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# Tiki

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416

1. ## Alcubierre Drive and dark energy

Perhaps it would be better to say 'Negative active gravitational mass is not so nutty.' After all, negative energies are allowable in quantum field theory and energy is a gravitational source. Pressure is also a source of gravity and tension is negative pressure, therefore if a system has sufficient negative pressure (ie. tension) then its active gravitational mass would be negative. Unlike the case for a vacuum, the speed of light in water can be exceeded in water.
2. ## Alcubierre Drive and dark energy

Negative mass matter is not so nutty. Part of the gravitational field produced by a stream of moving particles acts in the opposite sense to the gravitational field produced by a static mass! (It is the part of the gravitational field that is produced by the momentum of the moving matter stream. cf. equations of gravitomagnetism).
3. ## Can I make a cloud chamber?

Building a cloud chamber has been one of my 'gonna do' projects for quite a while. A peltier design is outlined here http://www.xionone.co.uk/howtomakeacloudchamber1.htm. There is also a video of such a cloud chamber in action https://www.youtube.com/watch?v=WtsDEkCAS_Q&feature=youtu.be.
4. ## Spiraling into a black hole? Really?

In a binary system the more massive component will quite often evolve to a white dwarf/neutron star/black hole before the smaller companion star begins to expand into a red-giant. As the smaller companion star expands it sometimes happens that its outer layers are no longer gravitationally bound and are pulled towards the more massive compact component. Conservation of angular momentum will cause the in-falling matter to spiral into an accretion disc. The particles in the accretion disc lose energy principally as a result of friction and eventually spiral in to the central compact object. Try googling Roche limit/lobe/sphere. I hope this helps.
5. ## Observed brightness of the moon

Perhaps it might help to think of a telescope as an instrument for mapping circles of 'sky' to circles on your retina.
6. ## A Collision at the Speed of Light

There is also a second error in Prof Gower's exercise.... 0x1=0 0x(1-1+1)=0 1-1+1=0 [oops, just cancelled the zeros. line 2 in Prof Gower's exercise] 1+1=1 whilst 1/0 is not a number , 0/0 is undefined and can be manipulated in an equation to provide any value.
7. ## Stacking, law of diminishing returns?

Gaussian noise cancels out quite nicely with more subs. Less well behaved noise though doesn't necessarily cancel at all and can even have the opposite effect.
8. ## Jupiter's Gravity

It would depend upon how the spaceship is moving.
9. ## These twins are hurting my head!

The speed of light being so fast in comparison to other objects that we encounter in everyday life leads to a great many counterintuitivisms when considering rapidly moving objects. In the case of the twins, no-one knows why they should age differently it's just that they do. It is an artefact of the way our universe is configured. Einstein predicted time dilation as a logical consequence of his SR postulates: the laws of physics are the same for all inertial observers (ie. not accelerating with respect to one another) and that the speed of light in a vacuum is constant regardless of how the source moves. To derive the same all you need are the two postulates, Pythagoras's Theorem and a carefully chosen thought experiment. The 'Special' in SR is special as in 'special case'. The special case being no acceleration and no gravity. With this being said, it is only possible to treat the twins within the framework of SR if we don't ask about the accelerated part of the journey. This leads to a paradox (cf. Olly's post on page 1 of this thread) as to which twin should be the oldest as within the confines of SR it is impossible to differentiate between the two. I agree, great analogy from David Levi. Concentrate on the 'space-time interval' and it's invariant nature and you will soon be ready to understand SR in terms of Minkowski-space (a geometric approach). Once you have Minkowski-space internalized then much of GR will be accessible without the need for heavyweight differential geometry/ linear algebra. Space-time Physics by E.F.Taylor and the great J.A.Wheeler is always worth a look. Lots of words and lots to ponder, a thought provoking read to say the least. I'm not sure I follow. Do you contend that time dilation is a consequence of following different world lines?
10. ## Energy, mass and velocity

