# Serious Scientific Answers to Absurd Hypothetical questions

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Lets be clear, I'm not saying that light sails would not work, I am disputing a very specific claim:

On 10/15/2018 at 1:35 PM, DAL59 said:

Actually, a 1 meter solar sail pushed by a gigawatt ground based laser could reach .3c.

So here we have a claim of a 1 meter solar sail (I can think of no way to interpret that except 1 square meter). Its also from a group that wants to essentially do a proof of concept within the general bounds of our current civilization and not too distant future technology.

15 hours ago, DAL59 said:

You could have a smaller aperture and a giant lens farther out.

Yes, you could, but you still need a giant lens. A series of lenses or accelerators could help, but again if its a true lens, its limited to near UV (>200 nm). This wavelength required ridiculous aperture sizes just to focus to 1 meter at 9 AU (lenses the size of earth). What you'd need for higher wavelengths are a series of aligned grazing incidence mirrors, close to perpendicular to the laser beam (which increases the size and mass of the "lens" considerably). This likewise presents a problem for the solar sail. One wants the lightest possible sail per unit area... and if you have to angle it to the beam, the mass of the sail per unit area (for light absorption) goes up by a factor of the tangent of the angle... not to mention the acceleration losses if you can't reflect the light straight back (as the decelerator staging stuff that was shown would require.

So basically, vacuum UV is out (still great potential for weapons), and we're looking at near UV/ UV-C/ about 250 nm as a realistic upper limit.

This throws the aperture size to ridiculous dimensions for a single aperture (larger than the earth, and it still only gets to 0.05 C by 9 AU). Putting a lens out at 9 AU to refocus it isn't even going to double the final velocity, because its going to be going past that 2nd lens at 0.05c before any acceleration starts. Lets say its final velocity was 0.1c at 18 AU, and the acceleration was constant. For the 2nd 9 Au, its average velocity would have been 0.075c, compared to 0.025c for the first 9 AU. To go from 0.05c to 0.1c would require that the acceleration triple after passing the 2nd aperture (we can consider a 2nd laser at 9 AU, or a refocusing lens at 9 AU as essentially equivalent). Plus, again, at UV-C wavelengths, those focusing mirrors would be ridiculously huge, one could need a whole series of reasonably sized lenses, stretching out many AU

14 hours ago, kerbiloid said:

For a civilization which sends interstellar ships?

The original link and claim that I was responding to basically refers to our civilization in the near future. A proof-of-concept probe, not the work of a civ routinely sending "ships"

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probably not. Also maybe they can make it modular, an expanding hexagonal grid or something like that.
I guess, it's one of the least tasks they should solve.

A hexagonal grid wouldn't work for vacuum UV (again, grazing incidence mirrors would be required, and then the target has to get all funky and much more massive with much less acceleration per unit energy reflected, with much less momentum imparted per photon because they aren't 180 degree reflections, but merely grazing deflections)

For near UV, that grid would have to be almost as wide as the Earth, and very very very precisely constructed.... not one of the lesser tasks at all.

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What makes to use such small sails? The sail should also be kilometers-sized.

I agree, it should be kilometer sized, that's why I objected to the initial claim of a 1 meter sail. Trying to make this work with a 1 meter sail gets pretty ridiculous

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I've assumed the ship manages 0.1 g acceleration iirc.
But ok, let it be 10 g for a sailed microprobe.
If you launch a 10 km sail, it will catch significant part of the emitted light even in several light years from the emitter.

Yes, a 0.1 g acceleration is much more reasonable, and much less likely to vaporize the probe...

But that 100 fold decrease in acceleration then requires a 10 fold increase in focusing range. I'm still very skeptical of your claim of catching a significant part of the light even several light years away...

1 light year is 63,241 AU... going from 9 to 90 AU increases spot diameter from 1 meter to 10 meters, 900 is 100 meters, 9,000 is 1 km. 1 light year away is 7km wide

By the time we get to several km away, we're talking a laser beam dozens/hundreds of km wide...

And this was assuming we could focus down to 1 meter at 9 AU... which again requires an aperature larger than the diameter of the Earth!!!!!!!!!!!!!!!!!!!!

