Thread: Would a deep space flight really require cryogenic freezing?

I may be misunderstanding relativity here, but as near as I can tell, the passengers on an self-accelerating space ship don't observe themselves to be time dilated, nor do they observe their space ship to be gaining relativistic mass, or their self-acceleration to be yielding diminishing returns as they approach C. However, they do observe length contraction. If you ever actually reach C, then length contraction would go to zero, and all destinations would be right next to you.

So, if the speed of light is just a little bit less than, 300 million meters per second, and you were riding on a rocket that had an unbelievable amount of fuel and capable of accelerating at a sustained rate of 10 meters per second per second (just slightly higher than 1 g of acceleration), then no trip to any destination, no matter how far away, even outside the Hubble Sphere, could ever appear to take longer than 347.22 days.

That is 30 million seconds (300 million/10 meters per second per second acceleration), 500,000 minutes, or 833.33 hours.

Of course, if you did that, you'd probably fly right by your target, so really we have to account for both acceleration and deceleration, and that would double the value to 694.44 days as the maximum time to both arrive and be able to come to a stop. So, really there's no need for cryogenic stasis or any of that other crazy stasis stuff in sci-fi scenarios, is there?

I'm not sure where you got that time from, but it's wrong. At 1g acceleration, it would still take 12 yrs to cross our galaxy. And that's without slowing down.

And you can't jut double the time needed to to make the trip without stopping to get the time with stopping.

The distance traveled and the time it takes is not linear. When I have some more time, I'll give an example.

Originally Posted by kojax

...So, really there's no need for cryogenic stasis or any of that other crazy stasis stuff in sci-fi scenarios, is there?

It's not that crazy. We can freeze embryos now, and bears do hibernate. Sleeping during a long trip sounds more plausible than carrying vast amounts of fuel. Even if you used something esoteric like antigravity you've still got to balance the energy books. You have to supply energy to lift a brick, and in similar vein you have to supply energy to lift a ship. Then you have to supply a whole lot more to move it very fast. If there was some trick way to get a ship moving very fast without adding energy, I fear you might find out that the ship temperature drops to absolute zero or even that atoms or particles lose integrity. It would be like moving an electron very fast but insisting the total mass/energy is pegged at 511keV. I just don't see how you can do it. Hibernating sounds a piece of cake compared to that.

Originally Posted by Janus

I'm not sure where you got that time from, but it's wrong. At 1g acceleration, it would still take 12 yrs to cross our galaxy. And that's without slowing down.

And you can't jut double the time needed to to make the trip without stopping to get the time with stopping.

The distance traveled and the time it takes is not linear. When I have some more time, I'll give an example.

I should have been clear about who's perspective I was taking. If you're a passenger aboard the ship, I think my analysis would be accurate. By the time you can accelerate to C, you will already have reached your destination.

However, if you're an observer watching from Earth (or from in the inertial frame the ship was in before it began to accelerate), the trip would take a lot longer. Maybe millions of years if the destination is far enough out. Also if you're an observer on Earth you would disagree as to how long it took the rocket to accelerate to C, because it would appear to accelerate slower once time dilation started to play a role. You would agree the ship arrived before reaching C, because it took it so long to accelerate (according to your perspective.)

Originally Posted by Janus

The distance traveled and the time it takes is not linear. When I have some more time, I'll give an example.

This may be where my confusion is. I'm assuming that the time you spend accelerating at a constant rate, and the speed you obtain is linear. That 30 million seconds at 1g (~10 meters/second^2) would get you to 300 million meters/second (or C) in 30 million seconds.

As for distance traveled, the distance between you and your destination keeps shrinking.

Even if you are traveling at C I dont think it would seem like the trip was so short. If you are going a distance of 12 light years at C wouldn't it still take you twelve years to get there. There are stars burning in the night sky that burned out or went nova millions of years ago. The light just hasn't gotten here yet. Or am I thinking to literally?

Originally Posted by kojax

Originally Posted by Janus

The distance traveled and the time it takes is not linear. When I have some more time, I'll give an example.

This may be where my confusion is. I'm assuming that the time you spend accelerating at a constant rate, and the speed you obtain is linear. That 30 million seconds at 1g (~10 meters/second^2) would get you to 300 million meters/second (or C) in 30 million seconds.

As for distance traveled, the distance between you and your destination keeps shrinking.

Oh... But then you run into the problem of inertia and the amount of energy it would take to move you because your mass is increasing

Originally Posted by kojax

I should have been clear about who's perspective I was taking. If you're a passenger aboard the ship, I think my analysis would be accurate. By the time you can accelerate to C, you will already have reached your destination.

However, if you're an observer watching from Earth (or from in the inertial frame the ship was in before it began to accelerate), the trip would take a lot longer. Maybe millions of years if the destination is far enough out. Also if you're an observer on Earth you would disagree as to how long it took the rocket to accelerate to C, because it would appear to accelerate slower once time dilation started to play a role. You would agree the ship arrived before reaching C, because it took it so long to accelerate (according to your perspective.)

