Interesting to note that when you are travelling close to c, even intergalactic vacuum requires a lot of mass ahead of you so the ship is not vaporised.
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.
Even ignoring galaxies, there's also the cosmic microwave background.
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.
the CMB is a bit mind bending; it's related to the question "if the universe is expanding, in the past it was smaller and smaller; so where is the center?"
Good read where you'll also learn about the "The Surface of Last Screaming":
I understand how there can be no center if the universe has positive curvature. But if the universe is saddle-shaped and non-infinite, surely there has to be a center?
I think not; consider pseudosphere, there is no obvious center. Its embedding in 3D space has infinite dimensions but the object itself has finite surface area.
Rather than think "surface of a balloon inflating", think "surface of an (infinite?) rubber sheet being uniformly stretched". It does intuitively seem like if there is 0 curvature and no "center", then the universe must be infinite though ...
- 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
That's not really right. There is a distortion where things appear more in front of you than they are at the time you see them, but that's only because they were at that angle, but by the time the light gets to you they're not where they appear.
Also there's nothing that would shift light to appear behind you other than your movement away from objects that you have passed.
I think this is why the Alcubierre drive will become critical even at well below light speed. Assuming I understand the physics correctly, the bubble of space around the ship is not moving (very fast) relative to the ship and so moving at fractional c will see an increase in flux but not the blue shift. You’re still hitting every spec of dust in your path, but not turn the entire cone of the universe in front of you into gamma rays.
At some point the warp drive might be cheaper than the shielding.
If an alcubierre drive can be created,even inside the bubble of normal space time you would probably die. This is because of the Unrue radiation that is created by compressing the space time in front of you. Also, it seems that even with the best theoretical approach you still need the mass equivalent of Jupiter in exotic matter to make it work for a small ship :/ Spacetime is extremely resistant to compression/expansion.
Think of it like walking in a rainstorm. Walk at a slow pace and most of the rain hits your head, but if you're moving very fast all of the rain will seem to be coming from the front because you are running into it instead of it running into you.
If you travel very close to c (relative to everything else) then due to length contraction everything else in the universe is 0 metres away. Likewise time appears stopped on everything else moving close to c relative to you, due to time dilation. (Is this right? I've never thought of it this way before).
Blue shift (aka increased apparent frequency) is caused by the velocity between you and the source, not the velocity of the source. That is how the speed of light works in General Relativity. If you move toward a photon is doesn’t get faster, it gets higher frequency. If that’s in the visible range it shifts toward the blue end of the spectrum.
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.
Even if the photon is coming from the side, just slightly ahead of you, you are ramming into it at near lightspeed. You can't add up speeds in newtonian way so energy goes (for you) into blueshift.
If you travel close to C, those photons will be blue-shifted into (depending on velocity) very energetic photons (x-ray, gamma spectrum) that may be rather difficult to shield against.
Special Relativity states that it's time that changes, not the speed of light. C is constant regardless of your reference frame. So, it's still C, but instead, it will take a lot longer to complete your project. You're definitely going to miss your deadline.
Unfortunately the photons hitting the solar panel at the front of your spaceship will slow you down by at least as much as you could turn them into thrust by shooting them out behind you (conservation of momentum) :(
Photons do have mass, because energy has mass & photons carry energy. They have no rest mass, which is why they are usually described as massless, but this isn't the whole story!
Radiation pressure. Remember, solar sails are a thing. At some point, you're blazing through spacetime so fast all that energy you're trying to share the same physical location with is impacting and ablating your hull material.
Could you collect photons from all around you and accelerate them out the back? There's no reason you have to place the solar panel at the front.
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.
No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration.
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.
> No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration.
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.)
I remember reading about a design that basically works like this. It’s sucks up all the atoms in front of the ship and uses them for acceleration. I think there is also a book about this where such a ship has a problem which doesn’t allow it to slow down. So they just keep accelerating. Wish I could remember the name.
This reminds me of the Lighthuggers in Alastair Reynolds' Revelation Space novels. They're powered by handwavey 'Coinjoiner drives' that sidestep the question of reaction mass or power sources, and so are able to accelerate to very close to the speed of light (hence the name). They have a few interesting features:
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.
Yes, but photons carry momentum even if they are massless, and you can't convert more than 100% of that momentum to energy for propulsion. So, if you achieve perfect efficiency, you can avoid them from slowing you down and still have to rely on internal energy sources to accelerate you .
Mmmh, I don't think so. At least efficiently. You might be able to decelerate emitting photons in front of you but they would escape to infinity unless you put them in orbit around a black hole. In that case perhaps.
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.
fundamentally? yes, of course. there's no physical laws or reason why you couldn't - just observe the usual caveats, such as being unable to achieve 100% efficiency, that sort of thing.
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?
If you happened to be a civilization that lived at the very edge of the universe, such that every star was in one direction and there was only black emptiness in the other, could you the accelerate indefinitely into the blackness without worrying about having mass to shield you?
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.