Question about Time

[falkor]

If somebody healthy named Bob lived just outside the event horizon of a black hole for all his life, generations of distant observers would have been born and died during Bob's single lifetime. The question is: according to Bob's clock, how long would he have lived for? Hundreds, thousands or millions of years maybe? Or would it seem to Bob just like any ordinary lifetime of 90 years or so?

 

[falkor]

Damn, that makes me so confused... if Bob's time is slowed right down then how can he not be aware of it!?

 

[SpeedFreek]

As an example, lets consider a more classic scenario first.<br /><br />A journey of 5 light years at .866 c.<br /><br />If you travel at 86.6% of c, a journey of 5 light years should take 5.55 years. An observer on your planet of departure would (after subtracting for the time-lag caused by the speed of light) see you take 5.55 years to travel 5 light years at 86.6% of c.<br /><br />But you would find that, at that speed, the journey would only take you half that time - 2.77 years. You wouldn't feel that time was passing slower as you travelled, but you would know time had passed slower when you got there in half the time you expected!<br /><br />Actually, due to an associated process called length contraction, at 86.6% of c you would see the distance shrink in your direction of travel, so your destination would only look 2.5 light years away, and so you would see no problem in reaching there in 2.77 years at that speed. <img src="/images/icons/smile.gif" /><br /><br />Now, onto Bob, sitting just outside the event horizon for a black hole. If time was slowed for him in a way that his lifetime lasted 10,000 years, he would in himself feel he had a normal lifetime, but the rest of the universe would be moving really quickly if he were able to observe it! If he could see the earth orbiting the sun, he might see it make over 100 orbits a year, but his watch would move as normal, to him. (It's actually more complicated than this, as any information reaching him would be time-dilated too, but this approximation is good enough! Basically, he might not be able to tell in any way that the rest of the universe was moving faster, only the rest of the universe could tell that he was moving slower!) He wouldn't feel like he had lived for 10,000 years, anyway. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[alokmohan]

In the first chapter of Kip Thormes book on black hole there is nice preface with rhis topic.

 

[weeman]

<font color="yellow"> Damn, that makes me so confused... if Bob's time is slowed right down then how can he not be aware of it!? </font><br /><br />Bob's time appears slowed only to other observers who are out farther from the event horizon. To Bob, the time ticks by at normal pace, yet when he rockets himself out to the other observers, all he finds is a bunch of space suits with skeletons in them <img src="/images/icons/wink.gif" /><br /><br />Much more time will have passed for the rest of the world. Bob might return to Earth and find that Mankind no longer exists, perhaps from extinction, or we've become so advanced that we've left Earth altogether. <br /><br />Basically, Bob will only be aware that time was slower for him when he sees how much time has passed for everything else. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

 

[alokmohan]

If you stay around a black hole, you live 4 billion years.I will quote Kip in subsequent post.

 

[mindmute]

hmm.. if one could get close to a black hole, and then <i>somehow</i> get away from it, then i guess it would be sorta like traveling into the future, as time went so darn fast while you were visiting the black hole.

 

[kyle_baron]

Hi alokmohan. I thought I'd mention to you that this month's Discover Magazine (November 2007) has a nice interview with Kip Thorn on page 51. I'll quote some of the interesting statements he made:<br /><br /><i>A big misconception is that a black hole is made of matter that has been compacted to a very small size. That's not true. A black hole is made from warped space and time......the stars matter is destroyed at the holes center, where space time is infinitly warped. There's nothing left any where but warped space time.<br /><br />General Relativity says wormholes could exist. When we combine General Relativity with Quantum Theory, we find moderatly strong evidence that wormholes can not exist after all. But we just don't know for sure yet.<br /><br />If any highly advance civilization attempts to make a time machine for backward time travel, quantum effects will cause the time machine to begin to self destruct explosively at the moment you activate it. We don't know if the explosion is strong enough to always destroy the time machine. We will have to have the full quantum theory of gravity (General Relativity + Quantum Mechanics, yet to be understood) to find out the answer.</i><br /><br /> <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[weeman]

<font color="yellow"> hmm.. if one could get close to a black hole, and then <i>somehow</i> get away from it, then i guess it would be sorta like traveling into the future, as time went so darn fast while you were visiting the black hole. <br /> </font><br /><br />Yes. It is, in a way, a form of time travel. The only thing that you can't do with a black hole is travel into the past. All you can do is travel to the black hole, wait a while, then travel back to Earth to find that you've taveled thousands to millions of years in the future. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>