The Case of the 5-Millisecond Cosmic Radio Burst (2014)
planetary.org
planetary.org
http://arxiv.org/pdf/1503.05245v2.pdf
which suggests the FRB dispersions are quantised to a standard unit. A lot of newsies picked this up last week and suggested it was proof of... something unusual.
The authors think it suggests the opposite.
Given the date, I can't tell if the paper is serious or not. But if it's meant to be serious, the effect seems real - possibly an artefact of a very small sample size, but it's hard to tell.
It would be useful to have access to the raw data to see the exact dispersion shape and frequency range, to check whether it matches - say - any standard radar chirp signal.
I still resent my Grade 11/12 physics teacher for insisting that there was no such thing as centrifugal force, and that what was really occurring was centripetal acceleration only. He basically decided that newtons third law would not exist in his classroom.
Why science instructors are so focussed on the inwards force (in the case of a tilt-a-whirl, it's the outer frame, in the case of orbiting a planet/spinning neutron star, it's gravity) to the exclusion of everything else never made any sense to me, and caused me know end of distress.
It probably came from the same place that suggested a charged item can attract a neutral object, but that a neutral object cannot attract a charged item. That made even less sense to me, but was right out of the book.
Needless to say, my high school physics instructor and I never really got along very well.
It makes an enormous amount of sense.
Telling someone there's no such thing as the centrifugal force in physics is a good starting point but you shouldn't stop there.
No, it's a terrible starting point. Maybe the worst possible starting point. "Here is something you know to be true that I am going to tell yo is false, creating confusion and a tendency to reject anything else I say as obvious nonsense."
End with the notion of fictitious forces (which do exist, so the name is terrible), don't start with them, unless you want to be like one of those dreadful people who talks about negative generalized temperature or negative generalized resistance to neophytes without first explaining the generalization, so you deliberately mislead people into thinking you are talking about the familiar, ungeneralized concept.
There is no evidence whatsoever that such introductions make anyone more motivated or able to understand, and a good deal of reason to expect they don't.
I've probably learned more about the topic from this HN thread than I ever did in grade 11 physics.
Forces push. Inertia is not a force. Inertia doesn't push.
What was completely left out of the entire conversation in High School physics was the concept of frame of reference. By leaving out even a 5 minute briefing on that, my physics instructor lost all credibility (from me, and probably half the class), claiming that something that actually existed, did not exist.
I disagree. It's not a real thing. If I pretend an acceleration that does exist doesn't exist, and then I realise that the only way I can make the mechanics work is by inventing a force to offset the force that's causing the acceleration I want to pretend doesn't exist, insisting that that invented force is "real" is just plain crazy.
It's a way to make the equations a bit easier. It's not real. There is nothing pushing anyone outwards. No atoms, no electromagnetic forces, nothing. There is no force pushing anyone away from the centre. To say that a force that simply does not exist, is "real", is just plain crazy or renders the word "real" meaningless.
Curious - which of the four forces are the source of the centripetal acceleration in a tilt-a-whirl?
There is a wall behind the person, right? We don't use the phrase "tilt-a-whirl" here in the UK (or at least, in my part of the UK), but I think I know the device you mean. This sort of thing: https://debrasanborn.files.wordpress.com/2013/07/flat550x550...
That wall exerts a force on the person, inwards. That's the force causing the person to accelerate (i.e. change velocity).
What we conventionally label that force depends on where you are in the world, I expect, but it's the interaction of the person's atoms with the wall's atoms. Electromagnetic, or electroweak if you want to go more fundamental.
The person's atoms, of course, are exerting a force on the wall's atoms, outwards. That outward force is not exerted on the person. The person gets an inward force.
The force exists because the person's velocity is tangential to the circle. Sideways, if you will. But there is something in the way. The curving wall. The wall gets ever so slightly compressed (much as when you put a book on a table).
(I've simplified a bit - the person's velocity is pointing tangentially, and if the wall vanished they would zip off in that direction; the bit of the wall in the way is what they push on, and obviously there's not a single point of contact, so the forces aren't quite so simple as "a force" that way, and "a force" this way, but when they've all been summed, you get what we see - a force towards the centre. Also, weight still exists, and I've ignored that completely)
PS: Apparently they aren't called tilt-a-whirls, but RoundUps. And they are a lot of fun.
https://www.google.com.sg/search?q=round+up+amusement+ride&s...
The goal is to help students break through assumptions and understand that our everyday experience results from interactions of forces, not just singular forces themselves.
A similar concept is the normal force. Everyone knows about gravity because they know what happens if they fall or drop something. But few people on their own will think of a force to explain why they are usually not falling.
It's like the joke that "it's not the fall that kills you, it's hitting the ground." Analyzed with Newtonion physics, that is literally true! My physics teacher used to say "gravity never killed anyone," which is the sort of counter-intuitive thing that drives teenagers crazy. But it's true--it's that nasty normal force you have to watch out for.
So, the definition of "force" he was trying to get across was different from the one created by Newton and used by every physicist and engineer? Why did he ignore Newtonian fictitious forces? Because he didn't want to explain them? If so, what was he doing there?
This is what happens when you have someone with no passion for a field teach out of a book/curriculum.
I am not a physicist but that seems wrong to me or at least imprecise. You can select a frame of reference that makes gravitational forces disappear locally but there is no frame of reference that makes them go away globally.
Relativity is a very tricky concept.
Of course gravity is a 'real' force but the ability to shift your frame of reference is a powerful reminder that gravity and acceleration can look the same until you exit your frame of reference.
One of Einstein's great contributions is the knowledge that this isn't actually true. If you're in a box with no windows it's literally impossible to tell if you're on the ground in a planet's gravity or if you're being accelerated in empty space.
But I agree, if you put the box within the event horizon of a black hole (aka near a point mass) the person inside the box might be able to tell, if they and the box were still intact. What I don't agree with is that splitting hairs on the divergence of the gravitational field is productive.
If you don't like the thought experiment commonly used to give people an intuition about the Equivalence Principle then come up with a better one. I don't think yours holds up either, I'm guessing that thanks to the Uncertainty Principle you'll also need infinities to generalize it.
The response to my fist comment I could not fully understand but the last part of it sounds to me like it implied that a frame of reference only comprises a single point in space which of course is not true.
All in all my objection was the you can not treat gravity like fictitious forces which is an understandable and easy to make mistake if you base your reasoning on the simple elevator gedankenexperiment. I have absolutely no objections to the way the equivalence principle is introduced to people. So I am not really sure what we are arguing about, it seems to me that we - at least mostly - agree on the matter.
> And you don't have to leave your frame of reference to see a difference between acceleration and gravity, they are distinguishable in any frame of reference.
You're clearly presenting an idealized situation, which is fine. You've chosen a non-idealized (or less idealized) gravitational field, which is also fine. But the convention when talking about this particular subject is to also idealize gravity and take it as being the same everywhere. If somebody reads this statement with the usual definition of gravity, they might get the wrong idea.
All it needs is some signal that the gravity you're using is different (less symmetric) than usual. It is an interesting idea, I'm just saying maybe present it a little differently so people like me don't get confused.
I thought about it - not very hard, admittedly - but I still don't see what you mean.