Serious Scientific Answers to Absurd Hypothetical questions

[sevenperforce]

In theory, there's a way for an object to "catch up" to itself, but it's rather hackneyed.

Consider an object traveling at a really ridiculously high percentage of c which emits a photon traveling at some angle θ. If that photon happens to pass by the edge of a very massive lensing object which bends its trajectory by more than 2θ, then I suppose it would be possible for that photon to be diverted back such that it would cross the path of the emitting object. In order for this to happen, the velocity vector of the object would have to be perfectly coplanar with the lensing object, and the increase in path length taken by the photon would have to be greater than the difference in speed between the photon and the emitting object.

[ARS]

If the energy of the star (aka total energy produced for the duration of it's life until it's death) around the magnitude of our sun is used to ignite a single explosion (all of that energy is used in a single, massive explosion instead of sustained fusion like our sun) and detonated at the center of solar system, would it be enough to destroy a planet like earth? What's the effect on other planets in solar system?

[Gaarst]

1 hour ago, ARS said:

If the energy of the star (aka total energy produced for the duration of it's life until it's death) around the magnitude of our sun is used to ignite a single explosion (all of that energy is used in a single, massive explosion instead of sustained fusion like our sun) and detonated at the center of solar system, would it be enough to destroy a planet like earth? What's the effect on other planets in solar system?

So a supernova? (Somewhat) Relevant XKCD.

Supernovae happening closer than a few tens of light-years have serious chances of making the Earth unhabitable. I'd be surprised if anything larger than a pebble is left of the Earth if one happened at the Sun.

[ARS]

2 minutes ago, Gaarst said:

So a supernova? (Somewhat) Relevant XKCD.

Supernovae happening closer than a few tens of light-years have serious chances of making the Earth unhabitable. I'd be surprised if anything larger than a pebble is left of the Earth if one happened at the Sun.

So does that means earth is gonna be completely obliterated? And what'll happen to the rest of the celestial bodies?

[Nikolai]

5 minutes ago, ARS said:

So does that means earth is gonna be completely obliterated?

Yes, possibly, but not because the Sun is going to go supernova; it's not massive enough for that.

Some five billion years or so hence, the Sun will turn into a red giant.  Its radius may grow large enough to swallow Earth.

https://www.universetoday.com/18847/life-of-the-sun/

[raxo2222]

How long supernova surroundings are fried by all sorts of radiation?

Lets say there is some sort of probe orbiting star at 100 AU, that became supernova.

How much time will pass before radiation levels fall to what it was before supernova?

 

[Diche Bach]

I seem to recall the window of future habitability for Earth is only around 1 or 1.5 billion years. The rate at which the sun swells up as it transitions into a red giant will be slow and will not culminate until considerably later (3 or 5 billion years hence or whatever the number is) but its increased energy output will have rendered the Earth uninhabitable and all life on Earth extinct well before that.

Still, would seem to be plenty of time for humanity to get its act together and found new homeworlds in various places around the galaxy. Even if only enormous O'Neill cylinder type of spacecraft with millions of people, animals and plants inside.

[bigcalm]

21 hours ago, raxo2222 said:

How long supernova surroundings are fried by all sorts of radiation?

Lets say there is some sort of probe orbiting star at 100 AU, that became supernova.

How much time will pass before radiation levels fall to what it was before supernova?

Hmm would you survive at 100AU.  Let's see, average energy from a type 1a supernova is 1.5*10^44 J.  At 100AU away, if we calculate a sphere of that volume, that'll give us the energy per square metre.  Energy / (4*pi*(100*150*10^9)^2) = 5.3*10^20 J / m^2.    That's not a trivial amount of energy - that's equivalent to the power output of the human race for a year hitting every square metre of your spacecraft.  Hope it's pretty sturdy!

With a Type 1a supernova, the main remnant radiation will be from the decay of nickel 56 to cobalt 56 to iron 56 (stable).  Nickel 56 has a half life of 6 days and Cobalt 56 has a half life of 77 days, giving a total decay half life of 83 days.  However.  It should be noted that the amount of Nickel 56 produced in a (type 1a) supernova will be vast - Wikipedia suggests 0.4-0.8 solar masses being produced, so the background radiation is going to take a considerable amount of time to drop. 

