Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Dick1038 on 13/11/2007 17:34:25
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The string theorists, in their endeavour to unite gravity with the other three forces, have posited the existence of gravitons. But general relativity states that gravity is merely a warping of spacetime and not really a force. Is there a conflict here? I'm confused.
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It wouldn't be unreasonable for the 'warps' to move around - like ripples. These ripples would take time to move and carry energy- wavelike behaviour. You could then endue them with a duality which would allow you to consider them as particles too.
Does that ease your Science conscience?
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If the force of gravity is thought of as being mediated by gravitons it give a good explanation as to while the force of gravity cannot be shielded against.
It is postulated that Gravitons have something like 10^-11 the mass of Neutrino's ( see Scientific American ) and light years of lead are required to shield against them
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General relativity is what is known as a "classical" theory in that it is smooth and uniform down to the smallest dimensions. Electricity and magnetism used to be treated in this way until experiments got very precise and theories examined in detail. It is now known that electricity, magnetism all the other forces in nature other than gravity are in discrete lumps ie "quantised" that is what quantum theory is all about.
It is considered that space, time and gravity must also be quantised at the smallest scales. But the scales at which this happen are almost unimaginably small. The graviton performs the same job in gravity as the photon does in electromagnetism that is it mediates the force between gravitating particles.
To get an idea of the scale over which this must apply let us think first about photons. The quantisation of photons depends on the frequency the higher the frequency the more energetic the photon. Under normal conditions photons only become really noticeable as quanta when they are about at light frequencies that is at around one thousand million million cycles per second.
So it is only when gravitating bodies are orbiting each other at frequencies of this order and faster that gravitational quantisation will start to become to be significant. The fastest rotating or orbiting bodies under gravity we know of are neutron stars and small black holes and they manage to orbit each other at around one thousand cycles per second so it is not until we get down to scales when massive bodies were interacting with each other on atomic scales a million million times smaller that gravitational and space time quantisation effects will become significant. the critical scales are at least as much smaller than the nucleus of an atom as a nucleus of an atom is smaller than a star!
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I dont understand how photons can mediate electromagnetism. you can block photons with lead, you cant block electromagnetism.
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You cannot shield a static magnetic field with a sheet of lead because if the field is static the frequency of the electromagnet radiation from it is zero Hertz.
Now set your magnet spinning so that the frequency rises, at 3.6 * 10^16 RPM it will radiate green light that will certainly be stopped by the sheet of lead.
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are you saying that, static magnetic fields emit photons without a frequency? And photons without a frequency cant be blocked by anything?
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No. Once again, this is a classic example of taking a mathematically convenient explanation and taking it to be the way things actually work. Mathematically, the oscillation of the magnetic field has a frequency of 0 Hz. Whether or not this field emits a photon is irrelevant because this photon has no energy (by E=hv) and so can't interact with other mass if it is really emitted. For all intensive purposes, the photon does not exist because it cannot be found. We can only speculate on what can be measured and such a photon could not be measured.
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I was expecting the case to be raised when the lead sheet is cooled so that it becomes supercondutive in which case it does in fact block the magnetic field.
the answer is of course that the field is not truly static, the magnet must move towards the lead sheet to generate the induced currents in it that produce an opposing field
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Is their ever a truly static magnetic field? If the domains in atoms are constantly moving around, even if they are generally all oriented in the same direction, there is some change in the surrounding magnetic field. In addition, in any space that is not a vacuum there are para or dia magnetic distortions in the field and then the medium is in constant motion (not at absolute zero) the field is dynamic.
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okay, so how does a photon mediate electromagnetism?
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The photon is a quantum of electromagnetic radiation. depending on the frequency this can be anything from long wave radio through microwaves and infra red to light xrays and gamma rays. when a charged body in an electric field or magnetically susceptable body moves in a magnetic fied it feels a force the energy transfer associated with this force is performed by photons.
For low frequencies and large bodies this is rather difficult to visualise but the case of an electron changing its orbit and emitting a photon is probably the most understandable.
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Electromagnetic photons can indeed be blocked by ferromagnetic material such as steel. It's done all the time when necessary in electronic circuits.
But let's get back to gravitons. How about when two black holes closely orbit each other?
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okay, so how does a photon mediate electromagnetism?
Those are "virtual" photons.
If the charges are equal, that is, in the case of repulsion, it's easy: virtual photons are emitted from a charge towards the other charge, and so the 2 charges recedes from each other.
In the case of 2 oppositely charged particles, that is, attraction, it's much more complicated:
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
If you can understand it, please explain me. [:)]
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Im still having trouble understanding this idea. If the strength of electric attraction between two objects is a product of its distance, and has nothing to do with what is in between the objects is true, then in order for photons to be able to carry this force they too must be unaffected by anything. They aren't, you can block them. Being able to block any photons makes the way they work inconsistent with how electric force works.