The time I gave was for the passenger. The trip time for someone on Earth would be 114385 yrs.


You can't just take 300,000,000 m/s and divide it by 9.8 m/s^2 to get the time to reach c according to the passenger.

Here's why:

First, you have to have to define what reference frame you are measuring its speed against. For convenience, we will use the Earth.

Now imagine that you are on the ship and are now moving at 0.25c relative to the Earth. We'll assume that it took 90 days by your clock to accelerate to this speed relative to the Earth. ( I chose 89 days because this is close to what you would get if you divide 0.25c by 9.8m/s^2)

You drop a buoy overboard, so that it retains a 0.25c speed relative to the Earth. You accelerate for another 90 days. You are now moving at 0.25c relative to the the buoy. How fast are you moving relative to the Earth? Your first guess might be 0.5c, but this would be wrong.

You are moving at 0.25c relative to the buoy and the buoy is moving at 0.25c relative to the Earth, and to get the correct value for your speed relative to the Earth, you must use the Relativistic velocity addition formula. Thus the correct answer is


= 0.47c.


If you drop a second buoy and then accelerate for another 90 days, you will now be moving at

= 0.64c relative to the Earth.

After another 89 days of acceleration, you will be moving at 0.77c

So in the first 90 days, you changed your speed relative to the Earth by 0.25c
In the second 90 days you changed it by 0.22c
In the third 90 days you changed it by 0.13c
In a fourth 90 days it will only change by 0.085c
In a fifth, 0.055 c

Etc.

Note how each 90 days of acceleration gains you a smaller and smaller gain in speed relative to the Earth.

So, after 450 days of acceleration, you will be moving at 0.91c relative to your initial "rest" speed. Even if you had been moving at 0.91c this entire time, you will have only moved 1.12 ly by your reckoning. The length contraction at 0.91c is 0.415. So according to the Earth frame, you will have moved 2.7 ly. (However, since you were not traveling at 0.91c for the entire time, you will have actually moved less.)

Ergo, in 450 days of your own time, you could not have traveled to even the nearest star.

Originally Posted by dmwyant

Even if you are traveling at C I dont think it would seem like the trip was so short. If you are going a distance of 12 light years at C wouldn't it still take you twelve years to get there. There are stars burning in the night sky that burned out or went nova millions of years ago. The light just hasn't gotten here yet. Or am I thinking to literally?

Relativity is of course all about perspective. As I understand it, travel at C would mean you would be everywhere at once from your own perspective, i.e. 0 travel time. Since every photon has to hit something at some point, I'd venture that achieving C would kill you instantly from your own perspective, while an outside observer would deduce that time has frozen on board your ship as you shoot along at C.

Originally Posted by kojax

So, really there's no need for cryogenic stasis or any of that other crazy stasis stuff in sci-fi scenarios, is there?

There is no need for cryogenic suspension if you manage to create an engine that can constantly accelerate at 1g for years, this is true. But most sci-fi writers understand the apparent impossibility of creating such an engine, so they either make their propulsion system a little more plausible (and thus a lot slower) and use cryogenic suspension, or they invent new physics to enable faster than light travel.

Not only does it get more and more difficult to maintain a constant acceleration of 1g as time goes on, but the problem is also exacerbated if you carry your fuel with you and you want to slow back down again.

Have a read of the article below - it explains the fuel problem and also explains why slowing back down again is not as "simple" as speeding up is!

The Relativistic Rocket

Once we build ecconomicly viable space habitats we will eventually reach the stars without the need for cryogenic sleep or faster than light drive. We will move naturally from mining the Ort cloud around the Sun to mining the Ort cloud around another star.

You also need to look at the mental aspect, people go crazy rubbing their shoulders against the same people all day all year long, and it doesn't help they'r in the same confined space.

Originally Posted by Sealeaf

Once we build ecconomicly viable space habitats we will eventually reach the stars without the need for cryogenic sleep or faster than light drive. We will move naturally from mining the Ort cloud around the Sun to mining the Ort cloud around another star.

I suggest you read the article which SpeedFreek linked in the post right above yours, if you have not already done so. Interstellar travel ain't happening, at least with any technology we can even dream about now. If we could build an engine like the one you would need for interstellar travel, then our energy problems would be solved. We'd be able to mine miles below the earth's surface, recycle the hell out of our waste, etc., all of which we would be doing long before going to the oort cloud or any other stars.

Originally Posted by Janus

You are moving at 0.25c relative to the buoy and the buoy is moving at 0.25c relative to the Earth, and to get the correct value for your speed relative to the Earth, you must use the Relativistic velocity addition formula. Thus the correct answer is


= 0.47c.


If you drop a second buoy and then accelerate for another 90 days, you will now be moving at

= 0.64c relative to the Earth.

After another 89 days of acceleration, you will be moving at 0.77c

So in the first 90 days, you changed your speed relative to the Earth by 0.25c
In the second 90 days you changed it by 0.22c
In the third 90 days you changed it by 0.13c
In a fourth 90 days it will only change by 0.085c
In a fifth, 0.055 c

Etc.

