Author Topic: Where are the UFOs?  (Read 4310 times)

rukawa

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Re: Where are the UFOs?
« Reply #30 on: December 03, 2017, 10:03:09 PM »
Accelerating at Earth's gravitational acceleration around 9.81m/s^2 you approach light speed of 3x10^8 m/s in 350 days if you have sufficient thrust and power.

I'm pretty sure your calc is wrong.  A confusing way to see this is a kind of Zeno like argument for relativity.

Warning: A long complicated overly complicated argument follows because I'm overly interesting in this and need to understand it.

Suppose I just accelerate to 0.5c. That would based on your calc take 175 days. Now lets consider you from the point of view of an observer travelling at 0.5c (O1)...he would think it would take another 175 days for you to get to 0.5c relative to him. Now consider a second observer (02) travelling at 0.5 c relative to 01...he would think it would take another 175 days to get to 0.5c relative to him. At this point what speed are you travelling at relative to earth rest frame?

Your speed relative to 02 is 0.5c.
Your speed relative to 01 is u = (0.5c+0.5c)/(1+0.5*0.5)= 0.8c
Your speed relative to rest frame is (0.5+0.8)/(1+0.8*0.5) = 0.928c

We can continue this calc:
(0.928c+0.5)/(1+0.5*0.928)=0.976
0.991803279
0.997260274
0.999085923
0.999695215
0.999898395
0.99996613
0.99998871
0.999996237
0.999998746
0.999999582
0.999999861
0.999999954
0.999999985
0.999999995

So you got to 0.92c and it took 525 days. Now this calc is not completely correct because you are not in the frame of the observers and so you will experience less time that each of them for each of your 0.5c speedups. But in each calc you are only within 0.5c of each observer and in this case time is reduced by at most a factor of sqrt(1-0.5^2) = 0.866. So it would still take you at least 525 days * 0.86 = 454 days. And to get to 0.999657123 it would take you 175days * 8 * 0.866 = 1200 days.

But you are right it does take a lot less than 44 years to reach the required speed. Based on calc above you need 18 speedups to get to 0.999999995..which takes at least 175*18*0.866= 2727.9 days. At this speed the galaxy is only 10 light years across and will only take 10 years to traverse. So its doable.




Dynamic

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Re: Where are the UFOs?
« Reply #31 on: December 04, 2017, 12:40:33 AM »
I was trying to consider it all from the human traveller's frame of reference as it's they who have to live for the length of time it takes in their frame of reference and it's they who will experience an acceleration similar to Earth's gravitational acceleration. To them, it doesn't matter so much how many generations may have lived and died on earth, as something 1,000 light years away will always take at least 2,000 year round trip time to visit and receive the signal back in Earth's frame of reference. For it to be viable for the travellers themselves, they need to complete it well within a human lifetime (unless it's a breeding population of explorers).

This completely ignores both the increasing power required to accelerate the same amount as they near light-speed and the time as observed from Earth, but considers survivability in terms of time and acceleration. It also makes the math simpler when all you want is a rough idea of whether a human body could withstand sustained acceleration to relativistic speeds within a human lifetime. Note that relativistic mass is a convenient fiction used to describe the additional kinetic energy and momentum required at relativistic speeds over and above those of Newtonian mechanics. As far as I understand, (Fermilab has a good YouTube video about this) relativistic mass is not a reality that the traveller would experience within their own frame of reference.

Yes, there is the problem of what 'speed' to each observer means, as both times and distances vary for each observer. My approximate answer may or may not be right but I think it indicates that survivable acceleration to relativistic speeds is plausible within the traveller's lifetime subject to a suitable source of acceleration.

rukawa

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Re: Where are the UFOs?
« Reply #32 on: December 04, 2017, 07:22:51 AM »
Quote
I was trying to consider it all from the human traveller's frame of reference as it's they who have to live for the length of time it takes in their frame of reference and it's they who will experience an acceleration similar to Earth's gravitational acceleration

Yes I was trying to do the same thing.

Yes, there is the problem of what 'speed' to each observer means, as both times and distances vary for each observer. My approximate answer may or may not be right but I think it indicates that survivable acceleration to relativistic speeds is plausible within the traveller's lifetime subject to a suitable source of acceleration.

What I did is kind of a poor man's integration. The accelerated rocket has its own proper time...but its proper time is instantaneously increasing at the same rate as an observer that happens to be travelling at the same speed at that particular moment. To figure out the real proper time in the rockets frame of reference you have to do an integration where the proper time is continuously changing. I'll work on that this week. But what I did kind of approximates that since I look at observers that are within 0.5c of the rocket at each stage of the calc...their proper time must be within a factor of at most 0.866 of the rocket itself and so I can figure out what the rocket is experiencing by looking at what each successive observer measures.

