# How fast could a human be propelled to light-speed non-fatally?

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#### thedoc

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##### How fast could a human be propelled to light-speed non-fatally?
« on: 18/09/2012 17:30:01 »
david wharfe  asked the Naked Scientists:

Dear Naked Scientists,

How long would it take to accelerate a human to light speed, without killing them through g-force etc? Or at least to a good theoretically possible fraction of lightspeed?

Many Thanks
David Wharfe

What do you think?
« Last Edit: 18/09/2012 17:30:01 by _system »

#### CliffordK

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #1 on: 18/09/2012 19:20:35 »
The first question is how great of acceleration could a person withstand and survive with adequate pressure suits and other devices.

Humans loose consciousness with an acceleration of about 9 or 10G.
Somewhere around 65Gs is considered fatal for an impact (which is of short duration).

For my calculations, I tend like to take 100Gs, or about 100*10m/s2, or 1000m/s2 in optimum conditions.  This may in fact be a gross overestimate as a person may be able to survive such high forces with adequate protection for a brief period, but not for an extended period.  But, perhaps one could ramp up the acceleration so it would not be quite the same as an impact event.

The speed of light is about 300,000,000 m/s.

You can simply divide the speed of light by your acceleration,  (300,000,000 m/s) /  (1000m/s2), and one gets 300,000 seconds, and one gets about 83 hours of acceleration to reach the speed of light (or thereabouts).

If you limited your acceleration to 10G's, or about 100 m/s2, then one gets 3,000,000 seconds, or about 35 days to reach the speed of light (or thereabouts).

Here I calculated that if one put a tunnel through the center of the moon, and an electromagnetic accelerator in the tunnel capable of constant acceleration, then after about 83 seconds of 100G acceleration, one would blast out of the other side of the moon at about 42km/s, or around 1/10,000th the speed of light.

#### David Cooper

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #2 on: 18/09/2012 20:59:35 »
There is an experiment which involves levitating a frog, and I'm guessing that it applies a force fairly evenly through the whole frog. Perhaps something like that, only a lot stronger, could be used to prevent a person from being squashed flat by acceleration forces and allow them to reach speeds close to the speed of light in an instant. I don't know if the application of such a levitation force would do damage to a person in itself though.

#### imatfaal

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #3 on: 18/09/2012 22:52:37 »
from a more practical point of view...

a. There is a maximum practical speed that can be obtained with a rocket.  A rocket works on throwing stuff out of the rear of the rocket - thus you need to carry with you all the stuff you want to throw out.

The mass of propellant M is defined with the Delta V (change in speed) the Ve (exhaust speed) and payload mass

$$M \approx P(e^{\Delta v/v_{e}}-1)$$

Even taking  $$\Delta v$$ to be 1/10 of the Speed of light 3*10^7 m/s and $$v_e$$ to be the theoretical max using future drives of 1*10^6 you would end up with a propellant mass needing to be ten million million times the payload.  And that is before taking relativistic effects into account .

2.  relativity!

it gets quite complicated quite quickly.  you need to decide whose frame the acceleration is measured in, who times the rocket etc, who measures distance.

If you accelerate constantly at 1g (lower case g is acceleration we feel on earth all the time - and NEVER to be confused with upper case G which is newtons gravitational constant) then after one year on the spaceman's clock he is going at 0.77c, but according to ground observer on earth he has been travelling for a year and 10 weeks.  After 2 years according to the spaceman he is doing 0.97c but the earthman thinks he has been gone for 3 years 9 months.  After 5 years the spaceman is damn close to the speed of light at 0.99993c - but the earthman has died as 83 years and 8 months have now passed on earth!

At 0.97c (ie after two years) the chances of anything surviving are practically zero - in fact they wont have even made it this far.  Anything in space hitting you when you are doing .97c will punch a hole straight through your space ship - and the background radiation of the universe will be blue-shifted upto very uncomfortable temperatures.
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#### wolfekeeper

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #4 on: 19/09/2012 03:14:06 »
There is a trick by which it's theoretically possible to accelerate at over 1000m/s^2, while experiencing only 1g.

Basically, you have to pack a flat disk in the nosecone of your vehicle with very dense material (black holes for example), and then adjust the crews position relative to that to nearly match the vehicle acceleration.

#### CliffordK

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #5 on: 19/09/2012 04:43:01 »
There is an experiment which involves levitating a frog, and I'm guessing that it applies a force fairly evenly through the whole frog. Perhaps something like that, only a lot stronger, could be used to prevent a person from being squashed flat by acceleration forces and allow them to reach speeds close to the speed of light in an instant. I don't know if the application of such a levitation force would do damage to a person in itself though.
So far it has been possible to levitate a frog at 1G, not a human at 100G, or 1000G.

