Guys, the problem here is that people are not identifying their reference frames. Jeff Root is in the frame of the ship, Faultline is in the frame of the Earth. Half of what you guys are saying is not even in contradiction! Example, the time it would take to get there is very different in the two frames, as I was well aware when I said that if you can convert your rest mass to energy, there is no limit on how quickly you can get anywhere (except of course the formidable technological challenge). I think this is, however, far more practical than "warping space and time", which is patently absurd as a practical means of space travel (where does the gravity come from?).

The time it takes to get there is instant for anyone who can reach lightspeed. But under the laws of physics, they still have to apply more and more force to continue accelerating as they approach the speed of light. Under 50% of lightspeed the effect is insignificant, but it continues to grow exponentially.

IIRC, frame of reference matters to the passage of time, but not to the force required to accelerate a body of mass to relativistic speeds.

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Here's something I find interesting. If you go closer and closer to light speed, and you get to the point where you need exponential growth of energy...

Wouldn't you need about that same amount of energy to STOP, as well?

A sudden stop would be devastating

Asimov went into some problems with going 0.6C speed, though. It doesn't have to do with energy, but instead, radiation. He says that cosmic radiation would go straight through the hull of ship (unless it was a very very thick hull).

The reason why is the same way as if you throw a rock at a vehicle going 100 MPH, you cause more damage than if it was sitting still.

__________________
There are few left who
Stare at the skies with wonder
Wishing to know more;
The clouds still drift by above
But the eyes below are blind.
--Laura Lundberg

Check out my writing, maybe.

Yes, since mass increases with velocity, it takes equal amounts of force to speed up or slow down. Acceleration and deceleration are the same thing to a physicist.

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My son is my universe.

That's a good point, stopping is a problem. The issue is not that you need to convert energy back to rest mass, if we are imagining advanced technology you could perhaps figure that we can go either way with rest mass. But the momentum you need to lose is going to be a nasty challenge. So I think the theoretical maximum efficiency would require that you first convert your rest mass to energy, and part with half of it to get the initial momentum you need, but then you have to part with virtually all of what you have left when you try to get rid of that momentum. So the payload must be a tiny fraction of the total mass of the original ship, and the original ship ends up as light beams going in both directions. But it wouldn't be that hard if you could convert rest mass to energy and back again (if!). Also, you'd have to keep the acceration down to a few g presumably, so it would take a while to make the trip for that reason. It would be most efficient for traveling across the galaxy, if you could solve the problem of...
I have a hard time seeing that this is a problem with "radiation", which is a term normally reserved for relativistic particles that wouldn't much care if you were going at 0.0001 c or 0.9 c. But dust would presumably be a huge problem, even at the lower speeds.

As for the force and the acceleration, these are also slippery topics that require careful definition of what is meant and in what frame. For example, an object that we on Earth think is moving at 0.99c toward alpha Cen will think that alpha Cen is moving at 0.99c toward it. If it applies a force that gives it 1 g of acceleration, on Earth we will barely perceive any change in its speed, since it's already so close to lightspeed, that we will instead see a change in momentum due to a change in relativistic mass. But the object will not only perceive itself to have its rest mass, not some huge relativistic mass. So for it, the force does go into changing its speed. But this is a speed measured relative to other objects that seem stationary in its own frame. It will not see alpha Cen approaching much faster than 0.99c, that's the way the velocity addition formula works. However, it will length-contract the distance to alpha Cen, which is a whole other matter, and is what Jeff Root is talking about.

Then either you or Asimov are wrong, or I read it wrong. I doubt Asimov's wrong, though, he's a genius.

Actually, I'm kidding. There've been a few times I've disagreed with Asimov's ideas and concepts, and have found a few things in error. So it's possibly Asimov got it wrong or didn't know enough about the issue at the time.

__________________
There are few left who
Stare at the skies with wonder
Wishing to know more;
The clouds still drift by above
But the eyes below are blind.
--Laura Lundberg

Check out my writing, maybe.

That is not quite what I said! I took pains to carefully
describe the difference between what you see, aboard the ship,
and what an observer not sharing your acceleration sees.

To reiterate: You accelerate at 1 g for as long as you want.
I don't know how you do it, because it is technically absurd,
but there's nothing theoretically wrong with the idea. If, as
I suggested, propellant magically appears in your fuel tank at
the instant it is needed, then the fuel mass is constant.

You feel normal gravity from the acceleration. As your speed
relative to nearby objects gets closer and closer to the speed
of light, the Universe appears to shrink in the direction you
are moving. So the distance you have to travel to reach your
destination continues to shrink as you continue to accelerate.
The longer you accelerate, the more squished the Universe is,
relative to you.

