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View Full Version : Intercept Speeds Over Light - What Happens?



FOTSGreg
02-07-2011, 05:36 AM
Let's say you have 2 forces moving through vacuum on an intercept course. Both forces are moving at sublight speeds and obeying the laws of physics, but their combined speeds add up to FTL.

What is seen by each side? What is experienced by each side especially as they come within weapons range of one another (ships are course controlled within combat ranges)?

Let's start the speeds at 101% of light and increase by 10% increments until the fleets can't possibly be moving faster than light.

STKlingaman
02-07-2011, 06:07 AM
Speculation is a great thing, if you can make people
believe what you're telling them, you can say most anything.
--Think what george lucas made people believe, with
the maneuverability of the Ty and X-wing fighters--
The whole light speed debate is really an unknown,
(much like what is in deep space 100's of light years away,
it's an educated guess - I'm thinking it's really blurry
when you're going that fast.

Even if two ship are battling at .7 of light speed, how fast
are the weapons they're firing traveling?
Even at .1 of light speed battle ranges would be
millions upon millions of miles.
It would be a total computer generated battle
moving that fast, the calculations for targeting
opposing ships would be . . . alot?


my two cents
... is there change?

benbradley
02-07-2011, 06:25 AM
Let's say you have 2 forces moving through vacuum on an intercept course. Both forces are moving at sublight speeds and obeying the laws of physics, but their combined speeds add up to FTL.

What is seen by each side? What is experienced by each side especially as they come within weapons range of one another (ships are course controlled within combat ranges)?

Let's start the speeds at 101% of light and increase by 10% increments until the fleets can't possibly be moving faster than light.
They don't go faster than the speed of light relative to each other. At relativistic speeds, speeds don't add up linearly. This is of course a consequence of Einstein's theory of relativity. I don't know how to explain it offhand, I just know that's what happens.

thothguard51
02-07-2011, 06:25 AM
A few things to consider...

I don't see why the combined speeds of both forces equaling FTL should matter in this equation. The only thing that should matter is the speed that each force has achieved. Unless you are going for some type of time displacement thingy being created between them...

As to combat range, assuming both forces are using the same type of weapons and one does not have an advantage, combat range will depend on the weapons range and targeting computers ability to accurately project.

To reach a range in which the weapons are effective, we have to assume both forces are acting in the same manner. Both slow down at the same time, but even at sub light speeds, slowing down in a vacuum is going to be time consuming.

What happens, if one force slows down and then speeds up again once they are sure the opposing force has slowed down sufficiently? I would think this would all be part of one force outmaneuvering the other, especially if one is being pursued.

I think the advantage would go to the force that can out maneuver and fire accurately first. Me, I would fire and move...fire and move. Very similar to what ships used to do once weapons reached the capability to fire over the horizon.

Does any of this make sense???

Albedo
02-07-2011, 07:55 AM
As Ben said, each ship would see its counterpart coming at it, then passing it, slower than light. Mind you, 'seeing' doesn't mean much at the clip they're passing each other. Your targeting system would need to be phenomenally fast, or by the time you react the enemy is a hundred thousand klicks behind you and your ship is an expanding cloud of dust.

Relativity, throwing intuition out the window since 1905...

movieman
02-07-2011, 06:32 PM
Your targeting system would need to be phenomenally fast, or by the time you react the enemy is a hundred thousand klicks behind you and your ship is an expanding cloud of dust.

Or you could just throw your garbage out the airlock and let kinetic energy do the job as the other ship flies through your garbage cloud.

Skyler
02-07-2011, 09:00 PM
Or you could just throw your garbage out the airlock and let kinetic energy do the job as the other ship flies through your garbage cloud.

Presumably, they're not so stupid as to fly through space with no protection from minor debris of that size. They wouldn't last a single lightyear.

movieman
02-07-2011, 09:57 PM
Presumably, they're not so stupid as to fly through space with no protection from minor debris of that size. They wouldn't last a single lightyear.

Minor debris? From a quick back of an envelope calculation, a polystyrene coffee cup travelling at 0.7c appears to have about the same energy as a ten ton truck at 300 kilometers per second.

A ship designed to protect itself against minute dust grains in interstellar space has little chance of surviving a polystyrene coffee cup.

DaveK
02-07-2011, 11:38 PM
It gets confusing because we're not used to thinking of moving at a large fraction of the speed of light. Say a ship is 10 light minutes away. It accelerates to 90% light speed toward us. (Damn good engines those lizard aliens have.) Well, we don't see that for ten minutes. In that time the ship has come to one light minute away. In one more minute we see the ship one light minute away. So in one minute we've seen the ship move from ten light minutes away to one. An apparent FTL speed of nine times that of light.