 

[Green Baron]

19 hours ago, Diche Bach said:

I seem to recall the window of future habitability for Earth is only around 1 or 1.5 billion years.

It might be be shorter until it gets too hot.

Find new worlds, i rather think we go the way of any other species loooong before that :-)

The whole process is short in a cosmic scale but still takes a few million years. The probe has to be patient.

Edited December 15, 2017 by Green Baron

[Diche Bach]

8 hours ago, Green Baron said:

It might be be shorter until it gets too hot.

Find new worlds, i rather think we go the way of any other species loooong before that :-)

The whole process is short in a cosmic scale but still takes a few million years. The probe has to be patient.

I look at it like this: humans are an incredible mixed bag, but life itself is pretty wondrous. As such, we are morally obligated to strive to insure the perpetuity of Earth life, by transporting it wholesale to new locales where it will persist eternal. Since we are the only capable steward of Earth life, we have to save ourselves too.

Sort of an evolutionary biologists reversed analogy of the Abrahamic Great Chain of Being! 

Given we have hundreds of millions of years to get our crap together and make it happen (and it does not require any technology that is literally impossible, just lots of money, time, energy and persistence) and we are already on the cusp of most of the technologies to allow it to happen, I think we will pull it off. Even if we go full retard and unleash a thermonuclear WWIII, I still think we will ultimately get it together and live up to our "The Precursors destiny." Nuclear war of maximum extent would ravage Earth and spool humanity (and most of Earth life) through a rather small bottleneck. Even so, my gut intuition the damage wouldn't be so drastic as to literally exterminate humanity nor life. As far as we know the Cambrian explosion itself maybe only took a million years (maybe less!) to get underway, and there is more than one "adaptive radiation" event in the geological record which appears to have ocurred relatively rapidly.

What I infer from that is: we humans could literally kill off most life on Earth and reduce our own numbers to miniscule levels (perhaps even as low as the Toba Catastrophe bottleneck of ~60,000 people!), but (a) we would likely hang in there, adapt and even rebuild from the wreckage of the past. (b) life would hang in there too. Project a few million years beyond that and we might even be right back to where we stand today, albeit perhaps with a lot of our persistent Imperial Ape foibles bred/trained/expunged out of our ontogenies?

Unless a bolide literally turns Earth into a molten slag heap, I suspect we will eventually save ourselves and our Earth family of lifeforms from extinction and go on to establish the first interstellar ecological networks.

Edited December 16, 2017 by Diche Bach

[DAL59]

https://alexrhowe.com/2017/11/22/what-if-rejects-10-3-turkey-day/

[USB4]

Is a atmospheric  motor carrying its own oxidizer verging on kerbin jet efficiency  possible?

[Green Baron]

First: i don't take things in here too serious any more, so the following is just a random thought of mine. And i am in writing mode before i go to town :-)

7 hours ago, Diche Bach said:

 As such, we are morally obligated to strive to insure the perpetuity of Earth life, by transporting it wholesale to new locales where it will persist eternal.

There are several things i feel a little uncomfortably with: first the moral obligation.

All life i know feasts on its resources until they are depleted and then disappears. That is what the physical basis dictates: the reaction takes place until energy is cut off or fuel is gone. From archaeology, looking at the development since the earliest neolithic, humans are exactly the same. They are a little "fitter" than the the other higher animals (not bactaria !) in filling more niches, but in the end they are reduced or a group disappears completely when things change and there is no place else to go. Today the bar is a little higher with all that technology, but that is only valid for the developed countries and the principle does not change. Even the countries that have been mentioned as "new moral leaders" in the press lately do not comply with the self imposed rules on green house gas emissions for example (just a recitation of the news, no political statement i hope). All i can deduct is that humans did, do and will further, like a stem of bacteria, take their basis away.

Second: leave the sh.. behind and start new.