The forces may not be as uniform as you think as it depends on magnetism which is related to the distance from the magnet.

#### Phractality

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #6 on: 19/09/2012 05:39:46 »
There is a trick by which it's theoretically possible to accelerate at over 1000m/s^2, while experiencing only 1g.

Basically, you have to pack a flat disk in the nosecone of your vehicle with very dense material (black holes for example), and then adjust the crews position relative to that to nearly match the vehicle acceleration.
Unfortunately, your nosecone mass would have to be many times greater than the mass of Earth, and you would need an engine powerful enough to accelerate that mass at over 1000 m/s^2. Also, the gradient of gravity near that mass would be so great that it would tear the person apart with tidal force.

A more feasible solution would be to first fill the person's lungs with CFCs so he can breathe liquid which would make the lungs less buoyant. Next, freeze the body solid inside a block of strong frozen material. The acceleration limit would be determined by the relative strengths and buoyancies of muscle, fat and bone. I am confident that 100 g would be achievable, but probably not 1000 g.
Imagination is more important than knowledge. Einstein

#### CliffordK

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #7 on: 19/09/2012 07:30:42 »
Keep in mind that an extreme high velocity aerobraking maneuver could be far more wicked than trying to create the initial acceleration.

We are nowhere near cryostasis.  Well, the freezing part is much easier than the reawakening.

Could we freeze and re-animate just a uterus?  One option would be to send fertilized eggs which presumably could withstand much higher acceleration (and deceleration) forces than a living adult human.

#### wolfekeeper

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #8 on: 19/09/2012 21:41:15 »
There is a trick by which it's theoretically possible to accelerate at over 1000m/s^2, while experiencing only 1g.

Basically, you have to pack a flat disk in the nosecone of your vehicle with very dense material (black holes for example), and then adjust the crews position relative to that to nearly match the vehicle acceleration.
Unfortunately, your nosecone mass would have to be many times greater than the mass of Earth, and you would need an engine powerful enough to accelerate that mass at over 1000 m/s^2.
Yes, but there's tricks you can pull with black holes that would give you an awesomely powerful engine; you can get extremely high energy from propellant. The hard bit is avoiding irradiating the passengers too strongly, but it may be possible.
Quote
Also, the gradient of gravity near that mass would be so great that it would tear the person apart with tidal force.
No, it depends on the arrangement of the black holes. You can control them by charging them up, and hold them where you want them. If you place them appropriately, you can dial in the tidal forces as you wish.

#### BishopE5

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #9 on: 22/09/2012 02:41:34 »
The first question is how great of acceleration could a person withstand and survive with adequate pressure suits and other devices.

Somewhere around 65Gs is considered newbielink:http://hypertextbook.com/facts/2004/YuriyRafailov.shtml [nonactive] (which is of short duration).

For my calculations, I tend like to take 100Gs, or about 100*10m/s2, or 1000m/s2 in optimum conditions.  This may in fact be a gross overestimate as a person may be able to survive such high forces with adequate protection for a brief period, but not for an extended period.  But, perhaps one could ramp up the acceleration so it would not be quite the same as an impact event.

The speed of light is about 300,000,000 m/s.

You can simply divide the speed of light by your acceleration,  (300,000,000 m/s) /  (1000m/s2), and one gets 300,000 seconds, and one gets about 83 hours of acceleration to reach the speed of light (or thereabouts).

If you limited your acceleration to 10G's, or about 100 m/s2, then one gets 3,000,000 seconds, or about 35 days to reach the speed of light (or thereabouts).

newbielink:http://www.thenakedscientists.com/forum/index.php?topic=42938.msg379155#msg379155 [nonactive] that if one put a tunnel through the center of the moon, and an electromagnetic accelerator in the tunnel capable of constant acceleration, then after about 83 seconds of 100G acceleration, one would blast out of the other side of the moon at about 42km/s, or around 1/10,000th the speed of light.

What if you accelerate by 1 G and allow the speed to remain constant, and there by reach equilibrium. Then, accelerate once more by 1G, and thereby technically you'd be at 2 G, but would experience 1G acceleration. Assuming all changes to acceleration are this way, would it be possible to withstand fatal G levels if you experience change in this way?

