Special Relativity math question, thanks in advance again

[WhatRoughBeast]

What about this one?
Fusion Rocket

Nope, sorry, at best that's Longshot.

And I apologize, but I forgot to specify that the photon drive is powered by 100%-efficient matter/antimatter conversion. The combination is the very best that current (mainstream) physics allows. All of the fuel is converted to energy, and all the energy goes to producing the maximum possible exhaust velocity. I mean, if you're going to wave your hands and specify a technology, why not go for the gold?

 

[Skeptic Ginger]

Nope, sorry, at best that's Longshot.

And I apologize, but I forgot to specify that the photon drive is powered by 100%-efficient matter/antimatter conversion. The combination is the very best that current (mainstream) physics allows. All of the fuel is converted to energy, and all the energy goes to producing the maximum possible exhaust velocity. I mean, if you're going to wave your hands and specify a technology, why not go for the gold?

I'm looking for as plausible as possible. That's why I appreciate this input.


How does the photon drive create propulsion if the photons are being bounced off a second ship? I'm sure I'm missing something.

 

[epepke]

Well,

1) You're going to need a good photon drive, with a huge amount of the spacecraft consisting of matter/antimatter fuel, in order to get to an appreciable fraction of c

2) It wouldn't help you much, as at that speed the background radiation will be blue-shifted enough to melt all known materials.

 

[Skeptic Ginger]

Could you explain #2 please? Would that be the case at .1c? And is this known or just another Van Allen Belt will kill us theory?

 

[epepke]

Could you explain #2 please? Would that be the case at .1c?

Not at .1c, but at that point, time dilation is only about 1 in 200.

And is this known or just another Van Allen Belt will kill us theory?

Look at that gamma curve. It not only affects time dilation and Lorentz contraction, but your energy and thus the energy, relative to you, of anything you hit, including photons coming your way. Do that enough, and even the cosmic background radiation will fry you. Of course, if you do it within a galaxy, the red-shifted hot light from nearby stars will probably cook you first.

 

[Skeptic Ginger]

So I got no problem. I only need to go ~4 light years from Earth and I believe at .1c that only takes ~40 years. Do I at least have that calculation approximately right?

 

[Roboramma]

So I got no problem. I only need to go ~4 light years from Earth and I believe at .1c that only takes ~40 years. Do I at least have that calculation approximately right?

Yep, but note that is the proper time on earth. From what you wrote above it sounds like you think the 40 years is the proper time experienced by those traveling: actually they'll experience slightly less than 40 years.

 

[Roboramma]

If you're looking to get the mass of the ship down, try a solar sail or a light sail (can be powered by lasers somewhere in our solar system). A nice way to get around the rocket equation: though not entirely, as since you speed up, the energy imparted by each photon becomes less and less. But at least you're no longer carrying your fuel with you.

For slowing down, use the destination star, or theoretically bring a mirror along and let your lasers bounce off the mirror and thus strike your sail from the opposite direction.

Using only starlight you'll have to build a pretty damn big sail to get the sort of acceleration you're looking for.

And for those interested, a 2010 japanese spacecraft testing solar sail technology: http://en.wikipedia.org/wiki/IKAROS

 

[quarky]

Nope, sorry, at best that's Longshot.

And I apologize, but I forgot to specify that the photon drive is powered by 100%-efficient matter/antimatter conversion. The combination is the very best that current (mainstream) physics allows. All of the fuel is converted to energy, and all the energy goes to producing the maximum possible exhaust velocity. I mean, if you're going to wave your hands and specify a technology, why not go for the gold?

What is the power source for obtaining and isolating the anti-matter?
Before we go anywhere, shouldn't we try for a colony a mile down in the Antarctic ice cap? Or undersea? Or on the moon?

Slight de-rail, but I find it odd how little overlap there is in those that long for the stars, and those that are still curious about the Earth frontiers. Both endeavors are compatible, and the exotic earth stuff should happen first, as a gesture of sincerity.

 

[WhatRoughBeast]

I'm looking for as plausible as possible. That's why I appreciate this input.

Kind words are always appreciated.

And plausibility in a postulated future society is up to you as a storyteller.

How does the photon drive create propulsion if the photons are being bounced off a second ship? I'm sure I'm missing something.

I'm sorry, but you've lost me. The only time I think I've alluded to bouncing drive photons off a second ship is when your're trying to cut it in half with your exhaust. Not a propulsion concept at all.

 

[WhatRoughBeast]

What is the power source for obtaining and isolating the anti-matter?

Left as an exercise to the author.

"Any sufficiently advanced technology is indistinguishable from magic."

 

[WhatRoughBeast]

Look at that gamma curve. It not only affects time dilation and Lorentz contraction, but your energy and thus the energy, relative to you, of anything you hit, including photons coming your way. Do that enough, and even the cosmic background radiation will fry you. Of course, if you do it within a galaxy, the red-shifted hot light from nearby stars will probably cook you first.

