Science question.

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that guy
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Re: Science question.

Post by that guy »

The biggest issue with most faster than light drives is that faster than light travel is interchangeable with time travel. You can't have one without the other. This wouldn't be time travel in a classical sense necessarily but it would allow you to see events which had not yet happened but whose outcomes you could still alter. This creates paradoxes. This stems from one of the key points of special relativity which is that there is no such thing as an absolute ordering of events. The same two events can occur in different orders in different frames of reference provided neither event can affect the other. This breaks down with faster than light travel as you end up being able to influence everything. This is a really vague explanation because a more accurate explanation is a total mind fuck.

Phas you may be interested to check out this http://en.wikipedia.org/wiki/Alcubierre_drive . It's a theoretical faster than light device that fits all the equations for general relativity without requiring time travel. It does this by distorting space so that there's less of it to travel through rather than making the object actually travel faster than light. It's kind of like creating a dynamic localized shortcut through space. This has all sorts of other potential issues as discussed in that wiki page most of which I don't fully understand. Note that from a general relativity perspective gravity warps space time much like an Alcubierre drive might so fundamentally the drive would not be that different to a gravity altering drive. Another interesting note is that the theory behind the drive requires the existence of negative mass.

Antimatter particles don't have negative mass. They typically have the negative charge from their counterparts but they have positive mass. If they had negative mass then when a positron and electron annihilated each other they would create no energy because their total mass would be zero. In fact they create a fuckton of energy. (Just noticed Jaedar already pointed this out)

In terms of spinning vs fancy-gravity gravity on ships. Note, DDL, that centripetal force is also dependent on distance (albeit linearly rather than quadratically) so someone in the perfect center of the ship will experience no gravity while someone 10 floors out will experience twice the gravity of someone 5 floors out. Also isn't it possible to create a mostly constant field from a quadratic force? I believe capacitors achieve this with the coulomb force (though when I try and solve the equations myself it appears not I seem to recall learning this once upon a time).

The reason two gyros spinning in opposite directions would be easy to spin is that their net angular momentum is zero. It's conservation of angular momentum that creates the gyroscope effect. Regarding DDL's comment about being hard to turn a spinning gyro. I would imagine a ship would have a central spinning body whose orientation would remain essentially fixed. Around this there would be a frame on which the thrusters could move. A plane contains gyroscopes for its navigation. When the plane turns the gyros don't (that's the point). If you made the gyros a lot bigger and crammed them full of people that gives you your centripetal force without impairing your turning ability.
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Re: Science question.

Post by that guy »

This website, although horribly designed, has a lot of interesting stuff on designing a sci fi universe: http://www.projectrho.com/public_html/rocket/index.php
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Re: Science question.

Post by Phasmatis »

That's pretty awesome, lots of stuff to read on there, thanks.
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Re: Science question.

Post by AEmer »

I really wish there was some way to experiment with momentum and body dynamics. It is counter intuitive as fuck to me. Like, I understand newtons laws. I can apply them. Conservation of mechanical energy means that, if I put energy into a rotation, I have to absorb a rotation as well.

Where it gets complicated is if I have objects that rotate within a frame of reference. If I apply a force that would twist the axis around which the rotation is going on, then there's a lot of momentum which is going to complicate that twist: Applying force to the axis means you're trying to (effectively) stop a rotation around the original axis and create a rotation around the new axis. Because the momentum will likely be preserved, the twist will likely do something completely different from what you expect it will do.

I realize that the force you need to apply is precisely what you need to stop the rotation and start the rotation at the new angle (plus a rotationless twist), summed up - but how that force looks under circumstances where you have several rotations going on is...well, it's complicated. Not to mention, if you have several rotations going on, how they interact when you twist them around is....guh.

Anyway. The whole 'internal rotation' thing is really important to space ships (I feel), because without a drag medium to stabilize the ship (like a plane or a boat, or even a car), all stabilization needs to be accounted for: You need to start rotating in a direction, and you need to stop rotating in that direction when you're facing the way you want to face. What this means is that, given a set of internal rotating objects that can be accelerated and decelerated at will, you should be able to change facing without expending impulse on the outside. Furthermore: If you have an object that is internally rotating in one way or another, applying outside impulse will make the ship behave in ways you don't expect! Which is really cool, because that means you can do weird things like moving the center of gravity and other such things, so that you can put your thrusters where you want them (like star trek nacelles or similar), rather than in places where they won't cause messed up rotation (...like on boring old rockets...).
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Re: Science question.

