Traveling to Alfa Centauri at 1/2 the speed of light

[McCartney]

Not a physicist or a mathematician, but reading about cosmology whenever I have a chance.

Alfa Centauri is 4.37 light-years away, so I conclude that it takes 4.37 x 2 traveling at 1/2 the speed of light. However, I am assuming that 4.37 x 2 is measured here on Earth. How much time has elapsed for the pilot of the spaceship?

In other words: How do we measure the distance to Alfa Centauri? Because of time dilation I am assuming it takes less time for the pilot to get there. I have heard of length reduction while moving, but i don't see how this applies to the distance to Alfa Centauri.

So the question is:

What is the real distance to Alfa Centauri?

I am confused.

 

[Vorde]

There is no 'real' distance. In relativity, everything is, well, relative. We would measure one distance/time, and a pilot onboard a spaceship would measure a totally different number (less time, or alternatively less distance). Both would be completely valid measurements.

 

[McCartney]

There is no 'real' distance. In relativity, everything is, well, relative. We would measure one distance/time, and a pilot onboard a spaceship would measure a totally different number (less time, or alternatively less distance). Both would be completely valid measurements.

I have a hard time visualizing what you say. I see that the clock in the spaceship is slower, but the pilot cannot tell. One woud think that for the pilot it would take 4.37 x 2 ly to get there. I guess a ly implies both the time and the space. However, if the pilot has to drive the spaceship for 4.37 x 2 ly to get there. The guy monitoring the flight on Earth will have to wait much longer. For the Earth bound person it would seem that Alfa Centauri is farther. Unless, the distance is shorter for the spaceship. I suspect the latter is correct, but cannot put it into words.

In other words, the people on Earth wait for 4.37 x 2 ly for the completion of the trip and the people on the spaceship have to wait less.

 

[Drakkith]

An observer on the ship would experience time at 0.866 the speed of an observer back on Earth. So the pilot of the ship would experience a local time for the trip to Alpha Centauri of 7.45 years instead of 8.74 years.
Look up Special Relativity on wikipedia to find all the equations to use, or look up a Special Relativity calculator on google (like i did) if you don't want to learn the math.

 

[McCartney]

An observer on the ship would experience time at 0.866 the speed of an observer back on Earth. So the pilot of the ship would experience a local time for the trip to Alpha Centauri of 7.45 years instead of 8.74 years.
Look up Special Relativity on wikipedia to find all the equations to use, or look up a Special Relativity calculator on google (like i did) if you don't want to learn the math.

OK, thanks.

So according to the Lorentz transformation two different observers will see different time and distance.

Does that mean that space travel will be much shorter than anticipated if traveling real fast?

Obviously it takes less time to get to Alfa Centauri than anticipated.

 

[Whovian]

One thing to note is that a lot of things are relative. Saying "something's going fast" means nothing, you'd have to say "something's going fast with respect to this other thing." If we have observer A and observer B moving quickly relative to each other (like an observer on Earth and an observer in the spaceship), observer A (on Earth, in this case) observes B's clock as ticking slowly, but observer B observes A's clock as ticking slowly.

But yes, space travel will be shorter than anticipated when traveling quickly with respect to the Earth and the destination. (For simplicity, let's assume the Earth and the destination are in the same frame of reference.)

 

[Drakkith]

OK, thanks.

So according to the Lorentz transformation two different observers will see different time and distance.

Does that mean that space travel will be much shorter than anticipated if traveling real fast?

Obviously it takes less time to get to Alfa Centauri than anticipated.

To the person doing the traveling, yes. If you increased your speed to 0.8c it would only take 60% of the time as a person on Earth would experience.

 

[Chronos]

And this time line does not consider the need for controlled acceleration and decelleration during the journey. Human physiology would greatly constrain how quickly you could reach .5c, then slow back down to zero.

 

[McCartney]

And this time line does not consider the need for controlled acceleration and decelleration during the journey. Human physiology would greatly constrain how quickly you could reach .5c, then slow back down to zero.

I believe the acceleration would further slow down the clock. Assuming the acceleration is a form of gravity. Is that correct?

