Ask the expert!

 

Click Here To Ask a Question

Sent by Ivan Robbins on Wednesday, November 15, 2006 at 10:04:16

Q: What is a wormhole?

A: Wormholes are solutions of Einstein equations that connect two separate regions of spacetime. The most famous example is the "Einstein-Rosen bridge" which describes a dynamical wormhole with spherical symmetry. Although wormhole solution are known, it does not necessarily means that these objects exist in nature. It has been proven that wormhole existence requires matter with negative energy, which has not been observed so far. (Answer by Marco Cavaglià)


Sent by Mike Gorski on Wednesday, June 25, 2008 at 19:19:32

Q: I understand that some people have proposed creating mechanisms that create black holes via particle collision. Doesn't this have the potential to be very destructive? I realize that someone might bank on Hawking radiation to cause the black hole to "evaporate", but that's still a theory. Failing that, wouldn't the black hole would fall to the center of the Earth, consuming matter as it fell, growing and destroying the Earth as we know it?

A: Black holes (BHs) from cosmic rays would have already killed us all since ultra high cosmic rays (UHECR) exists and would form BHs. If we can make BHs at the LHC at all, we should be ok. (Answer by Arunava Roy)


Sent by Joshua Nuijens on Tuesday, November 27, 2012 at 11:43:52

Q: It is possible that the magnetic field caused by a dead neutron star just seriously bend time space and our perception of it. That a black hole doesn't go anywhere and there is nothing but a bent piece of time space folded around this dead star and if to take away its magnetic field that space would unfold and a dead neutron star would remain where the black hole was. I am not saying there isn't a large bent piece of space on the another side of the black hole but nothing is getting by the this dense dead neutron star. The only way that this other side would gain matter in a vast volume if the neutron star would become too dense and implode on itself. That would just be another big bang with space time going in the opposite direction that we are on. I don't believe that is imploded first. I believe it become powerful enough that is bends time/space around it and we perceive it as going somewhere. There is a certain amount of space/area on the other side which is dictated by the size and strength of the dead neutron star magnetic field. Of course, this bent time space would not go back in time but be going in the opposite direction of our time space. The neutron star is still there and the depth of it in the black hole is all relative to its curve of the bend of time space. Another factor is dark matter. Wouldn't we need it on the other side of the black hole and is it affect by the dead neutron star? Only having it there would constitute a full expense and ordinary matter of the dead star would not keep it stable? (that's if it implodes) Implodes in my sense it followed by an outward extension of the bubble of the bend of time space. Dark matter would have to be collected to allow for this bubble bend to expand and remain stable. Does dark matter get collected by the gravity of the dead star or is dark matter a part of the fabric of time space itself? Does dark matter cluster and is more dense in certain areas? Does more planets mean more dark matter grouping? Maybe the collection of dark matter by the gravitational forces of the dead star weaken that portion of time space allowing it to bend easier.

A: These are very interesting questions. As far as we know, no, it's not possible that a black hole is actually a neutron star with a magnetic field. We of course don't know what is "inside" a black hole. Laws of physics forbid us to get information from beyond the black hole event horizon, and Einstein equations break down at the center of the black hole. It has been speculated that black holes could connect different regions of space-time (wormholes) but in general these wormholes would not be traversable and, anyway, these are more speculations based on particular solutions of Einstein's equations (with a lot of assumptions) than description of reality. As far as dark matter is concerned, from the point of view of gravity, it behaves like ordinary matter. The only property which is relevant is its mass. Thus dark matter clusters indeed in certains regions of space. (Answer by Marco Cavaglià)


Sent by Joshua Nuijens on Tuesday, November 27, 2012 at 11:45:21

Q: Why the core spins faster than the crust? At a point the earth's rotation was much faster, both core and out layers pretty much at the same speed. Both the moon and space itself causes drag on the surface of the earth. Once the moon comes into play, the earth's inertia is slowed down by the moon gravitational forces along with the drag of space itself. Since the center is liquid and in itself has inertia does not mean that both the core and outer layers are affected the same. The inertia on the core remains more at a constant while the outer layers are dragged, if you will, against the moon's forces causing the out layers to slow while the inner core is affected less. Of course, over a considerable amount of time. This would cause a different in rotation for both the solid outer and the liquid core. During the development of the earth, the outer crust begins to cool while the mantle cools, and the mantles electrons would a line in the direction of the magnetic poles. Due to the thickness of this direction line up of electrons in the crust and mantle wouldn't the stationary crust/mantle electrons create lines of flux and the core of iron act as a conductor in a sense? After the core begins to lap the crust/mantle a difference in magnetic fields (crust/mantle and core) will begin to conduct. The core is the conductor that stores energy and releases it in both a magnetic field and heat. At this point could we consider the earth as one massive inductor? Does the core after lapping the solid outer mantle only can stored so much energy that is cannot appose/rotate anymore due to the magnetic differences and comes to a stop? The core would change its rotation direction due to the opposition of fields. A total of two shift of rotation would have to be applied given what has been researched. The opposing core rotation would only last a faction of the time versus when it rotates the same direction. Is it a possibility that our rotation and the Sun's magnetic field play a part in the amount of energy the earth takes in and keep its core moving? Does the speed in which the planet rotation directly correlate with how much energy the planet can store? Is the reason that Venus has a greater gravitational forces is become the crust and it mantle have been slowed down so mush versus its core causing a larger difference against the opposing fields? Is the reason Mars has a less gravitational pull become it was not large enough to store enough energy to allow the core to spin and change direction? Maybe after its first cycle it didn't have the energy for its core to jump start again when it did change directions. That would be reason for it having an atmosphere for a given duration and the reason it die out long before earth will.

A: I am no expert of the origin and evolution of planetary magnetic fields, however it is certainly true that the gravitational drag of the Moon slows down Earth's rotation, and this has an important effect on the Earth magnetic field. Direct observations of the magnetic field over the past few centuries have shown that the Earth's south magnetic pole (the pole the "N" of a compass points to) appears to be moving slowly westwards. The origin of this effect is likely the differential motion of fluid in the Earth's outer core with respect the Earth's mantel, which may be due to the gravitational of the Earth's surface by the Moon. Electric currents produced by the coupling of convective and rotational motion in the Earth's liquid metallic outer core of iron and nickel then produce a dynamo effect. I don't think the Sun's magnetic field plays any role in this mechanism. Neither that a planet rotation correlates with how much energy it can store. In the solar system, there is no evidence of such a correlation. (Think for instance of Venus and the Earth, very similar planets but very different rotational speed.) I have to add, however, that the origin of planetary magnetic fields is very complex, and not at all understood yet. (Answer by Marco Cavaglià)


Sent by Sumontro Sinha on Tuesday, April 1, 2014 22:24:23

Q:How does one calculate proton capture cross sections for reactions between protons and heavy ions (Z>100)?

A: For low energy protons or neutrons the nucleus appears to be a disk of size pi*R^2 on side. For protons we must include the Coulomb repulsion between the proton an nucleus, referred to as the Coulomb barrier, which will reduce the pN cross section. Then at low energies, for a few MeV of the incident proton, one might use the following formula: sigma = pi*R^2 ( 1 - (V_coul/E)), where sigma is the cross section for the proton absorption, R is the nucleus radius (1.3 Fermi * A^(1/3)), A is the atomic weight, V_coul is the Coulomb potential barrier term (1/4*pi*epsilon) * Ze*e/R, Z is the atomic number, e is the electronic charge, and E is the energy of the incident proton, where E > V_coul for the reaction to occur. (Answer by Ahmed M. Hamed and Lucien Cremaldi)


This page has been viewed 1563 times.