View Full Version : Fuel constraints on relativistic travel

2006-Nov-27, 04:36 PM
After looking at the c-ship website(cba to find the link, just google it), I still have a few questions regarding what would happen to a ship travelling near the speed of light. The site was exceptional, yet it did not take into account the energy required to accelerate a ship at 100m/s squared for a long time. Although an observer on a nearby planet would not see the ship ever reach light speed , the site goes on to explain that the ship could (from the point of view of the ships crew) cross the galaxy in a few years. I understand the effects of time dilation fairly well, and am quite capable of working out the fuel constraints on such an endevour, and realise that the ISP of the fuel would need to be "astronomical". However, I was lead to believe that more energy would be required to maintain such acceleration as the ship approaches C. Is this only true from the point of view of the observer, or would the crew of the ship need increasing energies to continue their voyage? I read that the mass of a ship will increase as its velocity increases, but again, is this only from the point of view of the observer?

2006-Nov-27, 04:54 PM
Relativistic travel is not currently very practical. Your point about fuel (or reaction mass) is a good reason why.

2006-Nov-27, 07:38 PM
The other issue is radiation effects at relativistic velocities. Much shielding is required to mitigate tissue damage.

Bearded One
2006-Nov-27, 10:02 PM
I read that the mass of a ship will increase as its velocity increases, but again, is this only from the point of view of the observer? Yes, the crew on the ship would notice nothing out of the ordinary unless they looked outside.

grant hutchison
2006-Nov-28, 09:27 AM
This site (http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html) provides calculations for fuel consumption by relativistic rockets.

Grant Hutchison

2006-Nov-28, 12:06 PM
All the info is good, cheers! But I wouldnt want to be anywhere near the rear of that "GRASER" if its ever invented... :sick:

Mister Earl
2006-Dec-03, 02:52 PM
Yes, the crew on the ship would notice nothing out of the ordinary unless they looked outside.

Interesting! What would happen, if they did? Would the universe appear to have shrunk, or enlarged? What optical effects are there at relativistic speeds? I imagine that in front of them, time would appear to have been sped up, and behind them slowed or stopped. Travelling at relativistic speeds, is there a "red shift in time" so to speak?

grant hutchison
2006-Dec-03, 03:21 PM
There are two sets of distortions at work when you look out the window of a spacecraft moving near lightspeed.
First of all, there's relativity, which makes the Universe look compressed in the direction of your travel, and makes time appear to be running slowly outside your ship.
Secondly, there's the effect your motion has on the light signals you receive. This makes light from up ahead look more blue, and light from behind look more red. It also displaces the direction light appears to arrive from: so you have to look towards the front of your spacecraft to see objects that are slightly behind you. And because you're running into signals from ahead in very rapid sequence, time will appear to pass more quickly for objects up ahead, despite the slow-down dictated by relativity. Meanwhile, signals from astern will arrive more slowly, so time will appear to pass very slowly there.

When you look out the window you'll see a large proportion of the Universe apparently compressed into a bright, blueshifted region ahead of your spacecraft, with the rest of the sky filled with sparse, dim, redshifted objects which are all actually behind you. The faster you go, the smaller, brighter and bluer the region ahead becomes, with more and more of the sky compressed into that region.

Grant Hutchison

grant hutchison
2006-Dec-04, 01:23 AM
There's a rather neat geometrical way of visualizing the distortions induced by relativistic travel, which I propose to share with you now. :)
It incorporates both relativistic effects and aberration effects.

First, imagine that you are at rest in the middle of a sphere of evenly spaced stars at some constant distance from your viewpoint. A cross-section through your view might look like this: http://www.ghutchison.pwp.blueyonder.co.uk/relativity/betazero.jpg
You're at the centre of the radiating view lines, with stars visible in all directions at the same distance.

Now we construct the view you'd have if you were passing through the same viewpoint at 0.6 of lightspeed, from left to right. This value, 0.6, is generally called beta (β); it’s the ratio of your velocity to that of light. We also need to calculate the parameter gamma (γ), which is equal to 1/sqrt(1-β²); in this case, 1.25.
We scale our circle of stars by a factor of γ in your direction of travel. This produces an ellipse (which turns out to have eccentricity β). And we place our viewpoint at one focus of the ellipse, the one opposite your direction of travel.
And that’s it: the distorted ellipse of stars, and the shifted viewpoint, gives us the apparent angle and distance of the evenly spaced sphere of stars, as seen from a spacecraft travelling at relativistic speeds. It looks like this: http://www.ghutchison.pwp.blueyonder.co.uk/relativity/betapointsix.jpg
I’ve marked blueshift and redshift with appropriate colours; the cut-off between the two is marked by the intersection between the original circle and our new ellipse, which is also the point at which a star’s apparent distance in our moving reference frame matches its distance as measured in the rest frame.

Here’s the same construction for β = 0.9: http://www.ghutchison.pwp.blueyonder.co.uk/relativity/betapointnine.jpg
A longer ellipse, more stars shifted to the forward view, and a tighter region of blueshift (which nevertheless incorporates a greater proportion of the rest-frame sky).

Notice that the aberration effects (which are due simply to the fact that you are moving close to the speed of the "signal carrier", light) are quite dominant; the relativistic effects (apparent shortening in the direction of travel, slowing of relative time) appear as adjustments to the aberration distortion.

Grant Hutchison

2006-Dec-04, 09:46 AM
The fact that the universe flattens in front of you is the most interesting aspect of this, because it lead me to consider what it would be like to travel AT light speed. Although this IS impossible for particle with a rest mass such as an electron, it is worth considering what the POV of a photon would be. Would the universe be infinitely flat, and would all time have passed the instant the photon appears? I would assume that there must be no such thing as time from the perspective of a photon.

Also, this is the link to the c-shiip website. It contains video and still frames of the perspective of a traveller in a fictional craft (in a fictional universe for that matter, but with the same laws as ours).


grant hutchison
2006-Dec-04, 01:44 PM
The fact that the universe flattens in front of you is the most interesting aspect of this, because it lead me to consider what it would be like to travel AT light speed.Everything goes to zero or infinity when you plug lightspeed into the equations. An infinite time passes in the outside Universe while zero time passes "aboard" the photon. All of the sky is compressed into a single, dimensionless, infinitely blueshifted point directly ahead: objects within this dimensionless point show no parallax and no apparent diameter, and so appear infinitely distant.
Shrug. Make of that what you will. :) I think it's telling us that "passage of time" and "viewpoint" don't mean anything when you're a photon.

Grant Hutchison