I think it's called the Picard maneuver. ;) OK, the sublight version.

Check 4) in http://www.universetoday.com/81918/astronomy-without-a-telescope-apparent-superluminal-motion/

Skyler
02-08-2011, 01:06 AM
Minor debris? From a quick back of an envelope calculation, a polystyrene coffee cup travelling at 0.7c appears to have about the same energy as a ten ton truck at 300 kilometers per second.

A ship designed to protect itself against minute dust grains in interstellar space has little chance of surviving a polystyrene coffee cup.

I can only hope that they are prepared to cope with at least a piece of gravel. No engineer worth his salt would design a ship reach speeds that high without taking into account the possibility of flying through a cloud of at least pea-sized rocks. Even in interstellar space, the probability of encountering such a hazard is too high to be discounted.

movieman
02-08-2011, 01:18 AM
I can only hope that they are prepared to cope with at least a piece of gravel. No engineer worth his salt would design a ship reach speeds that high without taking into account the possibility of flying through a cloud of at least pea-sized rocks.

You're basically talking about building a vehicle which can survive flying through a hail of ten-kiloton atomic bombs, because the impact energy of a pea-sized rock at that velocity would be of that order; worse, in fact, because the atomic bomb might not explode if you shoot at it before impact, whereas the rock always will.

It's probably possible (e.g. push an asteroid in front of you), but I'd say it's at least borderline magic for any economically viable spacecraft... getting anything up to 0.7c with any forseeable technology is likely to be insanely expensive, let alone doing it with an asteroid stuck on the front.

Edit: actually, shooting the rock might be an option if you could manage to detect it, track it, determine a firing solution and hit it in a fraction of a second; that should cause an explosion ahead of your ship with only lighter debris to deal with. But achieving that also seems rather close to magic to me.

Amadan
02-08-2011, 01:24 AM
The climax of Alastair Reynolds's House of Suns has a sublight-but-relativistic-velocity chase scene. You might want to check it out.

FOTSGreg
02-08-2011, 03:38 AM
Thanks for the insight, guys. I'm more interested in what a fleet traveling at 50.1% of light speed "sees" via it's sensors/scanners/out the viewports (assuming they have them) than what happens when a weapon hits or they close within weapons range.

My original thinking was that the velocity relationships would scale the same way they do in standard kinetics (ie, if you have 2 cars traveling at 40mph each their combined closing velocity is 80mph). Do things not scale this way at really high velocities?

blacbird
02-08-2011, 03:54 AM
My original thinking was that the velocity relationships would scale the same way they do in standard kinetics (ie, if you have 2 cars traveling at 40mph each their combined closing velocity is 80mph). Do things not scale this way at really high velocities?

No, they don't, because of time-dilation effects that become large at relativistic speeds. But it's damn hard to explain/visualize. I once saw a science program about this very thing, what it would look like to travel at near-light speed, and essentially you lose all peripheral field of view as things get compressed into a tiny window of vision forward.

Always remember, nothing travels faster than light speed (in a vacuum), including two beams of light shining at one another. For the same time-dilation reason. As an obvious example, we are traveling toward the Andromeda Galaxy, but the light shining from stars there is traveling toward us at, of course, light speed. Therefore, if it worked like cars on a highway, that light would be traveling toward us at faster than light speed; and vice versa, vis--vis relativity. In fact, if you are driving a car toward another car, at night, and see the headlights, the same faulty logic would apply, relative to the speed of the light shining from the other car.

Obviously, it doesn't work like that, and that's one of the key things that guy . . . what's his name? . . . Eisen . . . Eisner . . . I forget . . . the one with the bad haircut and droopy mustache, one of the things he figgered out.

movieman
02-08-2011, 08:12 AM
There's a formula for adding velocities; I think it's (u+v)/(1+(uv)/c^2), where u and v are the velocities.

So if you were traveling at 0.7c relative to Earth and another spacecraft was approaching you traveling at 0.7c relative to Earth in the other direction, you'd see it approaching you at 0.94c, not 1.4c.

Edit: Note that there would be other interesting effects like red/blue shift so you wouldn't actually 'see' the ship as the light reflecting or emitted from it would be shifted far into the ultraviolet.

blacbird
02-08-2011, 10:03 AM
There's a formula for adding velocities; I think it's (u+v)/(1+(uv)/c^2), where u and v are the velocities.

