1

Physiology & Medicine / Re: Why do we get goosebumps when we are cold?

« on: 11/07/2019 15:29:31 »

It's a hold over from when our evolutionary ancestors had more bodily hair.  The "Goose bump" reaction would cause the hairs to stand more upright, which, in turn, made them more effective at trapping air near the body which insulated it from the cold.
It also made one look larger by puffing up the hair, hopefully scaring off would be attackers.   This is why people can get goose bumps in situations where they feel scared.

2

Technology / Re: Is renewable energy good for the environment?

« on: 06/07/2019 18:25:15 »

Whatever the merits of renewable energy, it adds a heck of a lot to global warming!

As does any method of "producing energy".  There will always be inefficiencies that create waste heat whether is by solar or digging up and burning coal.   But burning fossil fuels also adds to the problem by taking carbon that has been sequestered and putting in the atmosphere as CO2.   There is no "perfect solution" to global heating due to energy production, ( other than reducing the energy production itself),  but the lack of a perfect solution doesn't mean that some methods are less problematic than others.

3

Physics, Astronomy & Cosmology / Re: Can a test of reciprocal time dilation be constructed from this?

« on: 05/07/2019 17:41:19 »

I imagine the simple ones used by the police do not do the full computation and thus can only be used for non-relativistic speeds.  We're assuming the Alice-Cop is armed with the high end model here, and it would need to employ RVA to get an accurate speed for Bob.
Actual police radar measures but doesn't display the police speed relative to the background (she's not interested in her own speed), nor does it display Bob's measured speed relative to the moving police car.  Alice is interested in Bob's speed relative to the road/tracks, and it might need to determine this even if Bob is not moving straight at or away from the gun.  That's pretty complicated, but the gun is designed to display Bob's speed relative to the road.  That's all done with mathematics inside the device.  For the simple 1-dimensional case outlined in this post, the calculation is pretty simple, and yes, RVA needs to be employed to get an accurate figure.

Just to add to what Halc has said here.
a radar gun, such as used by Police calculates the velocity of an object relative to the gun by using the formula
v= cΔf/2f_s
where c is the speed of light, Δf is the difference in frequency between original transmission and reception, and f_s is the original transmission frequency.   This works well enough when v is small compared to c and is simple, as v is proportional to Δf.

The more correct and accurate equation is

v = c(1-(f_o/f_s)²)/2(1+(f_o/f_s)²)
where f_o  is the observed return frequency and f_s the original source frequency.

For comparison, a typical radar gun using the first formula and giving a velocity of 67.5 mph should return 67.5000135 mph using the accurate equation.  Hardly worth the extra work.

However, a standard radar gun using the first equation and giving an answer of v=0.25c  would be significantly inaccurate, as the correct value of v for the same difference between transmission and return frequencies (using the accurate equation) is 0.3c.

For police radar used in a moving car, the radar calculates the speed of the car by comparing the frequencies returned from objects stationary with respect to the road and those returned from the car.   It then just takes a simple difference. At low values of v, this simple difference is not going to differ significantly from the answer you would get using RVA.   Again, once the velocities begin to become significant relative to c, RVA becomes more and more important for getting the correct answer.

4

Physics, Astronomy & Cosmology / Re: What time is it on the Moon?

« on: 01/07/2019 17:26:52 »

You have to distinguish "local" time and "global" time on the Moon.

For Local time on the Moon, Noon is when the Sun reaches its highest point overhead.
- One day is the interval between two adjacent Noon events.
- One Lunar Day = 29.53 Earth Days.
- See: https://en.wikipedia.org/wiki/Moon#Appearance_from_Earth

Though to be accurate, that is the "Mean" solar day for the Moon.  Any given solar day can be from 29.13 to 29.93 Earth days long, varying over the course of the Earth's year.   This is due to the eccentricity of the Earth's orbit around the Sun and the slight variance this produces in its orbital velocity.

5

General Science / Re: Are "nucular" devices weapons of mass digestion?

« on: 29/06/2019 16:24:05 »

Well, they obviously would be the true "seeds of destruction".

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Physics, Astronomy & Cosmology / Re: How does gravity affect a caesium clock?

« on: 26/06/2019 02:41:47 »

Nobody claims that a cesium clock is perfect, only that they are very very accurate

On the contrary, time is defined by the frequency of a cesium clock, which makes it perfect by definition.