#### CliffordK

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #10 on: 22/09/2012 04:38:26 »
What if you accelerate by 1 G and allow the speed to remain constant, and there by reach equilibrium. Then, accelerate once more by 1G, and thereby technically you'd be at 2 G, but would experience 1G acceleration. Assuming all changes to acceleration are this way, would it be possible to withstand fatal G levels if you experience change in this way?

NO

Both NASA and the Airforce (presumably in different countries too) have centrifuges to simulate different forces of acceleration.

These circular centrifuges would simulate the linear acceleration one might experience in a rocket acceleration (or deceleration).  For example, the Blackout threshold of 9 or 10 G's can be determined quite easily with a centrifuge G-Force simulator, even with slow ramping up of the acceleration.

I would think that an impact event might be somewhat different from a controlled gradual acceleration.  However, experiencing high acceleration for days, or months would be very hard on a person.

#### wolfekeeper

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #11 on: 20/11/2012 00:20:06 »
Incidentally, you can accelerate people, even untrained people, at about 14g without killing them or them passing out.

If you accelerate them flat, rather than with them sitting up you avoid centrifuging the blood away from their brains, something that is rapidly fatal.

But you can only do that safely for a few minutes, even so.
« Last Edit: 20/11/2012 00:22:43 by wolfekeeper »

#### jumper2020

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #12 on: 03/01/2014 19:40:36 »
Everyone has got it wrong. What you would have to do is accelerate the ship to near the speed of light without any passengers aboard. You don't need to worry about cyrogenics or maximum g-forces. You would do so while orbiting earth from some distance and then drop a person into the ship when it is traveling at near the speed of light, or even say 75% of that speed. (I know this doesn't exactly answer the original question but there's no need to propel the person via this method)
« Last Edit: 03/01/2014 19:43:48 by jumper2020 »

#### David Cooper

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #13 on: 03/01/2014 20:52:40 »
How does that not kill them? [Mind you, in the question at the top it's about accelerating them to the speed of light, and if they could actually reach that speed, the length contraction would flatten them to the point that their arrangement of atoms would be destroyed and they would be killed anyway.]

#### CliffordK

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #14 on: 03/01/2014 21:02:04 »
You would do so while orbiting earth from some distance and then drop a person into the ship when it is traveling at near the speed of light, or even say 75% of that speed. (I know this doesn't exactly answer the original question but there's no need to propel the person via this method)
That would be like standing in front of the Schwerer Gustav 80 cm rail gun, and expecting to catch the projectile as it was launched at 1,800 mph

Except, of course, it wouldn't be traveling at 10x as fast as an Indy 500 race car, but would be traveling at 3 million times as fast as an Indy 500 car.

Furthermore, the centrifugal force on a body orbiting the Earth, Jupiter, or the Sun at close to the speed of light would be extremely high.  Far greater than the gravitational pull of the body, and one would have to expend an extraordinary amount of energy to maintain orbit.  The only body that one could effectively orbit at the speed of light would be a black hole.  Or, perhaps try to accelerate along some kind of an orbital rail, still the forces on the rail would be so high that it would tear it apart.

The centripital acceleration of an orbital object is:

$$a = \frac{v^2}{r}$$

Substitute in 299,792,458 m/s for v.
What orbit do you want?  The lunar orbit would be 384,400 km.

a = (299,792,458 m/s)2 / 384,400,000 m = 233,807,278 m/s2 = 23 million G.  You can subtract 1 G for Earth's gravity, and you still have to add a lot of force to maintain the orbit.
« Last Edit: 03/01/2014 21:03:40 by CliffordK »

#### evan_au

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #15 on: 03/01/2014 21:27:41 »
LOL
like a bug on a windshield....
or (more likely) like an atomic bomb exploding...
I calculate a 70kg human would explode with the equivalent of 850 Megatons of TNT if they were instantly accelerated to 75% of lightspeed.

#### jeffreyH

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #16 on: 09/01/2014 03:09:03 »
LOL
like a bug on a windshield....
or (more likely) like an atomic bomb exploding...
I calculate a 70kg human would explode with the equivalent of 850 Megatons of TNT if they were instantly accelerated to 75% of lightspeed.

Well no point fitting windscreen wipers to clear the dead bugs then. And I thought the wipers would be a neat touch.
Fixation on the Einstein papers is a good definition of OCD.

#### jumper2020

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##### Re: How fast could a human be propelled to light-speed non-fatally?
« Reply #17 on: 29/03/2014 02:16:37 »
Of course, using extant technology it would kill them. Teleportation to a zero acceleration ship wouldn't