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Post by Elwood on Jan 21, 2005 22:31:21 GMT -5
The second is known as Hawking Radiation. Bigboy, I've got a question for you - I've understood that Hawking Radiation is used to explain how a black hole may, over long periods of time, "evaporate" and cease to exist. But in order to do that, the particle pair that appears must always drop the antiparticle over the event horizon and allow the particle to escape. Why the bias towards an antiparticle falling in? I would have expected equal odds for either the particle or antiparticle to fall in, causing on average no net change in the black hole's mass. Do you know what I'm misunderstanding here?
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Post by Bigboy on Jan 22, 2005 6:41:15 GMT -5
You are absolutely correct - it is equally probable that either the particle or antiparticle will fall into the black hole. I actually made a mistake - the formation of virtual particle/antiparticle pairs actually does violate the second law of thermodynamics. However quantum uncertainty allows for this over extremely small timescales (the pair would normally anihilate practically immediately). [NB Antimatter has a normal positive mass.] However, when a particle from the pair crosses the event horizon of a black hole, they can no longer do this. The escaping particle maintains it's normal mass, but the particle that has fallen into the black hole must restore conservation of energy by giving itself a negative mass-energy (thus reducing the mass of the black hole). This is why the particles are considered 'virtual' until thier interaction with an object such as a black hole. It's all down to quntum fuzziness, but I'm not going to sit here and pretend I understand all the ins and outs of quantum field theory
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John
Junior Member
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Post by John on Jan 22, 2005 12:47:28 GMT -5
I have this quantum physics book by this guy named Brian Greene, I want to read it but I've never gotten around to it, but I really should.
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Post by Elwood on Jan 22, 2005 19:47:59 GMT -5
The escaping particle maintains it's normal mass, but the particle that has fallen into the black hole must restore conservation of energy by giving itself a negative mass-energy That helps - thank you! Do you happen to know if there is a reason that the particle that falls into the black hole is the one to give itself a negative mass-energy, as opposed to the particle that is escaping? I would be tempted to once again think that there might be equal probabilities that either the captured particle or the escaping one would assume negative mass-energy. And as for how the particle can simply switch from a positive to a negative mass-energy, is that just some difficult to discern oddity of quantum theory? Perhaps along the lines of how some particle pairs are able to instantaneously align certain characteristics, such as their spins, even at far distances? Another comment - much earlier in this thread you mentioned the Big Crunch. You're probably aware that this theory of the "end of the universe" has fallen out of favor, replaced by the mainstream Big Chill, and the more radical Big Rip, which postulates that the repulsive force of dark energy grows over time, eventually overcoming binding forces at smaller and smaller scales. I love this quote from Robert Caldwell of Dartmouth, lead author of the Big Rip theory, explaining when various structures eventually fly apart (from space.com): -When the Milky Way begins to fly apart, there are 60 million years left. -Planets in our solar system will start to wing away from the Sun three months before the end of time. -When Earth explodes, the end is momentarily near. At this point, there is still a short interval before atoms and even their nuclei break apart. "There's about 30 minutes left," Caldwell said, "But it's not quality time." ;D
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Post by Bigboy on Jan 22, 2005 20:35:28 GMT -5
Good questions! The short answers are both "I dont know!"
but after a quick Google I found this:
"If one particle of a pair of virtual particles falls into a black hole and the other one doesn't they can't react back to energy and the escaping one becomes a real particle leaving behind a lack of energy in the vacuum. Somehow this "hole" in the vacuum energy has to be filled again - even in quantum physics the law of energy preservation can't be disobeyed for a longer time. So this "hole" draws energy from that black hole. But what kind of energy does a black hole without any spin or charge have? It's mass! Thus a black hole loses some of its mass after Einstein's famous formula E = mc².
Maybe you have noticed that that's no proof that the energy has to originate from the black hole. In fact we can't really prove it without a lot of quantum physics (It depends on the tunnel-effect). You just have to swallow it as being true. Another, yet similar, way to explain the black hole's loss of mass is that the particle that gets sucked into the black hole gains a negative mass and thus the mass of the black hole decreases..."
That's basically the limit of my understanding (my A Level Physics doesn't really stretch to advanced quantum theory!), and for the moment I'm quite happy with the lay person's explanations.
I've heard of the other end of the universe theories, and they all seem as likely as each other to me! If you like a bit of Sci-Fi, you should track down 'Time' by Stephen Baxter for a good story involving the entire history of the universe (he eventually adopts the big cool - don't worry this shouldn't spoil the end of the book ;D)
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Post by Liansky on Jan 27, 2005 16:05:37 GMT -5
Hi there BigBoy. I don't know much about physics, but i am kinda interested in something really really stupid.
You said that e=mcsqaured thing can show energy being emited using mass. So if mass is 4 before the match is lit, and 2 after the match is burned, how do you calculate e?? What about c sqaured?? Is that speed or velocity.. is there a difference between speed and velocity....
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Post by Bigboy on Jan 27, 2005 17:46:47 GMT -5
The equation E=Mc2 describes the relationship between mass and energy.
E=Energy M=Mass c=Speed of light in a vacuum - this is a known constant value - approximately 3x108m/s (meters per second)
In your example a 4 gram match burns down to a 2 gram stump (we'll ignore the mass lost as smoke and ash to keep it simple). We can use those figures to calculate the energy emitted by the flame as heat, light and sound:
We know the amount of mass converted to energy = 4g - 2g = 2g or 0.002kg (kg is the base unit)
so we substitute the numbers into the equation:
E=0.002x(3x108)2 =0.018x1016 =1.8x1014j (joules) of energy. thats 180 million million joules.