When you push on a wall (a solid one) it moves a very little bit. The constituent elements of the wall will gain a small amount of potential energy due to their new positions. This gain in potential energy is the work done on the wall. The tiredness that you eventually feel in your arm is the consequence of the wall pushing back at you. It is just like compressing a spring. There is no work being done when you and the wall are in equilibrium. When you push an object that is free to move the force that you feel is inertial (the unwillingness of objects in nature to change their velocity) and the work done on the object manifests itself in the form of kinetic energy. I hope this helps.
11. ## Help with BlackHole question: Schwarzschild radius

I like this argument. The least curved path is of course a consequence of a lesser force. Orbits near black holes are in some ways very different from their Newtonian analogues. For instance in a Schwarzschild space-time there are no stable circular orbits for r<6M and no circular orbits at all for r<3M. OP's original question can be answered using purely Newtonian gravity. In fact, a distant observer would be able to deduce Kepler's third law if he were to observe a body in circular orbit around a distant black-hole.
12. ## Temperature of Space

When matter is heated it's mean kinetic energy increases. The change in velocity of the charged particles within the matter emit electromagnetic radiation as a consequence of this acceleration. This is known as thermal radiation. Electromagnetic radiation of say 3K means that the spectral density of the radiation is consistent with a black-body at a temperature of 3K. Try Googling 'Plank's law' for a better explanation. Yes. Think what it is like to toast yourself next to an open fire in a cold room.
13. ## Help with BlackHole question: Schwarzschild radius

A is approx. 1003 km distance from COG of M=1 hole B is approx.3,000,001,000 km distance from COG of M=10^9 hole 'A' therefore feels about 90,000 times more force than 'B'. Force is proportional to M/R^2 and Schwarzschild radius =2M = 3km approx.

15. ## elephant's trunk: where to aim

You can download little bits of sky with the 'DSS' button in CdC.
16. ## Setting-up time?

Thanks very much everyone. I appreciate the time that has been spent in providing so much detail.
17. ## Setting-up time?

Thanks for the replies so far. Thanks DP, I hadn't thought about the cool down time too much but I would be imaging with my 4" scope. It is the prospect of too much fiddling that has kept me away from imaging so far.
18. ## Setting-up time?

How long does it take to get set-up for a session of imaging? By set-up I mean; place the mount/scope on the patio, polar align, calibrate the guiding, point the telescope precisely (ie. plate solve) and focus. Assume a sampling rate of 2.75"/pixel and requirement of 5minute subs. Are there any particular mounts/software/gizmos that would make it quicker to get up and running? What sort of a time saving would a permanent rig in an observatory likely yield? Thanks, Paul
19. ## I've been thinking...

You could fall into a supermassive black hole without coming to any immediate harm whatsoever. If however you were to fall feet first into a stellar-remnant sized black hole then you also wouldn't feel a thing but for a very different reason. In the latter case, the tidal forces would increase at such a rate that you would be ripped apart so quickly that there wouldn't be time enough for the nerve impulses from your feet to reach your brain! I agree, space is cool. Black holes are also pretty cool as they provide a way for the curious to get their heads around GR.
20. ## Is our Universe 13.7 billion years old ? I'm not quite convinced

On a trivial (and local) level, time is simply that which is recorded on your wrist-watch. On a global level however, to be able to cogently define time would quite possibly be equivalent to solving the deepest cosmological questions.
21. ## Core collapse confusion

I am not a physicist either, but it might be helpful to think of the core as just a giant ashtray for the fusion processes which goes on above. If the core reaches a certain critical state then fireworks will happen but for most of the time, the core does not influence the rest of the star too much. IMO, the key idea to appreciate is a star in a 'steady' state: a balancing act which goes on between gravity, the kinetic energy (like gas pressure) of constituent particles and the rate of fusion reactions. eg. fusion rate decreases-> star contracts ( graviatational potential energy released) and kinetic energy increases, temperature goes up, fusion rate increases-> star expands (gravitational potential energy increases) and kinetic energy decreases, temperature goes down, fusion rate decreases-> star contracts.....
22. ## Core collapse confusion

