A better photon rocket...

[Spacescifi]

 

Theoretically if man could make photons faster, would that not make for a better photon rocket?

The idea is to shoot out photons at a much higher speed than they normally travel. See, with a normal photon rocket, the amount of photons required is enough to vaporize/nuke anything in it's wake.

With a much higher momentum (a million or perhaps a billion times faster than light), perhaps the photon quantity required would be less to produce sufficient momentum transfer? Which would mean hopefully that lighting up the FTL photonic drive would'nt nuke everything behind it.

EDIT: Provided FTL photons behave like photons, just faster, then I would expect them to diffuse through mass much faster. In other words, shine an FTL beam laser on a rock and instead of burning through it, it would evenly heat the whole thing. Like Star Trek phasers. Only difference beung that you would have to put the laser right on the rock, since firing it in the air would diffuse it across several kilometers, making it useless as a cutting beam.

 

I dunno, what do you think?

Edited June 20, 2019 by Spacescifi

[Ol’ Musky Boi]

I mean, if you could make FTL particles then sure, the higher the exhaust velocity the better your specific impulse. But that is a massive if.

[Spacescifi]

6 minutes ago, Ol’ Musky Boi said:

I mean, if you could make FTL particles then sure, the higher the exhaust velocity the better your specific impulse. But that is a massive if.

 

FTL particles amd FTL light behave differently.

They are'nt the same.

FTL particles I think would be inherently more dangerous as they would nuke anything they hit.

Edited June 20, 2019 by Spacescifi

[Ol’ Musky Boi]

I'm a little confused by your question. How can you make light that goes faster than light? If a particle exists that could go faster than light, it's not going to be light is it?

[cubinator]

Photons go at 299 792 458 m/s in a vacuum, there isn't really any known way around that. I bet, though that you could make a rocket that was a lot more efficient than a photon rocket if you used something not with greater speed, but with mass. If you had a rocket that fired any kind of particle, be it protons, neutrons, or whole atoms, out the back at 99.999 % c, it would give a huge amount of energy to your ship. Now, how to get particles moving that fast?

[Nuke]

you can use particle accelerators to push the energy levels up particles way up. you get close to c and you can keep pumping energy in, they wont go any faster but they will gain some relativistic mass. i figure they would give you significantly more thrust than just using lower energy particles. of course a several kilometer long particle accelerator built into space ship would be so massive as to nullify any gains. maybe an alternative to lasers for beamed power allowing for a large magnetic sail. if a small ship can put out a huge field it would work out well. 

Edited June 20, 2019 by Nuke

[Spacescifi]

19 minutes ago, Ol’ Musky Boi said:

I'm a little confused by your question. How can you make light that goes faster than light? If a particle exists that could go faster than light, it's not going to be light is it?

 

We can't. But I am not concerned with that.

This is more a what-if scenario. Since science fiction tends to both ask and answer that.

So not light. But behaves like light. Just faster. Call them quotons. The much faster cousin of photons.

I am only guessing here about the effects of FTL photons? Anyone else care to share?

[Scotius]

https://en.wikipedia.org/wiki/Tachyon

Tachyons would fit. But no one ever found a trace of a tachyon. So, here you have it - sci-fi FTL particle.

[Ol’ Musky Boi]

Well, your rocket would violate causality for a start. If you were floating a few light years behind the craft you would see the engines fire, before they actually fired, because presumably your exhaust would travel back in time (I think?). Since we're assuming FTL particles I don't know wether we can still take into account modern physics, but assuming that everything we know now is more or less correct minus the fact that tachyons/quotons actually exist, then the rocket still wouldn't be able to reach >C, because the energy needed to speed it up will still rise to infinity and even tachyons/quotons don't have infinite energy. It would make a great propulsion system though.