Post by DDL »

I really wish I understood relativity better.

Every time I read attempts to explain how FTL breaks causality, it always comes down to frames of reference. In my head, there's basically no problem whatsoever with FTL travel if you assume the entire universe is a single frame of reference: light prevents you SEEING events until the light reaches you, but the event has nevertheless happened. I.e. you could shoot a guy, take an FTL ship to an AU out, then look through a telescope and see him happily walking around, but he'd still be dead, just "not yet in this lightbubble": if you waited a few minutes you'd see yourself walk out and shoot him. There would be no way to influence events in the past, because everything happens when it happens, no matter when you actually SAW it happen.

What I really can't wrap my head around is how this breaks down when you no longer can assume the universe is a single frame of reference (which, I guess, relatively tell us it is not). Having light as a limit sorta...makes a referenceless universe BEHAVE as a single reference frame, but it is not (apparently) actually one. And I don't quite understand how this makes paradoxes possible.

Anyone physics-y willing to try and dumb it down for me?
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Re: Science question.

Post by that guy »

if you add me on skype I'll talk you through what I understand. Never know when I might be plagued by a chemistry question. Please note that I've only studied this at an undergraduate level and that was 5 or so years ago.

The fundamental thing that multiple frames of reference gives you is that light travels at the speed of light regardless of your frame of reference. This may sound a bit self proving but essentially multiple frames of reference are going to exist in any theory. You and I have different frames of reference. It's simply a question of what the implications of that are.

A simple thought experiment to disprove a single frame of reference would be to consider a car coming towards you at the speed of light (I'm driving the car). From your frame of reference the car is traveling at the speed of light. From the car's perspective you're traveling towards it at the speed of light (because everything is stationary, i.e. has a relative velocity of zero, in its own frame of reference). What happens if the car turns on its headlights. From a classical perspective since you are stationary and the car is moving at the speed of light then, in your frame of reference, each photon would get emitted at the speed of light and thus travel with the car. This would create a building mass of photons in front of the car sort of like a sonic boom. Already this sounds a little unintuitive but now let's consider it from the car's perspective. In the car's frame of reference the photon's are also building up at its headlights and so have a relative velocity of zero. However we've said that photons always travel at the speed of light. This is not the case in the car's frame of reference so that statement is violated. We could revise our statement to say light travels at the speed of light from a stationary frame of reference but then what defines stationary? The car isn't stationary but if you're on earth which is orbiting the sun then neither are you. Even if you were stationary in our solar system you'd still be orbiting our galaxy etc. The fundamental assumption of special relativity is that there is no special frame of reference such as the stationary frame of reference I'm talking about.

It's important to distinguish between a truth in a particular frame of reference and an observation in that same frame of reference. Everything that is an observation is a truth but there are an infinity of truths that are not observed. The speed at which the information of truths reaches you is unimportant. In the case of the car in the classical case you of course wont see the photons building up on the headlight because they haven't reached you yet but it's still happening and you will eventually observe it or the effects it has.

I'll let you digest that before I go further.
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Re: Science question.

Post by DDL »

hrm.. (can't skype: at work..obviously not currently working terribly hard, coz asking physics questions, but hey)

So with time dilation (which I think I'm happy with), at light speed, if you turned your car lights on....you wouldn't turn your car lights on because at lightspeed time wouldn't actually exist for you.

And at 'marginally below the speed of light', the photons from your car lights would be emitted at lightspeed, and thus while the actual distance between you and the front of your pulse of emitted photons (from an external, stationary observer) would accumulate at less than lightspeed (i.e. the light would be coming out at lightspeed, you're going 90% lightspeed so in a given timestep the distance between 'you' and 'emitted photons' would be 0.1C), you'd still observe the light travelling at lightspeed AWAY from you, because time is slower for you. Right?

Except then of course there's the problem of photons travelling toward you, which should be arriving at MORE than lightspeed no matter what, even with time dilation (C + X is always going to be bigger than C, no matter how small you make X). Instead we get blue/redshifting which falls into the whole wave half of wave/particle duality. And then my head breaks.