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If the future humans don't have a way to reflect 10 nm UV , they should use softer UV and bigger mirrors (and sails).

They won't. This isn't a problem of technology, its a problem of the fundamental properties of matter. As I've already shown, leaving the vacuum UV results in the "bigger mirror" being larger than the diameter of the Earth for the focusing required in the claim that I've been arguing against this whole time (please, no moving the goalposts!)

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I never used a word lens, only mirrors. ... [me saying that atmosphere bplocks vacuum UV] ??? I was describing in two posts the near-Sun (0.3 AU) s

I never said that you did. Keep in mind I primarily was addressing @DAL59's post, and also just laying out the difficulties involved in using low wavelengths (thus UV-C would need to be used, thus mirror/lens diameter becomes almost as large as the diameter of the Earth)

14 hours ago, p1t1o said:

Theres no reason that it has to be 1GW:1m2, but yeah, I did mention it wasnt really a practical plan.

But if the sail can handle it (and if you had, like, infinite/massive resources/money AND if you can convince the world to let you build what is effectively the largest weapon in the solar system) it can plausibly be accelerated very hard and can be used for interstellar travel.

It may be the case that the sail can be made to act like a diffraction grating - so incident radiation is not absorbed, not reflected but diffracted back (so you get the double momentum change and exceedingly little absorption) - this is btw how Xray lenses and "mirrors" work.

Well, the initial reason (1 GW/m^2) is because that's the specific claim I was arguing against. I haven't checked what accelerations that would even produce (but that requires very fuzzy assumptions as to the density of the sail, and how much reflects vs passes through vs is absorbed (I'm initially thinking 1 atom thick gold sheets are possible, but then how much light passes through at those thicknesses?)

Also, diffraction gratings are fine for focusing a beam by slightly deflecting rays (so they aren''t quite going parallel, but converge to a point many AU away).. but its not going to get you a 180 degree deflection.

They'd be great with X-rays for focusing a laser beam as a weapon, but your solar sail is not going to be able to make use of that light, its going to be vaporized.

Although you could use such a laser to heat propellant/achieve fusion of propellant for a probe, so that the probe is lighter and doesn't carry its own reactor... but its not going to be a solar sail at that point.

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45 minutes ago, KerikBalm said:

Well, the initial reason (1 GW/m^2) is because that's the specific claim I was arguing against.

Oh right, well thats fair.

It all depends though doesnt it, Id be hard pressed to say it was physically impossible, new materials are invented all the time, but for sure we cant do it today.

48 minutes ago, KerikBalm said:

Also, diffraction gratings are fine for focusing a beam by slightly deflecting rays (so they aren''t quite going parallel, but converge to a point many AU away).. but its not going to get you a 180 degree deflection.

Using a crystal lattice, diffraction angles >90deg can be achieved, but it does depend on the wavelength and on the crystal, Im not sure what the max theoretical deflection is.

I dunno how you can build a viable sail using this technique, but cant rule it out for some indeterminate time in the future, the physics works.

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Generally, the higher the frequency, the less you can deflect it:

Note that the higher wavelengths (blue/violet) are deflected/diffracted less than the lower wavelengths (red/orange).

This continues to hold true into the wavelengths that aren't visible (ie x rays, UV, IR, radio)

X rays and vacuum UV don't really look appealing for a solar sail... but they're great for weapons (many people who don't know what they're talking about speak of mirror armor for use against laser weapons... that's not going to work in space against lasers operating in the vacuum UV or higher frequencies... although it could work for planetary laser weaponry that needs to pass through the atmosphere)

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Im not talking about a diffraction grating, Im referring to Bragg diffraction, which is appropriate for Xrays. Angle of deflection is proportional to Lambda/d (where Lambda is wavelength and d is distance between crystal lattice planes). With wavelengths in the same scale range as lattice seperation, deflection angles can easily be quite large (again, >90deg is possible but Im not sure what the theoretical maximum is. I doubt very much that 180deg is possible with one deflection.):

Im still not saying that this would be easy to leverage in a solar sail though.

Edited by p1t1o

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Hmm, well, that's interesting...

