PDA

View Full Version : Human Acceleration Light speed?



Quickshift
2008-Oct-02, 04:47 PM
How long would it take for a person to accelerate to the speed of light with out destructive g-force killing them? I don't know how much g-force the human body can stand, I'm sure it could be calculated from there.

grant hutchison
2008-Oct-02, 04:50 PM
How long would it take for a person to accelerate to the speed of light with out destructive g-force killing them? I don't know how much g-force the human body can stand, I'm sure it could be calculated from there.Forever.
Under special relativity, there is no level of acceleration that will take a material object to light-speed in a finite time, because the required energy is infinite.

Grant Hutchison

PraedSt
2008-Oct-02, 05:05 PM
Er...not sure what you mean exactly.

Even if light-speed were possible, you could pick any arbitrary acceleration. All that would change would be the time needed to get you to your desired speed.

No destructive acceleration necessary

Quickshift
2008-Oct-02, 05:13 PM
I'm under the impression you can't accerate to LS, your either LS or not, even if you leave out the required energy and because relativity does it's time warp thing a finite time is unachievable? Am I on the same page?

Quickshift
2008-Oct-02, 05:17 PM
I need to rephrase, instead of level of acceleration, safe maximum rate of acceleration.

Metricyard
2008-Oct-02, 05:24 PM
I need to rephrase, instead of level of acceleration, safe maximum rate of acceleration.

Why not 9.8m/s?
Same as gravity, no one would notice the difference, except that it would take a very long time. A VERY long time.

phunk
2008-Oct-02, 05:28 PM
If you set your goals to something possible, say 99% of light speed, then it would take roughly a year at 1G.

PraedSt
2008-Oct-02, 05:33 PM
I need to rephrase, instead of level of acceleration, safe maximum rate of acceleration.

They've done some testing on this one. For pilots, astronauts and so forth.
You could look it up...probably in wiki somewhere.

Safe peak will obviously be much higher than safe sustained acceleration.

Find that number, then stick it in this formula:

Time taken= (Desired final velocity)/(safe sustainable acceleration)

I am assuming you are starting at rest.

Hope this helps

NEOWatcher
2008-Oct-02, 05:35 PM
Er...not sure what you mean exactly.
....
Even if light-speed were possible, you could pick any arbitrary acceleration. All that would change would be the time needed to get you to your desired speed.

Exactly...

2 parts to the question:
What is the highest arbitrary acceleration you can use without harming someone?
and
How long would it take at that acceleration?

I think phunk was close to answering by using 99%LS, but maybe 8G or something similar would be close enough.

Anyway; how long would it take under human acceleration (ie pedalling) like the title says? :lol:

Quickshift
2008-Oct-02, 05:40 PM
I see what you mean Neo, my title was a bit misleading

PraedSt
2008-Oct-02, 05:43 PM
Anyway; how long would it take under human acceleration (ie pedalling) like the title says? :lol:

Heh. Maybe a year on Discworld. Light travels slower there.

astromark
2008-Oct-02, 06:31 PM
Notice; We are not telling you the answer.... We are telling you that the question can be answered, but you have to do the work. Here on Earth we live in a One gravity world. The human body can withstand Nine g's briefly. A sustained acceleration that would be required to attain Light speed could not be more than four g's.
BUT this is NOT possible. The amount of energy required to accelerate matter to light speed is not available.

astromark
2008-Oct-03, 11:36 AM
I am still not actually answering the OP's question but is acceleration expressed as km / sec. multiplied by km / sec. to get you that rate of acceleration. Deciding at what rate is the human body safe for a sustained length of time at above the one g we are accustomed to is a question I do not have the answer to. It has been found that zero gravity has some dire effect on the space traveler. Some rehabilitation is required for recovery from extended space flight periods.

Quickshift
2008-Oct-03, 02:14 PM
Am I going in the right direction here?
I believe Phunk is correct in using the constant accelleration force of 1 g to keep our travelers happy. It would create a kind of gravity effect, provided the travelers are in a head first position in the direction of motion instead of the siting in the car position. I came up with the time of 1.77 years to reach 670,616,629.398 mph. which would be roughly equal to 0-60 mph in 5 sec. It would be shorter time to our travellers according to Theory of Relativity. Hitting a pothole at this speed would be worse than being a bug on a windshield (windscreen).

grant hutchison
2008-Oct-03, 06:33 PM
Trouble is, a constant 1g as measured aboard your spaceship will be a steadily decreasing acceleration as observed from outside: so your spaceship can never reach lightspeed. If β is the required velocity as a fraction of the speed of light, a is the constant acceleration aboard ship and c is the speed of light, then the time necessary to reach β as measured by a stationary observer is:

t = cβ/(a√[1-β])

The corresponding time aboard ship is:

t' = c.arctanh(β)/a

Both of these climb towards infinity as β approaches 1.