And, for clarity, "c" being the same thing it is in E + MC^2, that formula the Eisensteiner guy done made: the speed of light in a vacuum.

Skyler
02-08-2011, 11:52 PM
You're basically talking about building a vehicle which can survive flying through a hail of ten-kiloton atomic bombs, because the impact energy of a pea-sized rock at that velocity would be of that order; worse, in fact, because the atomic bomb might not explode if you shoot at it before impact, whereas the rock always will.

It's probably possible (e.g. push an asteroid in front of you), but I'd say it's at least borderline magic for any economically viable spacecraft... getting anything up to 0.7c with any forseeable technology is likely to be insanely expensive, let alone doing it with an asteroid stuck on the front.

That's fine. I'm just saying this could be a believability issue--it would be like building a cargo ship out of balsa wood and papier mache and sending it out to brave a hurricane. Is it possible? Yes. Would anyone invest in it? I can only hope not.


Edit: actually, shooting the rock might be an option if you could manage to detect it, track it, determine a firing solution and hit it in a fraction of a second; that should cause an explosion ahead of your ship with only lighter debris to deal with. But achieving that also seems rather close to magic to me.

Or, you could alter your course by a fraction of a degree. If you're far enough off to shoot it before you get there, you're far enough off to adjust your course at least enough to put you beside it rather than on a collision course.

High-speed high-resolution sensors would be a great help here, but I'm not seeing anything capable of detecting peas at dodging-distance at 0.7c. Just saying.

movieman
02-09-2011, 01:07 AM
That's fine. I'm just saying this could be a believability issue--it would be like building a cargo ship out of balsa wood and papier mache and sending it out to brave a hurricane. Is it possible? Yes. Would anyone invest in it?

It's more like building a cargo ship with a fifty-year lifetime that could survive a once-in-a-million-year storm. Real engineering is designed to survive likely hazards with typically a 30-300% margin, not a 10,000% margin. The more you overengineer the more you increase costs and given that you're probably talking about a 500:1 fuel to ship mass ratio for any realistic technology that could reach 0.7c and then decelerate every kilo you add requires maybe an extra half ton of fuel and the tanks to hold it.

Though that does create another possibility, I guess: put the fuel tanks on the front so any big impact will most likely destroy empty tanks. One of the space tourism rocket designs I saw some years ago would have put the empty tanks between passengers and ground on landing so if the landing went horribly wrong the tanks would take the impact and the passengers could probably walk away.


Or, you could alter your course by a fraction of a degree.Not at 0.7c... changing your course by 1 degree would require changing your velocity by about 2,000 kilometres per second. By the time you see something in front you're probably going to hit it.

Maybe a transforming ship could work: you would move modules around so that by the time you reach the rock it will pass through an empty space between modules in the ship. But your tracking would have to be very accurate.

benbradley
02-09-2011, 04:41 AM
There's a formula for adding velocities; I think it's (u+v)/(1+(uv)/c^2), where u and v are the velocities.

So if you were traveling at 0.7c relative to Earth and another spacecraft was approaching you traveling at 0.7c relative to Earth in the other direction, you'd see it approaching you at 0.94c, not 1.4c.

Edit: Note that there would be other interesting effects like red/blue shift so you wouldn't actually 'see' the ship as the light reflecting or emitted from it would be shifted far into the ultraviolet.
Oh, but the infrared and/or microwave would be shifted into the visible. You CAN see, but it's just a different band of the EM spectrum.

And here's something interesting, due to time dilation it would, to the inhabitants, going FTL. At some speed time slows down enough in the ship that it looks to the inhabitants like the ship is going FTL with respect to the rest of the universe, but in an unaccelerated frame (those of us sitting on the sidelines) it's not.

Astronomer
02-09-2011, 09:15 PM
What's needed is an active system that is mind-bogglingly explosively powerful enough to vaporize matter on contact before there's a hull breach.

I know: Just shield the front of the ship with a bunch of polystyrene coffee cups.

Skyler
02-09-2011, 10:46 PM
It's more like building a cargo ship with a fifty-year lifetime that could survive a once-in-a-million-year storm. Real engineering is designed to survive likely hazards with typically a 30-300% margin, not a 10,000% margin. The more you overengineer the more you increase costs and given that you're probably talking about a 500:1 fuel to ship mass ratio for any realistic technology that could reach 0.7c and then decelerate every kilo you add requires maybe an extra half ton of fuel and the tanks to hold it.

