# Janus

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1. ## Pedestrian crossing the street

The light clock experiment with 2 light clocks( red and blue) starting at the same point: The expanding circles highlight how the the light pulses travel at c. The same experiment, but now, we are "riding along" the blue light clock. From the inertial frame where the red clock is stationary, the blue clock ticks slower, and the from the inertial frame where the blue clock is stationary, the red clock ticks slower. Keep in mind this is the same scenario, just viewed from different inertial frames of reference.
2. ## Time dilation explained with laser range finder

It reminds me of one of "sovereign citizen" videos where they show up in court, ignore everything the judge says, and just keep repeating the same phrases from their "script" over and over again.
3. ## Time dilation explained with laser range finder

This is nothing but a bunch of half-baked ideas based on misconceptions, and all it shows is that you don't have the faintest understanding of Relativity.
4. ## Time dilation explained with laser range finder

The issue with time dilation descriptions like the one with the train and embankment is that they often miss a critical point, since they generally focus on the embankment observer alone. So let's consider both observers. We start with two colocated light clocks, 1 stationary to our reference frame and one in motion. In these animations, the yellow dot is the pulse of light bouncing between the mirrors. The expanding circles are radiating outward at c, and act as reference showing that each pulse is moving at c relative to the frame of reference. Animation 1 is the from the frame in which the red clock is stationary. As can be seen, the red light clock ticks faster than the blue clock. If, for instance, the round trip for the red clock takes 1 micro second, it takes longer than that for the blue clock to complete one round trip. But what if there were someone "riding along" with the Blue clock? What would be happening according to them? This is what Animation 2 shows Since light travels at c in all inertial reference frames, In this frame, it is the Blue clock that takes 1 microsecond per round trip, and the red clock that ticks slower. Keep in mind, we have changed nothing from the previous animation other than switching observers. And there is no reason to prefer Red's perspective of events over Blue's or vice-versa. Both are equally valid. The two frames just measure time differently. This is the essence of Relativity.
5. ## Anyone traveling to see the eclipse?

Someone else fact-checked this, and determined it was actually only 2 (of the 7 in the US). So maybe the world only has a 2 in 7 chance of ending tomorrow.
6. ## Anyone traveling to see the eclipse?

I also heard about at least one hotel in the path of totality pulling the same stunt that some pulled during the last eclipse, canceling reservations that had been made well in advance once they realized how much people were willing to pay for a place to stay in the eclipse path. In this particular case, it was a travel agency that had been booking "Eclipse packages", and had been making arrangements with this hotel starting 2 years ago. Suddenly, and just recently, the hotel informed them that they were canceling the contract. I missed the '79 eclipse due to clouds(even though I lived in the path of totality), and almost missed the one in '17 due to fog/low clouds(we had a lucky encounter with a sanitation worker who told us that by driving up a certain street and up a hill, we could get a clear view.)
7. ## Time dilation explained with laser range finder

Point 1 is wrong. Time dilation is not due to a change in difference between emitter and receiver. Such a difference change produces a Doppler shift, which is a separate effect. Time dilation is where two reference frames measure different time intervals between two events. So, for example, if you had two emitter/receiver setups. Each with a constant and equal distance between each respective pair, and these two setups were in motion, Then an observer at rest with respect to either of the setups would note that the time intervals between transmission and reception would differ between the setups. Point 2 is also incorrect. In such a mechanism, the purpose of the wound spring to to counter losses due to friction. It doesn't take any force to keep something moving at a fixed angular speed. If you were to remove friction from the example, the hand, once moving, would just continue to rotate on its own without any further input of force. This would not change due to the overall motion of the mechanism. The clock hand would be measured as moving slower by someone that the mechanism is moving with respect to, but this is due to their measuring time intervals differently, and not due to some mechanical effect acting on the mechanism. Point 3 is, again, incorrect. c is the "speed limit" in the universe because it is an invariant speed. If something ( like light) is moving at c, then everyone measures the light as moving at c with respect to themselves, regardless of the relative velocity differences between those doing the measuring the light. A universal speed limit equal to this invariant speed automatically follows due to its mere existence.
8. ## Sunshine on Titan