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Bragg diffraction occurs when radiation, with a wavelength comparable to atomic spacing, is scattered in a specular fashion by the atoms of a crystalline system, and undergoes constructive interference.

This could work for soft-X rays, but hard X rays have wavelengths smaller than atoms themselves, not the space between atoms in a crystalline lattice.

Still... going from 2e-7 to 1e-9 is going to be a big increase in laser range

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Btw. Sails.

The sailship should be growing its sail. It should keep oozing a sticky slime dripping from the fuselage to the age and becoming a huge light-reflecting bubble/bubbles of snots around the ship.

Bionics.

Edited by kerbiloid

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On 10/19/2018 at 2:59 PM, KerikBalm said:

Hmm, well, that's interesting...

This could work for soft-X rays, but hard X rays have wavelengths smaller than atoms themselves, not the space between atoms in a crystalline lattice.

Still... going from 2e-7 to 1e-9 is going to be a big increase in laser range

Thats ok because IIRC it is the electron clouds that the Xrays are interacting with, I think - wavelength would have to be smaller than an electron to pass through without interaction.

This appears to be correct, but maximum deflection is seen when wavelength = lattice spacing

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3 hours ago, kerbiloid said:

Btw. Sails.

The sailship should be growing its sail. It should keep oozing a sticky slime dripping from the fuselage to the age and becoming a huge light-reflecting bubble/bubbles of snots around the ship.

Bionics.

Spider DNA.

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If the Multiverse Theory is true, there should be a universe where it is not?

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Doesn't a multiverse imply a meta-universe containing all the other universes?

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3 hours ago, Mad Rocket Scientist said:

Doesn't a multiverse imply a meta-universe containing all the other universes?

Depends on which multiverse you're talking about. In Everett Many Worlds, all "universes" are just alternate history states of a single universe. On the other hand, certain branches of String Theory suggest possible existence of multiple branes within the bulk, which would result in some number of worlds, possibly finite, and in some models as few as two. In that case, bulk is the meta-universe you're suggesting.

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A true complete multiverse includes every possible state of any multiverse.
So, if there is something not included into the multiverse, this is not a true multiverse, this is only a sub-multiverse.

As well, any time position is a part of a true complete multiverse.
Otherwise the time would be something outside of the multiverse, and that would mean that this was not the true multiverse.

As any event is an opposition of "before" and "after" on some dimensional axis, and the true multiverse includes all possible dimensions, nothing can happen in/with the multiverse.

A true complete multiverse is absolutely static, with no events, no time, no changes. "We" are just "looking" at its landscape.

That's why all that "entropy may only grow" stuff sounds just funny in our true complete multiverse.

(As well as "big bang" and "age of universe" are just "our" local features).

Edited by kerbiloid

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11 hours ago, Gargamel said:

If the Multiverse Theory is true, there should be a universe where it is not?

What you refer to as "the multiverse theory" is actually called "the many worlds interpretation".

It does NOT simply state that "anything and everything you can possibly concieve and anything and everything that you cant possibly concieve, literally exists in reality somewhere"

What it states - or more accurately, IMPLIES - is that "all POSSIBLE timelines exist"

Ergo, if a timeline is IMpossible, it need not be present in a multiverse. And I would have thought that the paradox you mention above, is exactly that.

It also does not stipulate that there must be infinite universes, there may be a limit (in fact there is universal doubt surrounding infinity as a concept and if it can really exist in reality under ANY context).

And if there is a limit, then there must be things left out of the multiverse, things which do not happen anywhere.

The many worlds interpretation is NOT a "theory", its nowhere near that solid. Its much closer to being a thought exercise, like the rubber sheet analogy for distortion of space by gravity, which in fact, breaks down if translated literally into the real world (ie: space/time is not really a stretchy sheet).

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Also the "many worlds" stuff basically requires that anything you discuss has to only apply within that "world" or universe. To do otherwise basically renders it a "one possible world" because you are talking about a possibility that unifies them. You can only use the "many worlds" though experiment when the results in each world are independent of the results in another world.

