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.
re: " If the star is above that critical mass but spinning, the centrifugal force can help keep the star from collapsing right away"
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.
He's not entirely wrong.
The thing with centrifugal force is that by picking a different frame of reference, it completely disappears and the reason it arises is due to your frame of reference being different. Interestingly, however, is that it is possible to pick a frame of reference such that gravity disappears, so it too is a fictitious force in a way.
Interestingly, however, is that it is possible to pick a frame of reference such that gravity disappears, so it too is a fictitious force in a way.
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.
That's the whole point of the exercise: to show that these frames of reference are all relative. And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears 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.
> And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears globally.
I thought about it - not very hard, admittedly - but I still don't see what you mean.
I am not sure if I just misunderstand you, but we don't know if there was something like a singularity at the big bang and I would guess many doubt that there was. But if there was such a thing it was probably not at a specific point but everywhere. 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.
> 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.
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.
That is actually wrong, the equivalence principle requires careful wording. Imagine a point mass. Gravitational forces will always point directly towards it and therefore the direction of the force you measure within the elevator will vary a tiny bit in order to point exactly towards the point mass. It will also vary a tiny bit in strength because of the varying distance to the point mass depending on where you measure it within the elevator. On the other hand normal acceleration of the elevator will not show any dependence on where you measure the force.
Right. So sorry. "Constant" gravitational field. Happy?
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.
But that's an important point, there is no such thing as a constant gravitational field besides no gravity at all. I only used a point mass because it is easier to visualize but the actual mass distribution does not matter. Therefore the simple statement that it is impossible to distinguish gravitational and normal acceleration is not true unless you restrict yourself to an infinitesimal small volume.
The comment you jumped in to "correct" was based on the equivalence of inertial reference frames which you can take up with Newton, and the equivalence of force-accelerated and gravity-accelerated reference frames which you can take up with Einstein.
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 original comment correctly pointed out that you can make fictitious forces vanish by selecting a suitable frame of reference (which will not be a inertial frame of reference), he incorrectly stated that you can do the same with gravitational forces which you can not because they will at best vanish locally.
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.
There are a lot of planetary science (née geological) experiments using various instrumentation to figure out variations in the force of gravity over land masses. I wonder if they account for what you're describing?
Depends on how narrow or broad you interpret the term. I would say back to the origin of the cosmic microwave background 380,000 years past the big bang the story is pretty well understood. Back to ten to the minus thirty two seconds the model still looks reasonable to me. The time before that makes me feel uneasy, inflation looks to ad hoc to me as a laymen. And I really don't believe there was some kind of singularity with infinite density at the very beginning, but I doubt many do. You can of course never prove something in science but the model seems to provide a reasonable explanation of our observations. There are of course definitely issues and things not yet understood and other things may turn out to be incorrect but that is the nature of science.
My physics teacher did the same thing. When it came down to it, he was talking about vocabulary, not science. The definition of "force" he was trying to get across meant that there was no such thing as "centrifugal force".
Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense. Anybody who has ever ridden a tilt-a-whirl instinctively knows there is a force pushing you out (otherwise you would go falling to the ground when it went vertical).
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.
Physics teachers are so focused on the inward force because everyone already knows about the outward force. In other words, the whole point of the lesson is to contradict what teenagers think they know. (Obviously, some teenagers don't react well to that, since teenagers think they already know everything.)
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.
Right - I had no problem with the introduction of centripetal acceleration, in fact, after newton's laws, that made intuitive sense. It was the statement that "Centrifugal force does not exist" which upset me. Your explanation does a better job in a throwaway HN comment than he did in a formal physics class.
Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense.
If you tell someone there's no such thing as a centrifugal force and that (in rotation) there's only the centripetal force, you're half wrong. There's also ordinary inertia which is what is pushing you against the side of the tilt-a-whirl.
Telling someone there's no such thing as the centrifugal force in physics is a good starting point but you shouldn't stop there.
> 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.
Thanks very much - I think you are nailing it exactly. My physics instructor just blatantly said, "Centrifugal force - it's fiction. Doesn't exist. Centripetal acceleration does exist." He pretty much left it at that, so I tended to ignore everything else he said for the next couple years that didn't make sense to me, thought he didn't know what he was talking about.
I've probably learned more about the topic from this HN thread than I ever did in grade 11 physics.
Hm, that's a good point. I was over-generalizing from my own experience. Someone I respected started off that way, but he knew I would hang around for the full explanation and that I knew that he knew what he was talking about. It was just a hook.
Normally I'd not say anything, but this is a thread containing such gems as people divining the existence of forces on "instinct". There comes a point :)
But, the thing is, the centrifugal force does exist. It's a real thing, for the person in the tilt-a-whirl. Suggesting that instinct/observation is not an accurate method for identifying force does a disservice to observational science.
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.
It's a real thing, for the person in the tilt-a-whirl.
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.
Once again - I continue to get more value out of this HN thread than I ever did in high-school physics. You are distinguishing between a "real force" in terms of something that can be objectively measured irrespective of things like inertia, versus "real force" as measured as a function of the basic four-forces. What I think your saying, is just because a force "appears" in the equations in a rotating frame of reference, doesn't make it real.
Curious - which of the four forces are the source of the centripetal acceleration in a tilt-a-whirl?
Yes. I call a force "real" if there is a physical phenomenom causing it. An interaction. Electroweak, strong nuclear etc etc, or one of the convenient subdivisions that are often very helpful under various circumstances. This is what I mean by "real" force. If you have a force in your mathematical diagram that is not caused by one of these, it's not real. It's a mathematical dodge you're using for convenience of the maths, which is not by any means a bad thing, or wrong. It just doesn't mean that the invented force is real.
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)
Thanks very much. Seriously - take this explanation and forward it to every high school physics instructor everywhere. Countless generations will sing your praise.
PS: Apparently they aren't called tilt-a-whirls, but RoundUps. And they are a lot of fun.
> The definition of "force" he was trying to get across meant that there was no such thing as "centrifugal force".
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?
I ended up ignoring him for the next two years, thinking he was just a doofus who didn't know anything, as did many of my classmates. Probably missed out on a good physics education as a result.
http://en.wikipedia.org/wiki/PSR_B1919%2B21