# Near Light speed Spaceship?



## Ray McCarthy (Mar 31, 2015)

Not without  a "deflector shield", which isn't likely to help protect you from Cosmic Background Radiation and even starlight!

http://www.theregister.co.uk/2015/03/31/strange_red_glow_incoming_aliens_infrared/


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## BAYLOR (Apr 1, 2015)

Could They could build a thick lead inner hull to protect against the radiation ?


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## Venusian Broon (Apr 1, 2015)

BAYLOR said:


> Could They could build a thick lead inner hull to protect against the radiation ?



Complete overkill for protection - think of all the mass you'd be adding. 

Water in the form of an ice barrier/protection would be 1) much cheaper 2) weigh much less 3) still be pretty effective at stopping radiation 4) Easier to manipulate 5) Also be very useful if you are not travelling so that you could use it to generate all sorts of stuff - oxygen, fuel etc...


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## Ray McCarthy (Apr 1, 2015)

Probably either would get used up quickly.


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## goldhawk (Apr 2, 2015)

Venusian Broon said:


> Complete overkill for protection - think of all the mass you'd be adding.
> 
> Water in the form of an ice barrier/protection would be 1) much cheaper 2) weigh much less 3) still be pretty effective at stopping radiation 4) Easier to manipulate 5) Also be very useful if you are not travelling so that you could use it to generate all sorts of stuff - oxygen, fuel etc...



Water, because it is less dense than lead, will provide less protection.

And it's not just photons. There is interstellar dust. At near light speed, a single speck could transfer the energy of an atomic bomb. Travelling at speeds faster than 10% the speed of light would require massive shields to protect you.


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## Venusian Broon (Apr 2, 2015)

goldhawk said:


> Water, because it is less dense than lead, will provide less protection.
> 
> And it's not just photons. There is interstellar dust. At near light speed, a single speck could transfer the energy of an atomic bomb. Travelling at speeds faster than 10% the speed of light would require massive shields to protect you.



Actually water still has many advantages, although I admit it is not perfect - 

Placing a water tank (or an ice bulge) [a.ka. Alastair Reynold's Revelation space craft] in front of a ship is advantageous in comparison with a shield made of metal or another solid material because it eliminates the damaging embrittlement of solids under intense nucleonic radiation. (And relatively easy to put a new one together after you have reached your stop.) 

Water, with a higher proton density is actually is better weight for weight at shielding from cosmic rays than heavier materials (as well as its other advantages - good luck trying to mine lead effectively in a new system to repair your shielding) 

Ok, disadvantages - you will get secondary radiation, but then you won't be building a spaceship entirely out of ice - you'd need other layers of protection. Hopefully nowhere near as big after a big ice shield. 

Yes dust in the path poses a problem - but it's going to equally pose a problem for a hunk of lead. Quite how you stop a neutral bit of dust slamming into your craft at near light speed with some active system I don't know...


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## Dave (Apr 2, 2015)

Venusian Broon said:


> Complete overkill for protection - think of all the mass you'd be adding.


As you approach the speed of light then mass approaches infinity in any case. What difference would a few Kg make?


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## Venusian Broon (Apr 2, 2015)

Dave said:


> As you approach the speed of light then mass approaches infinity in any case. What difference would a few Kg make?



off the top of my head - time . Bigger ship will need bigger engines to push it at a constant acceleration. A ship with the same engines but smaller mass will be zippier and get up to speed much quicker.


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## BAYLOR (Apr 2, 2015)

Venusian Broon said:


> off the top of my head - time . Bigger ship will need bigger engines to push it at a constant acceleration. A ship with the same engines but smaller mass will be zippier and get up to be speed much quicker.



Someone needs to invent wormhole technology.


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## goldhawk (Apr 2, 2015)

Dave said:


> As you approach the speed of light then mass approaches infinity in any case. What difference would a few Kg make?



The relative mass as measure by an observer here on Earth goes up. As far as being in the ship, it stays the same.


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## Mirannan (Apr 2, 2015)

Regarding shield materials - it's a little known fact that elements of low atomic weight are actually better at stopping the main component of solar wind particles (protons and helium nuclei) than heavy elements such as lead are. The same would undoubtedly be true of interstellar gas.

The reason is that momentum exchange is most efficient when the two things colliding are of mass as close to equal as possible. Thus, the best materials for shielding against the proton and helium nuclei flux caused by relativistic flight would be various ices (water, ammonia, maybe methane) and possibly something like polythene or paraffin wax - either of which shouldn't be all that difficult to make from available materials.

The reputation of lead as a shield material comes from the fact that it's best at stopping high-energy photons such as X-rays and gamma radiation. In this case, the density is the important thing. (Which is why radiographers wear lead aprons.) Both of these are present in space, of course, but at relativistic speeds the flux would be trivial compared to the protons.

A bit of trivia is that other elements would be slightly better as a shield material than is lead because they are considerably denser. The difference varies. However, they are all at the bottom of the transition metals in the periodic table and are less practical for other reasons. They are all either more difficult to work with (tungsten, tantalum) or hugely expensive (gold) or both (platinum group metals). Something tells me that the NHS budget wouldn't stretch to radiographers' aprons made out of platinum.


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## Ray McCarthy (Apr 2, 2015)

The article states that due to relativity, ordinary light, infra red and even microwaves would be gamma radiation for the traveller, that the collisions are so energetic any such vessel would be generating a massive wave of light and radiation travelling ahead.

You'd need something like a plasma shield (possible) to destroy any dust in front. But the authors have no idea how the radiation can be dealt with. Any type of physical shielding would be worn away and also become very radioactive.


