Ask a physicist

[cookyy2k]

I would question the non of the fallout thing since yes while their is no radioactive waste left over annihalation turns the mass of the objects completly into gamma radiation at an extreem rate (E=mc[sup]2[/sup] and all). So the shear amount of radiation will be beyond current comprehension and that is bound to cause havoc with elecments in rocks and trees etc and make them radioactive leading to indirect fallout answell as a highly irradiated area.

Not really. I was talking about a bomb within an order of magnitude or two of the power of a current fusion warhead. The sorts of processes you're describing take place in high-energy laboratories, extremely high mass stars, and supernovae. Yes, there will be a huge burst of gamma radiation at the moment of the explosion, but that's not really fallout and a bomb of that magnitude is not going to cause much past temporary ionization.

The 8g of anti-matter we do have, plus 8g of matter required for anhilation would produce 1.4x10[sup]15[/sup]J of radiation. That is quite a bit and for only 8g of anti-matter.

If black holes are so dense that nothing can escape them, then why do they emit radiation?

I covered this one earlyer in the thread too, along with a link to Hawking's origional paper.

Where'd I leave my YoYo?
Also how exactly does one blind someone with science?

Don't know about the yoyo, but a laser is a good blinding devise.

So what do you think of the possibility of a multiverse, with our universe being the result of a collision between multiple universes? (note that astrophysics, and physics in general is just a hobby of mine, and I don't really pay a hole lot of attention to it.)

It is certainly an interesting hypothesis, that I have only ever really read a little on. One of the theories for gravily is that it is so weak due to it leaking from another dimention in a multivesre. It's not impossible, but also it's not really testable even on a theoretical standing since all our laws of physics work in our known universe, we don't even know if our laws hold up for all our universe of just a special patch around us so going for outside our universe thoughts is a little beyond me.

 

[ShadowKatt]

Galactic dynamics and astrophysics, eh? Alright, I have a question for you.

There is generally one scientific theory on how the universe started, that being the big bang. There are several theories about what brought about the big bang, contact between universal branes(M-theory), phase transition(strange theory), or universal creator(the religous theory). Any one of these are...somewhat valid, and they all share the common big bang. The end of the universe is another matter.

There are several theories about the end of the universe, which I'm sure you're already familiar with. There's the Big Crunch, whereas the universe ceases its expansion and collapses in on itself before exploding forth once more. Observed universal expansion discounts this somewhat however the rate of deceleration may be so small that we can't measure it at this point. Then there's the Big Freeze, which I subscribe to, where the universe continues to expand outwardly forever until entropy reduces the entire universe into an energeticly void expanse. Then there's the Big Rip.

This one is rather new to my experience, and of all the theories I've heard, it's hard not to call this one out as batshit insane=slash-stupid. The theory is that as the universe expands, it will come to a breaking point, like an elastic band, pulling and pulling and pulling until eventually it tears. At that moment, every particle in the universe will be ripped down to its smallest sub-atomic particles at the same time. Planets and stars will be vaporised, nebulae will disperse, and the entire universe will essentially end up identical to the Big Freeze, only a hell of a lot faster and more sudden and with nothing larger than the smallest possible particle existing.

I would like to know what physics support this theory and if such a thing would even be viable in the first place. What possible attractive universal forces would actually increase with increasing distance and cause a ripple effect that would destroy all matter? Or, have you not even heard of this theory before?

 

[MikailCaboose]

So what do you think of the possibility of a multiverse, with our universe being the result of a collision between multiple universes? (note that astrophysics, and physics in general is just a hobby of mine, and I don't really pay a hole lot of attention to it.)

It is certainly an interesting hypothesis, that I have only ever really read a little on. One of the theories for gravily is that it is so weak due to it leaking from another dimention in a multivesre. It's not impossible, but also it's not really testable even on a theoretical standing since all our laws of physics work in our known universe, we don't even know if our laws hold up for all our universe of just a special patch around us so going for outside our universe thoughts is a little beyond me.

And now for one more question, and I don't know if you have answered this in some way, oops.
But, what do you think about brane theory?

 

[cookyy2k]

Galactic dynamics and astrophysics, eh? Alright, I have a question for you.

