If I fly east to west across the international date line I cross back into yesterday. Given that it's easily possible to circumnavigate the world in less than 24 hrs, if I go around and cross the date line again can I go back to the day before yesterday?
Easy answer: no. The dateline is an artifact of the local time definition. You have the GMT time and +- 12h from it. Crossing the date line artificially puts you into "yesterday" in local terms, but it's not a time function, just local definition. All regions of earth have some form of local time and you define your current time in the reference of your local time. Therefore crossing the date line twice in 24h will not be an actual phenomena because you'll have to cross over to today at some point in the meantime again
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In the weeks leading up to dialing up the LHC to it's rated energy there was much discussion about the creation of microscopic black holes possibly leading to uncontrollable accretion of matter and the destruction of the planet. That evidently didn't happen, since I'm here typing. Can you tell us how seriously the doomsday black hole was considered at CERN and what hypothesis was the basis for it being a non-issue in the minds of those with their finger on the neutron star button?
It was considered thoroughly. If you want you can read the actual safety assessment report:
http://lsag.web.cern.ch/lsag/LSAG-Report.pdf
Not only were black holes considered, but also strangelets and other phenomena that people were worried about. The basics come down to this. The LHC collides particles at basically speed of light. If a black hole were to form (a microscopic one that is) it's evaporate before it could move even an inch. For it to be a problem we'd need to firstly throw out the hawking radiation theory and make the black holes stable. Now even if they were stable they'd be microscopic and for them earth would be almost as empty as empty space, they'd be hard pressed to accidentally stumble on particles to eat up. The second issue is that because the initial particles were moving at the speed of light the probability that the created black hole would have an initial velocity below earths escape velocity is minuscule. And if we stretch the probabilities even further and make them be stopped and have them gravitationally aggregate to earths core it'd still take billions of years for them to gain enough mass to actually start to be felt. If it shot out and went to the sun i.e. velocity below solar systems escape velocity, then that'd have a faster effect, but still billions of years from now.
And the most relevant piece of data is that the universe has been doing far higher energy experiments with earth for billions of years from cosmic rays hitting earth and we're still here so the whole question is somewhat moot
Oh and I don't know if you heard at the time, but CERN was taken to international human rights court by some crackpots who thought CERN was putting the whole human race into danger. The case was dismissed though
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I've got this piece of popcorn stuck between a couple of molars and I'm outta floss. What now? :redface:
You wait in basement until your fingernails grow long enough and then use those?
Not really a physics question so I'll defer it to others. The physics solutions are plentiful, not necessarily pleasant ones (i.e. hammer and chisel to extend the crack where the piece of popcorn is stuck).
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Where does string theory fit into all this ? I haven't heard much around the string theory lately though.
The reason is that string theory as it stands is something for mathematicians to have orgasms about. Its usability in physics hasn't been shown yet. The main reason being that it's too general a theory, it contains so many free parameters, that you could make the theory do pretty much anything you'd like except you have no clue how to tune it to make it do the things we observe in the real world. It's a mathematical generalization that probably has as a subset the current theories, but noone's really able to calculate anything with it and noone's found a way to falsify it (a requirement of any physics theory is a test to falsify it, until that exists it's just mathematics).
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So my 1st question is:
If we go with gravity as space/time being warped by mass, thereby curving the shortest distance between two points, why do two massive objects at "rest" relative to each other accelerate towards each other when released. This is not the same question as why a moving object follows a curved path when passing by another massive object.
My guess is that both objects are moving through time and their masses warp space/time such that as they move along the time axis they get closer in the space axes. Is that a reasonable model?
I have to admit, I'm not too fluent in general relativity therefore take it with a grain of salt, but in general I'd say that yes this is a correct interpretation. They both warp spacetime and as their fields merge they warp the space axis that connects them to be shorter and they move towards each other. The classical representation of 2D surface that is bent isn't a good illustration of 4D space, but even there the curvature is smaller in the axis between the objects than to the rest of space giving preferential movement towards each other.
My question B is:
Consider walking down the street, swinging one's arms and legs. The hands and feet are moving with some velocity relative to the head, so they are subject to time dilation (not much, but > 0). Does that mean that our hands and feet are essentially "behind" us on the time axis and we're looking back down that axis to see them?
Sure, any objects moving would experience minor time dilation, but it's extremely small. But indeed any object moving faster than other objects would in relative terms be behind in time
It's an actual effect going to space and coming back etc, but it is an effect that is actually used daily by for example also GPS satellites
They were going out of sync initially all the time until they incorporated time dilation from their orbital motion
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Liquid fluoride thorium reactor, yes? no?
Haven't studied it in details, but the fact is that the first and second generation nuclear reactors were basic designs out of necessity and in multiple cases served as multi functional tools to produce radioactive isotopes (for medicine) and weapons grade material for A-bombs. There are various new designs that really are only as power stations, that can "burn" through various materials and are far safer. The only downside is that they've not been built yet so their actual efficiency etc needs to be understood. I've only seen a few vids on the thorium reactors, but they look promising. My personal attitude is that the safest large scale energy production and also the cleanest is nuclear reactors, but they need further work to further increase their safety margins and materials they take. There are for example designs for integral reactors that start from one radioactive compound and the process is repeated many times over in various reactions until the excess material is iron (just too inert to continue) which will be radioactive for a short timeframe, but not excessively long. Also, people have to understand that the tons of material doesn't mean huge piles of material. Uranium is 13x heavier than water so a cubic meter holds 13 tons of it. And that can power a lot of households for quite a while.
What would be easier to teraform, mars or venus?
I'd say Mars. All it needs mostly is CO2 into the atmosphere to block in the heat and raise the surface temperature ~10 degrees or so and some oxygen producing plants. Venus is a mess with highly acidic atmosphere and temperatures in excess of 500C. Couldn't even thing how to start there
What is the theoretical maximum energy density of lithium-ion?
No clue
One would have to do some serious math for it and that's far out of my field to answer from the top of my head...
You said that it is possible that somewhere in the universe, it can be collapsing at the speed of light and we would not notice it. But isn't the universe expanding at the speed of light as well? Considering that we are seeing things that are billions of years old and our planet is billions of years old, would the universe stop expanding at that point?
Universe is not expanding at the speed of light or we'd not see any stars at all. What is expanding is the visible horizon though as in photons from the horizon just about make it to us the first time and they've traveled at the speed of light. The universe itself is expanding at a far slower rate. The Hubble constant for example determines this expansion as 68 km/s / Mpc (68 km/s speed per megaparsec distance to the object we measure so the further out the faster they move away). So the two are not directly related. The causal information between any two points in the universe can interact at the speed of light so if something happens at point A that effect can propagate to point B only at the speed of light. That's why we'd know about our Sun going Nova only ca 8 minutes after the fact (and probably too late as the exploding matter would be hot on the heels of the photons telling us about it).
At this point, is the big bang still the most prevalent theory?
Yup, still the best theory there is, all experimental evidence points towards an origin event. However the laws of physics break down ca 10^-43 seconds after epoch so anything before that is something we can't explain and / or observe. Therefore whether there was anything before big bang or what/where/when are hard questions to answer at this point...
How far away are we from a true quantum computer?
No clue
Again, this is more of a material science problem than physics problem. From physics point of view it's easy, as is warp drive
The devil's in the details of the implementation.