How fast could a spaceship travel in space assuming there is little friction and little pull by gravity?

 

[Vorde]

How fast could a spaceship travel in space assuming there is little friction and little pull by gravity?

Infinitely close to the speed of light, but never at the speed of light.

 

[Chronos]

I believe the acceleration would further slow down the clock. Assuming the acceleration is a form of gravity. Is that correct?

How fast could a spaceship travel in space assuming there is little friction and little pull by gravity?

It would be the same as for gravitational time dilation. At only a g or so [to accommodate human physiology], the effect would be negligible.

 

[McCartney]

Infinitely close to the speed of light, but never at the speed of light.

I am familiar with the concept that the mass becomes larger and supposedly infinite at the speed of light. But, at some point the acceleration needed to to go even faster must be enormous due to the increment in mass.

However, I assume once a respectable speed is reached there is no need for more acceleration to maintain speed due to negligible friction and gravity. So perhaps intergalactic space travel will be cheap (good mileage) in relationship to the enormous distances.

 

[Vorde]

Friction is practically absent in space, especially in the interstellar void, practically I don't really think friction is ever an issue. The thing is you need ridiculous, ridiculous amounts of energy just to get a sizable object up to a fraction of the speed of light. To accelerate something like a spaceship up to those speeds is still well outside our reach as a civilization.

 

[Chronos]

I recall having seen a figure of about 4 lbs of antimatter per light year [assuming highly efficient conversion to thrust]. Antimatter is not something you want laying loose in the cargo hold, so JIT generation would be essential.

 

[Drakkith]

I am familiar with the concept that the mass becomes larger and supposedly infinite at the speed of light. But, at some point the acceleration needed to to go even faster must be enormous due to the increment in mass.

Here's the thing. The amount of fuel and thrust needed to provide 1g of acceleration does NOT change for a person inside the spaceship under acceleration. (Ignoring the loss of mass as fuel is spent and ejected as exhaust from the engines) They will accelerate at a constant 1g until the ship stops accelerating or it runs out of fuel. The "increase" in mass due to high velocity is...not really accurate. Mass is generally considered to be something calculated in the rest frame of an object. Relativistic mass is an outdated term if what I've been told about it is true, and relativistic mass is what is increasing as velocity increases. In the frame of the spaceship no mass is ever gained.

However, I assume once a respectable speed is reached there is no need for more acceleration to maintain speed due to negligible friction and gravity. So perhaps intergalactic space travel will be cheap (good mileage) in relationship to the enormous distances.

Correct. Once you get up to speed you will travel effectively forever as long as you don't run into anything.

 

[SHISHKABOB]

Correct. Once you get up to speed you will travel effectively forever as long as you don't run into anything.

or require course corrections >.>

 

[twofish-quant]

I am familiar with the concept that the mass becomes larger and supposedly infinite at the speed of light.

Get rid of it, since it's wrong. There was a popular book in the 1940's that used it to explain relativity, but that's caused more confusion.

The reason you can't travel faster than light is that light always travels at the speed of light. If you shine a flashlight, the light is traveling away from you at speed C. If you run toward the light, it's still traveling away from you at speed C. No matter how hard you try, light is always going away from you at the speed C, so you'll never catch up to the beam of light.

However, I assume once a respectable speed is reached there is no need for more acceleration to maintain speed due to negligible friction and gravity. So perhaps intergalactic space travel will be cheap (good mileage) in relationship to the enormous distances.

The problem is that physics but biology and sociology. It doesn't take that much fuel to go into interstellar space. So if you don't care how long it takes to get to a star, you'll make it there. However, our bodies self-destruct after about 70 years which imposes a limit.

 

[twofish-quant]

To accelerate something like a spaceship up to those speeds is still well outside our reach as a civilization.

Maybe, but it looks like something that can be solved with a few hundred years of effort.

Personally, my guess is that "suspended animation" will turn out to be an easier problem than acceleration to 0.5c. Or you could do both. Once you are an ice-cube, then you can accelerate at 1000g without turning into jelly.