What am I missing, that's super obvious?
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Is the answer, hypothetical virtual photons carry electromagnetism, and virtual photons are completely different from electromagnetic photons, visible light, xrays etc. ?
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Im still having trouble understanding this idea. If the strength of electric attraction between two objects is a product of its distance, and has nothing to do with what is in between the objects is true, then in order for photons to be able to carry this force they too must be unaffected by anything. They aren't, you can block them. Being able to block any photons makes the way they work inconsistent with how electric force works.
What am I missing, that's super obvious?
1. Let's suppose you put something between two charges, able to block photons; it means that system must interact electromagnetically; let's say a metal plate. But then, the electric field in the space between the charges varies, it's not the same as before; so in this case you can't say that "the strength of electric attraction between two objects is a product of its distance, and has nothing to do with what is in between the objects".
2. Now let's suppose that what you put in between doesn't interact electromagnetically; but then, it can't block the virtual photons!
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Is the answer, hypothetical virtual photons carry electromagnetism, and virtual photons are completely different from electromagnetic photons, visible light, xrays etc. ?
Now I surprise you: there is no essential difference; the only difference is their duration.
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Let me try to explain the whole photon business as I understand it, thanks to many people on this forum (including lightarrow), and from my own independent reading. The oscillatory energy of Planck's 'harmonic oscillators', E, equals nhν. In otherwords, E is an integer multiple of hν (h is Planck's constant and ν is the frequency of the oscillation). The number 'n' is the number of photons (of energy hν) that any single oscillator has. If these oscillators are charged, then they produce oscillations in their surrounding electromagnetic fields with the same oscillatory energy, E, as the charged oscillator. These EM waves are light and when we say there are photons of light we mean that the oscillation in the electromagnetic field has an energy equal to 'n', the number of photons, times hν. Now, knowing this, I disagree that these photons and the so-called 'virtual photons' are essentially the same, unless it can be shown that there are harmonic oscillators inside of individual charged particles (electrons for example). By giving the mediators of the EM force the name 'photon,' we are comparing them to the photons of Planck, which are packages of energy of the value hν. This suggests that 'virtual photons' arise from oscillations of harmonic oscillators inside of the charged particles. But, charges need not oscillate to be attracted or repelled from each other. So, either they are not photons in that they are packets of oscillatory energy, or electrons have little harmonic oscillators in them like black bodies have atoms inside of them which generate these 'virtual photons'. This is not consistent with current particle theory wherein electrons are leptons, which are fundamental particles and cannot be broken down into anything smaller.
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Let me try to explain the whole photon business as I understand it, thanks to many people on this forum (including lightarrow), and from my own independent reading. The oscillatory energy of Planck's 'harmonic oscillators', E, equals nhν. In otherwords, E is an integer multiple of hν (h is Planck's constant and ν is the frequency of the oscillation). The number 'n' is the number of photons (of energy hν) that any single oscillator has. If these oscillators are charged, then they produce oscillations in their surrounding electromagnetic fields with the same oscillatory energy, E, as the charged oscillator. These EM waves are light and when we say there are photons of light we mean that the oscillation in the electromagnetic field has an energy equal to 'n', the number of photons, times hν. Now, knowing this, I disagree that these photons and the so-called 'virtual photons' are essentially the same, unless it can be shown that there are harmonic oscillators inside of individual charged particles (electrons for example). By giving the mediators of the EM force the name 'photon,' we are comparing them to the photons of Planck, which are packages of energy of the value hν.
Ok till here. Forget the rest. [:)]
The description with oscillators is a classical or semi-classical description. If you really want to know how photons mediate electric force, you have to jump to QED, Quantum ElectroDynamics; there you only talk about electromagnetic fields and how to quantize them. A classical description is not appropriate even just for the fact, as you have noticed, that charges don't need to oscillates to generate the electrostatic field.
According to Heisenberg's Uncertainty Principle (HUP) in the form ΔE*Δt ≥ h/2, particles can be created literally "from nothing", if their existence is very short (Δt little enough so that ΔE can be high enough to create a particle); so photons can be created "from nothing" if they lasts for very little time; but they can however interact with the other particles and so produce an effect, even if for such short time; the simpler of these effects is the electric force between charges; I know, it's paradoxical that we all exists because of such an effect!
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I have never understood this implication of HUP before but I think I have just sorted it out. As the time interval gets really small, the minimum energy change gets to be so large that a particle can't help but exist for that small moment of time.
Getting back to photons, the idea that I was trying to make clear in my previous post is that photons are not particles but a proportional measure of the amplitude (which can only be integer multiples of the minimum amplitude) of the oscillations of the SHO's in an object. The energy of a SHO is E = kA2/2 = nhν. The 'quantization' here is in the energy of an oscillation and so I am puzzled over how you can take this concept and make the leap to a 'quantization' in an electric field, which may very well not be oscillating. It seems to me that all of the different things that have been found to be 'quantized' involve oscillations...energy of light, energy levels of electrons (standing waves) in atoms, etc. Are you implying that electric fields are always non-constant, that they are always oscillating even if they are produced from a static charge?
