Question about relativistic acceleration:

Originally posted by googlefudge
I think you would be better off simply focusing the laser beams conventionally
using regular [if giant] optics.

Skimming laser beams over the suns surface looks good until you account for the
scattering and absorption caused by the plasma in the corona and the fact that
the sun is a wobbling a-symmetric irregular object that makes focusing very hard,
if not impossible.

Not to mention that if the laser beam carried enough momentum to generate thrust for the ship it would drive the laser source in the opposite direction due to Newton's third law. Also it probably wouldn't be the best thing to be in the way of it...

Originally posted by DeepThought
Not to mention that if the laser beam carried enough momentum to generate thrust for the ship it would drive the laser source in the opposite direction due to Newton's third law. Also it probably wouldn't be the best thing to be in the way of it...

All light generates thrust... ~1 newton per 300MW

I think your biggest problem is preventing your solar sail, and spacecraft, from being vaporised.

Originally posted by googlefudge
I think you would be better off simply focusing the laser beams conventionally
using regular [if giant] optics.

Skimming laser beams over the suns surface looks good until you account for the
scattering and absorption caused by the plasma in the corona and the fact that
the sun is a wobbling a-symmetric irregular object that makes focusing very hard,
if not impossible.

I was just using that to illustrate the point of the focal line. If you go away from the sun, say now the beams are 10 million km apart and still parallel, they still focus but further out. I know there would be problems actually skimming the surface.

I have worked on that lensing thing for years. One thing I figured out, if you have a spacecraft out at that 580 or so AU one thing that doesn't get bothered by ANYTHING in the corona and such, is Neutrino's.

If you have a big fat neutrino detector, you can use the sun as a million km wide lens to focus those guys.

So the idea is to look for neutrino emissions and scan back and forth like an atomic force microscope but on a slightly larger scale🙂

Same for electromagnetic radiation.

Interesting thing about EM radiation, the shorter the wavelength, the more gain the sun delivers pound for pound.

Another thing I figured out. There is a connection between the mass of a star and the lower frequency limit a star will focus.

Of course, for the sun, that limit is low indeed, something like 1/10th of a hertz, you can see if the wavelength is much larger than the sun, there won't be any focusing simply because the focusing lens is not big enough.

If you were looking at a black hole, the frequency at which you would notice gain would be a lot higher in frequency, lower wavelength.

So you could if you had a couple of spacecraft, shoot a sweeping spectrum of EM at a target star and the other spacecraft on the other side of the object, the spectrum reaching the detector spacecraft would see a spectrum where there would be gain after a certain frequency but none below that wavelength. So you could work out the mass of the object even if it were isolated and no objects in orbit around it.

Those are some of the things I worked out studying the effect.

Originally posted by sonhouse
Another thing I figured out. There is a connection between the mass of a star and the lower frequency limit a star will focus.

I don't understand this. Surely it just moves the focal point? The mass of the star doesn't significantly change the size of the lens, it only changes the 'thickness' of the lens.
Note also that the sun produces significant quantities of neutrinos, so your focusing neutrinos idea is not going to work. With light, we can stick an occulter in front of the sun, but you can't do that with neutrinos.
But even with light, I suspect you would not do very well unless you pick frequencies that the sun does not emit in significant quantities: and I am not sure that there are any such frequencies.

And finally, the lens will not be flat due to the effects of the planets etc, so you will need corrective optics as well.

I am not even sure that gravitational lenses have a focal point. They are not like a pin hole camera or standard optical lens in that they do not create a perfect image, instead the bending of light is greater towards the center, thus creating rings in the image.

Originally posted by twhitehead
I am not even sure that gravitational lenses have a focal point. They are not like a pin hole camera or standard optical lens in that they do not create a perfect image, instead the bending of light is greater towards the center, thus creating rings in the image.

They do focus but only in a line. If you looked at a ring of light, say a small slice of light from a star in a ring approaching the sun, all that light would converge on one point, more or less. But a ring from a slightly larger size would focus down the line and then keep going apart at the same angle.

One interesting thing I figured out, these focal lines poop out at the same distance as the distance between the objects, so light coming from Alpha Centauri, starts a first focus at 580 AU and light from the wavefront further from the center focuses say at 585 AU and so forth till the reducing bending angle is no longer able to focus light at all and that distance happens to be in the case of AC, a line of focus 4.4 light years long.