And of course, at near-lightspeed you observe everyone ELSE'S clocks going slower, because as far as you're concerned, THEY'RE the ones speeding. This also makes my head hurt.

Let's see. So, if (for the purposes of a random thought experiment) we have a vessel that travels at 0.9C, and we head off toward a star that is 1 light year away. At this speed, time dilation is about a factor of 2, so you'll see the clocks at your destination spinning at half speed, and everyone else will see yours spinning at half speed. The journey takes fractionally over a year, but for you this only feels like half a year, and your clocks reflect this (and indeed, for you, only half a year has elapsed). You'll also see your target's clocks have only spun half a year's worth, right? So does this mean that they've spun a quarter of a years worth? Coz you've still taken a year to get there....so what happens to the clocks at your destination, do you see them spinning faster (because you're approaching the place fast enough to get a 'speeded up version' of events in its light bubble), at the same speed? Slowing down and then suddenly spinning to the correct time, or ...what?

Oh dear, I've broken my head again.
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Re: Science question.

Post by Made in China »

Actually, you have some assumptions wrong.
As you approach your destination, you will see every photon their clock emits - meaning, you will see their clocks turn a year in half a year's time, while also shifting to blue due to your fast approach. Think of it like playing a movie in fast forward - they emit the same photons you see, only they are more condensed.
Also, note that as you grow closer, the clocks will tick faster, as light takes less time to travel to you. Just to add to the mind fuck.

On your previous question - light speed is a constant in every frame of reference, so even if you'll race towards it in a near-light speed vessel, it still won't seem to you faster than light speed. That's because (as far as I understand it) it doesn't have mass, so gravity and time doesn't bend in its frame of reference.

I think you have issues with understanding this because you still view light speed from a Newtonian standpoint, which sees velocity and space quite linearly. The whole relativity thing that Einstein came up with dictates that time and space can bend, while the upper limit will always remain light speed, due to its mass-less properties.
So instead of seeing the world going from 0 to infinity, physicists now view the world from 0 to light speed, which creates all of these mind fucks since infinity is given a numerical value and sci-fi authors constantly use FTL as space magic.
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Re: Science question.

Post by Jaedar »

So here's my question(I can't think of a good answer to DDL's): How can relativity say anything about what happens when we move at faster than c, when the inability to do just that is a rather fundamental part of the theory? It seems to be rather contradictory to me.

As part of my mechanics 3 course, there was some relativity, but it was mostly about relativistic recoils and such. I seem to recall it being mentioned at several times though, that you can show that moving faster than c would break causality by constructing some special frames of reference(with the lorenz transformation). I found a picture by googling, and it makes some sense to me but I am not sure I can explain it properly.

Image
This is basically the lorenz transformation(It's how you transform betweeen coordinate systems in a way that doesn't break under the fact that c is constant in every reference frame) in graphical form. A happens after B, and C happens after D. But in the FTL case, the order changes, this means that the person travelling ftl will see D happening before C.

That was a really crappy explanation, and it might not even be the kind of explanation that you wanted.
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Re: Science question.

Post by DDL »

Made in China wrote:Actually, you have some assumptions wrong.
As you approach your destination, you will see every photon their clock emits - meaning, you will see their clocks turn a year in half a year's time, while also shifting to blue due to your fast approach.
Right, so when they look at ME, they should see MY clock ticking faster too? Because after all, they're approaching me at 0.9xlight speed.

But then...because I'm travelling at 0.9x lightspeed, I experience time dilation, so my clock should appear slower, since I'm only going to experience half a year over the course of my year's travel.

So..if everything is relative (i.e. you cannot say whether I'm travelling at 0.9c toward a stationary object, or the stationary object is travelling at 0.9c toward me, or indeed if we're both moving at 0.45c toward each other, or even both moving 0.45c toward each other while ALSO moving 0.4c in a shared direction), how is time dilation apportioned?


Also,
Also, note that as you grow closer, the clocks will tick faster, as light takes less time to travel to you. Just to add to the mind fuck.
why? I'm not accelerating, I'm just moving at a constant, very fast speed. Otherwise this implies that clocks on the far wall tick ever so slightly slower than clocks on my desk.


