Oberth Effect and FTL Travel?

[Surefoot]

Causality isn't safe either way. We know that there are situations when it breaks down. In particular, near rotating black holes. So Sgr A* at the center of our galaxy already provides a causality breakdown.

We dont really know actually. What we know is gas reaching the event horizon reach near c speeds (think about the oberth effect of a black hole.. Yay). After that, all theories are open, no one knows for sure what is happening between the event horizon and the singularity itself. And we are not even sure the singularity would break causality... One thing is with enough materials we could exploit the gravity well of a black hole for generating energy (by throwing stuff around it), which is another "physics hack" i like.

Edited November 1, 2013 by Surefoot

[JebidiahsBigSister]

Note that space expanding is actually still a theory.

It's backed by numerous observations of far away-galaxies. It is only a theory in the sense that there is a "theory of Gravity". If you are aware of any legitimate alternate explanations published in a scientific journal I'd be interested in seeing such a link.

[Ralathon]

We dont really know actually. What we know is gas reaching the event horizon reach near c speeds (think about the oberth effect of a black hole.. Yay). After that, all theories are open, no one knows for sure what is happening between the event horizon and the singularity itself. And we are not even sure the singularity would break causality... One thing is with enough materials we could exploit the gravity well of a black hole for generating energy (by throwing stuff around it), which is another "physics hack" i like.

Actually, we know pretty well whats between the event horizon and the singularity. The singularity itself is where our theories break down due to infinite curvatures and distances that approach zero.

And it isn't really free energy. You're drawing on the kinetic energy of the black hole's rotation. While that will usually be extremely large, it is still finite. As you leech energy via the Penrose process you slow down its rotation until it transforms into a simple schwarschild black hole. A rotating black hole is more like a rechargeable battery with ridiculous energy densities.

[K^2]

We dont really know actually. What we know is gas reaching the event horizon reach near c speeds (think about the oberth effect of a black hole.. Yay). After that, all theories are open, no one knows for sure what is happening between the event horizon and the singularity itself. And we are not even sure the singularity would break causality... One thing is with enough materials we could exploit the gravity well of a black hole for generating energy (by throwing stuff around it), which is another "physics hack" i like.

True enough, and with cosmic censor in place it doesn't really matter. But theory also shows that if we dump enough charge into a black hole with enough angular momentum, the event horizon should vanish, exposing a naked ring singularity. At that point, we no longer have the luxury of saying that what happens below event horizon stays below event horizon, and causality violations become a factor for the rest of the universe. Closed time-like curves become a feature of our space-time and you can smoke your paradoxes in a pipe. Whether such a singularity exists is a question. But since everything suggests that it's allowed, the rest of the physics cannot possibly depend on global causality.

[Surefoot]

If you are aware of any legitimate alternate explanations published in a scientific journal I'd be interested in seeing such a link.

I saw some of them, based on interstellar matter effects (mostly plasma), i dont have the links anymore unfortunately, but as long as we dont have a direct measurement everything is possible, and those alternate theories are just as interesting as the one from Hubble. Actually, they allow for removing a lot of axioms we currently rely upon for the ever-expanding model, which is if we consider Occam's Razor, boosting their chances.

Actually, we know pretty well whats between the event horizon and the singularity. The singularity itself is where our theories break down due to infinite curvatures and distances that approach zero.

Well inside the event horizon a lot of weird stuff SHOULD happen given all our theories are correct (what happens to space time when you are *supposed* to go FTL..), keep in mind that even now the top scientists are not even sure themselves, and are still adjusting their views after recent (indirect) measurements. One thing is for sure, we'll never know since no information can escape intact...

And it isn't really free energy.

You're nitpicking: for practical purposes it's so huge we can consider it near infinite, much like our "renewable energies" (which depend on resources that will actually disappear in a few billion years). And maybe i wasnt very clear, what i meant by "free" is we just exploit the gravity well, which is an interesting way of exploiting a phenomenon we're usually fighting against.

Edited November 1, 2013 by Surefoot

[Surefoot]

At that point,(...) Closed time-like curves become a feature of our space-time and you can smoke your paradoxes in a pipe.

Makes me think of that scene in Airplane (the movie) where total pandemonium erupts aboard the plane, with naked boobs girls walking across and such

[JebidiahsBigSister]

i dont have the links anymore unfortunately, but as long as we dont have a direct measurement everything is possible, and those alternate theories are just as interesting as the one from Hubble.

I don't doubt that they are interesting to you. I prefer to base my reality (and my statements) on known scientific facts rather then unnamed "alternate theories". But then again I don't believe in such things as astrology, homeothapy or the tooth fairy and I think that makes me a more, rather than less, interesting person to be around.

[Surefoot]

I don't doubt that they are interesting to you. I prefer to base my reality (and my statements) on known scientific facts rather then unnamed "alternate theories". But then again I don't believe in such things as astrology, homeothapy or the tooth fairy and I think that makes me a more, rather than less, interesting person to be around.

If you could be less insulting, that would be nice. By "alternate theories" i mean peer reviewed, published papers, not whackjob nonsense. If thinking we actually have a rock-solid, ultimate foolproof theory at hand for the red shift, and it makes you sleep at night comfortably, all the better for you. In the mean time, maybe a bit of research on your side could be useful if you are genuinely interested in that topic instead of forum bashing. Science is all about being curious, with a heavy dose of skepticism, even for the "established" theories. (Please also note that a "static universe" model was also advocated by Einstein, who then had doubts about it - without any real substantiation, but still it's a legitimate hypothesis to keep in mind - with some refinements of course) PS: a quick google search gives me plenty of papers about self redshift, gravitational redshift, and other non expansion-related effects. Very interesting subject to research, actually. Maybe the truth is not so simple, maybe it's a combination of all of these: we are not 100% sure yet.