The second is defined by the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom.  But there is a world of difference between this standard and using it in a practical clock.  We can measure that frequency through the electromagnetic radiation emitted by caesium atoms.  However since those atoms ( not being at absolute zero), will be moving at various directions and speeds, that radiation will be Doppler shifted to a certain degree leading to a "smearing" of the measured frequencies.   There are means to reduce the effect(and others) this has but none of them is perfect.   Thus a practical caesium clock will not be 100% accurate. We have been able to push the uncertainty of these clocks to an extremely small value, but not down to zero.

7

Physics, Astronomy & Cosmology / Re: How does gravity affect a caesium clock?

« on: 25/06/2019 16:18:00 »

There are basically two kinds of clock those that depend on gravity a and those that depend on inter molecular forces such as crystals or balance wheel springs, these are affected by magnetic fields and to a lesser extent by gravity (of course the former is wholly dependent on gravity).
It is known that the intense magnetic fields of magnastars  distorts atoms would this screw up the caesium clock
 

It would likely screw up the electronics needed to get a reading from the clock, but I really don't see the point of this question.  Nobody claims that a cesium clock is perfect, only that they are very very accurate, and that any variation due to local conditions in normal situations (such as between sea level and a high mountain) are much much smaller than the effect we are trying to meaure ( time dilation).

If you could put a clock in orbit around the black hole at the center of our galaxy, it would be in free fall and experiencing 0g, and even orbiting just 1 AU from the BH, the tidal forces on it would be insignificant, in the order of  3 x 10^-8 g over 1 meter.   However, the time dilation factor would have such and clock losing nearly 3/4 of an hour per day as compared to a far distant clock.   You wouldn't need an cesium clock for this measurement, any reasonably priced watch would do to measure the effect.

8

Physics, Astronomy & Cosmology / Re: How does gravity affect a caesium clock?

« on: 24/06/2019 16:01:24 »

Relativity says either there is no "actual" frequency, or it is always whatever you measure at that point! Interesting that a mechanical wristwatch with an escapement wheel will keep perfect time anywhere in the universe but a pendulum clock will stop if g = 0.  Thus a pendulum clock does not measure time, but gravity!

However, while a pendulum clock sitting on the surface of the Earth and one sitting on the surface of a world with 4 times the Earth's mass and twice its radius would experience the same value of g, the one sitting on the more massive planet would tick slower when compared to the one on the Earth.

9

General Science / Re: What will be the radiation exposure from 5G networks?

« on: 22/06/2019 16:08:28 »

I get annoyed by the misuse of the term radiation I was bought up to believe that radiation was the emission of energy in the form of electromagnetic waves but nowadays there seems to be a confusion between waves and particles emitted from radioactive sources.
I blame Einstein and his disciples   

The use of the word "radiation" to refer to many types of emission predates Einstein, and was never limited to just EMR.
X-rays when discovered, were called a radiation( of an unknown type, thus the "X"). And while it turned out that they were a apart of the electromagnetic spectrum, this wasn't learned until later.  Cathode rays, another discovery predating Einstein, were another radiation, one that was later discovered to be electrons. 

If you were taught that radiation ever only referred to the electromagnetic spectrum, then you were taught wrong.

10

Physics, Astronomy & Cosmology / Re: Where were the electrons before the big bang?

« on: 22/06/2019 15:54:14 »

During the earliest stages of the universe, the fundamental forces were unified into a single force( or at least were all of the same strength).  This means that they did not even exist in forms that they have today.  As the universe cooled, different forces separated out from that mix, one by one. First gravity, then the strong force...   Thus when the "quark soup" was in sway, the electromagnetic force had yet to come into existence as a singular force, so the entity we now call the electron wasn't possible.
To use an analogy, its like when you freeze water.   It doesn't really make sense to ask what "form" or shape the crystal structure of ice took before the water started to freeze, as the crystals depend on the water having the properties to allow them to form.

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Physics, Astronomy & Cosmology / Re: Do Peking and Helsinki lie on a straight line through Earth's core?

« on: 22/06/2019 01:16:02 »

The red line represents the straight-line path.  The yellow sphere represents the extent of the outer core.


core.png (122.53 kB . 960x540 - viewed 277 times)

So the question is, What do you mean by "pass the core"?   Do you mean "pass through" the core, or do you mean "bypass" the core without entering it?  An if you mean bypass, do you mean it has to be close miss or does any miss count?  It is obvious that the line never enters the outer core, nor does it come very close.  so the answer to your question depends on exactly what the question is. 

