20110129, 05:22  #1 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
1011001011101_{2} Posts 
Does the photon ever reach the spaceship?
One of my minions came to me with this.
A spaceship is accelerating in a straight line at a constant 1g (local time frame within the ship) in a vacuum in flat space. At t=0 the position of the spaceship is x=0 and the velocity of the spaceship is v=0. The ship immediately starts accelerating at 1g at t=0. Some time t_{p}>0 later a photon is sent directly towards the spaceship from point x=0. Assuming GR and SR are true then ... 1) The photon reaches the spacecraft: (a) Never. Photons a wussies and can't catch me. (b) Depends upon the value of t_{p}. Did someone say "dilation"? (c) Always. Photons are crazy fast dude. 2) Show your working to prove either (a), (b) or (c) is true. [size=1]Yes, the answer is available with a search engine. Yes, you are cheating if you use a search engine.[/size] 
20110129, 13:09  #2 
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
10AB_{16} Posts 
1) (c)
2) I have no actual work or proof to show, I just guessed based on the fact that you can never go faster than c, so you shouldn't be able to outrun a photon. I'd like to see the real answer though. :) I googled to no avail. Last fiddled with by MiniGeek on 20110129 at 13:09 
20110129, 15:27  #3  
Jun 2003
7×167 Posts 
Quote:
This doesn't suffice to demonstrate that a spaceship could do this while maintaining constant (local) acceleration, but I'm pretty certain it could. 

20110129, 15:51  #4 
I quite division it
"Chris"
Feb 2005
England
31·67 Posts 
My reasoning:
The spaceship can never accelerate to c. The photon always approaches the spacecraft at c. Therefore (c). If the ship was, say, 1 light year away when the photon was released then it would take 1 year to catch up, from the point of view of the pilot. Despite the ship's acceleration. Of course, I'm bound to be wrong! 
20110129, 16:19  #5  
Bamboozled!
"πΊππ·π·π"
May 2003
Down not across
7^{2}×11×19 Posts 
Quote:
I won't reveal an answer but will hint that, as in so many cases in relativity, drawing a spacetime diagram will probably be very helpful. Paul 

20110129, 16:29  #6 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
5725_{10} Posts 

20110129, 16:42  #7 
Bamboozled!
"πΊππ·π·π"
May 2003
Down not across
7^{2}·11·19 Posts 

20110129, 16:50  #8 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
5^{2}·229 Posts 
I am interested to see what happens. I also disabled emails.

20110130, 15:11  #9  
Undefined
"The unspeakable one"
Jun 2006
My evil lair
5^{2}·229 Posts 
Quote:
Quote:


20110130, 16:30  #11 
Aug 2002
Buenos Aires, Argentina
2×661 Posts 
Since the Universe is expanding, after some time t[sub]z[/sub] the photon cannot reach the spaceship. The value of this t[sub]z[/sub] is an exercise for the reader.
Last fiddled with by alpertron on 20110130 at 16:32 
Thread Tools  
Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
How long will it be until MM61 is within practical reach of PRP/primality testing?  Stargate38  Operazione Doppi Mersennes  14  20200129 20:35 
photon teleportation puzzle  jasong  jasong  9  20150729 22:14 