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Thijs Versloot

Relativistic rocket: Dream and reality - 3 views

www.sciencedirect.com/...S009457651400040X

propulsion antimatter relativistic

shared by Thijs Versloot on 27 Mar 14 - No Cached
  • Thijs Versloot on 27 Mar 14
     
    An exhaustive overview of all possible advanced rocket concepts, eg.. "As an example, consider a photon rocket with its launching mass, say, 1000 ton moving with a constant acceleration a =0.1 g=0.98 m/s2. The flux of photons with E γ=0.5 MeV needed to produce this acceleration is ~1027/s, which corresponds to the efflux power of 1014 W and the rate of annihilation events N'a~5×1026 s−1 [47]. This annihilation rate in ambiplasma l -l ann corresponds to the value of current ~108 A and linear density N ~2×1018 m−1 thus any hope for non-relativistic relative velocity of electrons and positrons in ambiplasma is groundless." And also, even if it would work, then one of the major issues is going to be heat dispersal: "For example, if the temperature of radiator is chosen T=1500 K, the emitting area should be not less than 1000 m2 for Pb=1 GW, not less than 1 km2 for Pb=1 TW, and ~100 km2 for Pb=100 TW, assuming ε=0.5 and δ=0.2. Lower temperature would require even larger radiator area to maintain the outer temperature of the engine section stable for a given thermal power of the reactor."
  • ...2 more comments...
  • johannessimon81 on 27 Mar 14
     
    We were also discussing a while ago a propulsion system using the relativistic fragments from nuclear fission. That would also produce an extremely high ISP (>100000) with a fairly high thrust. Never really got any traction though.
  • dejanpetkow on 29 Mar 14
     
    I absolutely do not see the point in a photon rocket. Certainly, the high energy releasing nulcear processes (annihilation, fusion, ...) should rather be used to heat up some fluid to plasma state and accelerate it via magnetic nozzle. This would surely work as door-opener to our solar system...and by the way minimize the heat disposal problem if regenerative cooling is used.
  • Thijs Versloot on 31 Mar 14
     
    The problem is not achieving a high energy density, that we can already do with nuclear fission, the question however is how to confine or harness this power with relatively high efficiency, low waste heat and at not too crazy specific mass. I see magnetic confinement as a possibility, yet still decades away and also an all-or-nothing method as we cannot easily scale this up from a test experiment to a full-scale system. It might be possible to extract power from such a plasma, but definitely well below breakeven so an additional power supply is needed. The fission fragments circumvent these issues by a more brute force approach, thereby wasting a lot of energy for sure but at the end probably providing more ISP and thrust.
  • dejanpetkow on 03 Apr 14
     
    Sure. However, the annihilation based photon rocket concept unifies almost all relevant drawbacks if we speak about solar system scales, making itself obsolete...it is just an academic testcase.
jmlloren

Scientists discover how to turn light into matter after 80-year quest - 5 views

phys.org/...-05-scientists-year-quest.html

PHY MAD

shared by jmlloren on 19 May 14 - No Cached
  • Thijs Versloot on 19 May 14
     
    Theoretized 80 years ago was Breit-Wheeler pair production in which two photons result in an electron-positron pair (via a virtual electron). It is a relatively simple Feynmann diagram, but the problem is/was how to produce in practice a high energy photon-photon collider... The collider experiment that the scientists have proposed involves two key steps. First, the scientists would use an extremely powerful high-intensity laser to speed up electrons to just below the speed of light. They would then fire these electrons into a slab of gold to create a beam of photons a billion times more energetic than visible light. The next stage of the experiment involves a tiny gold can called a hohlraum (German for 'empty room'). Scientists would fire a high-energy laser at the inner surface of this gold can, to create a thermal radiation field, generating light similar to the light emitted by stars. They would then direct the photon beam from the first stage of the experiment through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can. Now this is a good experiment... :)
  • ...6 more comments...
  • jmlloren on 19 May 14
     
    The solution of thrusting in space.
  • dejanpetkow on 20 May 14
     
    Thrusting in space is solved already. Maybe you wanted to say something different?
  • Thijs Versloot on 21 May 14
     