12

Plant Sciences, Zoology & Evolution / Re: Who is better at climbing: spiders or geckos??

« on: 21/06/2019 17:03:20 »

In either case, you run up against the cube-square law.   What works for spiders at their scale just isn't practical when you start to go much larger.   The adhesion strength will be a function of the surface area touching the surface.   If you scale up a spider (or gecko) by 2 times, you increase that surface area by the square, or 4 times.  However, you also increase the volume, and thus its weight, by the cube, or a factor of 8,  The weight that needs to be supported has increased twice as much as the adhesion force holding it to the surface.  Keep going and the spider/gecko will no longer be able to "stick" to a surface enough to support its own weight.

13

Physics, Astronomy & Cosmology / Re: BREAKING: New potentially habitable exoplanet found around Teegarden's star

« on: 19/06/2019 16:30:55 »

I think the key here is "OLD red dwarf".   A planet orbiting a red dwarf in the habitable zone, by necessity, has to orbit close in, where it is going to be subject to a stronger solar wind.  With an old star, this means more time for that solar wind to strip the planet of any atmosphere it may of had.

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Physics, Astronomy & Cosmology / Re: What happens when a neutron star collapses, and what happens is a black hole collapses?

« on: 19/06/2019 16:23:12 »

Josh Ansell asked the Naked Scientists:
   
Love the show, it's really interesting and so are the kitchen sciences. My secondary specializes in science so sometimes we do the kitchen science experiments.

My question is: a neutron star is a collapsed star and a black hole is a collapsed red giant what happens when a neutron star collapses, and what happens is a black hole collapses?

What do you think?

Some stars, like our own Sun, become white dwarfs at the end of their life cycle.  These are essentially the cooling embers of a once active star.    However, There is a limit as to how massive a white dwarf can be. This limit, known as the Chandrasekhar limit is ~1.4 solar masses.   After that, the star cannot no longer support its mass by electron degeneracy pressure and the star would collapse into a Neutron star.   
Neutron stars also have a maximum mass. The Tolman–Oppenheimer–Volkoff limit, for cold, non-rotating neutron stars has been recently estimated to be 2.17 solar masses. 

If a white dwarf were to collect enough additional material it would collapse into a neutron star, and if a neutron star were to collect enough material it would collapse into a black hole.
These are not the usual methods by which neutron star or black hole are produced.  Both are produced by the collapse of stars at the end of their life cycle (just like white dwarfs.   What decides the end fate is how much mass is left after the collapse.  Our own star will blow off a good part of its mass before what is left shrinks to a white dwarf.  Stars much more massive than our own die by supernova, blowing off a large part of their mass.  Whether a neutron star or black hole is left behind depends on how massive the remnant is.  The more massive the original star was, the more likely it is to form a black hole.
All moderate to large mass stars that reach  either white dwarf, neutron star or black hole stages pass through a red giant stage first.   Red dwarf stars on the other hand never pass through a red giant stage. They just continue to burn fuel until they finally reach a blue dwarf stage. This takes trillions of years and no red dwarf has yet reached its end stage during the lifetime of the universe.

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Physics, Astronomy & Cosmology / Re: Is it possible to make the Kessel run in under 12 parsecs?

« on: 19/06/2019 01:32:22 »

The Naked Scientists podcast this week quoted Han Solo's boast that has been annoying science geeks for decades...
- That he "made the Kessel run in under 12 parsecs"
- This uses a measure of distance as a time...
- ...Not so different from members of the public interpreting light years as a time, when it is actually a distance

After years of criticism, the Star Wars franchise did a "save" in a recent prequel, where they spin a story that IIRC:
- They are at location in a nebula
- There is a safe but really long way to get where they are going
- But they are running low on fuel...
- So Han Solo takes a reckless short-cut which gets them to their destination before they run out of fuel

So this sets up a scenario to explain that they were really were using the parsec correctly, as a distance...

It wasn't fuel that they were low on, It was that their cargo, which was unrefined and unstable wouldn't have lasted the longer route without blowing up the ship.  The whole "Under 12 parsecs" thing dealt with calculating a hyper space course which was short enough to get to the refining station fast enough. I assume that this is predicated on the idea that the nebula was so full of junk that had to be avoided that a "straight line course was not possible, and the only known plotted courses meandered enough to make the total distance traversed greater than 12 parsecs.

But I agree that this whole bit was ad-hoc in order to explain way the line in the first movie.

16

Physics, Astronomy & Cosmology / Re: If a ball bounces off a train, what's its recorded speed?

