Student proves existence of plasma tubes floating above Earth

brenan85 · 1日前

Pretty sure some of her colleagues weren't nice with their dismissals.

Possible, but there's no reason to assume that. When unexpected and unexplained effects show up in data there can be honest skepticism within a group. These things can be some kind of error or unanticipated effect within the apparatus, so the burden of proof is to show that they are real.

In experiments, unexpected things are most often "whoops, this thing is broken or not working as well as we thought", not discoveries. Thus it is natural to think that something unexpected fits in that category unless given good reason to think otherwise.

My own experiments have had innumerable setbacks and disappointments, so the few times I reported to colleagues that things were working better than expected I was met with respectful skepticism until I could prove my case.

akkatracker · 3日前

In physics it's often used to refer to the speed of whatever wave is being discussed.

Ambiwlans · 5日前

Ha, just an enthusiast, don't put too much stock in my numbers. Glad to help give a rough idea, though.

Ambiwlans · 5日前

My spreadsheet says a F91.1 with a 478 kg payload would do 14,306 m/s, but that's without gravity loss correction so we're pretty close.

I've never sat down to deal with FH fuel consumption properly, but a quick stab says an expendable launch a 478 kg payload would do 17,022 m/s, which will be short of New Horizons after gravity losses.

I haven't tried 1.2.

Note that New Horizons used a Star-48V upper stage tacked on to the Atlas V

prookyon · 15日前

Comic ray muons have been used to create x-ray-like images of very large objects such as volcanoes and nuclear reactors.

I'm having a hard time finding the flux of actual x-rays at the Earth's surface, so I can't answer how long an x-ray image would take to form.

Some of the issues involved though are the lack of ability to lens them (you would have to lay the object you want to image directly on the film to get any kind of image, and it would still be blurry, or use something like a lead pin-hole camera, which would increase the exposure time tremendously), and the very low exposure rates leading to reciprocity failure if you tried to use film. The muons will also be depositing dose, possibly much more so, causing a terrible loss of contrast.

corrrrallll · 16日前

Sure, I don't actually know the history of the foot, I just made up this example to highlight some of the qualities of a bad standard and how they can be improved.

corrrrallll · 16日前

Good question, and I don't know the answer. It could have something to do with our ability to measure that pressure precisely enough.

corrrrallll · 16日前

I mention it in this comment with a link to the original paper. It looks like they measured how many cycles there were per year and converted it to an average number of cycles per second.

corrrrallll · 16日前

Thanks!

corrrrallll · 16日前

Yes, the frequency in space as viewed from earth does change due to general relativity. This has to be accounted for in the GPS system, which uses high-precision clocks on-board the satellites as its basis.

corrrrallll · 16日前

That would have changed the length of our time measurements by about 8%, which would have a huge effect. Suddenly the day would not be 24 hours, but 22.0623... hours. This would certainly disrupt every day life!

There are technological effects that would be very confusing as well. Take for example radio communications. If you want to communicate at 100 MHz, that's 100,000,000 cycles per second. A radio tuner built with the new standard tuned to "100 MHz" would be tuned to 91,926,318 cycles per old second instead.

And now you can see that even a much smaller difference could have an effect on electronics. Just changing one of the last few digits of the standard would have a noticeable effect on radio communications and backwards-comparability.

corrrrallll · 16日前

It is, but they're working on two possible replacement standards. It's actually hard to come up with a standard that is both more precise than the prototype kilogram and practical in its use.

The two potential replacements are defining it as some number of silicon atoms (effectively declaring an exact value for Avagadro's number), or some contraption called the "Watt balance" (effectively declaring an exact value for Planck's constant).

corrrrallll · 16日前

My expertise in time is not great enough to pursue this much further, but with the help of wikipedia I did dig up this paper on the original measurement of the second in Cs-133 hyperfine periods based on the then-definition of the second in terms of the year (not the day) based on astronomical photography.

They report 9,192,631,770±20 cycles at the year 1957. It does seem they measured more digits, for example for 1954.25–1958.25 they measured an average of 9,192,631,761 cycles.

Upon adopting this standard, they just threw away the uncertainty and declared it an exact definition.

corrrrallll · 16日前

The issue was that the original foot itself was imprecise: it was a bit different every time you went to use it. By using the coins we can have a new definition which is more precise and consistent.

We measured the current foot to the best precision we could with the new standard. I stopped at 3 digits, 9.19, because, say, that's the limit of precision of the coins themselves. If they are only self-consistent to 0.1%, then additional digits are just noise.

The point is to create a new standard that reflects the old one as well as possible for consistency, and then throw away the old one and don't worry about it anymore.

corrrrallll · 16日前

Standards are chosen based on the combination of their precision and the practicability of using them.

The second was first defined as 1/86,400th of a day (24 hours/day, 60 minutes/hour, 60 seconds/minute: 24×60×60 = 86,400), but that's a difficult thing to nail down to high precision in practice. As technology improved and we found more precise ways to measure time in a practical manner, we changed the standard to something better. In order to keep the new definition practically interchangeable with the old one, we used the best measurement of the old one with the new one as the definition. As better technology emerged, this process repeated.

