distant planets

[shamrock_uk]

Originally posted by Vaskez: Also, I think I've read that light travels slower near the immense gravity of a black hole and since our perception of time passing depends on the speed of light (because this determines electromagnetic wave propagation and other physical phenomena) if someone were to reside near a black hole without getting sucked in, they could effectively time travel.

Black holes are just messed up. Like if you were watching someone fall into one, you would see them get slower and slower and eventually they would appear to just stop right on the edge.

If you manage to get anywhere near it, you would, unfortunately be ripped to pieces by tidal forces long before entering it. Even if we imagine you're unlucky enough to fall into a really large black hole (like maybe the size of the solar system so the tidal forces aren't strong because they're spread out) then as you fall in, time outside the blackhole would appear to speed up to an infinite amount. This has the unenviable effect of shifting all the radiation in the universe to the nasty 'gamma-ray' end of the spectrum and you get to receive several billion years worth of it all at once Not that healthy all things considered

[Vaskez]

Shamrock - well, duh

I was just fantasizing...

[Iron Greasel]

Um. If time stops (looking from the outside) just before the event horizon, how do black holes get bigger? If we have a large asteroid just about to enter a black hole, and we look at it from the outside, just before it would enter the hole it stops as time gets too messed up. If time is stopped, the rock never falls in the black hole. Thus there shouldn't be any large black holes, as the small ones don't grow.

Unless of course Einstein was wrong.

[Sir Goulum]

How can time slow down?!

[Vaskez]

Originally posted by Sir Goulum: How can time slow down?!

AGH! If you do this one more time I'm gonna strangle you! Read the bloody posts! [img]tongue.gif[/img] [img]tongue.gif[/img]

Time depends on speed of light, gravity of black holes affects speed of light...read back a few posts...

[shamrock_uk]

Bah, well, if you're going to ask awkward questions... [img]tongue.gif[/img]

The answer lies in the fact that time isn't an absolute concept. To you, he may appear to be slowing, but for him, you're speeding up [img]smile.gif[/img] For the poor sod himself, he's just trundling towards the black hole in normal time.

I believe the reason that he appears to freeze on the edge is because once he's passed over the event horizon the reflected light from his body (how you normally observe things) can no longer reach you - thus (even if time wasn't relative) you never see him pass the event horizon. It's not that he hasn't fallen in, just that from your perspective time is running so slowly for him that he hasn't reached the event horizon yet. The light from his body gets slowed and stretched increasingly by the black hole as he falls into it ever faster - if I remember correctly I think he will actually turn invisible as the light moves out of the visual range of the spectrum.

[Sir Goulum]

Originally posted by Vaskez: quote: Originally posted by Sir Goulum: How can time slow down?!

AGH! If you do this one more time I'm gonna strangle you! Read the bloody posts! [img]tongue.gif[/img] [img]tongue.gif[/img]

Time depends on speed of light, gravity of black holes affects speed of light...read back a few posts... [/QUOTE]I was! It didn't tell me anything. [img]tongue.gif[/img]

Shamrock- Ok, I get it now... I think. So, it just appears that he's slowing down because the light is slowing down?

[shamrock_uk]

Originally posted by Sir Goulum: How can time slow down?!

Believe it or not, it's even been measured. We put an atomic clock on a rocket and made it travel fast and when it came back to Earth it had lost time.

As for the 'why', there's probably only a handful of people on the planet who could explain that - 'because Einstein said so' works for me

There are lots of thought experiments to help you get your head around it, involving light beams and moving trains and the like. I've tried googling for an easy link but there's always going to be a certain amount of pain involved in thinking about it [img]smile.gif[/img]

[Iron Greasel]

Originally posted by Sir Goulum: How can time slow down?!

Bacause otherwise you would notice that you were moving. If you travel at the speed of light holding a mirror on front of your face, you cant see a reflection. According to some old rule, if you travel steadily, without braking, accelerating or turning, you can't know whether you are moving or staying at one spot (without looking out). Travelling at the speed of light would break this rule. Einstein explained (though I think explaining just made it more complicated) this by doing freakish things to time. Apparently the universe is willing to bend a few rules to avoid breaking others.

There was other stuff too, about trains moving at the speed of light and so on, but I can't remember much of it.

[Iron Greasel]

Originally posted by shamrock_uk: Bah, well, if you're going to ask awkward questions... [img]tongue.gif[/img] The answer lies in the fact that time isn't an absolute concept. To you, he may appear to be slowing, but for him, you're speeding up [img]smile.gif[/img] For the poor sod himself, he's just trundling towards the black hole in normal time. I believe the reason that he appears to freeze on the edge is because once he's passed over the event horizon the reflected light from his body (how you normally observe things) can no longer reach you - thus (even if time wasn't relative) you never see him pass the event horizon. It's not that he hasn't fallen in, just that from your perspective time is running so slowly for him that he hasn't reached the event horizon yet. The light from his body gets slowed and stretched increasingly by the black hole as he falls into it ever faster - if I remember correctly I think he will actually turn invisible as the light moves out of the visual range of the spectrum.

So basically the "voluteer" falls in the hole, but the light reflecting from him doesn't? I guess that could make sense.