See, on that site That Guy linked to, they mentioned a phrase: "FTL, causality, relativity: pick two", and as far as I can see, it's just the relativity bit I'm having trouble with. If we take the universe as a fixed reference frame, it really doesn't matter what speed you travel: everything happens when it happens, no matter when you actually see it happening. Somehow relativity + FTL allows for sorta..bubble universes with different orderings of events to interpenetrate and create paradoxes. Which is what I'm having trouble with. And those time/distance diagrams really don't click for me very easily. :(
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Re: Science question.

Post by Made in China »

DDL wrote:So..if everything is relative (i.e. you cannot say whether I'm travelling at 0.9c toward a stationary object, or the stationary object is travelling at 0.9c toward me, or indeed if we're both moving at 0.45c toward each other, or even both moving 0.45c toward each other while ALSO moving 0.4c in a shared direction), how is time dilation apportioned?
Actually, I'm not sure about that one, and it had me quite confused, but I think I have your answer - it's about timekeeping. You keep your time on both your spaceship and the destination in regards to your point of origin, so you have an overbearing universe on top of your micro-universe of the spaceship, and that one includes both you, your origin and your destination.
In that universe, you move at 0.9C, and both your planet and the destination planet are basically stationary. That explains why you have time dilation while they do not. If the planet were to experience time dilation, everything it can observe in that universe should have the effects of special relativity, including your home planet, while your ship's velocity would be cancelled - meaning that their meeting would still occur at the same time to that planet.
It also means that they will see your clock ticking slower, in accordance with the messed up timekeeping.
DDL wrote:
Also, note that as you grow closer, the clocks will tick faster, as light takes less time to travel to you. Just to add to the mind fuck.
why? I'm not accelerating, I'm just moving at a constant, very fast speed. Otherwise this implies that clocks on the far wall tick ever so slightly slower than clocks on my desk.
Yes, but light will have less distance to travel the closer you get, so you'll receive more photons just because V=d/t=d*f, and therefore f(photons)=C/d.
During your trip you'll see clocks from 1 LY ago to 1.11 LY in the future (when you reach the planet), while only about a year on that planet has passed. I think you can understand why the planet will seem brighter to you as you get closer to it.

P.S.
As I've said, I'm not entirely sure about the first part. If anyone wants to correct me, fill free to do so.
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Re: Science question.

Post by DDL »

Yeah, but the whole POINT of relativity is that the "universe is static, you're moving" analogy is NOT the case, as far as I understand it. There is no static frame of reference: you moving toward something at a given speed is wholly and totally indistinguishable from you being stationary and everything else moving toward you at the same speed. Or at least, everything observable, because light bubbles again.

If you could just say "yep, universe is fixed", then you'd have one stable frame of reference, relativity wouldn't be a thing, and FTL would be fine.
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Re: Science question.

Post by gamer0004 »

I think even in classical mechanics, you can't distinguish between moving toward something at a constant speed and the that moving toward you. You can only measure acceleration, speed is relative to other objects and there is no fixed "centre" of the universe.
Relativity was once explained to me using a concept of light clocks, where two people moving at different speeds would record a different number of times a light beam flashes between two points, but I can't exactly remember how that worked.
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Re: Science question.

Post by Jaedar »

DDL wrote:Yeah, but the whole POINT of relativity is that the "universe is static, you're moving" analogy is NOT the case, as far as I understand it.
Indeed. It's all relative to the observer.
gamer0004 wrote:I think even in classical mechanics, you can't distinguish between moving toward something at a constant speed and the that moving toward you. You can only measure acceleration, speed is relative to other objects and there is no fixed "centre" of the universe.
It was a very classical notion that there existed some TRUE frame of reference, some universal point 0,0,0 and everything else was just relative to that.
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Re: Science question.

Post by Made in China »

@DDL
I've tried to put all of the factors in the question in one frame of reference, because that seemed the best course of action when talking about time. Time isn't constant across all space, and it's simply confusing discussing it without a static object.

OK, Wikipedia time.
It seems that in special relativity, each of the clocks seem to tick slower to each one. The effect is the same on both sides as long they don't change in acceleration, which the ship will have to undergo to land - but during travel, the clocks will be slower, not faster. After landing time dilation will only occur on the object the changed its velocity during its travel.
So, sorry for misleading you, I was wrong the first time.
So, I quit. Like you, it doesn't make much sense to me, and my old understanding of astrophysics has completely failed me.
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