Some ideas on interstellar space travel

[elcaro]

Science fiction is of course full of all kind of futuristic ideas about interstellar space travel and ways of propulsion, some more physically plausible then others.

But within the current realm of what is physical possible, what could interstellar space travel be like?

First you need a source of high energy propulsion that emits the most energy per kilogram of weight. So either nuclear fusion or anti-matter.

Second, the mission profile for a trip to let's say alpha centauri would be half the way accelerate at 1g, then decelerate with 1g. So, total trip time for a journey of 4,37 light years would be 3,58 years (on Earth it would last 6 years).

The ship, since traveling at high speeds we will encounter interstellar dust and/or particles, we need shields to deflect particles, and the ships would need a needle design. At half way the thrusters/engines need to be pointed in the opposite direction while the ship itself still facing forward.

The mission would not exist of only one ship but a small fleet of ships. It would be best to have an unmanned ship (or ships) before each manned ship, equipped with strong shields to clear the path from whatever particles might be on our trajectory through space.

At regular intervals small satelites will be deployed. Their function would be:

1) the earlier ones will serve as relay stations for contact with earth, and for exploring interstellar space.
2) the later ones (deployed near the top speed of the vessel approx. half way into the journey) will explore the solar system we are aiming for as they will arrive there before the ship does.

 

[BWV]

Possible, but feasible?

you can play with this calculator
https://spacetravel.simhub.online/

accellerating and decelerating a 25K kg ship to Alpha Centauri would take around 120 billion Kg of fuel

so likely need something more efficient than fusion

 

[elcaro]

Possible, but feasible?

you can play with this calculator
https://spacetravel.simhub.online/

accellerating and decelerating a 25K kg ship to Alpha Centauri would take around 120 billion Kg of fuel

so likely need something more efficient than fusion

Suggestions?

 

[BWV]

Suggestions?

Warp drive?

as a long time SF fan, have come to the depressing conclusion that manned interstellar space travel may well be an intractable problem

 

[Filip Larsen]

I am still inclined to think that if humans ever is to travel to other star systems it is more likely to happen as "seed ships", i.e. slow moving automated ships that transport "embryonic" humans to be "grown" at the destination only if the automation is able to establish a suitable environment. With the production systems such a feat requires, a von Neuman ship type (i.e. replicating) ship is not far off and will vastly increase the chances that humans can get more chances at more stars. Of course, replicating seed ships also requires technology such that embryos or similar "source material" can be infintitely replicated/reconstructed, and I'm not sure how feasible that is.

On top of this, even when just aiming to colonize the nearest stars its hard to see what the purpose such a project would aim for (more or less independently of mode of transport). With humankind's current track record of (lack of) social stability its hard to see humans as an interstellar coherent civilization. Once sustainably seeded, a colony is independent of Earth and even with information round-trips as low as 10 years or so it is difficult to imagine social coherence between Earth and any colonies can be maintained for very long. The only purpose I can see fit this if the colonies are to be humankind's last hope if Earth is facing a solar system wide catastrophe.

I'm with BWV in being pessimistic about the feasibility of interstellar travel in order to bring humans to other star systems. The complexity ratio between manned and unmanned mission seems to grow exponentially with "distance", so I would say it is orders of magnitude more likely that the first interstellar mission, if its every done, will be fully automatic, i.e. not manned. Compare, for instance, with the Breakthrough Startshot project which appears to be not that unrealistic even if serious challenges remain - not so with any idea of a manned interstellar mission I have so far heard.

 

[Melbourne Guy]

So, total trip time for a journey of 4,37 light years would be 3,58 years

Is that right? Surely it's more years travel time than the distance in LY

 

[Rive]

Please note that 4.37LY is the 'standing' distance. If you move, you will see it shorter.

 

[Melbourne Guy]

Please note that 4.37LY is the 'standing' distance. If you move, you will see it shorter.

Thank you, @Rive, that's Einstein's relativity, I presume. But does the OP's 3.58 year trip assume the ship has accelerated to very close to light speed? And doesn't it take the better part of a year to get close to lightspeed if your acceleration is 1 gee. Then the same at the other end to slow down?

Basically, I don't have the math, but 3.58 years of ship time in the scenario seems too quick? Or is the shorter distance about two years worth of travel at close to light speed?

 

[Filip Larsen]

I don't have the math, but 3.58 years of ship time in the scenario seems too quick?

It's more or less what the proper time aboard a rocket accelerating at 1 g, with a flip-over in the middle, when it has covered the original distance (as measured before or after the trip) of 4.7 ly. You can find an example of the derivation and some tables and discussion at https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html

 

[elcaro]

Is that right? Surely it's more years travel time than the distance in LY

No, since we accelerate at 1 g we will reach relativistic speeds, so on board there is time dilatation (time going slower on board then stationary observers on earth).

 

[PeroK]

No, since we accelerate at 1 g we will reach relativistic speeds, so on board there is time dilatation (time going slower on board then stationary observers on earth).