You mean blue-shifted, right?

 

[Skeptic Ginger]

Kind words are always appreciated.

And plausibility in a postulated future society is up to you as a storyteller.



I'm sorry, but you've lost me. The only time I think I've alluded to bouncing drive photons off a second ship is when your're trying to cut it in half with your exhaust. Not a propulsion concept at all.

It was in the links.

 

[Skeptic Ginger]

...
Before we go anywhere, shouldn't we try for a colony a mile down in the Antarctic ice cap? Or undersea? Or on the moon?

Slight de-rail, but I find it odd how little overlap there is in those that long for the stars, and those that are still curious about the Earth frontiers. Both endeavors are compatible, and the exotic earth stuff should happen first, as a gesture of sincerity.

In my story only a few people try their lot on a new planet. Not everyone does. Why did Europeans emigrate to the Americas? It's in our nature.

 

[WhatRoughBeast]

In my story only a few people try their lot on a new planet. Not everyone does. Why did Europeans emigrate to the Americas? It's in our nature.

Yes, but also a lot of them didn't do it voluntarily. Before Australia, America was where Great Britain sent criminals to get them out of the way. Ben Franklin wrote a very unhappy essay on the subject.

And let's not forget savage political and religious persecution, not to mention famine.

 

[Skeptic Ginger]

Yes, but also a lot of them didn't do it voluntarily. Before Australia, America was where Great Britain sent criminals to get them out of the way. Ben Franklin wrote a very unhappy essay on the subject.

And let's not forget savage political and religious persecution, not to mention famine.

But none of that has anything to do with the story I'm telling in my novel.

 

[epepke]

You mean blue-shifted, right?

Yup. Sorry.

 

[quarky]

Left as an exercise to the author.

"Any sufficiently advanced technology is indistinguishable from magic."

I guess the difference between sci-fi and fantasy is when an attempt is made to explain the amazing device.
I almost fear that the Steve Job's of the future will insist on wand-shaped devices of magnificent powers. And genetic engineers will be hired to make unicorns. Saucer-shaped flying devices will be unveiled...woo's hand will manifest its emotional needs.

I'm scaring myself a little.

 

[Skeptic Ginger]

I guess the difference between sci-fi and fantasy is when an attempt is made to explain the amazing device.
I almost fear that the Steve Job's of the future will insist on wand-shaped devices of magnificent powers. And genetic engineers will be hired to make unicorns. Saucer-shaped flying devices will be unveiled...woo's hand will manifest its emotional needs.

I'm scaring myself a little.

Fantasy is Harry Potter, Twilight, and the worlds of Anne McCaffrey. Sci Fi is Battlefield Earth, 2001 A Space Odyssey, and Isaac Asimov. There is a big difference. It's not about imagination, it's about the mechanisms you use to explain your imagination. It's science fiction, not science. In my novel the sci-fi just creates the setting for the story to take place in.

 

[WhatRoughBeast]

This is interesting:

Yes, although there are just a few problems, the biggest one being that of scale.

The numbers for radiation pressure are straightforward:

E = p x c, (p is momentum, the x is "times"), so


dE/dt = dp/dt x c, where dp/dt is force, and dE/dt is power.

in mks, the unit of force is the newton, which will accelerate one kg at one meter/sec^2. Since a one g acceleration is about 10 m/sec^2, to get a one g acceleration of a one kg mass, you need

dE/dt = 10 x 3 x 10^8 watts,

or just about 3 gigawatts of optical power.

For a 100 metric ton vehicle, you need 300 terawatts of beam power to get a 1 g acceleration. (This is why I said a photon drive makes a dandy ship's weapon.) And that's continuous power, not pulsed.

To answer an earlier question, you can improve the efficiency of the drive by using opposing mirrors. Make the vehicle with a flat mirrored base, and oppose it with a massive mirror. Light from the photon drive accelerates the ship. It then hits the opposing mirror and is reflected. Most of the reflected beam hits the mirrored base of the ship. When it is reflected it exerts radiation pressure on the ship, and the beam then travels to the mirror, and the cycle repeats ad infinitum.

There are two reasons why this is not as good an idea as you might think.

1) This is intrinsically limited, and not perpetual motion, because with each reflection the momentum imparted to the mirror is deducted from the photons in the beam. As the beam bounces back and forth it is continually red-shifted, imparting less and less momentum with each bounce.

2) This is not terribly useful for interstellar travel due to beam spread. As the ship and mirror separate, the beam gets wider and wider, and will eventually start to spill around the edges of the mirrors. Over very large distances (but very small compared to the proposed voyage), virtually all of the photon beam will miss the mirror. Also, keeping the secondary mirror aligned with the ship, at the same time it is being hit with a ginormous energy flux would be, as the phrase goes, non-trivial. Likewise cooling both mirrors. Unless you want to specify perfectly reflective mirrors at extreme power levels - oh, why the Hell not?