[Spacescifi]

9 minutes ago, Ol’ Musky Boi said:

Well, your rocket would violate causality for a start. If you were floating a few light years behind the craft you would see the engines fire, before they actually fired, because presumably your exhaust would travel back in time (I think?). Since we're assuming FTL particles I don't know wether we can still take into account modern physics, but assuming that everything we know now is more or less correct minus the fact that tachyons/quotons actually exist, then the rocket still wouldn't be able to reach >C, because the energy needed to speed it up will still rise to infinity and even tachyons/quotons don't have infinite energy. It would make a great propulsion system though.

I think you would see the FTL ray after it was fired. It would just hit you much faster than light would. If the the beam traveled a LY per min it would hit you in 5 min.

I never have really believed that FTL meant time travel backwards.

Forwards is one thing that we know can be played with.

Edited June 20, 2019 by Spacescifi

[Ol’ Musky Boi]

20 minutes ago, Spacescifi said:

I think you would see the FTL ray after it was fired. It would just hit you much faster than light would. If the the beam traveled a LY per min it would hit you in 5 min.

I never have really believed that FTL meant time travel backwards.

Forwards is one thing that we know can be played with.

Things that go faster than light are supposed to go back in time according to special relativity.

[Rakaydos]

1 minute ago, Ol’ Musky Boi said:

Things that go faster than light are supposed to go back in time according to special relativity.

Not quite. Going faster than light makes flow of time Complex. As in, the combined real numbers and imaginary numbers, Complex numbers.

With the right manipulations, you can make this output a negative time flow. but it's not straightfoward negative numbers.

Edited June 20, 2019 by Rakaydos

[Ol’ Musky Boi]

Just now, Rakaydos said:

Not quite. Going faster than light makes flow of time Complex. As in, the combined real numbers and imaginary numbers, Complex numbers.

Thanks for the correction, I don't know much about relativity and all this wibbly wobbly timey wimey stuff. Hurts me brain.

[wumpus]

3 hours ago, Ol’ Musky Boi said:

I mean, if you could make FTL particles then sure, the higher the exhaust velocity the better your specific impulse. But that is a massive if.

The Isp of a photon rocket is already infinite.  Photons have momentum, but zero rest mass and thus divide by zero in the rocket equation.  On the other hand, photon rockets have a miserable energy efficiency, so aren't used at all.

[Ol’ Musky Boi]

19 minutes ago, wumpus said:

The Isp of a photon rocket is already infinite.  Photons have momentum, but zero rest mass and thus divide by zero in the rocket equation.  On the other hand, photon rockets have a miserable energy efficiency, so aren't used at all.

Theoretically, but not practically. You are going to have to lose some mass to generate power to run your lasers or whatever you're using to shoot photons out the back of your rocket. Unless you are using beamed power, as in lights sails, in which case you would have infinite Isp. I think the OP was talking about the first case.

[kerbiloid]

As the "mass" entity is just a coefficient connecting energy and momentum, and it's absolutely unclear how does this coeffecient (aka "mass") appear in real world (say, there are hypotheses like it is a measure of whole Universe attraction, or so), I would expect that any practical realization of FTL could affect the mass as well, so currently the result is absolutely unpredictable.

[cantab]

In our current physical theories, particles with real mass must travel slower than c and massless particles at c. Particles with imaginary mass must travel faster than c but they have never been observed and probably don't exist.

If you change that, well who knows what implications it has for spaceflight. Reactionless drives may be possible. (In fact reactionless drives are possible in existing physics if negative mass objects exist.)

[Spacescifi]

1 hour ago, cantab said:

In our current physical theories, particles with real mass must travel slower than c and massless particles at c. Particles with imaginary mass must travel faster than c but they have never been observed and probably don't exist.

If you change that, well who knows what implications it has for spaceflight. Reactionless drives may be possible. (In fact reactionless drives are possible in existing physics if negative mass objects exist.)

 

Yes, we are discussing and walking on untrodden ground. We cannot go where no man has gone before, but we can sure talk about it.