Time dilation is symmetrical. You could equally say that time is going slower on Earth than in the rocket.

 

[elcaro]

Time dilation is symmetrical. You could equally say that time is going slower on Earth than in the rocket.

But the acceleration breaks the symmetry. The shortening effect of this kind of spacetravel is standard textbook theory.

 

[PeroK]

But the acceleration breaks the symmetry. The shortening effect of this kind of spacetravel is standard textbook theory.

The acceleration does indeed break the overall symmetry. Suppose, however, we had a row of clocks from the Earth to the destination. The crew on the rocket would measure each of these clocks to be running slow as they passed it: according to the relevant $\gamma$ factor at that point.

 

[BWV]

Thank you, @Rive, that's Einstein's relativity, I presume. But does the OP's 3.58 year trip assume the ship has accelerated to very close to light speed? And doesn't it take the better part of a year to get close to lightspeed if your acceleration is 1 gee. Then the same at the other end to slow down?

Basically, I don't have the math, but 3.58 years of ship time in the scenario seems too quick? Or is the shorter distance about two years worth of travel at close to light speed?

At constant 1G acceleration you could get to Andromeda Galaxy in about 28 years

https://www.omnicalculator.com/physics/space-travel

 

[Freixas]

The acceleration does indeed break the overall symmetry. Suppose, however, we had a row of clocks from the Earth to the destination. The crew on the rocket would measure each of these clocks to be running slow as they passed it: according to the relevant $\gamma$ factor at that point.

I know this is an old thread, but I don't think this is right.

I used my Minkowski diagram generator tool to model what I think you said. I assumed that the Earth, the clocks, and Alpha Centauri all have a velocity of 0 relative to the rest frame. I also assumed all these entities have synchronized clocks (from the point of view of the rest frame).

So here's what I come up with:

The blue curve is the traveler's worldline. The red line is Alpha Centauri's worldline. The green dashed lines are the clocks' worldlines.

The green times are the times on the clocks when the traveler passed by. The blue times are the times on the traveler's clock. The crew on the rocket would not measure each of these clocks to be running slow as they passed it, but I may have misinterpreted your statement.

The cyan lines represent the traveler's line of simultaneity at the first and last clock. Going by these, the traveler would consider Earth time as running slow. However, as the traveler's relative velocity reaches 0, the line of simultaneity would sweep the Earth's worldline very quickly so that when the traveler reaches Alpha Centauri, the line of simultaneity would cross Earth's worldline at 5.999.

Anyone interested in the software I used can download a copy at https://github.com/freixas/gamma/releases.

 

[PeroK]

The crew on the rocket would not measure each of these clocks to be running slow as they passed it, but I may have misinterpreted your statement.

You must have misinterpreted something. The clocks all have motion relative to the rocket, so must be measured to run slow by the rocket's crew as they pass each.

 

[Freixas]

You must have misinterpreted something. The clocks all have motion relative to the rocket, so must be measured to run slow by the rocket's crew as they pass each.

How does the rocket's crew determine that the clocks are running slower?

When they pass by one clock, I assumed that they grabbed the clock's reading as well as their own clock's. Then they wait for the next clock and calculate the delta of each pair of clock readings.

My diagram doesn't include the delta calculation. I'll do one here: the first two clock readings are 1.104 and 1.714 for a delta of 0.61. The traveler's corresponding tau readings are .946 and 1.294 with a delta of 0.348. So the rocket's crew, using this method, determines that their clocks are running slower.

Let's use a simpler case. We have two points 4 LY apart with a relative velocity of 0 and synchronized clocks. We place equally spaced clocks all along the route between the two points, all clocks also with a relative velocity of 0 and also synchronized with the two end points. A traveler passes by the starting point at 80% the speed of light. and sets his clock to 0 as he goes by.

There's no acceleration in this version, so it should be easier to do the calculations. If the traveler uses the clocks in the manner stated above, he will determine his clock is running at 60% the time given by the clocks he encounters. This should be obvious just by looking at the endpoints. The ending traveler time is 3; when the traveler arrives, the end point time is 5.

This is not to say that the traveler can't determine that the other clocks are running slower. All I'm saying is that he can't do it by using the method that I assumed you meant him to use.

Since you never explicitly stated how the clocks on the route would be used, I may certainly have guessed wrong. Perhaps you could clarify?

 

[PeroK]

How does the rocket's crew determine that the clocks are running slower?

By measuring its tick rate compared to their own.

When they pass by one clock, I assumed that they grabbed the clock's reading as well as their own clock's. Then they wait for the next clock and calculate the delta of each pair of clock readings.

The clocks are not synchronised in their frame. Hence that method is not valid to measure the rate of either clock.

 

[Freixas]

By measuring its tick rate compared to their own.

The clocks are not synchronised in their frame. Hence that method is not valid to measure the rate of either clock.

