I am imagining a windmill-type device with rotating solar sails - directly converting the speed of the photons into rotational speed. Would that not result in accelerating the device to near the speed of light? Could it then be used as a time machine? How long would it take to accelerate to speeds necessary to provide useful travel into the future, and would it be possible to protect the matter within from being ripped apart by centrifugal forces?

I am imagining a windmill-type device with rotating solar sails - directly converting the speed of the photons into rotational speed. Would that not result in accelerating the device to near the speed of light?
Accelerate the device to near-C? No.
Even if you're just talking about the tip speed of the sails it wouldn't work. Apart from increasing mass reducing whatever acceleration was applied there's also the fact that the faster it rotates the greater the mechanical stress - and subsequent failure to hold together.


Could it then be used as a time machine?
Huh? Why would it?


How long would it take to accelerate to speeds necessary to provide useful travel into the future
What do you mean "travel into the future"?


and would it be possible to protect the matter within from being ripped apart by centrifugal forces?
Do you think it would be even possible to stop the sails being ripped apart? What's it going to be made of?

Accelerate the device to near-C? No.
Even if you're just talking about the tip speed of the sails it wouldn't work. Apart from increasing mass reducing whatever acceleration was applied there's also the fact that the faster it rotates the greater the mechanical stress - and subsequent failure to hold together.

The point regarding mechanical stresses is well-made, and I think the biggest challenge to overcome, but with a constant stream of photons (say, from the sun), wouldn't it EVENTUALLY accelerate to near C? Perhaps not with sails, but a spherical device and a funnel focusing the photons? (Think of a spinning basketball, with a hand constantly applying new force.)



Huh? Why would it?


As matter approaches the speed of light, time slows down. An object inside said device, accelerating towards the speed of light, would experience time more slowly than the planet nearby. Anyone or thing exiting the device would return to a planet that had continued experiencing time at the original rate, thus resulting in travel to the future.



What do you mean "travel into the future"?


I simply mean traveling into a future point faster than the rest of the nearby planet.



Do you think it would be even possible to stop the sails being ripped apart? What's it going to be made of?

I do think that would be the biggest challenge, but a technological one rather than a challenge to physics.

Ah, got you. I was picturing some fixed wind-farm type thing and couldn't work out what you were on about. :o

Um, okay. What makes you think a spinning ship would be any better than a non-spinning one? Surely some of the applied energy would be wasted in spinning it?

And no, the constant stream of photons (even if they didn't reduce in "power" as you got further away from the Sun) have a fixed value for their energy. As mass increased the applied force would become negligible - no further acceleration. And I'm not sure that a solar sail would get much power at all in the outer Solar System, let alone interstellar space.

Yeah, the materials problem... got to be strong enough to withstand spinning of near-infinite mass tips at near-C velocities, which means that they'd maybe be too heavy to spin in the first place. It's for damn sure we haven't got any materials (or construction techniques) around today that would take that kind of punishment.
And then, of course, you also have the problem of making sure that if the sails are strong enough to not get torn apart how about the "bolts" that hold them onto the main body? And then, if the bolts are strong enough... will they rip out of the hull they're fastened to? Or rip out the hull plate if that's strong enough? Or pull out the bulkhead the plate's fastened to if that's strong enough? It's a nasty regression...
Unless we get some sort of (highly) exotic matter all we're going to do is build something so heavy (and probably more solid material than crew space) that it wouldn't move anyway.

Ah, got you. I was picturing some fixed wind-farm type thing and couldn't work out what you were on about. :o

Um, okay. What makes you think a spinning ship would be any better than a non-spinning one? Surely some of the applied energy would be wasted in spinning it?

And no, the constant stream of photons (even if they didn't reduce in "power" as you got further away from the Sun) have a fixed value for their energy. As mass increased the applied force would become negligible - no further acceleration. And I'm not sure that a solar sail would get much power at all in the outer Solar System, let alone interstellar space.


No, that's just it - a stationary object rotating near the speed of light. Rotational speed is still movement through space, right? So it would still experience time dilation, and would have a steady stream of fuel from the sun.



Yeah, the materials problem... got to be strong enough to withstand spinning of near-infinite mass tips at near-C velocities, which means that they'd maybe be too heavy to spin in the first place. It's for damn sure we haven't got any materials (or construction techniques) around today that would take that kind of punishment.
And then, of course, you also have the problem of making sure that if the sails are strong enough to not get torn apart how about the "bolts" that hold them onto the main body? And then, if the bolts are strong enough... will they rip out of the hull they're fastened to? Or rip out the hull plate if that's strong enough? Or pull out the bulkhead the plate's fastened to if that's strong enough? It's a nasty regression...
Unless we get some sort of (highly) exotic matter all we're going to do is build something so heavy (and probably more solid material than crew space) that it wouldn't move anyway.

:) Yeah, well - first things first. We can solve the materials problem later.

No, that's just it - a stationary object rotating near the speed of light. Rotational speed is still movement through space, right? So it would still experience time dilation, and would have a steady stream of fuel from the sun.
"Passenger" compartments in the blade tips? Aaah.


:) Yeah, well - first things first. We can solve the materials problem later.
F*cking typical! The old "I've done the hard work coming up with the idea, it's up to the engineers to make it now."
How many times have I banged my head against the wall on hearing that one? :D

"Passenger" compartments in the blade tips? Aaah.


F*cking typical! The old "I've done the hard work coming up with the idea, it's up to the engineers to make it now."
How many times have I banged my head against the wall on hearing that one? :D

:D Precisely.