    Thrusting until your fuel runs out is solved, in this way one can produce mass from, among others, solar/star energy directly. What I like about this experiment is that we have the technology already to do it, many parts have been designed for inertial confinement fusion.
  • johannessimon81 on 29 May 14
     
    I am quite certain that it would be more efficient to use the photons directly for thrust instead of converting them into matter. Also, I am a bit puzzled at the asymmetric layout for photon creation. Typically, colliders use two beam of particle with equal but opposite momentum. Because the total momentum for two colliding particles is zero the reaction products are produced more efficiently as a minimum of collision energy is waisted on accelerating the products. I guess in this case the thermal radiation in the cavity is chosen instead of an opposing gamma ray beam to increase the photon density and increase the number of collisions (even if the efficiency decreases because of the asymmetry). However, a danger from using a high temperature cavity might be that a lot of thermionic emission creates lots of free electrons with the cavity. This could reduce the positron yield through recombination and would allow the high energetic photons to loose energy through Compton scattering instead of the Breit-Wheeler pair production.
  • Thijs Versloot on 30 May 14
     
    Well, the main benefit from e-p pair creation might be that one can accelerate these subsequently to higher energies again. I think the photon-photon cross-section is extremely low, such that direct beam-beam interactions are basically not happening (below 1/20.. so basically 0 according to quantum probability :P), in this way, the central line of the hohlraum actually has a very high photon density and if timed correctly maximizes the reaction yield such that it could be measured.
  • johannessimon81 on 30 May 14
     
    I agree about the reason for the hohlraum - but I also keep my reservations about the drawbacks. About the pair production as fuel: I pretty sure that your energy would be used smarter in using photon (not necessarily high energy photons) for thrust directly instead of putting tons of energy in creating a rest-mass and then accelerating that. If you look at E² = (p c)²+(m0 c)² then putting energy into the mass term will always reduce your maximum value of p.
  • Thijs Versloot on 31 May 14
     
    True, but isnt it E2=(pc)^2 + (m0c^2)^2 such that for photons E\propto{pc} and for mass E\propto{mc^2}. I agree it will take a lot of energy, but this assumes that that wont be the problem at least. The question therefore is whether the mass flow of the photon rocket (fuel consumed to create photons, eg fission/fusion) is higher/lower than the mass flow for e-p creation. You are probably right that the low e-p cross-section will favour direct use of photons to create low thrust for long periods of time, but with significant power available the ISP might be higher for e-p pair creation.
  • johannessimon81 on 03 Jun 14
     
    In essence the equation tells you that for photons with zero rest mass m0 all the energy will be converted to momentum of the particles. If you want to accelerate e-p then you first spend part of the energy on creating them (~511 keV each) and you can only use the remaining energy to accelerate them. In this case the equation gives you a lower particle momentum which leads to lower thrust (even when assuming 100% acceleration efficiency). ISP is a tricky concept in this case because there are different definitions which clash in the relativistic context (due to the concept of mass flow). R. Tinder gets to a I_SP = c (speed of light) for a photon rocket (using the relativistic mass of the photons) which is the maximum possible relativistic I_SP: http://goo.gl/Zz5gyC .
LeopoldS

Relativistic positioning systems: Perspectives and prospects - 0 views

arxiv.org/1302.5782.pdf

RPS galileo PHY

shared by LeopoldS on 02 Mar 13 - No Cached
  • LeopoldS on 02 Mar 13
     
    Bartolomé call's workshop paper ... with some nice words also for the ACT "I want to congratulate the Advanced Concepts Team of the ESA and the Faculty of Mathematics and Physics of Ljubljana for this initiative." congrats Sante and Pacome again!
santecarloni

Relativistic Baseball - 2 views

what-if.xkcd.com/1

physics baseball science

shared by santecarloni on 13 Jul 12 - No Cached
LeopoldS liked it
  • santecarloni on 13 Jul 12
     
    What would happen if you tried to hit a baseball pitched at 90% the speed of light? --not sure it is correct, but it would be a lot of fun :)
johannessimon81

