Is near light speed achievable?

[JaredJames]

so we can't really imagine a realistic machine that could send a macroscopic object to such speeds

See above discussion concerning the "starwisp".

I may not like it, but it's certainly possible for getting a 1km array up to near relativistic speeds.

 

[jambaugh]

Getting 50,000 tons into space and then the construction of said materials for a 50,000km emitter lens is considered "reasonable engineering"?

What was the cost per kg to get stuff into space?

Depends on where you find it. If I were lord and master of the Earth and determined to build the thing, my first stage would be to establish a base on the moon and set up materials production. The "lens" is a wire mesh, not a big hunk of plastic. Getting 50,000 tons into space is a matter of lifting 5000 10ton spools.

But I challenge you to suggest a cheaper way to get a probe to the nearest star. Again I said "most" viable, not "super-easy-cheap-why-didn't-Columbus-just-bypass-America-and-go-to-Alpha-Proxima" viable.

 

[jambaugh]

Here's some math for the curious...

Figure the sustained acceleration you need to reach a given % of c (including over 100%) in simple Newtonian physics then take the hyperbolic tangent. That's the relativistic % of c.

Example you boost to what would be c in Newton's universe, that's about 8500 g-hours (almost a year accelerating at 1 g) and you will get to tanh(1)= (about) 0.76 or 76% c as seen by the people watching from your starting frame.

To figure the energy you need, you would take the hyperbolic cosine minus 1 times the mass (the minus 1 subtracts out the initial mass-energy). E.g. cosh(1) = 1.543 so you need about 54% of the payload mass times c^2 in energy to get up to 76% lightspeed.
That's about 48,900 TeraJoules per kg or 48,900 Tera-watt seconds per kg payload...
or 13.6 TW hours per kg or about 19 GW months.

Figure the output of a few industrial scale power plants sustained over a month to accelerate at 10 or 12 gees per kg payload.

That's delivered to the payload. With a beamed source (light or microwaves or whatever) one must figure one or two factors of 10 for losses and beam spreading.

It's not unthinkable but certainly would require we keep the payload size under say 20kilograms and a major industrial presence in space. And .76c may be too ambitious, more like .01c or .005c. Something we could imagine happening in the year 2200. I mentioned we wouldn't need to assume new tech (like cracking fusion) but we'd probably need orbital fusion power plants to get that kind of power output or some other energy production improvement.

 

[sr241]

Hi sr241,
First note that according to relativity, no matter how fast the rocket goes, the people inside always see light going at lightspeed (the constant c). This is elementary relativity.
_

c is speed of light in vacuum; that means light slows in a medium like air and water. so in air and water speed of light is going to be less than c, how can you correlate this with the said principle (speed of light is constant).

 

[JaredJames]

Well jambaugh, I concede the point to you.

As un-realistic the prospect of the starwisp appears to me, it really is one of the most viable methods of getting to the nearest stars.

I didn't realize just how 'bad' current propulsion systems were. They're really not up to much when it comes to interstellar travel.

I suppose the scale of the starwisp is a good demonstration of what it takes to get to the nearest star. Even if it is only in a very basic way.

 

[DaveC426913]

c is speed of light in vacuum; that means light slows in a medium like air and water. so in air and water speed of light is going to be less than c, how can you correlate this with the said principle (speed of light is constant).

As he said, this is elementary.

Speed of light in a vacuum is a constant, regardless of your frame of reference. If you want to get into propogation of light in a medium, that's a whole different kettle of fish that's got nothing really to do with this topic.

 

[sr241]

I think worm holes are not possible. If somebody in future does make worm holes it is again possible to bend time and time travel will be possible.

Since we don't see any time travelers around us. in the entire human future nobody has made a time machine. so science of relativity requires an entire revision.

 

[JaredJames]

I think worm holes are not possible. If somebody in future does make worm holes it is again possible to bend time and time travel will be possible.

Worm holes 'fold' space, not time.

Since we don't see any time travelers around us. in the entire human future nobody has made a time machine. so science of relativity requires an entire revision.

This is a flawed line of thinking. There are many possibilities out there which allow for time travel in spite of what you have said. For example, one hypothesis says we can only travel back to the point the time machine is turned on. So until we invent and turn on a time machine, we won't see evidence of time travel.