More than likely I am wrong about what happens to a photon once it becomes superluminal. And that's okay. I am only an aspiring scifi writer anyway.

 

The way I see it, spacetime is the box that light inhabits. Light diffuses slowly but surely across it. If the speed of light were faster, I see either one of these possibilities.

1. Superluminal light stays concentrated for much longer. Which means FTL LIDAR and laser cannon become a practical matter. Overpowered and may break many fictional popular settings likely, but so what? We are considering the implications.

2. Superluminal light diffuses (spreads) much faster across spacetime. Which means getting it to be a laser or LIDAR would be impossible even if you put your superluminal laser right on the target to be zapped.

 

3. Good for space propulsion.

 

 

 

Edited June 21, 2019 by Spacescifi

[natsirt721]

Are we considering the question 'what if light went faster' or ' what if my exhaust velocity was superluminal'?

Because the phrase 'superluminal light' doesn't really make sense.

If the former, as was mentioned you would observe the superluminal exhaust before you observed the vehicle start to accelerate. This isn't time travel, its just a violation of causality.

Check out the Causality section here:

http://www.projectrho.com/public_html/rocket/fasterlight.php

[Cunjo Carl]

I'm having the craziest deja vu with this, I swear I just wrote to someone about this but I can't find it here, so maybe it was somewhere else? Anyways, funnily enough, you actually want slower photons to make a better photon rocket.

Rocket engines convert stored energy into momentum (in context, our deltaV). If you look at how photon rockets work, you find:

Momentum Gained = Energy Used / c                 (where c is the speed of light)

Notice how we divide by c in this equation, so a faster speed of light means we actually get less momentum for a given energy we put into our photon rocket. Put in another way, it's because the speed of light is so fast that the TWR of photon rockets is so small. On the flipside, the Isp (momentum efficiency) of a photon rocket is equal to c/g0    (with the c on top). So scifi slower photons would mean worse Isp but better thrust. Because it's the thrust of photon rockets that hold them back, we want a lower speed of light for better photon rockets!

[Bill Phil]

36 minutes ago, Cunjo Carl said:

I'm having the craziest deja vu with this, I swear I just wrote to someone about this but I can't find it here, so maybe it was somewhere else? Anyways, funnily enough, you actually want slower photons to make a better photon rocket.

Rocket engines convert stored energy into momentum (in context, our deltaV). If you look at how photon rockets work, you find:

Momentum Gained = Energy Used / c                 (where c is the speed of light)

Notice how we divide by c in this equation, so a faster speed of light means we actually get less momentum for a given energy we put into our photon rocket. Put in another way, it's because the speed of light is so fast that the TWR of photon rockets is so small. On the flipside, the Isp (momentum efficiency) of a photon rocket is equal to c/g0    (with the c on top). So scifi slower photons would mean worse Isp but better thrust. Because it's the thrust of photon rockets that hold them back, we want a lower speed of light for better photon rockets!

Isp doesn’t really apply to photon rockets since specific impulse is the change in momentum per unit mass, and light is massless.

Of course you could solve for mass-energy equivalence, I guess. Or just use impulse per unit energy.

[Cunjo Carl]

2 hours ago, Bill Phil said:

Isp doesn’t really apply to photon rockets since specific impulse is the change in momentum per unit mass, and light is massless.

Of course you could solve for mass-energy equivalence, I guess. Or just use impulse per unit energy.

It does though! Because of einstein's mass-energy equivalence, any form of energy storage actually has mass. It's pretty paradoxical but when you solve for mass-energy equivalence as you say, despite the photon being massless the Isp of a photon rocket is exactly equivalent to a normal rocket shooting reaction mass out at the speed of light.

Mass-energy equivalence applies to all forms of stored energy from radioisotopes to batteries to chemical propellants. For a fun case-and-point, let's consider a photon rocket that operates by burning and holding onto 1kg of methalox, collecting the energy and using it to shoot photons out the back. What we'll find is our rocket becomes lighter by a tiny mass 'm', and our rocket gains a momentum of m*c. Again, this is the same mass efficiency as what we'd get from a normal rocket with an Isp of c/g0.