A counter point (pro-expansion) is explained here:

http://en.wikipedia.org/wiki/Olbers%27_paradox#Steady_State

I will quote a part which i find relevant to our discussion:

Since the speed of light is a constant value, regardless of the shift towards infrared frequencies, the universe is still sharply constrained to finite sizes in space as well as in time. Some models of an infinite (Steady State theory or static universe) solution of the universe are still viable, and Olber's paradox cannot sharply distinguish between them from some variants of the Big Bang model.

Enjoy

Edited November 2, 2013 by Surefoot

[SpaceSphereOfDeath]

How can causality be taken seriously, then, if it's violated by something as natural as a rotating neutron star, or by the natural expansion of space on a global scale?

I believe the reason it does violate causality is because A it's a black hole, not a neutron star. B a singularity i a point of INFINTE density and INFINTE gravitational pull, thus physics doesn't work. And C, physics likes to mess with us.

[Surefoot]

As for space expansion not violating causality, here is a good explanation:

http://curious.astro.cornell.edu/question.php?number=56

Rotating black holes create some weird effects (reference frame dragging..) which can still fit into the model without breaking causality (as far as i know, from the books a read some time ago on the subject). Only the singularity breaks everything...

[impwarhamer]

Only the singularity breaks everything...

Singularity be like: bitch I don't need no physics!

[DaveofDefeat]

The singularity isn't some mystical physic less object, its just a place that current mathematical models fail to describe.

[Person012345]

Time dilation isn't why you can't go FTL, it's just a phenomena you observe when going fast.

The simple way to put it is that, as the first post said, as you go faster you increase in mass and by the time you get near lightspeed the amount of energy you need to dump into the craft to accelerate it further (due to it's massive inertia) causes it to become so much "heavier" that it ends up not being enough. C is the point that you simply can never reach. The oberth effect may help you get nearer to C than ever, but because it doesn't provide infinite efficiency gains then it won't be enough. To reach C you need an infinite amount of energy.

[K^2]

The simple way to put it is that, as the first post said, as you go faster you increase in mass and by the time you get near lightspeed the amount of energy you need to dump into the craft to accelerate it further (due to it's massive inertia) causes it to become so much "heavier" that it ends up not being enough.

The problem with this explanation is that from ship's perspective it never gets heavier. As viewed by the crew, the ship can manage the same acceleration it always did no matter how fast they are going.

Of course, from crew's perspective, you can go as fast as you like. Faster than light by an arbitrary factor. The objects outside still moves at less than c relative to the ship, but distances get smaller, so it's the same thing in the end.

[WestAir]

I believe the reason it does violate causality is because A it's a black hole, not a neutron star.

Because a Neutron Star can induce *noticeable* frame-dragging, and has its own photon sphere, and can be centimeters away from falling within its own Schwarzschild Radius [meaning some of them can have ~10 Solar Masses], I figured the difference between a stellar black hole and a neutron star was unimportant to the discussion on causality.

K^2 corrected me a page ago, long before your retort.

Edited November 3, 2013 by WestAir

[Person012345]

But since everything suggests that it's allowed, the rest of the physics cannot possibly depend on global causality.

Does this mean I could have been right all along in rejecting causality as an argument?

I don't like paradoxes as arguments. Just because it makes no sense doesn't mean it's not true. I don't recall having ever seen any compelling evidence that cause must necessarily precede effect. Maybe I have and I've just forgotten it but I don't think so.

[Surefoot]

The reasons involve general relativity, lorentzian manifolds, and cauchy surfaces. My maths are too old now to remember how to work with this, but i remember it's an application of Occam's Razor, mathematically speaking (it's the more stable structure we can think of). Any model with more lightly enforced causality is unstable, so if they even pop into existence they tend to disappear and get overruled by current model, at least this is what i remember of it

[Bob012345]

Ralathon,

 

On October 30, 2013 at 6:56 AM, Ralathon said:

The Oberth effect says that more energy generated by the burn is deposited into the ship as it goes faster. 

So, this means that a ship that accelerates from 0 to 100m/s takes the same time and fuel as accelerating a ship from 1000m/s to 1100m/s. But due to the quadratic nature of the kinetic energy equation the latter takes far more energy.

The ship has more kinetic energy but it did not take more energy in the burn to sustain the change from 1000m/s to 1100m/s. The difference comes from the fact that the fuel has high kinetic energy to start with. 

[Lukaszenko]

Is there a relativistic equivalent of the Oberth effect?

Oberth makes your rocket more effective due to classical physics equations. When approaching C, surely the efficiency rises even faster than can be explained by Oberth...  

[magnemoe]

On 2.11.2013 at 11:33 PM, DaveofDefeat said:

The singularity isn't some mystical physic less object, its just a place that current mathematical models fail to describe.

This, density goes towards infinite, same with gravity,
In it self its no issues with singularities, you can run into them in KSP if you clip trough ground without getting destroyed and experience an singularity without relativity effects.
you will fall towards the singularity in the center of the planet or moon gaining speed and accelerate all the time and the acceleration will increase, in KSP you will usually shoot outward at extreme speed as your your position and acceleration is only calculated 20 times or something every second or something  and you are likely to en up with an huge rest speed. 
In real world you would not gain speed but tidal force will rip molecules apart. and they will be ejected in various directions depending on how close you get. this can be seen in KSP if you plot an trajectory trough the center of an planet or moon.

Because of  the light speed you get black holes. 
Now imagine if something stops the collapse after escape velocity is higher than light speed. No singularity but an objekt say  1000 times more dense than an neutron star, would the black hole behave different? the energy would still be trapped.