Do we really have any way to gauge the probability of encountering pebble-sized meteoroids in interstellar space? I don't think any of our sensors are capable of detecting stray rocks of that size in the area of space we're talking about.


Not at 0.7c... changing your course by 1 degree would require changing your velocity by about 2,000 kilometres per second. By the time you see something in front you're probably going to hit it.

Yep. So you'd pretty much have to be able to take the hit (or deflect it to the side somehow).


Maybe a transforming ship could work: you would move modules around so that by the time you reach the rock it will pass through an empty space between modules in the ship. But your tracking would have to be very accurate.

You'd have 9.53 milliseconds, assuming a speed of 0.7c, if you could detect it at 2000km. Good luck. =)

Skyler
02-09-2011, 10:51 PM
What's needed is an active system that is mind-bogglingly explosively powerful enough to vaporize matter on contact before there's a hull breach.

I know: Just shield the front of the ship with a bunch of polystyrene coffee cups.

I had no idea polystyrene cups were that powerful.

*looks at cup dubiously and sets it down with extra care*

Maybe I'll stick with cans.

Albedo
02-09-2011, 11:37 PM
I'm chuffed to know there's a role for me in the space navy! I'll be the guy in an armchair in the nose-cone, downing cans of beer and chucking the cans out to wipe out enemy ships.

movieman
02-10-2011, 03:09 AM
Do we really have any way to gauge the probability of encountering pebble-sized meteoroids in interstellar space? I don't think any of our sensors are capable of detecting stray rocks of that size in the area of space we're talking about.

Good question. As of a decade or two back the expectation was that you wouldn't find much other than hydrogen gas and occasional bits of dust, but I don't know whether any new research has changed that.

Rocks are probably going to come from asteroids hitting each other, and you shouldn't be finding many asteroids far outside of a solar system.

Edit: a quick google search for 'interstellar medium' seems to indicate that no-one's expecting a significant number of rocks out there. But rocks are probably harder to detect from Earth than gas and dust.

ColoradoMom
02-10-2011, 07:38 AM
The climax of Alastair Reynolds's House of Suns has a sublight-but-relativistic-velocity chase scene. You might want to check it out.

This was exactly the book that came to mind when I opened this post. That book rearranged my FTL thinking in a big way.

blacbird
02-11-2011, 02:36 AM
Good question. As of a decade or two back the expectation was that you wouldn't find much other than hydrogen gas and occasional bits of dust,

At near sub-light velocities, even these present enormous problems. The hydrogen atoms, in particular, become induced high-energy cosmic rays that would require extensive shielding to prevent them from frying any crewpeople inside such a craft.

movieman
02-12-2011, 06:14 AM
The hydrogen atoms, in particular, become induced high-energy cosmic rays that would require extensive shielding to prevent them from frying any crewpeople inside such a craft.

Mmm, that's a good point. I make it about 4x10^14 impacts per second on a human with no shielding; I haven't been able to determine how that compares to natural cosmic ray intensity in space, but it looks rather worrisome without significant shields.

PeterL
02-15-2011, 12:44 AM
Another problem with this speed is the energy required to get there. The mass of the spaceship increases at the same rate as the velocity. At the speed of light the massof the spaceship would approximate the mass of a large galaxy. I've seen what the mass of a hydrogen atom would become, and it would be along the lines of a bullet. Evasion is the only way around the speed of light.

movieman
02-15-2011, 03:45 AM
Another problem with this speed is the energy required to get there. The mass of the spaceship increases at the same rate as the velocity. At the speed of light the massof the spaceship would approximate the mass of a large galaxy.

Actually, at the speed of light the mass would be infinite. But that's OK because you'd need an infinite amount of energy to get there.

At 0.7c an external observer would think you had something like 40% more mass than at rest.

JimmyB27
02-15-2011, 05:30 PM
This is why I don't write science fiction... :tongue


The climax of Alastair Reynolds's House of Suns has a sublight-but-relativistic-velocity chase scene. You might want to check it out.
There's also one in Redemption Ark.

PeterL
02-15-2011, 10:30 PM
Actually, at the speed of light the mass would be infinite. But that's OK because you'd need an infinite amount of energy to get there.



Thatwould be true if the Lorenz transformations are correct. They may be, and if they are, then they are to best evidence that the speed of light is the speed limit.

Hallen
02-15-2011, 10:38 PM
http://www.youtube.com/user/ThePhysicsX

Watch the three topics on relativity. It's really bizarre and really cool. It will help you understand some of the weird factors regarding c.