The problem with lightspeed.

[Souper]

Here we have a ship about to accelerate to lightspeed. When it does, something strange happens; it burns up in atmospheric reentry.

Why? Let's have a look.

In space, there are only a few atoms per square centimeter, and most of these atoms are hydrogen. Normally, going at orbital speeds around Earth would mean these atoms aren't frequently hit enough to be encountered as drag.

However, when travelling at the speed of light, these atoms are being hit so often it's like going through REENTRY.

Except here, we have atoms that are so massively energetic, the ship would be incinerated and shredded almost instantly.

Do you know what this means? It means we are going to take a long, LONG time until we develop SAFE lightspeed travel.

Edited September 5, 2014 by Souper

[Ralathon]

I did not do the full maths to support this. But a quick wiki search shows me that the earth's atmosphere is about 1e13 times denser than the densest molecular clouds in the interstellar medium. Assume spaceships reach maxT at 1/100th normal pressure and this means the interstellar medium is 1e10 times as dense. This means that to get the same kinetic energy dumped into your spacecraft per square meter per second you'd need to go sqrt(1e10)=~ 1e5 times as fast. (This assumes interstellar hydrogen has the same weight as atmospheric nitrogen which it obviously doesn't. But we're working in orders of magnitude here, not worrying about decimals).

Spacecraft enter the earth's atmosphere at about 7.5km/s. So you'd have to go 7.5e8 m/s. That's faster than the speed of light so we'll have to use relativistic equations for kinetic energy here.

(y-1)*1e-10*mc^2 = 0.5*mv_earth^2

y = 0.5*v_earth^2/1e-10*c^2 + 1

1 - v_ship^2/c^2 = (1e-10*c^2/(0.5*v_earth^2 + 1e-10*c^2))^2

v_ship = sqrt(c^2- c^2(1e-10*c^2/(0.5*v_earth^2 + 1e-10*c^2))^2) = 0.97c

So you'd have to move at 97% of lightspeed before you really need to start worrying about the interstellar gas. By the time we have the technology to reach those kinds of speeds we should be able to implement something as simple as a big magnet to deflect ionized gas.

Really, compared to the technological problems associated with near lightspeed travel the interstellar gas is almost trivial.

[Mazon Del]

It would be interesting to see a mathematical comparison between the two. There is a far cry between one or two atoms per square inch (which if I remember right is the atom count INSIDE a solar system near a star, but outside it is predicted to be much lower) and the billions of atoms per square inch that exist for an atmosphere. Definitely you will need some sort of shielding, that just goes without saying.

[DasValdez]

Bussard. Ramjet.

Two birds, one stone.

[Mazon Del]

Bussard. Ramjet.

Two birds, one stone.

Hah! Well done.

[-Velocity-]

I did not do the full maths to support this. But a quick wiki search shows me that the earth's atmosphere is about 1e13 times denser than the densest molecular clouds in the interstellar medium. Assume spaceships reach maxT at 1/100th normal pressure and this means the interstellar medium is 1e10 times as dense. This means that to get the same kinetic energy dumped into your spacecraft per square meter per second you'd need to go sqrt(1e10)=~ 1e5 times as fast. (This assumes interstellar hydrogen has the same weight as atmospheric nitrogen which it obviously doesn't. But we're working in orders of magnitude here, not worrying about decimals).

Spacecraft enter the earth's atmosphere at about 7.5km/s. So you'd have to go 7.5e8 m/s. That's faster than the speed of light so we'll have to use relativistic equations for kinetic energy here.

(y-1)*1e-10*mc^2 = 0.5*mv_earth^2

y = 0.5*v_earth^2/1e-10*c^2 + 1

1 - v_ship^2/c^2 = (1e-10*c^2/(0.5*v_earth^2 + 1e-10*c^2))^2

v_ship = sqrt(c^2- c^2(1e-10*c^2/(0.5*v_earth^2 + 1e-10*c^2))^2) = 0.97c

So you'd have to move at 97% of lightspeed before you really need to start worrying about the interstellar gas. By the time we have the technology to reach those kinds of speeds we should be able to implement something as simple as a big magnet to deflect ionized gas.

Really, compared to the technological problems associated with near lightspeed travel the interstellar gas is almost trivial.

Wrong, you are using Newtonian equations (0.5*m*v^2) on something that REQUIRES relativistic equations. For the relativistic equations, see this- http://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies

In reality, the impact of the ISM into the front of a ship moving at relativistic speeds is a BIG problem. For protons and H2, it represents a source of radiation, but the REAL danger is interstellar dust. There is quite a bit of dust out there-

The dark areas are all dust clouds (gas is there too), though varying concentrations of dust exists throughout the galactic disk, everywhere you go.

A tiny dust grain can cause a huge amount of damage. In science fiction- the science fiction written by authors who know what they are talking about and want to maintain realism- interstellar spacecraft that travel a large fraction of the speed of light usually utilize a large shield to absorb impacts from dust.

DOH, how blind and dense can I get? I just noticed that you did switch to the relativistic equations later. I just assumed based off your earlier sentences that you used Newtonian all the way through- I would have never even started with Newtonian myself, so I just assumed you must not have known about the relativistic equations.

Oh well, the problem with dust remains.

Edited September 5, 2014 by |Velocity|

[Ralathon]

Wrong, you are using Newtonian equations (0.5*m*v^2) on something that REQUIRES relativistic equations. For the relativistic equations, see this- http://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies

Did you bother reading past the first paragraph? I did use the relativistic equations as soon as it became apparent that we're dealing with relativistic speeds.

In reality, the impact of the ISM into the front of a ship moving at relativistic speeds is a BIG problem. For protons and H2, it represents a source of radiation, but the REAL danger is interstellar dust. A tiny dust grain can cause a huge amount of damage. In science fiction- the science fiction written by authors who know what they are talking about and want to maintain realism- interstellar spacecraft that travel a large fraction of the speed of light usually utilize a large shield to absorb impacts from dust.

Yea relativistic dust grains are a problem and the gas will cause some radiation. But compared to the insane things you need to even get to that kind of velocity they really are trivial problems. We know how to deal with radiation. We know how to build a whipple shield. We don't know how to get something to 0.97c.

[lajoswinkler]

Interplanetary space is too dense for such endeavours. Interstellar, on the other hand, is pretty much fine.

[SargeRho]

I don't think a whipple shield would be very useful for such high velocity impacts, it'd probably be more useful to have a regular impact shield, and perhaps somehow carry a dust cloud ahead of you.

[Ralathon]

I don't think a whipple shield would be very useful for such high velocity impacts, it'd probably be more useful to have a regular impact shield, and perhaps somehow carry a dust cloud ahead of you.

If you have a thin whipple shield a few dozen kilometers in front of the craft it'll vaporize any dust particles as they hit the shield and give the resulting plasma cloud a few microseconds to expand. You can then use radiation shielding or a magnetic field to deflect the plasma.

It's mostly a matter of giving the debris cloud enough time to spread out so as not to put too much strain on the shielding. It is by no means easy. But compared to the kind of tech you need to ever reach those speeds it is almost trivially easy.

[problemecium]

I had a friend in high school who calculated that if you were to travel at about 16 orders of magnitude above the speed of light, you could pass from one side of a brick wall to the other without touching it.

Just tossing in my two cents xD

[OrbitusII]

The way I see it, interstellar and intergalactic gases would only cause appreciable drag if you were legitimately going faster than light by accelerating, something that I can't see happening. Ever.

Using something like an Alcubierre Drive would basically eliminate the interstellar drag issue because you're technically going slower than light, at least in your little spacetime bubble.