

Going off of the assumption that the Hawking drive can ignore the laws of physics and relativistic travel as we know them today by exceeding the speed of light within its own frame of reference, you're right, cryogenics for preserving the body from the ravages of time would seem unnecessary.

But perhaps they are being preserved from something else in their fugue state, like the forces exerted by a massive acceleration? Or possibly, since they must be tunneling through reality at some level, it's simply safer and easier to make the trip while in a state of suspended animation.

It could be just for logistical purposes. 1,000 people in cryo sleep do not need much space for food, drink etc. Now think of housing, feeding and entertaining that same 1,000 people for 3 weeks. Think - cruise ship.

Great explanation on relativistic travel. There are genuinely brilliant people on these boards!! I wonder about the crew clone workers, do they go into fugue sleep as well? Or perhaps are grow and used as needed (until they expire) and new ones are grown.

Psst . . . You didn't hear it from me but they harvest the passengers for genetic material for cloning while they are in fugue sleep.

I felt like this was self explanatory if you did high school level physics. also a lot of hard sci fi does deal with this concept. its often used as a conceit to allow characters to exist throughout a long period of time and witness multiple important events. the forever war (mentioned above) is a good example. some of asimovs and Clarke's short stories do a great job too.

The notion of accounting for time debt is one of the geeky/science details that I loved about this book

Relativistic time dilation isn't just SF geekery or interesting experiments with clocks and airplanes. It's a fact of life for aerospace programmers. Because GPS satellites circle Earth twice per day, they're traveling considerably faster than the surface of the Earth spins. At 20,000 km out from the surface of the Earth, they also experience about 1/4 the gravity of the surface of the Earth. Because both speed and gravity have relativistic effects on time, the clocks on satellites will "gain time" versus clocks on Earth at the rate of about 38 microseconds per day. Which doesn't sound like much, until you know that for GPS to work, the margin of error cannot be more than 50 nanoseconds (the amount of time it takes for a signal to travel to Earth's surface from the satellite). So GPS satellites must be programmed to compensate for relativistic time dilation, or they wouldn't work.

There's an article with more details about GPS satellites and time dilation here:
http://www.physicscentral.com/explore...

I feel like this conversation could have been a lot shorter if you'd all have just seen Flight of the Navigator. The author should have just called it Flight of the Navigator syndrome.

Sky wrote: "t' = 11 years = 132 months
t = 10 months

Solving for v gives...

v = c(1 - (10/132)^2)^(1/2) = 0.99712627 times the speed of light. This is his average speed going from and to the planet."

OMG. This is one of the coolest things that I have read this half of the century.

Thank you for being super awesome.

Ed wrote: "OMG. This is one of the coolest things that I have read this half of the century."

Yes! Take that, everything written in the last 12 years! Physics for the win.

I should probably point out that I'm not even taking into account the effects of gravity (and therefore inertial mass due to the acceleration of the ship) on time, however. But that's fine, because when it comes to napkin math, I believe in things like frictionless surfaces and perfect vacuums. Just throwing that out there. I'd hate to be a math tease.