Hacker News Reader

Comment by wallace_f | Hacker News Reader

Is it plausible for anything we'd build to travel .998c and survive the journey?

Just going that fast isn't really risky at all (assuming you don't hit anything, which isn't actually a safe assumption). The tough part is making something capable of a constant 1g of acceleration for a 7 year trip. Most of the craft we make only accelerate for the first small percent of a trip. We can do accelerations that high, but not for very long, and we can't even manage acceleration for such a long duration even at a small fraction of 1g.

My layman's reading of physics-for-layman books has taught me that if we were to travel at the speed of light, we'd convert into energy. However, I'm not so sure about 99.999999% the speed of light. That's really OK, as far as we know?

As far as we know, as long as you don't actually reach light speed (which is currently thought to be impossible anyway) you can pretty much get as close as you want and be fine.

Thanks. It's quite counter intuitive to think about. You'd be going along at 99.99999999999999999999999999999999% and then hit that last bit and poof now I'm photons.

Well, achieving that "last bit" takes an infinite amount of energy, so it's not surprising that that has some effect.

Yes, as long as you don't hit anything or care about the time dilation you will experience.

You are technically travelling at 99.99999% of the speed of light right now, relative to something.

What determines my speed limit? Is it set at 0 here on earth? What if I travel to another galaxy, this one was going at .5c relative to me when I was on earth. Then after trillions of years, inflation causes all of the galaxies to separate beyond sight from me in this new galaxy. Now, the centre of this galaxy appears at rest. Does my velocity get "reset?"

inflation/expansion of space is not limited to the speed of light.

I know. That understanding is already illustrated in my question... I don't see what that has to do with answering it?

Special relativity says that no matter what your velocity is relative to Earth you can proclaim that you're actually at rest and it's the other guy that's moving. So your velocity is always continuously being "reset". And in each of the frames of reference of everything in existence nothing else is traveling faster than the speed of light.

Assuming spacetime is not expanding, let's say 1 galaxy is moving away from us at .9c. I leave our galaxy and catch it by travelling at .99c. Then a citizen of that galaxy, at rest, accelerates to .9c. So his velocity relative to earth is .89c? I think I need to go watch a youtube video on special relativity. Intertial reference frames are really weird.

The 1905 Special Relativity Theory (1905 SRT) version of reciprocity implies that the other guy in a different inertial frame than your own will be the one that is moving. The consequence of this implication is the "Twin Paradox" (i.e. both twins in different inertial frames have been accelerated and are both aging slower than the other twin). A different version of reciprocity that implies that the accelerated twin is moving and the apparent movement of the twin that has remained stationary is only an aberration (or mirage) is axiomatic because movement first requires acceleration.

You are actually traveling faster than the speed of light relative to some distant galaxies. The "speed limit" in special relativity refers to an inertial reference frame (in general relativity, this would be a reference frame with no/negligible spacetime curvature). As soon as you have significant gravity/acceleration relative to things with mass, the answer becomes much more complicated (and not fixed).

If you go fast enough doesn't any incoming radiation just get blue shifted into gamma rays? Just running into cosmic dust at the speed of light would not be fun.

Isn't that the whole relativity theory: that if you emit a photon while at celerity 1c, it looks like it goes away at 1c both from you and from a still reference? hence blue is still blue at 1c?

I'm talking about radiation from other stars or what ever. Imagine if every star in the star field in front of you shifted from visible light to gamma.

I'm sure there is a study out there that describes the exact amount of radiation exposure you would receive at reletivistic velocities due to blue shifting.

The spectrum of light depends on the frequency of the vibrations in the electromagnetic field, not the velocity of the wave. For both red and blue light the wave is traveling at 1c but the peaks of the wave are at 700 nanometers or 400 nanometers depending on the color. And under Special Relativity distances distort so that you do in fact see light shifted to higher frequencies/shorter wavelengths in the direction you're traveling.

The speed doesn't change but the wavelength does. That's why we see light from other galaxies redshifted.

>Just running into cosmic dust at the speed of light would not be fun.

That's why you need a navigational deflector dish. http://vignette1.wikia.nocookie.net/memoryalpha/images/b/b4/...

Also, and I'm not an astrophysicist, but I believe going fast _relative to Earth_ doesn't make it any more or less likely that you're going to hit anything. Moving at .998c relative to Earth and hitting something that is stationary relative to Earth is the same thing as not moving and being hit by something going .998c relative to you.

EDIT: Right, as chris_va pointed out, we're all traveling at .998c relative to something out there.

> Also, and I'm not an astrophysicist, but I believe going fast _relative to Earth_ doesn't make it any more or less likely that you're going to hit anything.

OTOH, given that it is (like most of the stuff around it) orbiting the center of mass of the Milky Way Galaxy, shouldn't Earth be moving relatively close (within a very small fraction of light speed) to the speed of most of the interstellar dust, etc., in its immediate galactic vincinity, such that moving at 0.998c relative to earth is also moving at a very similar fraction of c relative to lots of other things you are going to potentially hit on a journey from earth to a nearby star system?

Of course. Thanks. This is why they don't put me in charge of our next interstellar trip.

>doesn't make it any more or less likely that you're going to hit anything.

I believe that would be wrong. Also not an astrophysicist but I would guess that most of the stuff in our galaxy is moving at speeds which are, on average, relative to our galaxy, and moving at some average rotational speed around it. Of course there could be stray bits moving quite quickly on through.

So moving at c relative to the planets and solar systems swirling around our galaxy probably would make you more likely to hit something.

You have no idea how scary traveling at near-lightspeed actually is. Isaac Arthur has a great video covering it. https://www.youtube.com/watch?v=s6BQSgidbmc

> The tough part is making something capable of a constant 1g of acceleration for a 7 year trip.

Isn't that just a building with an engine in the basement? Seems like one of the harder parts would be making sure it can survive freefall at launch, turnaround and arrival.

> Isn't that just a building with an engine in the basement?

No, it turns out that the requirements of generating thrust (which require generating energy and using it to toss something out the back of your ship) end up running into hard limits in the delta-V you can carry based on the available thrust-generating technology. Surviving 1g of constant acceleration in easy (well, I mean, it requires a certain degree of structural strength, which as mass gets large--which it will even with something delivering a minute payload--becomes challenging because of square/cube issues), but building something that can actually generate 1g of constant acceleration for 7 years is a non-trivial engineering challenge.

> Seems like one of the harder parts would be making sure it can survive freefall at launch, turnaround and arrival.

No, surviving free fall is a non-issue. Literally, that means surviving not having external forces acting on it. There's nothing to it.

Not a physicist, but wouldn't a spacecraft standing on Earth's surface already feel 1g because of Earth's gravity (https://en.wikipedia.org/wiki/Equivalence_principle)?

You can look up Project Daedalus's report online. They considered all these issues in great detail and came up with an affirmative answer.

> Is it plausible for anything we'd build to travel .998c and survive the journey?

The first half is actually a bigger problem than the second half.

Reply on news.ycombinator.com