Grant Hutchison

Sam5
2008-Oct-03, 06:54 PM
How long would it take for a person to accelerate to the speed of light with out destructive g-force killing them? I don't know how much g-force the human body can stand, I'm sure it could be calculated from there.

Leaving out "relativity", I've read that it will take a little less than a year to reach light speed while accelerating at a constant 1g.

But considering "relativity", it can't be done at all.

Quickshift
2008-Oct-03, 06:57 PM
Trouble is, a constant 1g as measured aboard your spaceship will be a steadily decreasing acceleration as observed from outside: so your spaceship can never reach lightspeed. If β is the required velocity as a fraction of the speed of light, a is the constant acceleration aboard ship and c is the speed of light, then the time necessary to reach β as measured by a stationary observer is:

t = cβ/(a√[1-β])

The corresponding time aboard ship is:

t' = c.arctanh(β)/a

Both of these climb towards infinity as β approaches 1.

Grant Hutchison

All I can say is thanks for expanding on the answers.
I asked the question out of curiosity nothing more, it's all good!

PraedSt
2008-Oct-03, 07:09 PM
Quickshift Noticed you using Mph...be careful with that. We've lost one probe already... :)

Quickshift
2008-Oct-03, 07:53 PM
quickshift noticed you using mph...be careful with that. We've lost one probe already... :)

lol

mugaliens
2008-Oct-03, 09:05 PM
Er...not sure what you mean exactly.

Even if light-speed were possible, you could pick any arbitrary acceleration. All that would change would be the time needed to get you to your desired speed.

No destructive acceleration necessary

The question was, "How long would it take for a person to accelerate to the speed of light...?"

Grant got it right. The answer is, "forever."

PraedSt
2008-Oct-03, 09:45 PM
The question was, "How long would it take for a person to accelerate to the speed of light...?"

Grant got it right. The answer is, "forever."

Yeah :)

I'm trying to console myself with the fact that if you took any speed other than c, my answer would be fine!

Neverfly
2008-Oct-03, 09:55 PM
Yeah :)

I'm trying to console myself with the fact that if you took any speed other than c, my answer would be fine!

Your answer was fine actually. You included the qualifier "IF light speed were possible".

The Lode
2010-Feb-05, 03:55 PM
I did some calculations and at 2.8G or 27.45862m/per second squared, you could travel over 6 LY in only 1.5 years. I wonder if this would really work if the 1.5 yars meant ship's time or if the numbers really mean nothing in the real world?

noncryptic
2010-Feb-05, 04:53 PM
I did some calculations and at 2.8G or 27.45862m/per second squared, you could travel over 6 LY in only 1.5 years. I wonder if this would really work if the 1.5 yars meant ship's time or if the numbers really mean nothing in the real world?

It really is 1.5 years in the ship's time (assuming your calculations are correct).

And in the time of the rest of the universe it would take much longer.

And both are in the real world.

That's what the whole twin paradox thing is about.

speedfreek
2010-Feb-05, 05:53 PM
This article explains the concepts involved:

The Relativistic Rocket
http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

neilzero
2010-Feb-05, 07:46 PM
The Newtonian formula is v = at If a = 10 meters per sec per sec that likely is tolerable, very long term = a bit more than the gravity at Earth's surface = no one has tried 10 meters per second long term continuously. If t = ten million seconds; then the velocity v is 100 million meters per second = 1/3 the speed of light. Ten million seconds = 2777 hours = 116 days, ship time. The next 1/3 would take longer, back on Earth time, perhaps 250 days, but 116 days ship time. Protecting the ship from collisions with sub atomic particles or a grain of sand, is however impossible with present technology, so you can't survive much more than 1/3 the speed of light. Even 1/3 requires heroic efforts, that greatly increase the cost. Neil

joema
2010-Feb-06, 02:12 AM
How long would it take for a person to accelerate to the speed of light with out destructive g-force killing them? I don't know how much g-force the human body can stand, I'm sure it could be calculated from there.
To simplify things I assume you mean nearly light speed, as no material object can actually reach that speed.