 

[Ryan_m_b]

However, I assume once a respectable speed is reached there is no need for more acceleration to maintain speed due to negligible friction and gravity. So perhaps intergalactic space travel will be cheap (good mileage) in relationship to the enormous distances.

It really depends on future economics and sociology to determine what is regarded as "cheap" but I doubt anyone will ever think interstellar (never mind intergalactic) space travel would be cheap¹!

To accelerate a vehicle with the mass of a space shuttle to 0.5C would take at minimum 22.5ZJ. That would require the perfect harvest and perfect transfer to momentum of all the solar energy hitting Earth for one and a half days! Or to put it in another perspective, all energy produced by human civilisation pumped into this vehicle for ~45 years! And then you have to do that all again to slow it down².

¹The reason I say it wouldn't be considered cheap is because throwing around that kind of energy is damn near apocalyptic. Any civilisation that didn't treat it with a healthy amount of fear isn't going to last long. If we throw in shedloads of handwavium and propose that a future space effort could simply send a small, cheap probe to a large asteroid and have it replicate and reassemble the asteroid into a very thin, Earth sized solar array with a several-hundred-petawatt laser to shine on the vehicles laser sail whilst we might think we've made a gateway to interstellar travel a more careful look would reveal we've made a device capable of snuffing out all life on Earth easily. A nation that attempts this would be regarded by other nations as well as one IRL that tried to stock all its planes with nuclear warheads that detonate if the plane is mildly damaged.

²That was all back of the envelop so take those numbers with a tiny pinch of salt just incase I made a basic mistake (though I don't think I have).

 

[Vorde]

Maybe, but it looks like something that can be solved with a few hundred years of effort.

Personally, my guess is that "suspended animation" will turn out to be an easier problem than acceleration to 0.5c. Or you could do both. Once you are an ice-cube, then you can accelerate at 1000g without turning into jelly.

I've often thought of similar techniques, or the general idea of figuring out how to accelerate the whole body at once to eliminate an acceleration barrier. I think the larger problem in terms of a civilization-wide thing (apart from figuring out where to get the energy), is the acceptance that interstellar (and potentially, even intergalactic) travel will require the fragmentation of our civilization through time.

Traveling long distances would either require suspended animation for long periods of time (which obviously splits up the time of the astronauts and the people at home) or travel at relativistic speeds, which due to time dilation would also split up the timelines of the two parties.

In a way, the best way to insure the survival of our civilization would be interstellar travel.

 

[McCartney]

To accelerate a vehicle with the mass of a space shuttle to 0.5C would take at minimum 22.5ZJ. That would require the perfect harvest and perfect transfer to momentum of all the solar energy hitting Earth for one and a half days! Or to put it in another perspective, all energy produced by human civilisation pumped into this vehicle for ~45 years! And then you have to do that all again to slow it down².

22.5 Zetta joules!

Wow!

I found this energy conversion by using e=mc^2

One of Einstein's great insights was to realize that matter and energy are really different forms of the same thing. Matter can be turned into energy, and energy into matter.
For example, consider a simple hydrogen atom, basically composed of a single proton. This subatomic particle has a mass of
0.000 000 000 000 000 000 000 000 001 672 kg
This is a tiny mass indeed. But in everyday quantities of matter there are a lot of atoms! For instance, in one kilogram of pure water, the mass of hydrogen atoms amounts to just slightly more than 111 grams, or 0.111 kg.
Einstein's formula tells us the amount of energy this mass would be equivalent to, if it were all suddenly turned into energy. It says that to find the energy, you multiply the mass by the square of the speed of light, this number being 300,000,000 meters per second (a very large number):

= 0.111 x 300,000,000 x 300,000,000
= 10,000,000,000,000,000 Joules

Not bad and only water is needed, ha, ha.

http://www.worsleyschool.net/science/files/emc2/emc2.html

I guess we cannot forget we are barbarians and highly primitive. All the great scientific achievements are very recent and within the last few hundred years. It would be a different story of these achievements were 10,000 years old. I guess by then we could be more civilized.

 

[McCartney]

In a way, the best way to insure the survival of our civilization would be interstellar travel.

I guess I meant to say interstellar instead of intergalactic travel.