To me that says if you have a solar powered spacecraft you can ride the waves at least 4 light years in the exact direction leading away from the sun on that line between the sun and AC. And all stars have this projection of light.

I think there are other implications of this, my son in law, a physicist living in Natal Brazil (world cup going on there to their disgust🙂

Anyway we are writing a paper on this subject.

Like you said, there are questions about the basic physics involved but if my analysis is correct there are definite very interesting implications.

One fundamental question would be does the light actually focus. I think it does. I think if you follow the Einstein rings if you could you would find the rings getting smaller and smaller till all or most of the light converges on one place.

I think it is just the chance occurrence of our placement in the universe that all we see is rings. We are just not lucky enough to be in the right place to see the converged light.

For instance, we already know we see huge magnifications in distance galaxies from microlensing where light gets a huge boost we can see but only lasts for a few days or less and they get information about stars in distant galaxies we could never have seen without such lensing.

I think the universe of light is much more involved than we can see, spikes of light coming off every star in every direction, some spikes only 4 light years long in the case of Alpha Centauri and 8 light years long in the case of Sirius and so forth. Magnify that by all the nearby stars and you have a beautiful vision of light doing things we didn't expect.

All assuming my analysis is correct. I fervently hope it is!

Originally posted by sonhouse
For instance, we already know we see huge magnifications in distance galaxies from microlensing where light gets a huge boost we can see but only lasts for a few days or less and they get information about stars in distant galaxies we could never have seen without such lensing.

I am not certain what you are saying here, but I am pretty sure its wrong. We do see lensing effects with distant galaxies, but it is hardly a temporary phenomena.
I am also fairly certain you are wrong about your focusing ideas.

Originally posted by googlefudge
All light generates thrust... ~1 newton per 300MW

I think your biggest problem is preventing your solar sail, and spacecraft, from being vaporised.

1 g ~ 10 m/s^2. Assuming a 1,000 tonne spaceship that's 10,000,000 Newtons of thrust. So 300 MW * 10,000,000 = 3 Peta Watts of energy. Which is the of the order of a thousand times the output of all the world's power stations. I think that this idea is not a runner...

Originally posted by sonhouse
They do focus but only in a line. If you looked at a ring of light, say a small slice of light from a star in a ring approaching the sun, all that light would converge on one point, more or less. But a ring from a slightly larger size would focus down the line and then keep going apart at the same angle.

One interesting thing I figured out, these focal l ...[text shortened]... t doing things we didn't expect.

All assuming my analysis is correct. I fervently hope it is!

The basic problem is that you've calculated (I'll take your word for it) a focal point at 580 A.U.. Given the heliopause is at 200 A.U. that's an awfully long way out.

Originally posted by DeepThought
1 g ~ 10 m/s^2. Assuming a 1,000 tonne spaceship that's 10,000,000 Newtons of thrust. So 300 MW * 10,000,000 = 3 Peta Watts of energy. Which is the of the order of a thousand times the output of all the world's power stations. I think that this idea is not a runner...

It's an enhanced solar sail.

Drop the acceleration to 0.01m/s and you are 'only' looking at the Terra-watt range.

What is the maximum theoretical velocity achievable via purely slingshot mechanisms?

Originally posted by twhitehead
What is the maximum theoretical velocity achievable via purely slingshot mechanisms?

Assuming you can use a rotating black hole then within the ergosphere faster than light relative to an asymptotic observer. The exit velocity can be arbitrarily close to the speed of light, relative to the singularity.

In the more realistic case of a satellite gaining speed from a planet orbiting the sun, up to twice the orbital velocity of the planet.

Originally posted by DeepThought
In the more realistic case of a satellite gaining speed from a planet orbiting the sun, up to twice the orbital velocity of the planet.

So not near light speed. 🙁

Originally posted by twhitehead
So not near light speed. 🙁

You gain velocity from a gravity slingshot by approaching a massive body from behind
[ie you are both going in 'roughly' the same direction] so that you take as long as
possible to catch up, so you accelerate towards it [due to it's gravitational attraction]
for as long as possible. And then you head away from this massive body by as quick
a route as possible so that you are slowed down by as small amount as possible.