I agree that the clocks are not synchronized relative to the traveler's frames (well, except at the start and end of the trip).

I'm trying to picture what method you had in your mind. Let's say the traveler becomes co-located with one clock exactly at the point that it begins a tick. When the tick ends, the traveler will no longer be close to the clock. How does he measure when the tick finished so he can compare it to his own clock? And couldn't whatever method he uses just as easily be applied to a clock on Earth? How does scattering clocks along the way help?

Time dilation is symmetrical. You could equally say that time is going slower on Earth than in the rocket.

I went back to your original statement.

You could say that, if you were to look at an instant in time for the traveler, he is in an inertial frame such that he could view Earth's time as running slower, but it's tough to talk about "slower" in reference to an instant. In the next instant, the traveler is in a different inertial frame and can't make a consistent measurement of Earth time.

If the traveler defines "simultaneous" as any event whose t coordinate with respect to the traveler's instantaneous inertial frame is the same as the traveler's own t coordinate, then along this line of simultaneity, the Earth's clock would sometimes run slower and sometimes faster than the traveler's own, given the trip to Alpha Centauri described above.

The traveler begins with his time running at the same rate as Earth's and ending in the same situation. If time runs slower on Earth for some portion of the trip, it has to run faster for some other portion.

 

[PeroK]

I'm trying to picture what method you had in your mind.

We don't need to specify a specific method. We can assume that the tick rate of a moving clock can be measured. Otherwise, we're not going to be able to do much physics.

You could use the transverse Doppler effect, if nothing else. Or, two measurements using a Isoceles triangle, which equalises the light travel time. These are practical technicalities that do not have a direct bearing on the theory.

In particular, your assertion that you cannot measure the tick rate of a moving clock is false.

 

[Freixas]

In particular, your assertion that you cannot measure the tick rate of a moving clock is false.

Interesting comment since I never said that. I just asked what method you would use.

You suggested using the transverse Doppler effect. Or isosceles triangles. Fine. But I said "And couldn't whatever method he uses just as easily be applied to a clock on Earth?"

That aside, I'm interested in how one decides on simultaneity when one observer is accelerating.

The traveler reaches a clock and is co-located. Let's say both the traveler and the clock set their origin to (0, 0) with time units being in clock ticks. The traveler continues accelerating but keeps an eye on the clock, noting when it reaches its next tick and simultaneously recording his own time.

He knows the tick actually occurred in the past. To keep it simple, he calculates (x, t) coordinate for the tick in the clock's rest frame since that stays constant. For the clock, the tick began at (0, 0) and ended at (0, 1). For the traveler, his clock tick also began at (0, 0) and he wants to know his corresponding (x', t') when the resting clock was at (0, 1).

But the traveler has so many inertial frames to choose from! And when the traveler and the clock aren't co-located, you have to decide what you mean by "corresponding".

My method is to draw a line of simultaneity for the traveler. Since he is accelerating, this line of simultaneity changes every instant. So the traveler's (x', t') corresponding to the clock's (0, 1) occurs when the traveler's line of simultaneity crosses the clock's (0, 1) position.

If we can agree that the traveler's instantaneous line of simultaneity can be used to establish a correspondence between the traveler's time and any other observer's time, then we can find the correspondence between the traveler's time and Earth time at any point on his trip. And the correspondence sometimes finds Earth time running sometimes slower and sometimes faster than the traveler's.

If we don't agree on the use of the line of simultaneity, then I don't see how the clock's (0, 1) position can be matched to any point on the traveler's worldline.

Here's a diagram. The acceleration is 1g, but the ticks are in half years to go a nice curve. The yellow line tells us where the traveler sees the tick complete, but the cyan line shows the corresponding location of the traveler along the line of simultaneity. If we don't use the line of simultaneity, then we know where (0, 1) is but have no idea of which point on the traveler's worldline it corresponds to. Without this, we can't calculate whether the clock rates are slower or faster.

 

[PeroK]

That aside, I'm interested in how one decides on simultaneity when one observer is accelerating.

Then you should post your question in the relativity forum. This is science fiction and fantasy.

 

[sbrothy]

Suggestions?

I've probably mentioned it before 'cause I'm a really big fan but if you want to keep it at least somewhat plausible look up Zubrin' s Nuclear Salt-Water Rocket.

EDIT:
Wrong quote. Here.:

"
Zubrin then goes on to speculate about a more advanced version of the NSWR, suitable for insterstellar travel. Say that the 2% uranium bromide solution used uranium enriched to 90% U235 instead of only 20%. Assume that the fission yield was 90% instead of 0.1%. And assume a nozzle efficency of 0.9 instead of 0.8.

That would result in an exhaust velocity of a whopping 4,725,000 m/s (about 1.575% c, a specific impulse of 482,140 seconds). In a ship with a mass ratio of 10, it would have a delta V of 3.63% c. Now you're talkin..."
- -Atomic Rockets