Remember that when you are travelling to that galaxy 18 billion light years away, you’ll hit every photon it sent our way over 18 billion years in just about 45 years.
It turns out that there is a rest frame in which this background looks the same in all directions. But if you keep accelerating in one direction, the CMB coming from there will keep blueshifting: from microwave to IR, to visible, and eventually hard gamma radiation.
I was really surprised when I heard this for the first time a while ago. Because it means that our universe does have a _preferred_ frame.
Relativity means that there are no special frames where the laws of physics are different.
Good read where you'll also learn about the "The Surface of Last Screaming":
https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Linew...
https://en.wikipedia.org/wiki/Pseudosphere
If you mean object with finite dimensions and negative curvature and no center, there seems to be such a thing, see Fig. n066200e:
https://encyclopediaofmath.org/wiki/Negative_curvature,_surf...
EDIT
Rozendorn's idea seems to be still an open question:
https://mathoverflow.net/questions/52851/the-geometry-of-nad...
- all those photons are blue-shifted and becomes hard gamma rays.
- once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you, which means you get hit not only by gamma photons coming from the galaxy you're aiming at, but gamma photons coming from half the universe!
once you get close enough from c, all the stars in the universe appear to be either right in front of you, or right behind you
Also there's nothing that would shift light to appear behind you other than your movement away from objects that you have passed.
At some point the warp drive might be cheaper than the shielding.
I don't understand that one, could you please expand on how it works ?
If you move fast enough, all of the ultraviolet light coming from behind you becomes visible light. A little faster and all of the ultraviolet light in front of you becomes X-rays. And all of the hard x-rays become gamma rays.
As a programmer, this gave me this warm feeling...
What photons do have is momentum, and so due to conservation of momentum, hitting a photon will slow you down.
Relative velocity is a hell of a drug.
Force would be negligible of course. But if you are in space, you are surrounded by energy in the form of starlight; it would be weird if that energy could not be used for work.
Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
This means that even say a solar sail has a limit on acceleration at a certain speed the photons hitting the front of the sail would be blue shifted enough to counteract the acceleration of photons hitting you from the back, even if you convert them to useful energy it wouldn’t matter since you’ll reach equilibrium.
Given 100% conversion rate you should getting energy via photovoltaic effect, so you use that to accelerate them.
> Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move.
But that's just "friction". Assuming a photonically uniform environment at rest, just buffer energy at rest, accelerate for a bit, goto 10. Eventually you'll get anywhere.
(Assuming a non-uniform environment, you're probably close to a star, in which case gravity outweighs photon pressure anyway.)
* momentum , impulse , force
1) Uniquely (to my knowledge for starships in hard-ish scifi) they're are 'aerodynamically' shaped, or at least long and pointy to minimize friction with interstellar mass.
2) Their front surfaces are covered in a thick layer of water ice which acts as an ablative shield against said mass and radiation.
3) The fact that they can accelerate at over 1G means that they can land tail-first on an Earth-type planet.
- Maneuver, like a sailboat.
- Decelerate. Shut off your thrust and dump the light-absorbing covers to reveal mirrors, for 2x the deceleration. Photobraking?
If we will ever have the technology to instead create proper massive particles at scale you could eject those in orbit around some other massive object and then re-steal their angular momentum to accelerate yourself. But that I believe would be an extremely lossy process so you would lose most of the energy.
now, the question of how to design an actual working solution? i don't know; and basically, it depends. you would then also have the problem of whether it was possible to implememnt that solution, using real matter and physical objects...
unfortunately, that was probably the question you really wanted answered, right?