We use Cs-133 now because it is very feasible to make very high-precision clocks based on its hyperfine levels. The 9192631770 number was based on the best measurement of the previous standard, and so on backwards until we were using the day standard.

Edit: found some info on the exact measurement that led to this number here

Edit 2: I tried to trace down the trail of standards all the way back to the beginning of the second to create a single expression which leads us to the number 9192631770 entirely from definitions and measurements:

(# of oscillations of the 133Cs hyperfine transition in the ephemeris year 1957 as measured by Markowitz and Hall) / ((# of seconds per solar day as defined by the Babylonians around 300 BCE) * (# number of solar days in the instantaneous tropical year 1900 January 0 12h as calculated by Newcomb in 1895))

In practice they did not deliver their results in these forms, but simply as the conversion between seconds and the day/year/Cs-cycle as they measured or defined them.

To make up a more tangible example, imagine we are dealing with distance, and the standard we're using now is the King's Foot. The King's Foot is a terrible standard, because you have to go to the King and actually touch his foot to measure things, and his foot changes size depending on how much he's walked today, how fat he's gotten, how hydrated he is, etc, so it's never consistent.

So to make a standard everyone can use, we note that Pound Sterling coins are very uniform because they are stamped out by a machine the same way every time. So we would like to use these coins as a standard measure.

However, we don't want to just start using "coins" as a standard, because we're in a world where everyone has been using King's Feet. Everyone's used to it: rope is sold by the King's Foot. We need to keep a unit of distance the same as the King's Foot, but just redefine a New Foot in terms of coins instead of the man's actual foot.

So we take some coins to the King and see how many of them end-to-end make up one King's Foot. We see it's 9.19 coins.

9.19 isn't some great clever number, it's just the way it happened to come out. However we don't want to round it off to 9 or 10 when we define the New Foot, because then society and trade will be in chaos and confusion over which foot is which. So we adopt 9.19 Pound Sterling coin diameters as our New Foot. Now anyone can use coins to measure things very precisely in terms of their familiar unit, the foot.

If some day we mass-produce a more precise object, we can measure how many of those make up 9.19 Pound Sterling coins, and have an even better definition of the foot for everyone to use.

Silpion · 16日前

Whoops, my search didn't bring up those threads.

Your second link is particularly enlightening, thank you. I didn't realize how much the populations were in favor of pressing the war to victory. I guess I don't really understand the psychology of war that well, and assumed the people were less willing to continue to send millions of their young men and boys to die horribly.

As for the "how" question, it seems clear it would have to start with Germany, but yeah, they'd probably have to take some bad terms. I guess in hindsight any terms they could have negotiated would have been better than Versailles, but they would of course not have known that outcome.

It's clear that I'm going to need to take some time eventually to understand much better the politics of why countries decide to fight wars. How complicated can it be? ;)

On the machine gun/defensive war question, you give some reasons why war in 1914 wasn't so different than from in the 1860's. Clearly it was very different though, so what was the real reason it stagnated so badly?

ltsaGiraffe · 16日前

I just lost my entire post.

I literally just had the same thing happen to me!

The browser plugin "Lazarus: Form Recovery" autosaves your form entries, so if you lose the window you can pop what you wrote back in.

ltsaGiraffe · 16日前

I just lost my entire post.

The browser plugin "Lazarus: Form Recovery" autosaves your form entries, so if you lose the window you can pop what you wrote back in.

admin-mod · 16日前

Physics is only single-blind in my experience: the reviewer knows who the authors are. This certainly true in PRL (where this paper was published), as I've had referees refer to me by name in their reports.

admin-mod · 17日前

To be clear, I'm making no judgement either way with regard to whether this is appropriate.

But I'm curious, how is this paper as important as you say? The Higgs discovery was published years ago. Isn't this just a new analysis that incrementally decreases the mass uncertainty?

admin-mod · 17日前

Very interesting. It looks like there were some changes to the length guidelines in December, but it still says 3500 words for PRL (about 4 pages).

I can only guess that the editor made an executive decision to grant exceptions for these cases.

admin-mod · 17日前

It's funny that this got published in PRL. They used to have a hard limit of 4 pages for their papers, but in the age of digital distribution they relaxed it to a 3500 word limit plus unlimited space for author lists, references, etc. This usually means 5–6-page papers, but this one is 33 pages, 24 of which are author list.

What I'm confused by is how they got 7 pages of text into a PRL. They are pretty harsh about their limits and I've never heard of an exception.

booOfBorg · 19日前

Okay, cool, so it sounds like if I budget a total ΔV of 2.0 km/s for all maneuvers between Jupiter transfer and low Europa orbit, then that's reasonable if I'm willing to spend some months/years doing a series of gravity assists within the Jupiter system?

While i have you, does the 1.44 km/s launch/landing burn for Europa <-> Low Orbit already include some gravity losses from some reasonable finite TWR, or is it some high-thrust ideal?

Thanks for your help! I'm a big fan of your work too.

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