It's a good question -- the simple answer is we don't know exactly because there's little human physiological data on long-term exposure to high G.

Obviously humans can withstand 1G, and given a torch ship http://www.projectrho.com/rocket/rocket3i.html or other hypothetical advanced drive system, it would take about 1 year at 1G acceleration to reach nearly light speed.

The G force humans can withstand varies based on time, body orientation and support structure. You can take 40 Gs for a short period -- John Stapp proved this on his own body: http://en.wikipedia.org/wiki/John_Stapp

From http://www.projectrho.com/rocket/rocket3b.html :
"Human subjects in Germany during World War II were subjected to 17 g's for as long as 4 minutes reportedly with no harmful effects and no loss of consciousness.

However 4 minutes @ 17 G isn't nearly long enough to reach light speed. To approach light speed you need acceleration for many weeks or months.

Thus the question becomes what's the highest G force tolerable by humans for that time (weeks to months), given optimum life support.

From the graph on the above page, it appears humans might be able to survive 4 G for long periods, if in a reclining position. At 4 G it would take about 1/4 a year or three months to approach light speed.

However using liquid immersion and liquid breathing, it appears humans might survive 20 G long term. This is advanced technology, but not science fiction: http://en.wikipedia.org/wiki/Liquid_breathing

At 20 G you'd reach nearly light speed in about 2.5 weeks. That's probably the limit using current materials and physiological knowledge.

The subject of human tolerance to sustained high G was addressed in the science fiction short story "Sky Lift", by Robert A. Heinlein: http://en.wikipedia.org/wiki/Sky_Lift

tonybone
2010-Mar-23, 02:32 PM
if you just do the maths it would take 353 days, 19 hours, 45 minutes and 22.99 to reach the speed of light falling through a (perfect) vacuum at earths gravity.
of course this assumes that you have a really big hole about 30,600 times deeper than the distance between the Sun and Earth, and big enough to hold the tanks about the size of 2 olympic size swimming pools for air, and the other 2 for expelled air.

grant hutchison
2010-Mar-23, 06:37 PM
if you just do the maths it would take 353 days, 19 hours, 45 minutes and 22.99 to reach the speed of light falling through a (perfect) vacuum at earths gravity.
of course this assumes that you have a really big hole about 30,600 times deeper than the distance between the Sun and EarthWell, that's what you get if you do the simple Newtonian calculation. It's not how gravity works in an inverse-square field around a point mass, nor how it works under general relativity. Nor is it what you get if you do the calculations for constant proper acceleration under special relativity.

Grant Hutchison

Noclevername
2010-Mar-25, 02:28 PM
BUT this is NOT possible. The amount of energy required to accelerate matter to light speed is not available.

Of course it's possible to get to 99% lightspeed, as Quickshift revised. It's not practical. We'd drain our entire civilization of resources doing it.

nice1
2011-Apr-20, 08:53 PM
Another concept may be to embrace light as a medium i.e. light takes x years to travel from a remote star to earth. Maybe we can focus that light as a beam i.e. through a cylical chamber. As light passes through the cylical chamber, and during this constant flow of energy, maybe we can accelerate at or faster than the speed of light as the light passes through the chamber.

There must be some great minds which can provide theories on this within the forum.

time to evolve is ever present.

WayneFrancis
2011-Apr-21, 05:28 AM
Why not 9.8m/s?
Same as gravity, no one would notice the difference, except that it would take a very long time. A VERY long time.

Actually not that long to get a good fraction of c....its just the very last bit that would take forever and then only to an external observer.

If you could maintain ~1g for what the local observer would experience 1 year then you'd be close to the speed of light but then you'd also be time dilated to the extreme and everything in the direction of travel would have high length contraction.

WayneFrancis
2011-Apr-21, 05:32 AM
Exactly...

2 parts to the question:
What is the highest arbitrary acceleration you can use without harming someone?
and
How long would it take at that acceleration?

I think phunk was close to answering by using 99%LS, but maybe 8G or something similar would be close enough.

Anyway; how long would it take under human acceleration (ie pedalling) like the title says? :lol:

Personally I'd rather spend 1 year at 1g then 1.5 months at 8g...I'm more likely to wake up after the 1 year :P

Tobin Dax
2011-Apr-24, 08:32 AM
In case Wayne (or anybody else) missed it, necromancy alert: Posts in this thread are 1-3 years old.