Note how each 90 days of acceleration gains you a smaller and smaller gain in speed relative to the Earth.

So, after 450 days of acceleration, you will be moving at 0.91c relative to your initial "rest" speed. Even if you had been moving at 0.91c this entire time, you will have only moved 1.12 ly by your reckoning. The length contraction at 0.91c is 0.415. So according to the Earth frame, you will have moved 2.7 ly. (However, since you were not traveling at 0.91c for the entire time, you will have actually moved less.)

Ergo, in 450 days of your own time, you could not have traveled to even the nearest star.

I hadn't thought of this. It kind of makes sense, because it prevents two objects from reaching speeds higher than C relative to each other by accelerating in opposite directions, but I don't understand why it works the way it does. Basically, you can perceive yourself to accelerate at 1 g for long enough to have reached C, but .... not be at C?

I remember learning that formula. I've just never found a situation to use it in before now, so I've never had to reason it out. Is it better to try and resolve it using length contraction, or time dilation, or both?

What happens if you hit a drifting egg sized pebble while cruising a 0.77 c ?

Originally Posted by kojax

I hadn't thought of this. It kind of makes sense, because it prevents two objects from reaching speeds higher than C relative to each other by accelerating in opposite directions, but I don't understand why it works the way it does. Basically, you can perceive yourself to accelerate at 1 g for long enough to have reached C, but .... not be at C?

I remember learning that formula. I've just never found a situation to use it in before now, so I've never had to reason it out. Is it better to try and resolve it using length contraction, or time dilation, or both?

You have to take into account time dilation, length contraction and the relativity of simultaneity.

Imagine a railway car. Someone fires a bullet from the rear to the front at velocity v. For this person, the time it takes to cross the car is L'/v, where L' is the length of the Car. Call this t'. This means that if there were clock's at the rear and front of the car that are synchronized in the car frame, the bullet would leave the rear clock when it read 0, and arrive at the front clock when it reads t'.

Now assume that you are watching this car travel past you at a velocity of w. You will also see the bullet leave the rear clock when it reads 0 and arrive at the the front clock when it reads t'.

So how fast does the bullet move relative to you?. Here's what you have to consider:
The clocks on the train run slow due to time dilation.
The two clock's will not be synchronized. (the front clock does not read 0 when the bullet leaves the rear of the train, but some time between 0 and 't)
The length of the car will be shortened due to length contraction.
The distance traveled by the car while the bullet travels from back to front.

The first two mean that you have to take both the rate at which the clocks run and the difference in their times to determine T (the time you measure for the bullet to travel the length of the car.) For example, assume that the time on the front clock is 4 sec when the bullet arrives, that the time difference between front and rear clocks is 1 sec in your frame, and the time dilation factor is 3. This means that when the bullet leaves the rear clock, the front clock already reads 1 sec, and thus 3 seconds tick off in the car during the trip of the bullet. This also means that 9 sec passes for you during the passage of the bullet.)

The second two give you the distance D that the bullet travels with respect to you in time T. It will be equal to the distance traveled by the train in T at velocity W plus the length contracted length of the car. (using the same example as above, A time dilation of 3, means a velocity v of 0.94c, This means that D will equal 8.46 light seconds plus L'/3)

The velocity of the bullet with respect to you will be T/D.

Originally Posted by Janus

The clocks on the train run slow due to time dilation.

This is finally starting to make sense. Your present self never appears to be time dilated as it accelerates now, but all of your past selves appear to have been time dilated when they were accelerating.

At least, that's how the buoys play out. Each buoy you left behind is time dilated with respect to you (at least you observe them to be time dilated. They would observe that you are the time dilated one.)

So, if you could see your past selves all lined up next to each other, you would note that from your present perspective, your past selves were always producing less thrust than you are now (even though they were consuming the same amount of fuel as you are consuming now.) Because a time dilated rocket produces less thrust than a non-time dilated (but otherwise similar) rocket.

I thought of another thought experiment. Suppose multiple rockets are being launched from Earth, one after another, and these rockets periodically stop thrusting for a moment every now and again (momentarily allowing an observer on the rocket to view the other rockets without general relativistic effects). Whenever they looked back at one of the other rockets behind them, they would observe that the other rocket (which is moving slower relative to Earth right now) is time dilated, and its thrusters appear to be less effective. Also whenever they look ahead at a rocket in front of them (which is moving faster relative to Earth right now), they also observe its thrusters to be less effective.

Really, the frame they are in right now is the one where those thrusters appear most effective.

Originally Posted by icewendigo

What happens if you hit a drifting egg sized pebble while cruising a 0.77 c ?

You're toast. If the egg weighs 57 grams and is traveling 231000000 meters per second relative to the space ship, then the kinetic energy would be on the order of 10 to the 18th power joules which is about a 23 or 24 megatons of tnt explosion.