That was possible in the past as long as there was ground to move to, since iron age mostly in connection with killing the ones who are already there. But the masses cannot leave earth, a few specially trained and highly educated astronauts do for a short while and depend on intensive medical care afterwards. So even they did never leave earth with its technology and infrastructure, had they they would have died sooner than "normal". Much sooner. There simply is nothing to feed on elsewhere and space is deadly and no technology lasts a human lifetime. Dreams of billionaires and fantasies of people living from the public hand (Nasa), and all their more or less hopeful followers as well as colourful movies will not change this.

Third: eternal. This word does not exist in my treasury of words ... even the universe itself is not assumed to exist "eternally". The limits physics, chemistry and biology sets for our individual existence as well as the further evolution process pushes the whole idea beyond my poor limited imagination. Or, simpler: it'll be the first time if humans didn't go the way of all the other species before.

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Sort of an evolutionary biologists reversed analogy of the Abrahamic Great Chain of Being! 

I don't believe in god, so the reverse is as hard to imagine for me.

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Given we have hundreds of millions of years to get our crap together and make it happen

We surely don't ! We don't even manage it in our individual lifetimes to "get our crap together", and nonody really cares for the future of our children. They will have to find their own ways. Like getting rid of all the debt that was piled up in the last 12 years.

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As far as we know the Cambrian explosion itself maybe only took a million years (maybe less!) to get underway, and there is more than one "adaptive radiation" event in the geological record which appears to have ocurred relatively rapidly.

Nope. There is no "explosion". We are looking at 100-150 million years from first Ediacaran fauna to the first fish, and the limits may still shift. I'd like to see the source for that, it is probably not from geoscience/palaeontology ... sorry !

There is a rich marine fauna and biocenosises in the early Cambrian, but it didn't "appear". It looks in itself on ~100My, that are themselves standing on the back of 3 billion years of biologic evolution.

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What I infer from that is: we humans could literally kill off most life on Earth and reduce our own numbers to miniscule levels

For the part: mission accomplished. Wait, there is still something moving ... *splat* ... ok, done :-) For the second part: yes i think that's well possible.

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(perhaps even as low as the Toba Catastrophe bottleneck of ~60,000 people!),

I wouldn't build a logical chain on this. This is a fixed idea of a few American journalists and even scientists, but not supported by data (correct me !) and not accepted widely (no correction necessary :-)). There are no traces of this in Europe for example. Humans were already scattered all over the world in different species, a genetic bottleneck may have happened locally, but again, data isn't really siding with "Toba catastrophists" :-)

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Unless a bolide literally turns Earth into a molten slag heap, I suspect we will eventually save ourselves and our Earth family of lifeforms from extinction and go on to establish the first interstellar ecological networks.

I don't :-)

Have a nice one, i am going to hit the town now.

Edited December 16, 2017 by Green Baron

[kerbiloid]

10 hours ago, Diche Bach said:

we will eventually save ourselves and our Earth family of lifeforms from extinction

And the first but the most important step on this way is to feed a cat. Cats are to hold the humans on their way.

11 hours ago, Diche Bach said:

we are morally obligated to strive to insure the perpetuity of Earth life, by transporting it wholesale to new locales where it will persist eternal

Or make an adaptive thing which will do this itself.

Spoiler

 

11 hours ago, Diche Bach said:

Nuclear war of maximum extent would ravage Earth and spool humanity (and most of Earth life) through a rather small bottleneck.

Probably this bottleneck would be much wider than XIX century humanity, and had at least the same level of "normal life" and technologies.

11 hours ago, Diche Bach said:

go on to establish the first interstellar ecological networks.

or get adopted by it.

[Zeiss Ikon]

18 hours ago, USB4 said:

Is a atmospheric  motor carrying its own oxidizer verging on kerbin jet efficiency  possible?

This goes back to the jet engine on Titan using an oxidizer in the fuel tanks.