Direct sunlight ~100,000 lux 0.1% of that 100 lux, which is equivalent to an heavily overcast day, and brighter than that of the hallway lighting of a typical office building. A moonless clear night is ~0.002 lux
9. ## Ants On High Gravity Planets

Ants can do this because of something called the square-cube law. When you increase the size of something by a given factor you increase its surface area and the area of its cross-section by that factor squared, and it volume (thus its mass) by that factor cubed. The strength of limbs are dependent on the area of their cross-section. Which, as the size increases increases at a slower rate than the mass/weight of the animal. Increase an ant to the size of a man, and it wouldn't even be able to support its own weight. Conversely, shrink a man down to the size of an ant, and they'd put a ant to shame in terms of strength. So in terms of higher gravity, this just means that smaller animals would generally fare better.
10. ## Today I Learned

TIL that I was born on a continental divide. I was born in the Mesabi range in MN, which I learned is part of the Laurentian continental divide.( a dividing line between which way water flows to the ocean.) It is one of 6 in North America: Great, Arctic, St. Lawrence, Laurentian, Eastern, and Great Basin. In addition, where I lived was also where the St. Lawrence divide meets up with the Laurentian divide.
11. ## Age limit for political positions- Split from: An observation on gun control

Approval ratings in today's political climate are a bit meaningless. How many disapprove because they think he's "too far to the Left?" and how many because they think he's "too far to the Right" How many give him a poor rating based on just one single issue? etc.
12. ## Force/ gravity/ mass/ acceleration

Again, F=ma merely gives a relationship between force, mass, and acceleration. It does not have anything to do with push or pull. It gives the magnitude of the force required, and isn't concerned as to the nature of, or how this force is provided. Here is another equation : F= mv2/r It tells you how much force is needed to constrain a mass to moving in a circle with a radius of r if it has a mass of m and has a speed of v. It makes no difference as to how that force is applied. It can be by a rope anchored at the center of the circle, a rocket engine mounted on the mass applying inward thrust, by the gravity of a central mass, or by the friction between a car's tires and the road.
13. ## Force/ gravity/ mass/ acceleration

It is really important to grasp what the variables mean in each equation. In F=ma, we are talking about the amount of force needed to give a mass of m an acceleration of a With F = GMm/d^2 we are talking about the gravitational force acting between masses M and m at a center to center distance of d. To make this clearer, F is often written as Fg Now if we were considering how much acceleration mass m would undergo as the result of gravitational attraction between m and M, Then we we are saying that Fg is assuming the role of F in F=ma or that F=Fg thus we can substitute ma for Fg to get ma = GMm/d^2 cancel m on both sides of the equation and get a = GM/d^2, which tells us that the acceleration of m due to the gravitational attraction is independent of the magnitude of m's mass.
14. ## Rotation effect on gravity?

As I already noted, 28,437 kph falls a bit below that for even a near surface orbit. To be lifted off "into space", using the Kármán line (at 100 km altitude) for the boundary of space, you would need to be moving at 28,498.5 kph. In which case, you would rise to a height of 100 km (the apogee of your orbit), and then drop back down to perigee at the surface, rise up to apogee... To leave the Earth's vicinity entirely, you'd need to be moving at 40,253 kph ( escape velocity)
15. ## Rotation effect on gravity?

1. It is meaningless to say "spin at 28,437 km", as rotation needs to be measured as angular velocity. (deg/hr, rad/sec. etc.) I know that it is common to express this in terms of tangential speed( in this case, at the equator) But it is sloppy and can lead to misunderstanding. For example if the Earth had a tangential speed of 28,437 kph at the equator, then at 45 north latitude it would only be 20,105 kph As swansont has pointed out there is a speed where the centripetal force (the force required to keep an object moving in circle) and the gravitational force balance out. This would result in the object going into orbit around the Earth. Gravity is still in play. In fact it is gravity that would prevent someone standing on the equator from just shooting out into space at in a straight line instead of just hovering over the Earth. And by the way, your number is a bit low, the equatorial speed would need to be 28463 kph And because of what I alluded to earlier, only someone on the equator would even go into orbit. People elsewhere will feel lighter, and the ground would seem to tilt a bit under their feet ( And even this is an over simplification which assumes the Earth maintains its present shape. If the Earth was indeed spinning this fast, its very shape would change, making it much more of an oblate spheroid.
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