Nothing, not even and including Light

[Vladimir]

Couldn't find the original so I'll make a new one. Just read this article and remembered a thread in the Frontroom about Black Holes (or Dark Matter stars, but that's another subject). It appears that light can travel so fast, that it can actually travel backwards! :dizzy2:

http://www.spaceflightnow.com/news/n0605/14light/

[Avicenna]

What's the difference between travelling backwards and changing the direction when reflected? Their mirror analogy doesn't explain anything to me.

[Vladimir]

What's the difference between travelling backwards and changing the direction when reflected? Their mirror analogy doesn't explain anything to me.

You should have read the explanation of Black Holes as Dark Matter stars I found. I'm still recovering.

[Csargo]

Black Holes meh.

[Avicenna]

Black holes are neutron stars, not dark matter stars... at least that's what I've read and heard.

Dark matter is mysterious stuff. Black holes are just results of supernovas.

[InsaneApache]

Nope. Black holes are not neutron stars.

[Avicenna]

They're roughly the same, just originate from a larger red supergiant compressed into a similar or smaller size.

[Byzantine Mercenary]

yeah and all dark matter is, is an attempt to cover up the fact we still don't understand the universe :laugh4:

[InsaneApache]

They're roughly the same, just originate from a larger red supergiant compressed into a similar or smaller size.

Nope.


Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form from burnt-out stars, they do not glow. The collapse of the star causes the matter to be converted into mostly neutrons, hence the name neutron star.

Some neutron stars emit radio waves that pulse on and off. These stars are called pulsars. Pulsars don't really turn radio waves on and off--it just appears that way to observers on Earth because the star is spinning. What happens is that the radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.


Once a giant star dies and a black hole has formed, all its mass is squeezed into a single point. At this point, both space and time stop. It's very hard for us to imagine a place where mass has no volume and time does not pass, but that's what it is like at the center of a black hole.

The point at the center of a black hole is called a singularity. Within a certain distance of the singularity, the gravitational pull is so strong that nothing--not even light--can escape. That distance is called the event horizon. The event horizon is not a physical boundary but the point-of-no-return for anything that crosses it. When people talk about the size of a black hole, they are referring to the size of the event horizon. The more mass the singularity has, the larger the event horizon.

So they are not roughly the same.

[Geoffrey S]

The principle of creation is similar (ie. large star collapses inwards to form a far smaller area, creating a hugely dense object) but they aren't made of the same material. The core of a neutron star consists of neutrons, but I'm not sure what a black hole is made of; anything at such high densities is purely theoretical, since it can't be recreated here on earth.

[Avicenna]

They do exist though, since you can observe the field of gravity around them.

As Geoffrey said, they're created the same way. I think it's a red supergiant that's over 200 solar masses that supernovas that creates a black hole, and something in the region of 50-200 that creates a neutron star. It's all a matter of size.

Not just neutrons, neutrons and protons. It's a massive nucleus.

Neutron stars do glow actually, but not like a main sequence star. They glow because the particles forming them were very hot at the time of the supernova, and squashing them together, their combined heat makes them stay at very high temperatures without requiring a fuel such as hydrogen. Think of how things normally cool. It happens when they are surrounded by cooler objects, and so the heat dissipates. But in a neutron star, all the particles are as hot, so it will glow for quite a long time until all the heat energy is converted to light.

[Viking]

Way cooler than dark matter, is antimatter. Something that we actually know exists. :idea2:

[Avicenna]

Gah, why just antimatter? Why not go on with other antiparticles, such as positrons, anti-neutrinos, antiquarks, or even dark energy? I guess we're not sure that dark energy exists though.

[Vladimir]

The principle of creation is similar (ie. large star collapses inwards to form a far smaller area, creating a hugely dense object) but they aren't made of the same material. The core of a neutron star consists of neutrons, but I'm not sure what a black hole is made of; anything at such high densities is purely theoretical, since it can't be recreated here on earth.

That could be the reason some people think it's made of Dark Matter. Supposedly this theory is validated mathematically, although I'm not sure how it is.

[Quietus]

What's the difference between travelling backwards and changing the direction when reflected? Their mirror analogy doesn't explain anything to me. They were saying if you fire a 'pulse of light' into a fiber optic cable, as the 'pulse' enters the cable, two pulses are instantly propagated at the other end (one moving forward, another moving backward).

Hence the 'pulse' moving forward meant it travelled faster than light. The other 'pulse' moves backward from the other end as well.

The mirror and 'special materials' were only examples of light slowing down and different speeds in a medium.

The difference is when light is reflected it is 'travelling' the whole length with speed respective to the medium. In their example the light instantly appeared at the other end (forward and backward).

What I don't know understand is what's the difference between the 'pulse of light' and a laser. I know what a laser is.


As the pulse exited the laser, it was split into two

[Avicenna]

A pulse could be in all directions I guess, being just normal light. Radiated, so that not all is travelling in the same direction. If you use a laser, it's all going to go in one direction.

Just an idea, it's probably wrong.

[Viking]

Gah, why just antimatter? Why not go on with other antiparticles, such as positrons, anti-neutrinos, antiquarks, or even dark energy? I guess we're not sure that dark energy exists though.


All that is exactly anti-matter.

[Avicenna]

Ah, thank you for that. I'm quite new to the world of complex physics, just a few weeks in.

@Quietus: sorry, I gave the wrong meaning. I asked a friend of mine this weekend, a pulse of light is when it goes on and off, and on single flash is a pulse. A normal laser is continuous, but even if it was a pulsed laser, they just had to say the word 'pulse'. Laser is light anyway, so it might well be laser.

[_Martyr_]

I would say the pulse being refered to is something similar to Dirac's Delta Function in a laser. An extremely short burst of laser light...