That's one of the fundamental problems of the ontological argument, which basically just tries to define god into existence.

https://en.wikipedia.org/wiki/Ontological_argument

specifically, here:

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If a maximally great being exists in some possible world, then it exists in every possible world.

Nope, doesn't work that way, you've broken the independence of worlds.

One can't use a trick of definition so that the results of any possible world apply to all possible worlds. If so then regarding this particular property there is just 1 possible "world" where its either true or false.

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I like the idea that there might be countless billions of alternate universes where the only difference is the precise configuration of the twisted paperclip on my desk.

Edited by p1t1o

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#define getAllPossibleStatesOfTheUniverseIncludingAllTimelines() MULTIVERSE

That's all.

Edited by kerbiloid

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Let's say Proton's do decay as is theoretically described, and you have a spaceship that can travel close to the speed of light. As in, really close. For you, the universe just speeds on by as you're cruising along, and after an absurdly long time, you slow back down. But, the protons in the universe are in the middle of decaying now. Do you decay as well; or do relativistic effects from you technically belonging to a time long passed prevent that from happening? Letting you see the end of the universe.

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Less time passes for you than for the outside universe, so you arrive at the end of the universe with all your protons intact.

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2 hours ago, andrewas said:

Less time passes for you than for the outside universe, so you arrive at the end of the universe with all your protons intact.

You also has the minor issue that the interstellar gas will hit you as the output of an particle accelerator.
But yes, we known in over 100 years that nuclear processes including breakdown slows down at high speed.
Cosmic radiation produces secondary radiation at the edge of the atmosphere, this induces particles to short half life to survive down to the surface, still some of it reach the surface, reason is time dilation

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Given a vacuum chamber on earth, so no air resistance.   You place a portal (from the game) on the ceiling, and one on the floor.  An object is then dropped into the floor portal, and falls through the upper one back into the lower one.   We've all done this in the game, falling for ever.

Given that gravity will be imparting it's acceleration for an infinite length of time, would the object ever reach the speed of light?  Could it surpass it since the energy is being 'supplied' by an outside source?

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3 minutes ago, Gargamel said:

Given a vacuum chamber on earth, so no air resistance.   You place a portal (from the game) on the ceiling, and one on the floor.  An object is then dropped into the floor portal, and falls through the upper one back into the lower one.   We've all done this in the game, falling for ever.

Given that gravity will be imparting it's acceleration for an infinite length of time, would the object ever reach the speed of light?  Could it surpass it since the energy is being 'supplied' by an outside source?

No, the portals would just heat up the box wall, and or push outwards and break them.

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1 minute ago, DAL59 said:

No, the portals would just heat up the box wall, and or push outwards and break them.

Let's assume the portals are static, and will not be affected by the object passing through them.

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2 hours ago, Gargamel said:

Let's assume the portals are static, and will not be affected by the object passing through them.

Then the question is effectively "Can gravity acting on mass over a drop of infinite length accelerate that mass to the speed of light" right? Given the link below I don't think it will. At least seems that way according to the page.

On the other hand, you have literally created a situation where it should be possible to basically create infinite energy. So.... I don't know.

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3 hours ago, Gargamel said:

Given a vacuum chamber on earth, so no air resistance.   You place a portal (from the game) on the ceiling, and one on the floor.  An object is then dropped into the floor portal, and falls through the upper one back into the lower one.   We've all done this in the game, falling for ever.

Given that gravity will be imparting it's acceleration for an infinite length of time, would the object ever reach the speed of light?  Could it surpass it since the energy is being 'supplied' by an outside source?

No. It doesn't matter what is providing the acceleration (even if it is a massive, physics breaking, free energy machine) you cannot accelerate a massive object to, or past, the speed of light.

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Exactly. It is the box that is accelerated because it stands on some surface on the earth. Relative to earth you just oscillate a bit. Relative to the box you will accelerate infinitely to just below the speed the of light in an infinite time (neglecting all weight, friction, heating, whatever, so scifi/gaming stuff :-)).

IRL, equally as you are accelerated towards the earth the earth is accelerated towards you. Fortunately, there is no way to transport a body in a gravitational field from a to b in no time and so cheat the counter reaction ;-).

Edited by Green Baron

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