The good news is that we have millions of years to figure this out. This issue reminds me a lot of the Galactica series that was on TV a few years ago.

 

[McCartney]

Get rid of it, since it's wrong. There was a popular book in the 1940's that used it to explain relativity, but that's caused more confusion.

The reason you can't travel faster than light is that light always travels at the speed of light. If you shine a flashlight, the light is traveling away from you at speed C. If you run toward the light, it's still traveling away from you at speed C. No matter how hard you try, light is always going away from you at the speed C, so you'll never catch up to the beam of light.



The problem is that physics but biology and sociology. It doesn't take that much fuel to go into interstellar space. So if you don't care how long it takes to get to a star, you'll make it there. However, our bodies self-destruct after about 70 years which imposes a limit.

The concept of traveling with a colony and the need to reproduce constantly to get there is possible (assuming the newborns are trained and there are enough people on board to maintain diversity). However, there could also be a cargo of female eggs and sperm for the long trip.

 

[Ryan_m_b]

22.5 Zetta joules!

Wow!

I found this energy conversion by using e=mc^2

Not bad and only water is needed, ha, ha.

http://www.worsleyschool.net/science/files/emc2/emc2.html

I guess we cannot forget we are barbarians and highly primitive. All the great scientific achievements are very recent and within the last few hundred years. It would be a different story of these achievements were 10,000 years old. I guess by then we could be more civilized.

If you follow that through you'll see that you'd have to perfectly convert 25,000 tonnes of matter to get the energy needed (ignoring the fact that that's the bare minimum because you're obviously not going to get perfect energy to momentum). The only way to do that is if we devise a way to mass produce antimatter but then we've got apocalyptic problems worse than what I outlined above.

I guess I meant to say interstellar instead of intergalactic travel.

The good news is that we have millions of years to figure this out. This issue reminds me a lot of the Galactica series that was on TV a few years ago.

If we last millions of years. And galactica? Man that show was bad. Essentially just took the United States and threw it into space with absolutely no clever thought what so ever.

The concept of traveling with a colony and the need to reproduce constantly to get there is possible (assuming the newborns are trained and there are enough people on board to maintain diversity). However, there could also be a cargo of female eggs and sperm for the long trip.

This statement is about as handwavy as what I've put above. Firstly we're going to have to figure out how to build sustainable, closed-ecosystems before we start going anywhere and that is utterly non-trivial. We've never come close. Secondly the number of people necessary to maintain the human noo/technosphere is huge, potentially millions. Modern society is highly complex and filling all the technically specialised roles required just to sustain it will take a nations worth of people. Figuring out how to build a minimum sized maximally redundant labour force is an interesting exercise that would have to fit with a new socioeconomic model (see below) and possibly include as much automation as possible.

Lastly sending off this nation in an ecosystem to boldly go will most likely end in failure unless you figure out a way to build a socioeconomic model sustainable over hundreds-thousands of years. The last thing you need in a small closed system is a revolution, war or economic crisis that ends up with a small percentage having a monopoly on the life support.

The interesting thing about all of this is that if you figure out good ways to do it they will apply on Earth just as well, if not better. And if you reallllllly wanted to live in space and got the funds why bother going interstellar? Spending your entire life living on the inside of a hollowed out asteroid with the vague knowledge that your distant descendants will see another sun up close isn't as attractive a prospect as just settling in the solar system.

IMO these conversations are always heavily muddled by a pioneering cultural narrative that doesn't apply to space in the same way it does on land. Fallaciously we see space colonisation as analogous to international/continental colonisation (the fact that we use the word spaceship speaks volumes towards this) and until we recognise the flaws in this way of framing the discussion I doubt we'll be able to properly address it.

 

[Whovian]

Not bad and only water is needed, ha, ha.

http://www.worsleyschool.net/science/files/emc2/emc2.html

And anti-water.

 

[McCartney]

If you follow that through you'll see that you'd have to perfectly convert 25,000 tonnes of matter to get the energy needed (ignoring the fact that that's the bare minimum because you're obviously not going to get perfect energy to momentum). The only way to do that is if we devise a way to mass produce antimatter but then we've got apocalyptic problems worse than what I outlined above.