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## goldhawk (Apr 2, 2015)

Ray McCarthy said:


> The article states that due to relativity, ordinary light, infra red and even microwaves would be gamma radiation for the traveller, that the collisions are so energetic any such vessel would be generating a massive wave of light and radiation travelling ahead.
> 
> You'd need something like a plasma shield (possible) to destroy any dust in front. But the authors have no idea how the radiation can be dealt with. Any type of physical shielding would be worn away and also become very radioactive.



No, it would not become radioactive. Even gamma rays won't convert a stable element into a radioactive one.

And the photons will strip away the material; they will break the molecular bonds and cause the atoms to fly off.


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## Mirannan (Apr 2, 2015)

goldhawk said:


> No, it would not become radioactive. Even gamma rays won't convert a stable element into a radioactive one.
> 
> And the photons will strip away the material; they will break the molecular bonds and cause the atoms to fly off.



Gamma rays wouldn't, perhaps - although there is some chance that some of the shield material would be turned into metastable nuclear isomers. However, sufficiently energetic gamma rays might break up nuclei and cause radioactivity that way. And I'm quite sure that bombardment with high-energy protons and helium nuclei is likely to lead to some transmutation, also leading to radioactivity.


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## Ray McCarthy (Apr 2, 2015)

goldhawk said:


> No, it would not become radioactive.


I'm not convinced!
I think secondary effects cause radioactivity, but that appears to be a minor issue!
It's an interesting paper.



> At relativistic speeds, the boffins note, cosmic microwave background (CMB) photons will “appear in the spacecraft frame as highly energetic gamma rays”, which, striking the spacecraft's hull would cause “effects ranging from ionization and Compton scattering to *pair production” (the creation of matter/antimatter pairs)*.
> That still leaves things such as the interaction with the CMB to deal with: “one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter”, the scientists write.




Your "deflector shield":


> A relativistic spacecraft that actually struck something the mass of a baseball would generate an impact energy of 30 megatons, but the researchers presume anyone building such a machine would also work out how to avoid such catastrophic collisions before they tried.





> *Limits and Signatures of Relativistic Spaceflight*
> Ulvi Yurtsever∗
> and Steven Wilkinson†
> Raytheon Company, El Segundo, CA 92195
> ...



From paper (text selectable PDF)


> We will assume that advanced technology can mitigate the harmful effects of ionization and Compton scattering, and therefore concentrate on pair production as the obstacle that is most likely to resist removal via technology. [1]
> The first speed level where pair production will pose a challenge for relativistic spaceflight engineers is photonnucleon interactions in the hull of the spacecraft. Let us consider a hull made of ordinary baryonic matter. In this case, a single CMB photon will create an electron-positron pair via its collision with a nucleus in the hull if its energy exceeds the rest mass of an electron and positron.



They go on to explain why even CMB (Cosmic Background Radiation) is a problem.

Also why we would see them coming ( in my First Contact SF, the possibility of a Stealth approach is regarded as impossible).


> *Signature of CMB Scattered from a Relativistic Spacecraft*
> The possibility of detecting radiation associated with distant relativistic spacecraft has been discussed in the literature before [6–8]. These discussions mostly focus on detecting radiation from spacecraft engines or light from nearby stars reflecting off the spacecraft. Our approach is different in that we do not speculate on possible propulsion technologies but are interested in how a large relativistic object would interact with the interstellar/intergalactic medium and mainly with the CMB radiation. As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology.


Also in my SF my propulsion is  plasma in a linear accelerator. The electrons are separated from plasma and sprayed out the front to destroy any dust. The plasma is generated from  water using Fusion power, which powers the 8km+  Linear Accelerator. I hadn't realise CMB can be a problem, but my ship only accelerates to about 0.5C or 0.6C then uses "Jump Drive" and decelerates. The plasma "torch" would be extremely visible.



> *Summary*
> We have explored the physical interaction of a relativistic spacecraft with the interstellar and intergalactic mediums. Our main  discussion focused on the interaction with the CMB where very little information currently exists and seems to have been overlooked. Central to our discussion was not how to obtain relativistic speeds but the consequences of traveling that fast. We discussed the interaction with baryonic matter in terms of high speed collisions to present a complete picture. In general one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter. Our assumption that matter-matter interactions can be dealt with when civilization can build relativistic spacecraft may prove false and may be a barrier that will prevent space travel with a large γ .
> We looked at two special reference frames, the spacecraft frame, and the rest frame of the CMB to understand how the CMB is distorted or acquires an aberration from those view points. The scattering of the CMB from a relativistic spacecraft is very similar to the Sunyaev-Zedovich effect where the inverse Compton scattering instead occurs at a macroscopic level. Our calculation for what an observer on earth could detect predicts a very unusual signature that is unlikely to be caused by any naturally occurring object in the known universe. This result is independent of propulsion technology, but the ability to detect the signal from Earth depends on available detector technologies. We are currently working to predict how far can we see this signature given our current capability.


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## BAYLOR (Apr 2, 2015)

goldhawk said:


> The relative mass as measure by an observer here on Earth goes up. As far as being in the ship, it stays the same.



But the mass prevents the ship from getting to light speed , doesn't it?


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## goldhawk (Apr 2, 2015)

BAYLOR said:


> But the mass prevents the ship from getting to light speed , doesn't it?



Yes but the mass only increases for observers not on the ship. To an observer on the ship:

1. the mass does not increase;

2. the speed of light is just as far away as it was before the flight started.


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