Cosmological model without accelerated expansion proposed - 1 views

www.nature.com/...e-may-not-be-expanding-1.13379

science cosmology

shared by johannessimon81 on 07 Aug 13 - No Cached
  • johannessimon81 on 07 Aug 13
     
    Redshift in this model is partially produced by a change in the masses of elementary particles (and atoms)
  • ...3 more comments...
  • Thijs Versloot on 25 Aug 13
     
    It seems to solve the problem of infinite energy density at the singularity in any case. I would love to see a way of experimentally verifying this, although most people seem to believe it is wrong. I read the following quote though by Dirac to Pauli "we all agree your idea is crazy, but the real question is it crazy enough to be correct?"
  • H H on 25 Aug 13
     
    As far as I can see, this is not untestable per se, rather an explanation to the redshift that is equivalent to accelerating expansion. It is not that the theory is untestable, rather just another way of looking at it. Kind of like that its sometimes convenient to consider light a particle, sometimes a wave. In the same way it could sometime convenient to view the universe as static with increasing mass instead.
  • Thijs Versloot on 26 Aug 13
     
    Well the premiss "matter getting heavier" may be up to falsification in some way or another. Currently, there is no absolute method to determine mass so it might even be plausible that this is actually the case. I don't think it is related but there is a problem with the 1kg-standards (1 official and 6 copies) where the masses seem to deviate.
  • johannessimon81 on 26 Aug 13
     
    It should not be impossible to verify a change in mass(es) over time. For example the electron cyclotron frequency scales ~e/m while the Hydrogen emission frequencies scale with ~m*e^4. Using multiple relationships like that which can be easily and accurately measured an increase in the mass of fundamental particles should - in principle - be detectable (even if the mass of the earth increases at the same time changing the relativistic reference frame).
  • Thijs Versloot on 26 Aug 13
     
    The Watt balance and a definition using the Planck's constant seems to do the trick and is currently being discussed. Would the electron charge not be problematic as it is related to Coulombs which depends on Amperes which is defined by Newtons which hence depends back on the mass again?
Ma Ru

The quantum mechanics of time travel through post-selected teleportation - 3 views

arxiv.org/1007.2615

PHY time travel science

shared by Ma Ru on 26 Jul 10 - Cached
  • Ma Ru on 26 Jul 10
     
    Giving the title, I think the comment is not necessary...
  • pacome delva on 27 Jul 10
     
    Haha, nice article : One of the best-known versions of non-general relativistic quantum versions of time travel comes from Wheeler, as described by Feynman in his Nobel Prize lecture [16]: 'I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, "Feynman, I know why all electrons have the same charge and the same mass." "Why?" "Because, they are all the same electron!" And, then he explained on the telephone, "Suppose that the world lines which we were ordinarily considering before in time and space - instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time - to the proper four velocities - and that's equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron."
Francesco Biscani

The End Of Gravity As a Fundamental Force - 6 views

science.slashdot.org/...Gravity-As-a-Fundamental-Force

phy

shared by Francesco Biscani on 11 Jan 10 - Cached
LeopoldS liked it
  • Francesco Biscani on 11 Jan 10
     
    "At a symposium at the Dutch Spinoza-instituut on 8 December, 2009, string theorist Erik Verlinde introduced a theory that derives Newton's classical mechanics. In his theory, gravity exists because of a difference in concentration of information in the empty space between two masses and its surroundings. He does not consider gravity as fundamental, but as an emergent phenomenon that arises from a deeper microscropic reality. A relativistic extension of his argument leads directly to Einstein's equations."
  • ...8 more comments...
  • Dario Izzo on 12 Jan 10
     
    Diffcult for me to fully understand / believe in the holographic principle at macroscopical scales ... potentially it looks though as a revolutionary idea.....
  • pacome delva on 13 Jan 10
     
    never heard about it... seems interesting. At first sight it seems that it is based on fundamental principle that could lead to a new phenomenology, so that could be tested. Perhaps Luzi knows more about this ? Did we ever work on this concept ?
  • Luzi Bergamin on 13 Jan 10
     