If we last millions of years. And galactica? Man that show was bad. Essentially just took the United States and threw it into space with absolutely no clever thought what so ever.

This statement is about as handwavy as what I've put above. Firstly we're going to have to figure out how to build sustainable, closed-ecosystems before we start going anywhere and that is utterly non-trivial. We've never come close. Secondly the number of people necessary to maintain the human noo/technosphere is huge, potentially millions. Modern society is highly complex and filling all the technically specialised roles required just to sustain it will take a nations worth of people. Figuring out how to build a minimum sized maximally redundant labour force is an interesting exercise that would have to fit with a new socioeconomic model (see below) and possibly include as much automation as possible.

Lastly sending off this nation in an ecosystem to boldly go will most likely end in failure unless you figure out a way to build a socioeconomic model sustainable over hundreds-thousands of years. The last thing you need in a small closed system is a revolution, war or economic crisis that ends up with a small percentage having a monopoly on the life support.

The interesting thing about all of this is that if you figure out good ways to do it they will apply on Earth just as well, if not better. And if you reallllllly wanted to live in space and got the funds why bother going interstellar? Spending your entire life living on the inside of a hollowed out asteroid with the vague knowledge that your distant descendants will see another sun up close isn't as attractive a prospect as just settling in the solar system.

IMO these conversations are always heavily muddled by a pioneering cultural narrative that doesn't apply to space in the same way it does on land. Fallaciously we see space colonisation as analogous to international/continental colonisation (the fact that we use the word spaceship speaks volumes towards this) and until we recognise the flaws in this way of framing the discussion I doubt we'll be able to properly address it.

I like your post, but if you were to tell a well educated person in the year 1830 that we would travel by airplane regularly, have internet, TV, electricity, running water, cell phones, and a computer that runs the entire house he would probably say it is impossible.

I believe we are still quite primitive.

 

[Ryan_m_b]

I like your post, but if you were to tell a well educated person in the year 1830 that we would travel by airplane regularly, have internet, TV, electricity, running water, cell phones, and a computer that runs the entire house he would probably say it is impossible.

I believe we are still quite primitive.

I said this recently to someone but it absolutely baffles me that people think this is worth mentioning. Firstly: Yes I am aware that technological and social change have made the present a very different place to the predictions of the past. Secondly: The fact that change happened in the past is no indication as to what will change in the future. Thirdly: blithely dismissing legitimate observations about the practicalities and obstacles of an endeavour with techno-optimistic enthusiasm is not productive and often seems insulting. Fourthly: regardless of what we do in future you can't change the fundamentals of how much energy it takes to do work. No matter what it will always take at least 1 calorie to raise 1 ml of water by 1 degree. It will always take at least 500KJ to accelerate 1kg by 1000mps. And zettawatts of energy will always have the potential to wipe out life on Earth!

I hope it doesn't sound like I'm having a go, I'm not trying to*. But it frustrates me no end when a rational and productive discussion is interjected by "now is different to the past therefore the future will be different to the present therefore your argument is invalid" when it is rarely applicable.

*Edit: Just to underline this point I really don't mean to sound aggressive or anything. This is just a common frustration.

 

[McCartney]

I said this recently to someone but it absolutely baffles me that people think this is worth mentioning. Firstly: Yes I am aware that technological and social change have made the present a very different place to the predictions of the past. Secondly: The fact that change happened in the past is no indication as to what will change in the future. Thirdly: blithely dismissing legitimate observations about the practicalities and obstacles of an endeavour with techno-optimistic enthusiasm is not productive and often seems insulting. Fourthly: regardless of what we do in future you can't change the fundamentals of how much energy it takes to do work. No matter what it will always take at least 1 calorie to raise 1 ml of water by 1 degree. It will always take at least 500KJ to accelerate 1kg by 1000mps. And zettawatts of energy will always have the potential to wipe out life on Earth!

I hope it doesn't sound like I'm having a go, I'm not trying to*. But it frustrates me no end when a rational and productive discussion is interjected by "now is different to the past therefore the future will be different to the present therefore your argument is invalid" when it is rarely applicable.