« on: 18/06/2019 15:54:40 »

This principle is what is taken advantage of when we use "gravity assists" by passing probes close to planets.  You don't have to throw the ball towards the train to get a Boost in speed.   If for instance, you were to toss the ball at 30 km/hr in the same direction as the trian's motion while in front of the train such that the train, moving at 50 km/hr catches up to a hits the ball "from behind", then the the ball and train collide with a relative speed of 20 km/hr, and the ball rebounds at 70 km/hr relative to the platform, for a gain of 40 km/hr.
With planetary gravity assists, we don't "bounce" the probe off the planet, but use the planet's gravity to change the probe's velocity.  In an ideal situation,  we would plan the probe's path around the planet to be a parabola, so that it makes a prefect 180 turn around the planet.   This isn't practical in real life, as in pretty much every case, the peirapis (closest approach to the planet's center) ends up being under the planet's surface.   Thus we have to settle for a hyperbolic trajectory and a less then maximum velocity exchange.   
We are also limited to using the " toss the ball in the same direction as the train's motion" method.   All the planets orbit the Sun in the same direction, and when we launch a probe, we take advantage of the Earth's orbital motion to get it up to the required velocity(with respect to the Sun).  This means the probe is going to be moving in the same direction as the planet when it get there.

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Physics, Astronomy & Cosmology / Re: Do only spinning objects in space have gravity?

« on: 16/06/2019 06:10:14 »

Everything with mass has gravity, spin has nothing to do with it. 

As to why objects in space such as planets, asteroids and moons spin, it would be pretty unusual for a object to have zero spin.
To put it simply, they were all formed by stuff falling together and collecting.   All this stuff is going to be moving at different speeds and directions.   It would take all these various collisions between objects to exactly sum up to zero for the final object formed to end up with no net spin at all.

18

New Theories / Re: How is this explained in Special Relativity?

« on: 11/06/2019 20:03:50 »

If you are considering A "tick" as when the light pulse leaves the central source and a "tock" as when the pulse reaches an end mirror,  Then you have to consider how Bob measures the time between "tick" and "tock".   Measuring the time between "ticks" or between "tocks"  is straight forward, because the interval is being measured by a single clock (either at the source or end mirror).    When Bob measures the interval between "tick" and "tock", he has to use two clocks, One that measures the time at the source when the pulse leaves, and one at the end mirror when the pulse arrives.  Now as long as Bob can consider these clocks as being synchronized to each other, then he can also assume that the time measured by an end mirror clock at a "tock" is the same as the time measured by the clock at the source.   

So according to Bob, a tick can start at zero for all clocks like this.

lightclock1.png (7.5 kB . 633x356 - viewed 357 times)
And and a tock occurs when all clocks read one, like this:

lightclock2.png (12.05 kB . 633x356 - viewed 351 times)

Now according to Alice,  A tick starts like this:

lightclock3.png (7.46 kB . 633x356 - viewed 353 times)
The horizontal leg is length contracted ( we are assuming a 0.5 c relative velocity between Bob and Alice)
And while the both the source clock and vertical leg end clock both read zero., the horizontal end clock already reads 0.5 ( relativity of simultaneity).   

A tock for the horizontal leg occurs when the end clock runs into the expanding light pulse.  In between tick and tock, all clocks will have advanced by 0.5.  The source and vertical end clock read 0.5 and the horizontal clock reads 1.

lightclock4.png (10.55 kB . 633x356 - viewed 351 times)
Both Bob and Alice agree that the pulse leaves the source when the clock there reads 0 and reaches the end clock when the clock there reads 1,  but they disagree as to what time it is on the other two clocks when that happens.  if Bob is located at the source clock, He will say that the end clock reading 1 and his clock reading 1 are simultaneous, while Alice will say that those events are not simultaneous.

Thus according to Alice, a tock ( the pulse reaching the end mirror) occurs when Bob's source clock reads 0.5, however according to Bob, when his source clock read 0.5, a tock has yet to occurred.   You can't make a direct comparison between the Tick-tock interval for the two frames because a tock represents different pairs of events for the Two frames ( end clock reading 1 and source clock reading 0.5 for Alice, End clock reading 1 and source clock reading 1 for Bob.)    The only way to make a proper comparison is to deal with a round trip for a pulse, either leaving the Source at a tick and returning on the successive tick, or measuring between tocks at the end clocks.  This way both Bob and Alice agree as which events the interval is being measured between.