Yes, if and only if the atmosphere is a reducing one, that is, hydrogen or hydrocarbon vapor, at least in significant fraction.  Compress this, inject your oxidizer, add a little spark, and use a turbine to extract enough energy to run the compressor (and a bypass fan if you want still more efficiency at mostly subsonic speeds).

if you mean a bipropellant engine, then no way, no how.  The best "efficiency" you can possibly get from a bipropellant rocket would require an exotic fuel/oxidizer compound (I don't recall if it's flourine/hydrogen, or oxygen triflouride/acetylene -- the latter is surely hotter burning, but requires unobtainium containment and plumbing since chemical glassware will spontaneously ignite in oxygen triflouride, and the combustion temperature is prone to melting the combustion chamber).  However, no exotic is enough better than hydrolox to be worth the trouble, and in terms of system efficiency (which is the real measure for things like what it costs to launch a payload, and what it costs per kilometer for a winged cargo carrier), it's very hard to beat kerosene/LOX, or if you need better storability, kerosene/nitrous oxide or kerosene/HTP (high test peroxide).

None of those bipropellant systems has an Isp that even approaches the efficiency of a no-bypass turbojet, which is just about the least efficient form of gas turbine propulsion.  The peak of chemical bipropellant systems is below 400 seconds, while it's pretty easy to build a turbofan that will exceed 10,000 seconds (by ignoring the mass of oxidizer/remass it takes from the atmosphere, and calculating using only the fuel).

[USB4]

27 minutes ago, Zeiss Ikon said:

This goes back to the jet engine on Titan using an oxidizer in the fuel tanks.

Yes, if and only if the atmosphere is a reducing one, that is, hydrogen or hydrocarbon vapor, at least in significant fraction.  Compress this, inject your oxidizer, add a little spark, and use a turbine to extract enough energy to run the compressor (and a bypass fan if you want still more efficiency at mostly subsonic speeds).

if you mean a bipropellant engine, then no way, no how.  The best "efficiency" you can possibly get from a bipropellant rocket would require an exotic fuel/oxidizer compound (I don't recall if it's flourine/hydrogen, or oxygen triflouride/acetylene -- the latter is surely hotter burning, but requires unobtainium containment and plumbing since chemical glassware will spontaneously ignite in oxygen triflouride, and the combustion temperature is prone to melting the combustion chamber).  However, no exotic is enough better than hydrolox to be worth the trouble, and in terms of system efficiency (which is the real measure for things like what it costs to launch a payload, and what it costs per kilometer for a winged cargo carrier), it's very hard to beat kerosene/LOX, or if you need better storability, kerosene/nitrous oxide or kerosene/HTP (high test peroxide).

None of those bipropellant systems has an Isp that even approaches the efficiency of a no-bypass turbojet, which is just about the least efficient form of gas turbine propulsion.  The peak of chemical bipropellant systems is below 400 seconds, while it's pretty easy to build a turbofan that will exceed 10,000 seconds (by ignoring the mass of oxidizer/remass it takes from the atmosphere, and calculating using only the fuel).

Sounds reasonable. Similar, but different, I found out Nasa in the 70s had a piston engine powered by monopropellant that apparently could go for about a hour and a bit, so I started modelling roughly (I imagine a 2 cylinder engine is basically what it looks like but without spark plugs) what I think that could look like. Unfortunately, the one photo they have is both blurry and from the front. here it is.

Edited December 17, 2017 by USB4

[shynung]

On 12/11/2017 at 3:27 PM, ARS said:

 

 

Is this level of maneuverability even possible? Say, with drones or missiles?

[Zeiss Ikon]

11 hours ago, USB4 said:

Sounds reasonable. Similar, but different, I found out Nasa in the 70s had a piston engine powered by monopropellant that apparently could go for about a hour and a bit, so I started modelling roughly (I imagine a 2 cylinder engine is basically what it looks like but without spark plugs) what I think that could look like. Unfortunately, the one photo they have is both blurry and from the front. here it is.

Based on the photos of the parts and description of the cycle, that appears to be a uniflow type hot gas expander running on the hydrazine decomposition gas.  The engine itself would (with suitable changes in seal materials) run on compressed air, steam, external combustion of fuel and oxidizer, or any monopropellant you like.  Hydrazine has the advantage of being long-term storable (disadvantages are short catalyst operating life, high toxicity, and only so-so energy content -- peroxide would be a better choice, but it decomposes in storage over long periods, such as a Mars transfer).  Uniflow engines are simple (the piston directly actuates the inlet valve, exhaust is through a port similar to a 2-stroke gasoline engine), light, and powerful compared to conventional valved single- and double-acting steam engines.  In terms of familiar devices, this engine operated the same way as the air engine from an Airhogs toy aircraft, or one of the "two stroke to steam engine" conversions you can find all over YouTube.