But you are right. Acceleration to speeds close enough to light speed to make the galaxy seem really small are possible within a lifetime. So its possible to traverse the whole galaxy in a person's lifetime. Pretty cool.

oddballstocks

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Re: Where are the UFOs?
« Reply #33 on: December 04, 2017, 07:37:58 AM »
Quote
I was trying to consider it all from the human traveller's frame of reference as it's they who have to live for the length of time it takes in their frame of reference and it's they who will experience an acceleration similar to Earth's gravitational acceleration

Yes I was trying to do the same thing.

Yes, there is the problem of what 'speed' to each observer means, as both times and distances vary for each observer. My approximate answer may or may not be right but I think it indicates that survivable acceleration to relativistic speeds is plausible within the traveller's lifetime subject to a suitable source of acceleration.

What I did is kind of a poor man's integration. The accelerated rocket has its own proper time...but its proper time is instantaneously increasing at the same rate as an observer that happens to be travelling at the same speed at that particular moment. To figure out the real proper time in the rockets frame of reference you have to do an integration where the proper time is continuously changing. I'll work on that this week. But what I did kind of approximates that since I look at observers that are within 0.5c of the rocket at each stage of the calc...their proper time must be within a factor of at most 0.866 of the rocket itself and so I can figure out what the rocket is experiencing by looking at what each successive observer measures.

But you are right. Acceleration to speeds close enough to light speed to make the galaxy seem really small are possible within a lifetime. So its possible to traverse the whole galaxy in a person's lifetime. Pretty cool.

This is fascinating.  The question is then what sort of thrust is required? How much energy do you need for each acceleration?

From what I've read there are two issues.  The first is the raw power, the second is overcoming Earth's pull.  So in theory if you wanted to do this you'd need to build the ship in space.  You wouldn't need to waste as much energy escaping the Earth.  Unfortunately we don't have the tech to build spaceships in space currently.
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Pelagic

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Re: Where are the UFOs?
« Reply #34 on: December 04, 2017, 07:45:39 AM »
The milky way is 100000 light years a across in distance. That means from our point of view the trip across the milky was would take 100,000 years if you travelled at max speed. But from the point of view of a rocket travelling at lets say 0.99999999 of the speed of light relative to us the time take to travel across the milky way would be:

100000*sqrt(1-0.999999999^2) = 44 years. So the real question is how hard is it to get someone to travel at that high a speed. You need very high rates of accelerations for very long periods of time....my guess is much much longer than the trip itself. It would be a really stupid idea to do this with human beings. Its most sensible to send robots.

Anyways you don't have to travel across the whole Milky Way. The nearest star is 4 light years away. I'm guessing you can find a planet with life within 1000 light years. But lets say you can't get close to the speed of light and so you can't take advantage of time dilation. Still....a few thousand years of travel might be reasonable for a robot designed by sufficiently advanced civilization. What you would probably do is send trilllions of very small robots that could accelerate to very high speeds using some nuclear process for energy and maybe could even reproduce in space and you would have these robots include some representation of the coordinates of your solar system relative to some galactic coordinate frame. It would be complicated but possible.

I don't know if you're familiar with Breakthrough Starshot or not but their plan to send craft to Alpha Centauri is quite similar to this. Using Earth based lasers and light sails to accelerate the craft to 20% of the speed of light, they estimate the trip will take 20 years. Basically like firing a shotgun blast of many small craft hoping some make it to their destination and can send back a signal of what they're seeing to us.

https://www.space.com/32546-interstellar-spaceflight-stephen-hawking-project-starshot.html

rukawa

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Re: Where are the UFOs?
« Reply #35 on: December 04, 2017, 07:54:28 AM »
Quote
I was trying to consider it all from the human traveller's frame of reference as it's they who have to live for the length of time it takes in their frame of reference and it's they who will experience an acceleration similar to Earth's gravitational acceleration

Yes I was trying to do the same thing.

Yes, there is the problem of what 'speed' to each observer means, as both times and distances vary for each observer. My approximate answer may or may not be right but I think it indicates that survivable acceleration to relativistic speeds is plausible within the traveller's lifetime subject to a suitable source of acceleration.

What I did is kind of a poor man's integration. The accelerated rocket has its own proper time...but its proper time is instantaneously increasing at the same rate as an observer that happens to be travelling at the same speed at that particular moment. To figure out the real proper time in the rockets frame of reference you have to do an integration where the proper time is continuously changing. I'll work on that this week. But what I did kind of approximates that since I look at observers that are within 0.5c of the rocket at each stage of the calc...their proper time must be within a factor of at most 0.866 of the rocket itself and so I can figure out what the rocket is experiencing by looking at what each successive observer measures.