    The paper is quite long and I don't have the time right now to read it in detail. Just a few comments: * We (ACT) definitely never did anything in this direction? But: is there a new phenomenology? I'm not sure, if the aim is just to get Einstein's theory as emergent theory, then GR should not change (or only change in extreme conditions.) * Emergent gravity is not new, also Erik admits that. The claim to have found a solution appears quite frequently, but most proposals actually are not emergent at all. At least, I have the impression that Erik is aware of the relevant steps to be performed. * It's very difficult to judge from a short glance at the paper, up to which point the claims are serious and where it just starts to be advertisments. Section 6 is pretty much a collection of self-praise. * Most importantly: I don't understand how exactly space and time should be emergent. I think it's not new to observe that space is related to special canonical variables in thermodynamics. If anybody can see anything "emergent" in the first paragraphs of section 3, then please explain me. For me, this is not emergent space, but space introduced with a "sledge hammer." Time anyway seems to be a precondition, else there is nothing like energy and nothing like dynamics. * Finally, holography appears to be a precondition, to my knowledge no proof exists that normal (non-supersymmetric, non-stringy, non-whatever) GR has a holographic dual.
  • Luzi Bergamin on 13 Jan 10
     
    Update: meanwhile I understood roughly what this should be about. It's well known that BH physics follow the laws of theormodynamics, suggesting the existence of underlying microstates. But if this is true, shouldn't the gravitational force then be emergent from these microstates in the same way as any theromdynamical effect is emergent from the behavior of its constituents (e.g. a gas)? If this can be prooven, then indeed gravity is emergent. Problem: one has to proof that *any* configuration in GR may be interpreted as thermodynamical, not just BHs. That's probably where holography comes into the play. To me this smells pretty much like N=4 SYM vs. QCD. The former is not QCD, but can be solved, so all stringy people study just that one and claim to learn something about QCD. Here, we look at holographic models, GR is not holographic, but who cares... Engineering problems...
  • LeopoldS on 13 Jan 10
     
    is there any experimental or observational evidence that points to this "solution"?
  • Luzi Bergamin on 13 Jan 10
     
    Are you joking??? :D
  • pacome delva on 14 Jan 10
     
    I was a bit fast to say it could be tested... apparently we don't even know a theory that is holographic, perhaps a string theory (see http://arxiv.org/abs/hep-th/9409089v2). So very far from any test...
  • LeopoldS on 14 Jan 10
     
    Luzi, I miss you!!!
  • Luzi Bergamin on 14 Jan 10
     
    Leo, do you mean you liked my comment on your question more than Pacome's? Well, the ACT has to evolve and fledge, so no bullshitting anymore, but serious and calculating answers... :-) Sorry Pacome, nothing against you!! I just LOVE this Diigo because it gives me the opportunity for a happy revival of my ACT mood.
  • pacome delva on 14 Jan 10
     
    haha, today would have been great to show your mood... we had a talk on the connection between mind and matter !!
pacome delva

Heaviest ever antimatter discovered - physicsworld.com - 1 views

physicsworld.com/...45544

phy antimatter

shared by pacome delva on 29 Mar 11 - No Cached
LeopoldS liked it
  • Physicists at the Relativistic Heavy Ion Collider (RHIC) in New York say they have created nuclei of antihelium-4 for the first time – the heaviest antimatter particles ever seen on Earth.
Thijs Versloot

GPS satellites suggest Earth is heavy with dark matter @NewScientist - 0 views

www.newscientist.com/...is-heavy-with-dark-matter.html

GPS darkmatter

shared by Thijs Versloot on 03 Jan 14 - No Cached
  • Thijs Versloot on 03 Jan 14
     
    At a meeting of the American Geophysical Union in San Francisco in December, he reported an average figure that was between 0.005 and 0.008 per cent greater than the value for Earth's mass established by the International Astronomical Union. A disc of dark matter around the equator 191 kilometres thick and 70,000 km across can explain this, he says. Harris has yet to account for perturbations to the satellites' orbits due to relativity, and the gravitational pull of the sun and moon. Maybe relativistic GPS could improve this even further? As a side note however, the Juno spacecraft flyby showed an gravity acceleration which matched the calculations, casting doubts on the earlier calculations.
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