1) Fusion stops the gaseous part of the star ( ie. the bit surrounding the core) from falling inwards under the force of gravity. Over time and with the fusion of heavier elements the core gets heavier and more dense. Eventually a point is reached whereupon electron degeneracy pressure is no longer able to support the core and a collapse ensues. 2) Degenerate matter gets hotter when you compress it. In a larger star there is more matter to support , therefore greater pressure, and compared to a smaller star, a higher rate of fusion. 3) / 4) As the core collapses, instead of fusing, iron will either be smashed to bits (protons and neutrons) by thermal photons (at about T=10^10K, density=10^12 kg/m^3) or at even higher subsequent temperatures, a proton residing in an Fe nucleus will capture an electron and will become a Mn nucleus which will in turn capture another electron to become a Cr nucleus etc. This certainly won't go on forever as the timescale of a stellar core collapse is of the order of milliseconds. A massive amount of energy is absorbed in these processes which makes the collapse all the more sudden. It is thought (and debated) that when the collapsing core reaches a critical density ( something like 5x 10^17 kg/m^3) it becomes suddenly rigid and bounces back to a density of about 2x 10^17 kg/m^3. This is thought to be the progenitor of an outward bound shockwave that meets in-falling stellar material. Of all the jaw-dropping sights that we are able to marvel at through the eyepieces of our telescopes, it is that barely discernible supernova that I like best. What a pyre. Unfortunately, I have only seen one to date.
23. ## Noctilucent Cloud from London this morning July 19

There are a few ways to estimate the cloud-range. eg. angular distance per pixel= angular distance across photo / no. of pixels across photo angular height of NLC = (height of NLC above horizon in pixels) x (angular distance per pixel) From trigonometry, tangent( angular height of NLC)= height of NLC / distance to a point directly below the NLC If we suppose that the earth is flat and NLC's reside at 50 miles altitude, then distance to a point directly below the NLC in miles = 50 x tangent( angular height of NLC). You can calculate the range( hypotenuse) using Pythagoras. All approximate but good enough for a rough idea. So approximately how far away are the clouds?
24. ## Special Relativity - Could we see it ?

The whole of science abounds in such results. Quantum mechanics is perhaps the quintessential such result. Einstein famously speculated about what it would be like to fall off a roof and came up with GR; he also wondered what it would like ride along next to a ray of light and came up with SR.......
25. ## Special Relativity - Could we see it ?

It cant . It does in the hypothetical situation that is under consideration. Only if you were to reinvent the aether, otherwise there is nothing to 'push' through. Since we have dispensed with SR then why not further extend the thought experiment by reinventing 'nothing' and 'aether'. Isnt it strange that "medium" always ends up in parenthesis ! Not always. https://en.wikipedia.org/wiki/Neutrino_detector#Cherenkov_detectors From the Wikipedia 'neutrino-detector' article :"Ring-imaging" Cherenkov detectors take advantage of a phenomenon called Cherenkov light. Cherenkov radiation is produced whenever charged particles such as electrons or muons are moving through a given detector medium somewhat faster than the speed of light in that medium. In a Cherenkov detector, a large volume of clear material such as water or ice is surrounded by light-sensitive photomultiplier tubes. A charged lepton produced with sufficient energy and moving through such a detector does travel somewhat faster than the speed of light in the detector medium (although somewhat slower than the speed of light in a vacuum). The charged lepton generates a visible "optical shockwave" of Cherenkov radiation. This radiation is detected by the photomultiplier tubes and shows up as a characteristic ring-like pattern of activity in the array of photomultiplier tubes. As neutrinos can interact with atomic nuclei to produce charged leptons which emit Cherenkov radiation, this pattern can be used to infer direction, energy, and (sometimes) flavor information about incident neutrinos. Cherenkov won the 1958 Nobel Prize for Physics for his work on the radiation that now bears his name. Cherenkov is not doing anything contrary to the so called speed of light barrier, it is not exceeding c. I almost totally agree. Nothing has ever been known to exceed the speed of light in a vacuum. The speed of light in water (approx. 2/3 the speed of light in a vacuum) is sometimes exceeded by certain particles (in the water) that absorb sufficiently energetic neutrinos. IMO, Cherenkov radiation is very counter-intuitive but really quite amazing.
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