Getting started, 1kg of burning methane will look like:

CH₄ + 2O₂ ->  CO₂ + 2H₂O + 16.7MJ of heat

Assuming by magic we collect all the energy, the products of the above reaction will be lighter than the reactants by 16.7MJ/c^2  (1.9E-10kg) because of the mass energy equivalence. I think that's amazing! The chemical reaction actually changes the total mass, despite no particles ever leaving the system. Moving on to the next step, launching the collected energy out the back of the rocket as photons will provide us a momentum of:

Momentum Gained = Energy Used / c
            .056 kg*m/s = 16.7MJ / c   
            .056 kg*m/s = 1.9E-10kg*c   (alternatively)

And finally, we notice that our rocket has lost 1.9E-10kg of mass, and gained 1.9E-10kg * c of momentum! (.056 kg*m/s = 1.9E-10kg*c) Like we claimed at the start despite the photon being massless the photon rocket has the same mass efficiency as a normal rocket with an exhaust velocity of c (or an Isp of c/g0) . Crazy stuff, right?

Edited June 21, 2019 by Cunjo Carl

[Ol’ Musky Boi]

Nice math! A good photon rocket is just about propellant mass fraction then, in which case you're not going to do much better than a quasar drive...

Spoiler

 

[Cunjo Carl]

47 minutes ago, Ol’ Musky Boi said:

Nice math! A good photon rocket is just about propellant mass fraction then, in which case you're not going to do much better than a quasar drive...

  Reveal hidden contents

 

A quasar drive would really take the cake for drymass, but the Isp should be right up there! It's funny to think for how desperate rocket engineers are to shave every kg off their rocket, that this one caries an entire black hole.

[Spacescifi]

4 hours ago, Cunjo Carl said:

It does though! Because of einstein's mass-energy equivalence, any form of energy storage actually has mass. It's pretty paradoxical but when you solve for mass-energy equivalence as you say, despite the photon being massless the Isp of a photon rocket is exactly equivalent to a normal rocket shooting reaction mass out at the speed of light.

Mass-energy equivalence applies to all forms of stored energy from radioisotopes to batteries to chemical propellants. For a fun case-and-point, let's consider a photon rocket that operates by burning and holding onto 1kg of methalox, collecting the energy and using it to shoot photons out the back. What we'll find is our rocket becomes lighter by a tiny mass 'm', and our rocket gains a momentum of m*c. Again, this is the same mass efficiency as what we'd get from a normal rocket with an Isp of c/g0.

Getting started, 1kg of burning methane will look like:

CH₄ + 2O₂ ->  CO₂ + 2H₂O + 16.7MJ of heat

Assuming by magic we collect all the energy, the products of the above reaction will be lighter than the reactants by 16.7MJ/c^2  (1.9E-10kg) because of the mass energy equivalence. I think that's amazing! The chemical reaction actually changes the total mass, despite no particles ever leaving the system. Moving on to the next step, launching the collected energy out the back of the rocket as photons will provide us a momentum of:

Momentum Gained = Energy Used / c
            .056 kg*m/s = 16.7MJ / c   
            .056 kg*m/s = 1.9E-10kg*c   (alternatively)

And finally, we notice that our rocket has lost 1.9E-10kg of mass, and gained 1.9E-10kg * c of momentum! (.056 kg*m/s = 1.9E-10kg*c) Like we claimed at the start despite the photon being massless the photon rocket has the same mass efficiency as a normal rocket with an exhaust velocity of c (or an Isp of c/g0) . Crazy stuff, right?

 

So lemme get this straight.

A mass conversion rocket that converts it's fuel mass 100% into slower than light photons would have rocket thrust but expend the fuel mass at chemical rocket rates?

Which means why even bother?