There is generally one scientific theory on how the universe started, that being the big bang. There are several theories about what brought about the big bang, contact between universal branes(M-theory), phase transition(strange theory), or universal creator(the religous theory). Any one of these are...somewhat valid, and they all share the common big bang. The end of the universe is another matter.

There are several theories about the end of the universe, which I'm sure you're already familiar with. There's the Big Crunch, whereas the universe ceases its expansion and collapses in on itself before exploding forth once more. Observed universal expansion discounts this somewhat however the rate of deceleration may be so small that we can't measure it at this point. Then there's the Big Freeze, which I subscribe to, where the universe continues to expand outwardly forever until entropy reduces the entire universe into an energeticly void expanse. Then there's the Big Rip.

This one is rather new to my experience, and of all the theories I've heard, it's hard not to call this one out as batshit insane=slash-stupid. The theory is that as the universe expands, it will come to a breaking point, like an elastic band, pulling and pulling and pulling until eventually it tears. At that moment, every particle in the universe will be ripped down to its smallest sub-atomic particles at the same time. Planets and stars will be vaporised, nebulae will disperse, and the entire universe will essentially end up identical to the Big Freeze, only a hell of a lot faster and more sudden and with nothing larger than the smallest possible particle existing.

I would like to know what physics support this theory and if such a thing would even be viable in the first place. What possible attractive universal forces would actually increase with increasing distance and cause a ripple effect that would destroy all matter? Or, have you not even heard of this theory before?

I've not heard this one before however I can see from your setting it out one major factor that could support this. As space expands so do the gaps between nucleons, nuclii/electrons and so on. eventually their could come a time when the nuclear restoring force in the nucleus or the bonds between molecules are just too long to be held together and then it all falls apart.

I favor the big freeze also, the universe at current is expanding just enough not to collapse back together and should keep going that way. It's the lack of knowlage about dark matter, which is my field, that is stopping us from saying with certanty which is going to happen.

And now for one more question, and I don't know if you have answered this in some way, oops.
But, what do you think about brane theory?

I have answered this in some way actually in that I've said I just don't know enought about string theory to makje sense of the papers or research material and don't really have the time to learn the entire field since it wasn't covered in my UG days and I ahven't touched it in PG either.

 

[LordOrin]

Cool thread.

Why are scientists sure of the existence of dark matter, as opposed to there being something wrong with the way we're measuring the mass of galaxies?

And how do we know the extra mass isn't coming from black holes that we haven't detected?

 

[FoolKiller]

Why is Pluto no longer a planet?

 

[cookyy2k]

Cool thread.

Why are scientists sure of the existence of dark matter, as opposed to there being something wrong with the way we're measuring the mass of galaxies?

And how do we know the extra mass isn't coming from black holes that we haven't detected?

My research field again.

We arn't 100% sure of dark matter but it's certainly not miss measurement of galaxy masses since the luminous matter in the Galaxy just doesn't have the right shape, densit or size to produce what we're seeing. It can be put down to stuff we can't see/measure being theie but that's dark matter.

Black holes along with gravity acting differently on larger scales are the two contendors against DM. Now the things I always say are a little biased since all my work assumes DM but here goes. The cold dark matter model we use today in galactic dynamics makes several predictions that have all been observed to be true since the predictions were made so it's not just fitting the numbers to the observation. the way in which galaxies form in CDM leaves a lot of dark matter fragments in the Galaxy (in the Milkyway 100-1000 fragments of a few thousand to tens of millions of solar masses) these act to asymmertarise the galactic gravitational potential and this has been observed to be true. these clumps may be undiscovered black holes but around 4 of these clumps have been see with stars in very close proximity to where they are predicted to be, a black hole massive enought to cause the potential would be destroying these stars so visible from it's acreation disc. Another is that these fragments are distortable due to the fact they're made of man constituant parts, this has been observed when clusters as old as the Galaxy suddently have a blip in their orbit, this is thought to be the DM fragment having ebing broken by a gravitational event and so effecting the cluster.

Now their are 2 major problems in CDM to be resolved. 1)we predict many more fragments than we've ever seen, though as discussed they're really hard to see. 2) the main halo should have infinate potential at r=0, this isn't the case obviously thought their is a super massive black hole at the centre so the potential is very high.