Again, this thread isn't the place for these topics.

 

[jambaugh]

Well jambaugh, I concede the point to you.

As un-realistic the prospect of the starwisp appears to me, it really is one of the most viable methods of getting to the nearest stars.

I didn't realize just how 'bad' current propulsion systems were. They're really not up to much when it comes to interstellar travel.

I suppose the scale of the starwisp is a good demonstration of what it takes to get to the nearest star. Even if it is only in a very basic way.

Yea, this is why I'm really really really skeptical of claims of extraterrestrial visitors.

 

[mugaliens]

Here's some math for the curious...

I doubt it was the most efficient, but Tsar Bomba was the largest thermonuclear (fusion) weapon produced to date, so it might be the start of an interesting comparison:

Mass: 27,216 kg
Yield (design): 420 PJ

Yield/kg: 1,543,266,580,930 J/kg, or 1.5 TJ/kg

I think that's saying something concerning the feasibility of using onboard propulsion, even fusion, if we don't develop something radically different than our best designs to date.

Assuming 50% of the energy of that detonation were able to be converted into accelerating a 1 kg payload, just how fast would 0.75 TJ accelerate 1 kg!

My math's a bit rusty, but ignoring relativistic effects for the moment, just to see where in the ballpark we land, v=sqrt(2*Ek/m), so that's sqrt(2*0.75 TJ/1), or 1,242,282 m/s, which remains 0.4% the speed of light.

I would think the actual useful conversion would be closer to 1%, rather than 50%, unless we figure out a way to use fusion to obtain relativistic velocities in propellant mass over a long time. Even http://en.wikipedia.org/wiki/HiPEP" exhaust velocity is microscopic by comparison.

Even so, a suitable habitat for just one person traveling to the nearest star would have to be as large as the ISS, close to 400,000 kg of mass, so...

...ouch, my head hurts. Don't think we're going to make it, not in my lifetime. Or my great-grandkids'.

 

[jambaugh]

Even so, a suitable habitat for just one person traveling to the nearest star would have to be as large as the ISS, close to 400,000 kg of mass, so...

...ouch, my head hurts. Don't think we're going to make it, not in my lifetime. Or my great-grandkids'.

I've often wondered if the means to travel (in person) to nearby stars would require techniques of energy and material manipulation which would render us (or whoever) independent of planetary living and even of reliance on stars.

Here's another thought...(or plot component for good SciFi novel) suppose that high energy cosmic rays were actually specks of reaction mass from one of the many interstellar craft plying the heavens.

 

[sr241]

if we were able to recover energy from retarding phase like a regenerative breaking in cars, would it be a more energy efficient way to travel in space. what are the hindrances in such regenerative breaking in a reaction type space craft?

 

[JaredJames]

if we were able to recover energy from retarding phase like a regenerative breaking in cars, would it be a more energy efficient way to travel in space. what are the hindrances in such regenerative breaking in a reaction type space craft?

The key here, is regenerative braking. Cars aren't recovering the energy and then using it to go faster. They don't accelerate to 10mph, recover energy (without slowing down), accelerate to 20mph (using that energy) and so on. They accelerate to 10mph, brake and slow down, and then can use any recovered energy to help get them back to 10mph.

They take the energy, store it, and then use it again later. Regenerative brakes don't make cars faster. The kinetic energy gained during acceleration is converted to a useful form during braking as opposed to heat. You can't have that kinetic energy and recover it at the same time.

So before this goes further, I can see you are thinking of using it with a mass recapture system as you proposed before.

The way a spaceship 'brakes' isn't the same as a car.

 

[sr241]

i mean spaceship accelerates to a certain speed say 50% of c and cruise on that speed and when necessary to stop at a destination then regenerative breaking is used, then would it be a much more efficient way to travel in space

what are the current hindrances in using a regenerative breaking in reaction type rocket engine, like in ion thrusters?

 

[russ_watters]

The hinderance is that rockets don't have brakes!

Now if space travel ever gets past the exploration stage and into the transportation stage, you could launch rockets railgun style and do mag-lev train style regenerative braking. But clearly, that requires infrastructure installed at each "station".