You do not see any change in your mass or the mass of your
spacecraft. Your acceleration does not change. You do not
need to change your thrust to compensate for anything.

I, watching from the sidelines, not sharing your acceleration,
see you getting closer and closer to the speed of light, but
never reaching it. The kinetic energy of your mass relative
to me becomes enormous.

My grandchildren see you reach your destination, 50 light-years
away, when you are still middle-aged. Their grandchildren see
you return from your trip while you are still young enough to
enjoy a couple of decades of retirement.

It was close, but not close enough.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

What cut-rate school of magic did you attend?

Everyone knows that there are specific limits on what magic
can and can't do. For example, the Wicked Witch of the West
had a magic broom that was quite compact and lightweight, but
which could carry her any distance at high speed, with no need
for fuel. In a different reality, with a different kind of
magic, Superman and (my personal favorite) Supergirl are able
not only to leap tall buildings in a single bound, but fly
through the air, through Space, or through the Earth at almost
unlimited speed for who-knows-how-long, with no apparent need
for pit stops. But they can't just click their heels together
and find themselves in a different location. At least not
without a decent pair of silver shoes. Or ruby, in the movie
version.

That misinformation is precisely what I was saying is wrong.

If you want to accelerate/decelerate toward your target, and
maintain a constant 1 g artificial gravity onboard your ship
by means of that acceleration, then your thrust must decrease
as the fuel is used up. The proper mass of the ship does not
increase as its speed relative to local objects increases.

The term "proper mass" is standard relativity terminology.
Your proper mass is the mass you measure yourself to have.
My proper mass is the mass I measure myself to have. If you
tell me your proper mass, and I measure how fast you are moving
relative to me, I can calculate what your co-ordinate momentum-
energy must be-- relative to me. I can confirm that calculation
by allowing you to smash into a block of iron, and measuring the
energy released as heat. If your speed relative to me is close
to c, your nuclei will collide with the iron nuclei, scattering
gamma rays, muons, and radioisotopes everywhere. It would be
a proper mess.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Jeff Root, I think you are taking this too personally.
Superman, and Supergirl are fictional charactors. But their super powers are not based on magic, but science. I once saw a book in the Sci-fi section of Barnes and Noble Bookstore, entitled 'The Science Behind Superman'.

I don't clearly remember all the details because it's been years since I saw it, and even then only glanced through it at the time.
Superman and Supergirl were from a planet called Krypton.

Inhabitants from Krypton are able to fly because of a combination of scientific (but fictious) factors. Our more dense atmosphere, much lighter gravity and yellow Sun giving more energy and strength then their red dwarf sun. Kryptonians have much denser body structure with greater muscle mass, and anti-magnantic properties. Their vision acusy is greater due to lower light level of a planet once orbiting a red dwarf, which becomes super on Earth.

So I am to understand that the long known laws of relitivity are wrong, and your personal opinion is correct?

To reword your own quote, What cut rate school of Science did you attend?


This is true of much slower Earth bound velocities, but in space getting near the speed of light, from the perspective of local (but outside) observers, the proper mass of the ship does indeed increase greatly the closer it gets to the speed of light.

Now an onboard crew or observers won't know the difference, except the 4 1/3 year trip to Alpha Centauri would only take the amount of time of a single afternoon from their perspective. Let's say they stay for 6 months to explore, then return to Earth.
To the crew on board the ship, the whole mission took 6 months and two days. But nine years will have elapsed on Earth.
These facts are not new. but have been known physics for longer then either of us have been alive.

But now you are saying that physics are wrong and you are right.
Next you are going to tell me that Einstin and Hawkings are wrong because they attended the wrong cut rate school of magic.
Now that is what I would call a proper mess.

__________________
Thanks for turning down the volume. Now I can hear the voices in my head again.

There's no such thing as proper mass from the perspective of people who are not moving with the ship, unless they are simply calculating what the people moving with the ship see, in which case they'd get the normal nonrelativistic answer. Proper mass is the same thing as rest mass, as I understand Jeff Root's meaning. By the time the ship reaches relativistic speed, the proper mass will be enormously smaller than it was when the ship started its trip (by virtue of being ejected in the thrust after as much as possible was converted to energy). This is very challenging technologically, since it means you need to achieve a few gees from what was initially a very heavy ship.

Actually, I don't think what Jeff Root is saying is controversial, he's just making the interesting point that relativistic flight is generally described from the point of view of the people staying behind. In the frame of the ship, it takes on some surprising elements that make it sound generally less impossible. What is being disputed by Planetwatcher is actually a misunderstanding of the meaning of acceleration as being used by Jeff Root, as I mentioned above. Planetwatcher means the rate of change of the relative speed between the ship and the Earth in the Earth frame, while Jeff Root is in the frame of a hypothetical object moving at the instantaneous speed of the ship, where both time and the change in speed of the ship are very different from what is seen from Earth. Planetwatcher and Faultline are not recognizing this alternate definition of the acceleration, and it is leading to a lot of confusion and even some consternation, but I trust everyone will be nice!