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Fields on their own can be static like gravitation and electrical. It is only when other objects MOVE in fields that you get the interaction that involves quantised energy packets like photons.
Going back right to the beginning of this discussion. Remember that physicists were strudying why and how warm bodies emitted electromagnetic radiation. We live in a dynamic universe and even the atoms of apparantly solid bodies are moving and jostling about (even at the absolute zero of temperature)
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I realize that...I even think I posted to the effect that there is no stationary magnetic field. But of course, theory and practice are not always the same. We can postulate about a stationary field and then extrapolate to changing fields. I am going to read more about QED but, mostly to answer the question of how energy packets come into play in motion that might not be harmonic (like current through a static field). Do they?, I am just thinking of different situations where things move in static fields. Is it all motion through a static field that involves quanta?
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I have never understood this implication of HUP before but I think I have just sorted it out. As the time interval gets really small, the minimum energy change gets to be so large that a particle can't help but exist for that small moment of time.
Getting back to photons, the idea that I was trying to make clear in my previous post is that photons are not particles but a proportional measure of the amplitude (which can only be integer multiples of the minimum amplitude) of the oscillations of the SHO's in an object. The energy of a SHO is E = kA2/2 = nhν. The 'quantization' here is in the energy of an oscillation and so I am puzzled over how you can take this concept and make the leap to a 'quantization' in an electric field, which may very well not be oscillating. It seems to me that all of the different things that have been found to be 'quantized' involve oscillations...energy of light, energy levels of electrons (standing waves) in atoms, etc. Are you implying that electric fields are always non-constant, that they are always oscillating even if they are produced from a static charge?
Unfortunately I cannot say to know QED; I only know that in this theory the fields themselves are quantized, they are made of photons; so there is no need of oscillations of any kind.
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Except that photons are the packets of oscillatory energy that a harmonic oscillator has. Photons are not objects but are like Joules: they are units of energy (the conversion factor is hν J = 1 photon). Saying that electric fields are made of photons is like saying it is made of cubits or seconds...preposterous. Of course, I understand that I am the last person to claim he understands QED but if this is the conclusion of the theory, it can't be valid. Maybe someone can explain the theory better...it is an experimentally proven theory (accurate to 10-12 according to Wikipedia). I still think that anything that is quantized has to have something to do with a SHO just as every SHO's energy is quantized.
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I am not quite certain what you mean when you use the term SHO but it looks likely that you mean simple harmonic oscillator. This is a simplification of what is going on because photons do start and stop and for a quantum to be a wave packet it must contain a range of frequencies it is possible for some quanta of electromagnetic radiation to be extremely broad band like impulses.
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Except that photons are the packets of oscillatory energy that a harmonic oscillator has. Photons are not objects but are like Joules: they are units of energy (the conversion factor is hν J = 1 photon). Saying that electric fields are made of photons is like saying it is made of cubits or seconds...preposterous. Of course, I understand that I am the last person to claim he understands QED but if this is the conclusion of the theory, it can't be valid. Maybe someone can explain the theory better...it is an experimentally proven theory (accurate to 10-12 according to Wikipedia). I still think that anything that is quantized has to have something to do with a SHO just as every SHO's energy is quantized.
One way of quantizing the free electromagnetic field is by defining the same creation and annihilation operators that is used in the quantum SHO (I also think this same technique works for quantum fields in general, but I've learned it with respect to E&M). Using a "creation" operator on a state give you an extra hbar*ω or energy (which corresponds to one 'photon'). Using an "annihilation" operator on a state takes away this same amount. In that way, you can make units of energy, which are associated with photons.
Why are these "particles"? I think it's because if you go to measure a photon, you're actually measuring a packet of energy from the field. The way a detector acts on the field is to usually annihilate some energy from the field (destroy a photon). In this way, single photons can be "absorbed" by the detector, and they're kind of like particles.
It's wrong to say that these photons are like "Joules." A Joule is a unit of measurement. These SHO states are wave functions, and physicists are used to describing particles by wave functions, so what's the problem with interpreting photons the same way?
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No one tried to answer What is a graviton, so I will give it a bash
Graviton is a postulated quantum, or carrier of the gravitational field, analogous to the well-established photon of the electromagnetic field. Gravitons, like photons, would be travelling at the speed of light and would be emitted only by highly accelerating, extremely massive objects such as stars. Graviton is postulated to be its own antiparticle, to have zero charge and rest mass, and a spin of 2. Since gravitons are apparently identical to their antiparticles, the notion of antigravity is questionable