And finally. I really really hope DM theory is correct. My work relies on it but more importantly all the work in the field for the last 50 years relies on it. If it turns out to be wrong we're being put back 50 years and worse still funding bodies and research councils will be slow to trus us again after 50 years of wasted money if DM theory is wrong.

Why is Pluto no longer a planet?

Pluto unfortunately failed to meet the criteria that was decided upon to designate something a planet. When it was discovered their was no hard and fast rules as to what a planet was so they decided it's a planet.

The criteria are:

(a) in orbit around the Sun
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (is roughly spherical).
(c) has cleared the neighbourhood around its orbit.

Now as you can see the Earth meets all these, we orbit the sun, it is roughtly spherical and we have no major objects in our orbital path.

Pluto however came up against a few problems. It does orbit the sun, but it's orbit is verry erratic and not in the same plane as the rest of the panets. It doesn't meet b, it's egg shaped, it's mass is not sufficient to produce something roughly spherical. And it definately fails on c since it's in an asteroid belt.

Thus it was classified a dwarf planet. If we'd made the definition to allow pluto planet status we would have to add 4 other objects as planets and probably many more down the line.

 

[Hungry Donner]

Why is Pluto no longer a planet?

When Pluto was discovered it was unique, and because of this it became a planet. However recently we have discovered that there are a lot of similar objects out there. For the simple sake of consistency either these objects had to be considered planets too or Pluto had to be demoted.

This isn't the first time that scientists have dealt with this situation. When Ceres was first discovered it was considered a planet, but later it was discovered that there were other similar bodies. Now Ceres is recognized as the largest member of the asteroid belt, just as Pluto is recognized as a prominent member of the Trans-Neptunian Objects (or Plutoids).

If you look at the solar system as we know it today, with quite a large number of these TNOs flying about, Pluto no longer looks unique at all. However there are four prominent rocky planets in the interior of the system and four very prominent gas giants further out. Pluto isn't really on the radar.

 

[llafnwod]

I would question the non of the fallout thing since yes while their is no radioactive waste left over annihalation turns the mass of the objects completly into gamma radiation at an extreem rate (E=mc[sup]2[/sup] and all). So the shear amount of radiation will be beyond current comprehension and that is bound to cause havoc with elecments in rocks and trees etc and make them radioactive leading to indirect fallout answell as a highly irradiated area.

Not really. I was talking about a bomb within an order of magnitude or two of the power of a current fusion warhead. The sorts of processes you're describing take place in high-energy laboratories, extremely high mass stars, and supernovae. Yes, there will be a huge burst of gamma radiation at the moment of the explosion, but that's not really fallout and a bomb of that magnitude is not going to cause much past temporary ionization.

The 8g of anti-matter we do have, plus 8g of matter required for anhilation would produce 1.4x10[sup]15[/sup]J of radiation. That is quite a bit and for only 8g of anti-matter.

Yes, it is, but that's just a number. What I was saying was that an antimatter bomb roughly comparable to a current fusion bomb would not irradiate the surrounding area. An antimatter bomb capable of the transmutation of radioactive elements out of base metals and minerals would be less a bomb and more a world-ending cataclysm, in which case I don't think you have to worry too much about fallout.

 

[cookyy2k]

I would question the non of the fallout thing since yes while their is no radioactive waste left over annihalation turns the mass of the objects completly into gamma radiation at an extreem rate (E=mc[sup]2[/sup] and all). So the shear amount of radiation will be beyond current comprehension and that is bound to cause havoc with elecments in rocks and trees etc and make them radioactive leading to indirect fallout answell as a highly irradiated area.

Not really. I was talking about a bomb within an order of magnitude or two of the power of a current fusion warhead. The sorts of processes you're describing take place in high-energy laboratories, extremely high mass stars, and supernovae. Yes, there will be a huge burst of gamma radiation at the moment of the explosion, but that's not really fallout and a bomb of that magnitude is not going to cause much past temporary ionization.

The 8g of anti-matter we do have, plus 8g of matter required for anhilation would produce 1.4x10[sup]15[/sup]J of radiation. That is quite a bit and for only 8g of anti-matter.

