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Below is a curated list of some of the most interesting and highest quality science news and discussions on Physics Forums. News and discussions are added weekly. Also check the Hot Threads page for discussions choosen algorithmically.

In this paper the following arguments are given why the horizon problem is a fake problem. It raised by me the following question: Why is it assumed in the formulation of the horizon problem that the observed thermal equilibrium from the CMB has to be explained by thermalization right after t=0?

One of the most frequent questions we get is the process and background of being a physics major. Often, we have students asking what are the requirements of obtaining a physics degree, and what can one do with such accomplishments. I am hoping that, in a series of postings on this topic, we get to go over and demystify the whole process of what one can expect...

"Most textbooks attribute cancer-causing mutations to two major sources: inherited and environmental factors. A recent study highlighted the prominent role in cancer of replicative (R) mutations that arise from a third source: unavoidable errors associated with DNA replication."

I'm sure we have all seen the frequent questions from students who don't understand the difference between potential and voltage in a circuit. They seem to think that zero voltage has physical significance. Even using the phrase "voltage drop" is not sufficient, because they say, "What happens when it drops to zero," leading to the concept that voltage is a consumable...

Snow flakes are highly structured, they form from water vapor which has very little structure. I must be misunderstanding entropy, my interpretation of it is that isolated system must evolve into more chaotic less structured states, which is not what happens with snow flakes. Can someone explain?

Plain old standard model baryons, but 5 at the same time, and with crystal clear peaks in the decay to ##\Xi_c^+ K^-##. Each peak in the figure is a particle never seen before, and the significances of those peaks are about 20 sigma (10 sigma for the 5th one). A broad 6th state might hide around 3200 MeV...

A rectangle is divided into a finite number of sub-rectangles. The sides of the sub-rectangles are all parallel to sides of the large rectangle. Each sub-rectangle has at least one side with integer length. Prove that the large rectangle also has at least one side with integer length.

And we can watch different related species at different steps in the transition. In this case, the reason is not the availability of food, but the lower number of predators on land. "At low tide, blennies are commonly found swimming in rock pools around the edges of the island. But when high tide moves in, they climb up to dry land and shuffle around the rocks until the tide retreats. The researchers found that this is most likely..."

I have camera traps out on the African bush, that are monitoring the responses of leopards to artificial scents. The cameras are in steel boxes to protect them from elephants, the boxes are bolted to brackets 150 mm long and the brackets are bolted to steel poles at a height of either 500 or 700 mm above ground level. The poles are 40 x 40 mm mild steel angle, either 2mm or 3mm thickness...

CHALLENGE:

Prove that if ##P(x), Q(x), R(x)## and ##S(x)## are all polynomials such that ##P(x^{5}) + xQ(x^{5}) + x^{2}R(x^{5}) = (x^{4} + x^{3} + x^{2} + x + 1)S(x)##, ##x - 1## is factor of ##P(x)##.

Prove that if ##P(x), Q(x), R(x)## and ##S(x)## are all polynomials such that ##P(x^{5}) + xQ(x^{5}) + x^{2}R(x^{5}) = (x^{4} + x^{3} + x^{2} + x + 1)S(x)##, ##x - 1## is factor of ##P(x)##.

It is usually referred to as Phonons for sound waves in solid. But, where it gets confusing, is in gases and air. Some still call it Phonons, others say, Phonons can only be used in solid states. So what is the Quantum of Sound in Gases/Air?

Beginning with a cube with each side of length 1", drill a 1" diameter hole all the way through in each of the 3 perpendicular directions. Find the remaining volume.

It is often said that Poincare was the last universalist, i.e. the last mathematician who understood more-or-less all mathematics of his time. But Hilbert's knowledge of math was also quite universal, and he came slightly after Poincare. So why was Hilbert not the last universalist? What branch of math he didn't understood sufficiently well to deserve this title?

"China is aggressively attempting to displace the US as the world's scientific superpower, and the US is aggressively attempting to abandon it's scientific superpower status, while Europe seems all but recovered from WWII. ....". I was wondering how the rest of you on PF feels on this regard.