When dealing with a vertical pulse, to be strict, you should also only consider the total round trip.  However, since both Bob and Alice will agree that both source clock and end clock remain in sync with each other, we can "cheat" and only consider just the one way trip and not encounter any problem.   And while this "cheat" works in this case, it is technically a cheat, one that only works in this particular case and can not be applied to the horizontal light clock.

19

New Theories / Re: what if a neutrino was not a particle?

« on: 11/06/2019 03:20:46 »

Neutrinos don't travel at the speed of light as the do not have zero rest mass. They have 1/2 spin and interact via the weak sub-atomic force.  They also come in three varieties, Electron, muon and tau.  All of these characteristics point to it being a particle and not a "hole in the space/time fabric". 

20

New Theories / Re: How is this explained in Special Relativity?

« on: 10/06/2019 20:06:23 »

You are coming up against the relativistic effect known as the "relativity of simultaneity".  Bob, if he sends light pulses to two mirrors, parallel to the relative line of motion and opposite each other, then, for Bob, those pulses will reach those mirrors at the same time.  For Alice, the light will reach the trailing mirror first and then the leading mirror.  Bob and Alice will not agree as what event are simultaneous if they are separated along the line of motion.   
But now consider the reflections.  Again according to Bob, they take an equal time to return and meet back up with him again at the same time.  For Alice, the trailing refection takes longer to catch up to Bob than the leading reflection take to run into Bob.  The end result is that both pulses return to Bob as the same time.
The upshot is that One "tick" must be a round trip and it makes no sense to only consider only one leg of the trip.
Length contraction come into play in that it assures that the parallel pulses make the round trip in the same time as the perpendicular ones do. 

The end result looks like this from Alice's frame:

length_con2.gif (127.71 kB . 640x100 - viewed 473 times)
( I only am showing one of the parallel mirrors here, as this is an existing animation I had already made, but adding the other mirror would not change the end result.)
From Bob's frame, His clock would behave like the stationary one in this animation, while Alice's clock moved off to the Left, being length contracted and ticking slow.

Thanks Janus, I am familiar with the Relativity of Simultaneity. That is why I posted the question, because of an issue with it.

If we consider the scenario with just one light clock first. Alice on the platform observes Bob's clock to be ticking slowly because the photon traces a longer, diagonal path between tick and tock. The conclusion in Einstein's interpretation of relativiity is that "time slows down for the relatively moving observer".

Now, conider the same clock turned on it's side. What does Alice see. Well, she sees the clock tick faster in one direction than in the other - your gif demonstrates it clearly. So, she sees "time" speed up in one direction and slow down in the other.

No, As I said before, is doesn't you really can't think of it that way when considering  the one way travel for light.
 Let's put it this way,   Let's say we put ideal clocks at Bob's mirrors.  According to Bob's frame, those clocks are synchronized to each other.  and to a clock at with Bob, halfway between those clocks.  If, as measured by Bob, those mirrors are 300 meters apart, then according to him if the pulses leave his clock when it reads 0, then will arrive at the mirrors when all three clocks read 0.5 microsec (we'll round c up to 300,000,000 m/sec), and returns to Bob when all three clocks read 1 microsecond.  However, according to Alice, All three of those clocks are not in sync, the Lead clock lags behind the middle clock and the trailing clock runs ahead.  So even though it takes less time for the pulse to reach the trailing clock, its "head start insures that it reads 0.5 microsec when the pulse reaches it, and while it takes longer for the pulse to reach the leading clock, the fact that lagged behind the other clocks means that it will also read 0.5 microseconds when the pulse arrives, even though this pulse arrived later than the pulse arrived at the trailing clock.  If you were to run a string of clocks between the mirrors,  each one recording its time when a light pulse passed it, Both Alice and Bob would agree as to what time was on any given clock when that light pulse passed it, even though they would not agree as to which two clocks pulses fired in opposite directions where passing at any given moment.   

Now, put the two clocks in together. For the floor to ceiling clock "time slows down", but for the end-to-end clock, time speeds up as the photon goes to the rear of the train (because the mirror is moving towards it) while time slows down when the photon is going in the opposite direcction (because the mirror is moving away from it).

Surely pardoxical, no?

No.  It doesn't make any sense to say that Time "speeds up" for a pulse going one way, as Bob and Alice will not agree on the simultaneity of events that are separated along the line of relative motion.  while one pulse takes less time going in one direction according to Alice, you cannot make a one for one comparison between this and Bob's measured rate of time.