Say the match burned for 10 seconds: Power=E/t, so the match burned with an output of 18 million million Watts - compare that to your average 60Watt light bulb!
There is a difference between speed and velocity: Speed only has magnitude - it's just a number describing how fast something is going. Velocity had magnitude and direction - it's a vector that describes how fast something is going and in which direction.
Hope that helps!
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Post by Liansky on Jan 27, 2005 18:27:06 GMT -5
Thanks, that helps a lot cause i was just a bit stumped with csqaured. However, I'm a bit confused with these aspects....
you say that c = 3x10(power 8)m/s
But then you use < 9x10(power 8) > in the eqaution.
also, starting with eqaution, E=0.002x(9x10{power 8}){power 2}, my calculator came up with 1.62 15(this is a business calculator). Now, if i disregarded the sqaure, then i would come up with your answer. Would you mind specifying the steps?
Please forgive if this sounds lame, but take into consideration that i never did physics, nor did i do maths homework.
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Post by Bigboy on Jan 27, 2005 18:33:36 GMT -5
Oops - well spotted - I made a mistake. I got ahead of myself and squared the 3 too early - I'll edit it.
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Post by swimmajor on Mar 10, 2005 11:41:11 GMT -5
some theries say that if you were travleing light speed time would stop.. or be non exsistance... techincally, if you were to travel at the speed of light, time wouldnt so much stop, as it just wouldnt exist. same as light, by going as fast as it, we wouldnt be able to see it because it wouldnt be able to travel the distance between you and the source. also you would have to take into account the possibility that going the speed of light would in fact wipe the slate clean of everything. infinite mass/energy = no room for anything else.
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Post by Bigboy on Mar 10, 2005 19:07:08 GMT -5
This wouldn't happen - actually increasing your mass just by moving faster would defy the Second Law of Thermodynamics (you cant just create mass/energy). What happens is the fact that you are travelling at near LS warps the space around you in the same way as a large mass does, and therefore increases your effective mass. Your (effective) mass would approach infinity at LS, but there isn't be enough energy in the universe to accelerate you to that speed.
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swimmajor
New Member
When the world ends..............there will be so much to look forward too
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Post by swimmajor on Mar 11, 2005 11:18:26 GMT -5
Your (effective) mass would approach infinity at LS, but there isn't be enough energy in the universe to accelerate you to that speed. and that seems to be the main problem with ever actually reaching LS, if there was infinite energy to be used, then it would be theoretically possible to reach LS, but its not like it matters, cause by taking all the energy to reach LS, would destroy everything so it could be used. oh well right?
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Post by Bigboy on Mar 12, 2005 7:38:48 GMT -5
Yep. Except there isn't an infinite amount mass/energy in the universe...
But that does leave to option of a wormhole for time travel - which is the method DD uses.
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Post by donniedarkorules on Mar 18, 2005 20:28:43 GMT -5
Not so - the equation E=Mc 2 does not lead to this conclustion. You can graph this equation, but c is the speed of light in a vacuum - which is constant at 3.8x10 18m/s. Plotting speed against mass and energy here is meaningless, because the equation describes the relationship between mass and energy (so you could calculate the energy released by a match, by comparing the mass of the match before you burn it to the mass of the resulting ashes, by applying the difference in mass to the equation). E=Mc 2 does not describe a relationship between velocity and mass. The first thing to get your head round is that space-time is four dimensional - time is absolutely equivalent to the 3 physical dimensions we are familliar with as far as relativity is concerned. Ok - now for the rubber sheet analogy Imagine space-time as a rubber sheet, and a mass (like a planet) is a bowling ball on that sheet. It is clear that the bowling ball 'warps' the sheet - that mass warps space-time. You were correct about the clock experiment - time is experiences at a slower subjective rate inside a gravity well - the deeper the well (the more space-time is warped or curved) the slower subjective time gets (relative to outside the gravity well). At a singularity (a point of space-time that is warped to an infinite degree [I'm not sure if the term 'infinite' here is entirely correct but it's as good as any]) time essentially stops (from the point of view outside the black hole). There are four solutions to relativity that result in a singularity: -The singularity at the big bang -The big crunch. -A black hole - a point of finite (but large) mass in an infinitely small volume. This results in an infinite density (for this explenation this can be considered equivalent to an infinite mass in a non zero volume) and thus warps space time to an infinite extent - a singularity. nb The big crunch is probably equivalent to a super-duper massive black hole. The fourth solution invloves a non zero mass travelling at the speed of light - the reason for this is that a mass travelling at speed warps space-time more than a stationary mass. This extra warp effectively increases the mass of the moving object. As the object approaches the speed of light it warps space to such an extent that if the speed of light was attained the mass of the object would indeed effectively become infinite and a singularity would result. However you are correct that because the effective mass of the object increases to an infinite number as it approaches the speed of light, it is impossible to get there because there is not enough energy in the universe to accelerate such a large mass. If you were to graph the speed of an object against the relative 'rate' of time experienced by the moving object (compared to that experienced at a stationary point in space), then at the speed of light the curve would hit zero. The idea that at faster than light travel you go backwards in time comes from the supposition that by continuing the curve past light speed, it results in a negative number for the 'relative rate of time' . *bangs head against wall several thousand times* Wow. I thought I was within the realm of understanding quantam physics, but I guess I am not. *bangs head against wall several thousand more times*
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Post by Bigboy on Mar 19, 2005 11:09:43 GMT -5
That's Reletivity, not Quantum Physics
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