Text 28690, 89 rader
Skriven 2007-05-07 17:16:00 av JIM HOLSONBACK
     Kommentar till en text av JOHN HULL
Ärende: Muslems
===============
-=> On 05-05-07 19:09 JOHN HULL wrote to JIM HOLSONBACK <=-

 JH> JIM HOLSONBACK -> VERN HUMPHREY wrote:
 JH> Hello, Vern.

 -=>> On 05-04-07 11:11 VERN HUMPHREY wrote to JOHN HULL <=-

---<   >
 VH>> If you accelerate at 32.2 ft/sec/sec, how long will it
 VH>> take you to reach a speed of 186,000 miles per second?

 JH> speed of light: c= 186,000 mi/sec x 5,280 ft/mi = 982,080,000 ft/sec

 JH> Since for constant acceleration, v=at  and t=v/a

 JH>  t= 982,080,000 ft/sec / 32.2 ft/sec^2 = 30,499,380 sec,

 JH> and 30,499,000 sec / (60 sec/min x 60 min/hr x 24 hr/day) =  353 days.

 JH> (Close to Jeff B's SI answer, but he rounded up g from 9.8 to 10.0)

 VH>> Then applying the forumla D = 1/2 G t^2 how far would you have come?

 JH> Here's an easier way: Since the v=at velocity change is linear wrt time
 JH> for a constant acceleration, the avg velocity while accelerating was
 JH> half the speed of light.

 JH> 0.5c x 353days/365 days/year = 0.48 light years in 353 days.

 VH>> Remember, having reached the speed of light, you must now
 JH> DECELERATE to
 VH>> make landfall on your planet -- which will take just as long as
 JH> it took
 VH>> you to reach light speed.

 JH> Same as above, but in reverse.  Another 353 days at -1g accel.

 JH> Even I know that you can't accelerate a mass greater than that of a
 JH> photon to the speed of light using conventional means.

When did anyone say we had to use "conventional" means?  Heck, with what
are now conventional means, we can't get a space payload up to anywhere
near the speed of light.  All Jeff and I did was to work out the math
for the problem as Vern had presented it, using Vern's assumptions.

 JHu> The problem is time dilation.  Even if you can reach near light
 JHu> speed (over .75C) a 100 year journey (apparent passage of time
 JHu> to the travelors aboard ship) would cause several thousand
 JHu> years to pass on earth.

That's just way off.  You might want to read up a bit on the "Lorentz
factor". I calculate that the Lorentz factor for v = 0.75c is 1.52.  The
velocities have to get a _lot_ closer to c in order to have the effects
you are talking about.  Your scenario would require a Lorentz factor of
around 20-30. That doesn't happen until around 99.9% of lightspeed is
reached.

There's a fun little animated exhibition, "Time Dilation Demonstration
Applet" link  at the bottom of the Wikipedia article on time dilation -
the URL for it is

http://www.walter-fendt.de/ph11e/timedilation.htm

Going there, and setting the travel speed to 0.8c, for the 5-light hour
sample trip to Pluto,
travel time as seen from earth = 6.25 hours
travel time as seen on ship    = 3.75 hours

Keeping v=0.8c and scaling up to a hypothetical planet 50 light years
away, we get
62.5 years time as seen from earth
37.5 years time as seen on ship

Noting that these times do not include accel - decel times and speeds

 JHu> In theory, enough time would pass on earth that it is likely
 JHu> that we would discover some means to exceed light speed or fold
 JHu> space, and we would be able to go to the originial destination
 JHu> of the spaceship long before it actually arrived.

Now we're getting into Sci-Fi territory and too far in the future for me
to worry about.   I have a feeling that "wormhole" theory isn't going to
work out.

- - -  JimH.

... What is mind? No matter! What is matter? Never mind! - Homer S.
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