Such an engine is not self-starting, and the test operation must have involved hazmat suits, because the person starting the engine would have been exposed to hydrazine vapors (unless they used an onboard starter that I missed in quick-scanning the article).

[DDE]

13 hours ago, USB4 said:

Sounds reasonable. Similar, but different, I found out Nasa in the 70s had a piston engine powered by monopropellant that apparently could go for about a hour and a bit, so I started modelling roughly (I imagine a 2 cylinder engine is basically what it looks like but without spark plugs) what I think that could look like. Unfortunately, the one photo they have is both blurry and from the front.

Monopropellant emergency power units are a thing in aviation, as are monopropellant torpedoes.

3 hours ago, shynung said:

Is this level of maneuverability even possible? Say, with drones or missiles?

Drones aren't that much tougher. Missiles may achieve something like that during their active phase, but they very quickly lose energy once their short-lived motors burn out.

[ARS]

On 17/12/2017 at 9:04 PM, DDE said:

Drones aren't that much tougher. Missiles may achieve something like that during their active phase, but they very quickly lose energy once their short-lived motors burn out.

Assuming that it's in space, without any air resistance that will compromise structural integrity of the craft, and we ignore fuel limitation, with the booster configuration depicted on the video (dual full 360 degree independent thrust vectoring on vertical axis, allowing the craft to execute roll, yaw and pitch with those pair of boosters to rapidly change movement vector with sudden maneuver), is it possible to create a space fighter that moves and fight like the one usually depicted in space battle movies?

[DDE]

2 hours ago, ARS said:

Assuming that it's in space, without any air resistance that will compromise structural integrity of the craft, and we ignore fuel limitation, with the booster configuration depicted on the video (dual full 360 degree independent thrust vectoring on vertical axis, allowing the craft to execute roll, yaw and pitch with those pair of boosters to rapidly change movement vector with sudden maneuver), is it possible to create a space fighter that moves and fight like the one usually depicted in space battle movies?

Not with such accelerations, but...

[DAL59]

 

 

[Diche Bach]

Excuse the additional tangent on the main theme of the thread . . .

@Green Baron You're too cynical! Earth life IS wondrous!

 

[ARS]

So here's my question:

There are 2 hypersonic fighter jet, they have the same max speed, performance, etc. But they have different propulsion method

Craft 1 use hypersonic scramjet engine attached on rear section that provide thrust that push the aircraft forward

Craft 2 use an advanced phlebotinum engine powered by handwavium (basically fictional engine), that propel the craft without actually provide thrust, but accelerate all parts of the craft at the same time forward

Both craft travel at the same speed.

Craft 1 moves by being pushed by the engines like normal jet configuration

Craft 2 moves by accelerating all of it's parts at the second time forward

My question is, does craft 2 has better performance in terms of withstanding the air resistance? Since in craft 1, the whole airframe is being pushed, but in craft 2, the whole airframe parts is moving at once

[DDE]

2 hours ago, ARS said:

So here's my question:

There are 2 hypersonic fighter jet, they have the same max speed, performance, etc. But they have different propulsion method

Craft 1 use hypersonic scramjet engine attached on rear section that provide thrust that push the aircraft forward

Craft 2 use an advanced phlebotinum engine powered by handwavium (basically fictional engine), that propel the craft without actually provide thrust, but accelerate all parts of the craft at the same time forward

Both craft travel at the same speed.

Craft 1 moves by being pushed by the engines like normal jet configuration

Craft 2 moves by accelerating all of it's parts at the second time forward

My question is, does craft 2 has better performance in terms of withstanding the air resistance? Since in craft 1, the whole airframe is being pushed, but in craft 2, the whole airframe parts is moving at once

It removes the issue of strctural stress from acceleration, but not air resistance.

Plus, depending on how the magic engine works, the first one may have an easier time maneuvering through thrust vectoring.