The optimum speed gain would be from approaching from behind, swinging round and
leaving in the opposite direction as that gives you the maximum closing time and
minimum escape time.

However this is seldom actually possible/practical and so gravitational sling shot
manoeuvres typically involve approaching from behind, and then escaping 'sideways'.

However you can daisy chain sling shot manoeuvres one after another picking up speed in
each encounter. And if you approach a massive body from behind and leave at an angle
to it's direction of motion then you will gain speed regardless of how fast you are going.

You could be going 90% the speed of light and this will still apply.

However, you will get diminishing returns, and the faster you are going the less you
will deflect going past the massive body, and thus the smaller the effect will be.
[you can fire your engines to alter your escape trajectory, but if you have to use them
too much you would have been better off simply using your engines rather than slingshots]


Your best bet for picking up serious speed is therefore to pick really really massive objects
to use as sling shots. Preferably very fast massive objects.

And as it happens we have such things in the galaxy.

Not all stars in the galaxy orbit at the same 'sedate' speed that our sun does.
There are stars on highly elliptical orbits which are travelling at tens or hundreds of times as fast,
There are binary stars which orbit each other at huge speed, and there are stars whizzing around
Sagittarius A [The black Hole in the centre of the galaxy] at a significant fraction of the speed of
light.

So your best bet for getting serious speed, is to daisy chain a whole sequence of encounters
with stars picking up speed to slingshot you towards the galactic centre, and then pick up some
real speed off of the stars orbiting Sagittarius A and off of the black hole itself.
Before shooting off out of the galaxy on an escape trajectory at a good fraction of the speed
of light, heading off to Andromeda, or wherever the heck you are going.

However, if you are looking to pick up speed solely inside the solar system...

There is going to be a limit where you can no-longer divert your course back inside the solar system
after your last sling shot and you will sail off into space. Your best bet is to bounce around the inner
solar system picking up energy from Earth and Venus before sailing off on a final escape trajectory
which takes in a gravitational assist from all the gas giants in sequence.

Which is essentially what the Voyager spacecraft did.

So while you could theoretically go faster, THAT is the ballpark of how fast you could go with gravitational
slingshots... We can do better with ion drive thrusters. Plus for those you don't have to wait for a once
in ten thousand year planetary alignment or some such.

Originally posted by DeepThought
The basic problem is that you've calculated (I'll take your word for it) a focal point at 580 A.U.. Given the heliopause is at 200 A.U. that's an awfully long way out.

If you look at our theoretical laser just skimming the surface and we have eliminated absorption from corona and other stuff, the famous number Einstein came up with, 1.75 arc seconds is what the laser beam would be deflected going by the sun when the beam just skims the surface.
This is all mass VS diameter thing. Nothing to do with heliopause. Actually, if you had a black hole the mass of the sun, the deflection would still be 1.75 seconds of arc at 700,000 kilometers above the black hole which might be only 20 miles across or so. It would just get a whole lot larger deflection closer in.
So if you have two beams, one on each side of the sun they will have that 1.75 arc seconds of deflection and as the beams leave the vicinity of the sun, they are going into a weaker and weaker gravity field or bending of space so the beams go more or less in a straight line after the deflection.

If you just do the trig with a triangle of a line going straight through the sun and out into space and another line going from the center to the surface and the third line with that 1.75 arc second of deflection, do the trig yourself, you find the beams meet at about 54 billion miles out, 580 AU or about 85 billion kilometers. It's a VERY skinny triangle.

When I first did the trig, I thought the triangle was so skinny I wouldn't be able to figure it out on a calculator, I originally thought the angle too small to calculate with any accuracy but if you take a 90 degree angle and minus 1.75 arc seconds you are left with an angle of 89.999504 degrees and if you take the tangent, it comes out at about 117,000 ( I haven't done this in years so just going by memory) so the vertical line from center of the sun to the surface, you multiply by 117,000 and that comes out at about 580 AU. The line from sun center to surface is the radius which comes out at about 440,000 miles or about 700,000 kilometers. That's how I figured 580 AU.

Tan (90 degrees -1.75 arc seconds)= ~117,000 * 700,000 Km (radius of sun), comes out to about 81 billion kilometers or 54 billion miles or 580 AU.