The problem with radiation is caused by impacts with individual hydrogen molecules at interstellar speeds (0.1 to 0.3 c; not relativistic by most definitions of the word); the impact is supposed to be energetic enough to convert the protons into radiation. Also the less frequent but more damaging dust impacts will also release much of their energy as radiation as well. This is why I always put big shields on the front of my imaginary ships...

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Ah, that explains it, it's not cosmic radiation it's radiation caused by the impact. Thanks eburacum45 for the clarification. Asimov fan? We'll miss him.

Thanks for clarifying!

I stand behind my statement that some of the things Asimov believed were pretty much... erm... questionable. Mainly in the area of psychology. I still love him, though. Asimov rocked.

__________________
There are few left who
Stare at the skies with wonder
Wishing to know more;
The clouds still drift by above
But the eyes below are blind.
--Laura Lundberg

Check out my writing, maybe.

I don't know anything about his psychological theories, or do you mean attitudes that emerged in his fiction? What are some of the more outlandish ideas that he had?

Oh yeah?? Tell them that face-to-face!

Most accurately, the Superman mythos is fantasy. It has many
elements of science fiction, but it isn't based on science.
There is no boundary between fantasy and science fiction.
The Superman mythos clearly falls within both domains. But
since the super powers are fundamentally inexplicable and in
conflict with reality, they are indistinguishable from magic.

However, a strong argument can be made for the influence of
science on the fantasy, even from the beginning-- much like
Mary Wollstonecraft's 'Frankenstein' story.

Superman creators Siegel and Shuster were heavily influenced
by Philip Wylie's 1930 novel 'The Gladiator', about a superman
named Hugo Danner. Danner had powers very similar to those of
the early Superman, but he wasn't from another planet--he was a
human being biochemically altered by his scientist grandfather.

I suspect that everyone born in the USA since the 1930's knows
that. Superman is probably more widely-known and identifiable
than just about any other person or character, real or imagined.
Right up there with Moses, Jesus, Julius Caesar, Mohammed, and
Abraham Lincoln.

Actually, the rationalizations for the various super powers
have changed many times over the decades. Often when a new
writer takes over, some of the fundamental ideas change.
Sometimes old ideas just don't make sense anymore, or the
writer just comes up with a new idea, and incorporates it
into a story.

"Anti-magnetic properties." Hmm. Is Superman a superconductor?
Give him a good orchestra for a few days, and we can find out!

No, you are to understand that you haven't described the the
long-known laws of relativity correctly.

No. You used the term "proper mass" because I used it. But you
didn't use it correctly. The proper mass of a thing is the mass
measured by an observer co-moving with that mass. An observer
not co-moving with the mass measures its co-ordinate mass, or
relativistic mass.

Similarly, a clock shows passage of proper time in the clock's
inertial frame, but co-ordinate time, or relativistic time, to
an observer not co-moving with the clock.

The figure of "a single afternoon" is way, way off. I could
probably calculate the correct number, and I'm sure you can find
it on the Internet, but I would estimate that a spacecraft
accelerating at 1 g halfway to Alpha Centauri and decelerating
at 1 g the second half of the trip would take about five years
in the crew's proper time. The more distant the destination,
the greater the time-saving for the crew. It would be possible
to reach the Andromeda Galaxy in a single human lifetime if the
spacecraft could maintain a constant acceleration of 1 g.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

I agree that the major savings would only be for very long flights, if you were restricted to a few "gees". You'd need some kind of "inertial dampers" to counter the g effects, perhaps some type of artificial gravity, though I don't see how you'd do it. So that may be the worst limitation for travel to nearby stars, not the relativistic issues.

Not allowing for deceleration, the journey times as measured on-board ship when accelerating at one gee are given below;
Alpha Centauri 2.3 years
Centre of Milky Way 11 years
Andromeda 15 years
Edge of observable universe (defined as a point 13.7 billion light years away) 25 years

so if you could build a craft which accelerated continuously for years on end, then you would be able to go literally anywhere in your lifetime.
However to do this you would need practically all the mass of the universe converted into fuel and propellant.