Yes, it is, but that's just a number. What I was saying was that an antimatter bomb roughly comparable to a current fusion bomb would not irradiate the surrounding area. An antimatter bomb capable of the transmutation of radioactive elements out of base metals and minerals would be less a bomb and more a world-ending cataclysm, in which case I don't think you have to worry too much about fallout.

So you're honestly telling me radiation doesn't effect the nucleus of an atom with the potential of changing it... Ever heard of K capture? That will result in a change of element... and since their is radioactive isotopes of iron, oxygen, iodine and prety much any element it's a fair possibility that radioactive isotopes can result from high energy gamma ray flashes. It's the same idea as photon jumps electron to higher energy level, electron decays back and emits a new photon... gamma ray photons can do this with nucleons instead of electrons and completly destabalise nuclii.

 

[llafnwod]

snip

Yes, it is, but that's just a number. What I was saying was that an antimatter bomb roughly comparable to a current fusion bomb would not irradiate the surrounding area. An antimatter bomb capable of the transmutation of radioactive elements out of base metals and minerals would be less a bomb and more a world-ending cataclysm, in which case I don't think you have to worry too much about fallout.

So you're honestly telling me radiation doesn't effect the nucleus of an atom with the potential of changing it... Ever heard of K capture? That will result in a change of element... and since their is radioactive isotopes of iron, oxygen, iodine and prety much any element it's a fair possibility that radioactive isotopes can result from high energy gamma ray flashes. It's the same idea as photon jumps electron to higher energy level, electron decays back and emits a new photon... gamma ray photons can do this with nucleons instead of electrons and completly destabalise nuclii.

I know. Atomic nuclei are destabilized by high energy radiation. But an explosion of a non-world ending magnitude simply isn't going to create enough radioactive material to constitute a radiological hazard, and no matter-antimatter explosion would create enough to threaten anything outside of its considerable blast radius.

 

[LordOrin]

Cool thread.

Why are scientists sure of the existence of dark matter, as opposed to there being something wrong with the way we're measuring the mass of galaxies?

And how do we know the extra mass isn't coming from black holes that we haven't detected?

My research field again.

We arn't 100% sure of dark matter but it's certainly not miss measurement of galaxy masses since the luminous matter in the Galaxy just doesn't have the right shape, densit or size to produce what we're seeing. It can be put down to stuff we can't see/measure being theie but that's dark matter.

Black holes along with gravity acting differently on larger scales are the two contendors against DM. Now the things I always say are a little biased since all my work assumes DM but here goes. The cold dark matter model we use today in galactic dynamics makes several predictions that have all been observed to be true since the predictions were made so it's not just fitting the numbers to the observation. the way in which galaxies form in CDM leaves a lot of dark matter fragments in the Galaxy (in the Milkyway 100-1000 fragments of a few thousand to tens of millions of solar masses) these act to asymmertarise the galactic gravitational potential and this has been observed to be true. these clumps may be undiscovered black holes but around 4 of these clumps have been see with stars in very close proximity to where they are predicted to be, a black hole massive enought to cause the potential would be destroying these stars so visible from it's acreation disc. Another is that these fragments are distortable due to the fact they're made of man constituant parts, this has been observed when clusters as old as the Galaxy suddently have a blip in their orbit, this is thought to be the DM fragment having ebing broken by a gravitational event and so effecting the cluster.

Now their are 2 major problems in CDM to be resolved. 1)we predict many more fragments than we've ever seen, though as discussed they're really hard to see. 2) the main halo should have infinate potential at r=0, this isn't the case obviously thought their is a super massive black hole at the centre so the potential is very high.

And finally. I really really hope DM theory is correct. My work relies on it but more importantly all the work in the field for the last 50 years relies on it. If it turns out to be wrong we're being put back 50 years and worse still funding bodies and research councils will be slow to trus us again after 50 years of wasted money if DM theory is wrong.

Makes sense. I guess being able to make accurate predictions with a model is pretty compelling evidence, or at least encouraging.

There was one bit I didn't understand. Can you explain again what the second problem with CMD is?

 

[Chromatic Aberration]

Hiho - A fellow physicist here.

I'am currently about to start working for my Master Thesis and plan to be done with that in about a year from now on and then go on and do my phD somewhere. I was wondering if you could perhaps share your experiences in finding/getting your current phD student position. Anything in that regard would be quite helpful

 

[cookyy2k]

There was one bit I didn't understand. Can you explain again what the second problem with CMD is?

