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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am reading the book SpaceTime and I have some questions for those that think they can answer in a coherent manner. It is not that I don't understand (well, some things I don't), but I am hoping I can get a clearer answer.
For example, if light does not escape from a black hole, does the photon actually stop moving and hence the speed of light at the horizon is zero or does the wavelength just expand to a frequency of zero? If the latter, does the photon keep moving forward?
Another question: if one is holding a mirror in front of himself and is running at the speed of light does the reflection disappear or again does the wavelength shift?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
For example, if light does not escape from a black hole, does the photon actually stop moving and hence the speed of light at the horizon is zero or does the wavelength just expand to a frequency of zero? If the latter, does the photon keep moving forward?
Actually EM wave falling into a hole increases it's frequency (but speed is unchanged), until it hits the horizon and gets ultimately added to black hole's mass. See gravitational red and blue shift:
Gravitational time dilation counters this so we can't see frequency change if we are riding the photon (but it can be seen from a distance for example).
Another question: if one is holding a mirror in front of himself and is running at the speed of light does the reflection disappear or again does the wavelength shift?
You can imagine a bunch of things 'going wrong' when you reach speed of light, time is stopped, your mass is infinite, so the mirror thing could be imagined rather artistic.
Wavelength of light in front of you would be 'infinite' ,as your speed is equal to c, your mass is infinite and you have stopped the time (things that are pretty impossible in our plane of existence).
Conbining all that I guess you won't be able to see anything in the mirror...
Registered Member #175
Joined: Tue Feb 14 2006, 09:32PM
Location: Sudbury, ON
Posts: 111
The speed of light in and around a black hole is always 3*10^8 m/s, same as everywhere else (barring density issues, and rounding up.) So what happens? Look at it with from the particle perspective; ballistic trajectory, decayed orbits , that stuff. And yes, or so the theory goes, there is going to be a 'light sphere' just in or around the event horizon, where space curves such that the photons have a stable orbit around the black hole.
For the second question, you can't go at the speed of light... so let's imagine you're holding the mirror in front of you, and going at .99999999c. So, does the wavelenth shift? Nope. You and the mirror are in the same frame of reference, so your reflection will appear AS NORMAL. Behind you, on the other hand... redshift galore, and all the other fun optical abberations expected from such speeds.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Black hole theories upset many people’s notions – personally I think Hawking’s theories may show some odd correlations and contradictions. However in classic terms the entire earth could only form a theoretical black hole with a radius of about 8.86mm as R=(2GM)/c^2.
In terms of Einstein’s ideas, the classic light clock problem may be illuminating.
Ultimately people like to hang on to traditional models even if they may be subject to deemphasis in mainstream theoretical sciences.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
One of those contradictions, elucidated by Stephen Hawking, is that black holes not only absorb matter and radiation, but emit radiation and matter as well. If I remember correctly, there are two mechanisms at work....
1. Blackbody radiation. The black hole has a "temperature" and emits radiation characteristic of blackbody radiation. In the coordinate frame near the event horizon, the photon could look like a gamma ray, but by the time it "climbs" out of the BH gravitational well, it is redshifted into the IR of mm-wave band. Hence, the BH, over time, will lose mass and "evaporate" when it falls below its critical mass. There's no escaping the Second Law of Thermodynamics! ;)
2. Virtual particle/antiparticle pair generation. Near the event horizon, particle/antiparticle pairs can be spontaneously generated. One is swallowed up by the BH and the other is ejected into some type of orbit around the BH.
I love studying modern physics, but unfortunately many of the papers that I've seen in the modern literature are heavy on arcane mathematical treatment and light on understanding... Or maybe I'm just a bit slow and fossilized!
wrote ...
Another question: if one is holding a mirror in front of himself and is running at the speed of light does the reflection disappear or again does the wavelength shift?
You will see no change in your reflection if you are running along a line that passes through you and the mirror, because for you and the mirror, light always travels at speed c. And you will never be able to reach speed c.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
He gets to the end of the first paragraph before saying something completely wrong. "if we couldn't see vibrations of electron energy, the moon would be completely unnoticed. " What? no tides? I stopped reading there.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
gravityboy wrote ... The Raisin bread model of expansion is also a farce, it can't expand because if the universe were a loaf, it would have to be an infinite sized loaf, or an infinite amount of loaves with no room left to expand.
I think his head got stuffed with an infinite sized loaf that left no room for brains.
I'm wary of any theory that tries to describe the fundamental structure of the universe in terms of everyday objects like loaves of bread and dodecahedrons. Everyday objects exist entirely inside the structure of the universe, so you can't make a theory of the structure of the universe out of them, any more than you can make pi by adding integers together, or make the Haynes manual for your car out of the parts of your car.
A good theory of the universe would have to be made out of something finer and more abstract than material objects. Einstein made his theory of relativity out of thought experiments, and pure thought is pretty fine stuff. You don't need any mathematics at all to understand it though: all you need to take on board is that the speed of light always measures the same no matter where you are or how fast you're travelling.
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