Unless...
if a reactionless drive were possible then there would be no need for propellant.
Another idea is putting a wormhole in your fuel tank, allowing fuel to be pumped from a stationary source continually into your tank. This would mean you would not need to carry alll the fuel for your voyage with you, cutting down on the amount of mass that needs to be accelerated to an incredible extent.
This concept has a few problems, though; one being the likelihood that a traversible wormhole will possibly mass the same as a comparable black hole, so in order to allow the flow of a respectable amount of fuel the hole will have to mass as much as a large asteroid or small moon. This would mean a corresponding increase in the fuel requirement, leading to a corresponding increase in the diameter of the hole, which needs to be even more massive...

depending on the fuel used this wormhole-refueling arrangement would need to be extraordinarily massive if it is to acheive a continuous 1 gee of acceleration.

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How long would it take for an alein to get here, ?
We could not know the answer to this. Becouse,.
Are there any ?
Would they want to come here ?
They might have been.
We might be the ansectors of them.
There are none, to come.
What was the question?

Okay, I used the term proper mass because Jeff Root did. Actually his useage was the first time I ever heard of it.
The onboard ship time of one afternoon was one I ran accross long ago, and not associated with a 1 G acceleration, but the notion of achieving 99% of light's velocity right away, and asuming the slow down could be also right away.
I believe those calculations were done without considering the time and distance of acceleration, or slow down.

Throwing in the 1 G. acceleration requires a lot more math figureing then the more simple passage of time from the ship's perspective assuming maximum speed is achieved right away. However, it is also more realisticly achievable then a jump to near light speed, and a jump back to conventional speeds.

However Jeff Root is still appearing to take some of this personally. No one here is attacking persons, but the ideas only. As Ken G. suggested let's keep it nice.

I won't dispute where the idea of Superman came from, but saying something not immedatley explained is magic is putting an awful fast label on it.
If I were to visit a Pigmy tribe and flick a bic lighter, they would declare it as magic, but we know it's not.

I know an incadecent light bulb uses scientific principles. I couldn't fully explain how it works but that doesn't make it magic. Just unexplained.
Call Superman mythos whatever you want. But it is still fiction, because it is not true to fact. The means of his powers is also fiction. However it does utilize some kind of science, even if that science is fiction as well.
The "Anti-magnetic properties," as explained in the book seemed more discriptive of anti-gravitational properities, but it never called it such.
Quite likely because anti-gravity would make it impossible to land, or to approach another object with mass, such as a building, or car, or airplane.
That is what the book called it.

E=MC2 is the great equation, taken a number of different ways. Most simply put. Energy is the result of a mass'es sub-atomic particles moving at the speed of light times itself. It says nothing about converting part of it's mass into thrust energy as the speed of light is approached, and it says nothing about arriving at a distant destenation nearly instantanously.

I'm going to use another post for the rest of this matter, it is quite lengthy.

__________________
Thanks for turning down the volume. Now I can hear the voices in my head again.

__________________
Thanks for turning down the volume. Now I can hear the voices in my head again.

Thanks for these numbers, I haven't verified them but they gibe with the expectation that constant acceleration is best for very long-distance travel. Also, you make the key point that the longer you want to maintain that acceleration, the more mass you have to start out with so you can convert it to energy. Most likely, relativistic travel rapidly becomes unfeasible because of this initial mass problem. Thus Jeff Root's basic point that it is not the increasing relativistic mass that is the real problem as seen in the reference frame of the ship, there is nevertheless a problem with the ratio of payload mass to fuel mass. That ratio should scale with the Lorentz factor you ultimately achieve, which is basically the ratio of the number of light years of the locations in eburacum45's calculation, to the calculated elapsed times. (so you need about 5 billion times the mass of your payload to cross the universe in a lifetime, if my figuring and eburacum45's calculations are correct. But only a thousand times to get across the galaxy, if you have the energy conversion mechanism, and that might not be so much different from conventional rockets?)

As a point of information; I didn't actually do the calculations myself- anyone who has encountered my own essays into mathematics will tell you I am generally at least an order of magnitude out. These figures came from Michael Clive Price; but they agree with other similar figures elsewhere on the 'Net.

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Pragmatically speaking, acceleration at one g will attempt to move your ship faster than the speed of light after only a few years but actually only increases its relativistic mass as its speed increases imperceptibly. Navigation and obstacle dodging become increasingly difficult as the speed nears that of light so 0.8c might be as fast as one would care to go, so trans-MW and transuniverse times may actually take much more time as measured on the ship.

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

The mass business was a key element of this thread, and the answer looks a lot different in the frame of reference of the ship. Nevertheless, speaking practically, we are all in agreement that the two biggest obstacles are how fast you can accelerate, which limits the value of trying to move relativistically if you are going to fairly nearby stars, and how you can avoid being destroyed by space particles. Putting these together, it certainly appears that interstellar travel would be fairly nonrelativistic and so would likely take decades at least. Looking on the bright side, there would not be so much trouble with the time being different for the folks left at home!