Ah right, for the main galactic DM halo and the sub halo DM fragments their is a theoretical density function that fits current observations, however at the centre of each of these the density function become infinate and so the gravitational potential should be infinate, obviously this is unphysical but then other proposed density functions that do not have this problem do not recreate what we observe well enough to be considered correct.

Hiho - A fellow physicist here.

I'am currently about to start working for my Master Thesis and plan to be done with that in about a year from now on and then go on and do my phD somewhere. I was wondering if you could perhaps share your experiences in finding/getting your current phD student position. Anything in that regard would be quite helpful

The most important things are do your research on the exact research fields of each department you wish to apply to to make sure what you want to do fits in, next find a PhD supervisor at that uni that researches into the area you want to go into and send an email explaining what you want to do and asking if they think it would be a good project. They will probably contact you back and adjust it sightly and say to apply. Then when you apply you are able to put a name in the "proposed supervisor" box then the application will go right to that supervisor to decide instead of just a departmental admisions tutor.

If they give you an interview, some do some just go off applications, then make sure you know your project inside and out. My interview was them asking questions about my project until I couldn't answer anylonger to see just how deep your understanding goes. They will ask job interview style questiong but they concentrate mostly on your understanding of your project because that tells them how suited to research you are.

 

[infohippie]

Special relativity assumes there is no privileged reference frame and therefore no universal time or absolute motion. How do we know there is no privileged reference frame? Have there been experiments done to demonstrate this?

 

[cookyy2k]

Special relativity assumes there is no privileged reference frame and therefore no universal time or absolute motion. How do we know there is no privileged reference frame? Have there been experiments done to demonstrate this?

I assume by this you're asking if their is an "ether" an absolute rest frame in which everything else moves? This was the thinking before relativity and was actually what relitivity set out to disprove, the speed of light stuff came later. Hence the name relitivity, everything is relitive to everything else, their is no absolute standard of rest.

An experiment was done on board a train travelling at constant velocity, the nice thing about this was it was a light experiment called the michelson interferometer, this is a fairl basic experiment but it showed the results are the same nomatter which frame you're in and this experiment plus maxwell's equations for light are what lead to the constant speed of light idea.

Things that show this are thought experiments like the twin paradox which I've described in this thread earlier. But these have problems in special relativity that are corrected by general relivity.

If their was an absolute standard of rest light would always have a constant velocity relative to this as opposed to all rest frames, so you'd be going back to Galilean relativity not the Eintein model.

 

[Valkyrie101]

What happens if a laser beam collides with a mirror? Would it burn through (I'm talking about proper, exciting lasers here) or be totally reflected?

 

[Salad Is Murder]

Can god make yo mama so fat that even he can't lift her?

 

[LordOrin]

There was one bit I didn't understand. Can you explain again what the second problem with CMD is?

Ah right, for the main galactic DM halo and the sub halo DM fragments their is a theoretical density function that fits current observations, however at the centre of each of these the density function become infinate and so the gravitational potential should be infinate, obviously this is unphysical but then other proposed density functions that do not have this problem do not recreate what we observe well enough to be considered correct.

Interesting stuff. Thanks!

 

[Dr_Matt]

Now in the case of superconductivity the electrons pair up into cooper's pairs which have spin 0 and so pauly exclusion no longer applies. Due to quantum mechanical effects the cooper pairs require a minimum amount of energy to excite them, if this energy is greater than the thermal energy (kT where k is boltzman's constant and T is temperature) the electron pairs will not be scattered by the lattice, thus having 0 resistance.

Superconductors display zero DC resistance, but they still have resistance for alternating currents due to the inertia of the Cooper pairs. Another bizarre feature is the heat transport properties of superconductors - although heat transfer in metals is by electrons, metals in the superconducting state actually display very low thermal conductivity, which is useful if rather counterintuitive.

Why is Pluto no longer a planet?

At the risk of igniting that argument again, Pluto technically never was a planet. Until the IAU meeting a couple of years ago where all this was discussed and settled, there wasn't a proper definition of what a planet actually is.

Oh, and I'm also a physicist. Good luck with the studies, guys.