But you are right. Acceleration to speeds close enough to light speed to make the galaxy seem really small are possible within a lifetime. So its possible to traverse the whole galaxy in a person's lifetime. Pretty cool.

This is fascinating.  The question is then what sort of thrust is required? How much energy do you need for each acceleration?

From what I've read there are two issues.  The first is the raw power, the second is overcoming Earth's pull.  So in theory if you wanted to do this you'd need to build the ship in space.  You wouldn't need to waste as much energy escaping the Earth.  Unfortunately we don't have the tech to build spaceships in space currently.

I feel like a huge issue will be shielding. If you are travelling at close to the speed of light relative to the rest of the galaxy then space dust will be hitting you at nearly the speed of light. And the space dust will also appear to have higher apparent mass. So if you manage to shrink the galaxy from 100000 lc to 10 lc by travelling near speed of light...you also managed to make all the dust particles appear to be 10000 times more massive, with 10000 times greater density (since the galaxy is now much smaller) and hitting you at near the speed of light. I feel like you would get shredded in fractions of a second.

Jurgis

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Re: Where are the UFOs?
« Reply #36 on: December 04, 2017, 09:11:03 AM »
That's why all advanced civilizations travel as neutrinos. 8)

cwericb

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Re: Where are the UFOs?
« Reply #37 on: December 04, 2017, 09:55:08 AM »
Think one may have landed in Washington DC back about a year ago.

Dynamic

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Re: Where are the UFOs?
« Reply #38 on: December 04, 2017, 03:35:30 PM »
The power to accelerate and the shielding are enormously difficult challenges for human interstellar travel near light speed.

With sufficient power it might be possible to deflect particles. Charged particles could be deflected by magnetic fields much as with the Earth - the cause of the aurorae. Photonic momentum transfer may be able to deflect uncharged particles just enough, or a number of mechanical shields could be flown ahead of the spaceship, some of which would deflect enough particles and some of which might be sacrificial, absorbing them but deteriorating over time. If robotic craft were shown to be sufficiently protected it might be possible to send humans. I imagine we would need enormously powerful fusion reactors or antimatter matter storage and annihilation to provide sufficient power for propulsion and protection, which are bound to be a very long time in reaching sufficient specifications. Solar sails/laser sails may be sufficient to attain much of the required acceleration away from Earth but again enormous technological advances would be essential to approach a reasonable rate of acceleration. At this stage it's very hard to guess whether these challenges are surmountable. I would have thought they are given enough time and technological advance.

Jurgis

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Re: Where are the UFOs?
« Reply #39 on: December 04, 2017, 07:12:37 PM »
The power to accelerate and the shielding are enormously difficult challenges for human interstellar travel near light speed.

With sufficient power it might be possible to deflect particles. Charged particles could be deflected by magnetic fields much as with the Earth - the cause of the aurorae. Photonic momentum transfer may be able to deflect uncharged particles just enough, or a number of mechanical shields could be flown ahead of the spaceship, some of which would deflect enough particles and some of which might be sacrificial, absorbing them but deteriorating over time. If robotic craft were shown to be sufficiently protected it might be possible to send humans. I imagine we would need enormously powerful fusion reactors or antimatter matter storage and annihilation to provide sufficient power for propulsion and protection, which are bound to be a very long time in reaching sufficient specifications. Solar sails/laser sails may be sufficient to attain much of the required acceleration away from Earth but again enormous technological advances would be essential to approach a reasonable rate of acceleration. At this stage it's very hard to guess whether these challenges are surmountable. I would have thought they are given enough time and technological advance.

By the time tech is advanced enough for interstellar travel, there won't be organic humans. We will be uploaded, replaced by manufactured bodies if needed, etc. The age of organic humans is close to the end, give or take 100 years or so, possibly less.

Sending current organic lifeforms into space is a folly. Space is totally inhospitable for humans. The only reason we send humans to space right now is (1) we cannot transfer ourselves into non-organic bodies yet; (2) we don't have sufficiently smart AI to send it instead of sending us. Either (1) or (2) or both will be solved within 50 years ... if we don't blow up ourselves in the meantime.

This is one reason Elon's Mars colonization plan is a bit quixotic. By the time we have good enough tech to send human colony safely to Mars and sustain it there, we will be close or past not needing to send organic human colony at all. There's a margin of time where organic human colony may exist, but it's very short period.