*Edit: Just to underline this point I really don't mean to sound aggressive or anything. This is just a common frustration.

Your frustration is quite acceptable.


The ceiling is not as high as in the past for technological advancement. But, you must admit that something as trivial as GPS would be considered a fairy tale in the year 1830. Interstellar travel seems very difficult, no doubt. But, I suspect you cannot imagine what kind of technology we will have in 100,000 years. In terms of the 4-5 billion years left for our sun 100,000 years is nothing.

Our civilization is extremely new.

But, then again we humans may become extinct.

 

[Ryan_m_b]

Your frustration is quite acceptable.The ceiling is not as high as in the past for technological advancement. But, you must admit that something as trivial as GPS would be considered a fairy tale in the year 1830. Interstellar travel seems very difficult, no doubt. But, I suspect you cannot imagine what kind of technology we will have in 100,000 years. In terms of the 4-5 billion years left for our sun 100,000 years is nothing.

Our civilization is extremely new.

But, then again we humans may become extinct.

Whether or not I can accurately predict future technological and social developments has little bearing on any discussion now. Putting it another way: I acknowledge that the technological and social unknown unknowns that may or may not crop up in both the near and far future could drastically change the basis for this conversation...now what?

You see I think that the majority of the time when people say something like "you don't know what future technology may be like" the unspoken subtext is that they believe that future developments will prove them right. It's akin to saying "you are wrong because I believe the future will prove you wrong". I'm not saying you are doing this but it's a common thing. In addition, regardless of what happens in the future I believe I am right in everything I have said because it applies to our known situation.

 

[3rdHeaven]

We only have about 10,000 years left give or take a bit before humans are extinct on this planet, so we better hurry up with that new technology considering we been pretty much dragging our feet for 200 million years and look how far we go :)

 

[TheTechNoir]

We only have about 10,000 years left give or take a bit before humans are extinct on this planet, so we better hurry up with that new technology considering we been pretty much dragging our feet for 200 million years and look how far we go :)

...What??

Source please of the fact that humans will be extinct in 'about' 10,000 years give or take a 'bit' (is a bit 1 year? 1 billion? etc.) and also a source that human beings have been on this planet for 200,000,000 years.

EDIT: Please a reliable source too. I don't want something with the words 'alien visitation' in it or anything.

 

[Ryan_m_b]

We only have about 10,000 years left give or take a bit before humans are extinct on this planet, so we better hurry up with that new technology considering we been pretty much dragging our feet for 200 million years and look how far we go :)

The human species is three orders of magnitude younger than 200 million years and we've got no way of knowing when we will go extinct.

 

[3rdHeaven]

It's an estimate, of course it could happen before or after

http://www.universetoday.com/11430/the-end-of-everything/

the point is we better hurry up with that advance technology because we really have a long way to go in a relatively short amount of time left for us :)

 

[Ryan_m_b]

It's an estimate, of course it could happen before or after

http://www.universetoday.com/11430/the-end-of-everything/

the point is we better hurry up with that advance technology because we really have a long way to go in a relatively short amount of time left for us :)

The Carter Doomsday Argument is a probabilistic argument based on a series of assumptions. Plug in different assumptions and you get different results. It is no more a predictor of the future than the Drake equation is for alien life, it is simply a proposition (and not an uncontroversial one) for how to look at the data we have or can speculate.

As for "long way to go" I don't understand what you mean. Human society doesn't have absolute goals in mind (beyond perhaps increasing prosperity and survival), there's no list of criteria that we have to achieve before we go extinct.

P.s. Welcome to the forums

 

[TheTechNoir]

Ryan got it. I'm also curious still where you got the figure of 200,000,000 from. Anywhere I know of puts even Apes at around 10 times younger than that figure, and Neanderthals at almost 100 times younger. The only thing I can think of is http://en.wikipedia.org/wiki/Anatomically_modern_humans you got 200,000 mixed with 200,000,000 based on this definition of human. Fortunately there is no evidence of there being humans we know them walking around running from dinosaurs.EDIT: I do agree that we MAY have a long way to go with a relatively short time ahead of us, but it's purely speculation.