I'm curious on just how much modern physics can be understood qualitatively, without equations. I know that people can understand F=ma with just words. For example, the acceleration an object experiences is directly proportional to overall force pushing or pulling on the object. The more force the more acceleration and vice versa. Of course, this ignores the fact that its a differential equation...

It's strange to me that multiplication and division are considered fundamental operations.

It makes sense for me that addition is a fundamental operation but multiplication is just like a function or algorithm that takes several numbers and apply additions. This is true even for multiplication with real number.

It makes sense for me that addition is a fundamental operation but multiplication is just like a function or algorithm that takes several numbers and apply additions. This is true even for multiplication with real number.

What do I mean by inhibit, you know taking patent or taking permission to use its license takes some time and money and if we think in large scale, today, almost every company spends lots of time to protect their tech. My point is maybe all these things slows the improvement in technology. I think if everything were free to use things would get faster in terms of tech...

Suppose that there are 2 companies. One of them is yours and other company publishes a product which can affect your situation in marketplace.Is it ethical to buy the product and resolve it by reverse engineering to get an idea to counterattack? It's not about producing similar technology but getting an idea to help you to think differently.

In a nutshell, does Newton's "action = -reaction" law apply to massless particles? If a spaceship directs a condensed light beam on its own heat-resistant photon sail, what would happen?

"The "scicomm" effect may be a contributing factor to people being less willing to accept science, but I don't think it's completely at fault. I think you have to go deeper and look at phenomena like confirmation bias. A reasonable person who is willing to be swayed by the balance of evidence will consider details behind scicomm statements." - Choppy

Yes, 7 planets around TRAPPIST-1, all about Earth-sized with quite precise radius estimates from transits, and approximate mass measurements from transit timing variation. 3 are too hot for liquid water (b,c,d), 3 of them in the habitable zone (e,f,g), 1 (h) is too cold. The three planets seen before were the innermost two and the outermost one, so all potentially habitable planets are new. Probably tidally locked (all?).

This paper; Non-extensive Statistics Solution to the Cosmological Lithium Problem, offers a plausible solution to the cosmic lithium problem that has baffled astrophysics, and BBN aficionados, for decades. Unlike prior efforts, it does not invoke any BTSM particles or new physics, just a twist on statistical modeling which looks entirely reasonable...

I'd have to think Jupiter from Mars would be the brightest but after that it's hard to say. Any guesses?

Woit over at his blog "not even wrong" considers string theory based multiverse to be fake physics. Is string theory based multiverse fake physics?

Say you've got four 4s -- 4, 4, 4, 4 -- and you're allowed to place any normal math symbols around them. How many different numbers can you make? It's best to think of a number and then try to make it.

Supposing immigration is heavily restricted in the future will there be an increased demand for home-grown US STEM in the next 4-8 years? How will these new rules impact the job market for US citizens? What sectors do you think will experience the biggest surge in demand?

"Producing metallic hydrogen has been a great challenge to condensed matter physics. Metallic hydrogen may be a room temperature superconductor and metastable when the pressure is released and could have an important impact on energy and rocketry. We have studied solid molecular hydrogen under pressure at low temperatures..."

All natural life uses the same four bases in its DNA: A paired with T and C paired with G. Scientists worked on adding more bases. Just putting them into DNA is not hard, the challenging part is to keep them there: They should not get removed/replaced during reproduction. This has now been achieved...

Are there fundamental limits on the accuracy for measuring both position ##q## at time ##t## and momentum ##p## at time ##t+\Delta t##, with tiny ##\Delta t##? If yes, why? If no, why can't one then measure (in principle) both ##q## and ##p## arbitrarily well at the same time ##p## (which is not allowed by Heisenberg's uncertainty relation), by taking ##\Delta t## sufficiently small and noting that any measurement takes time?

We're all pretty smart people on this forum, but what little things does your mind, for whatever reason, just never want to learn? For me, it's the stupid trig functions. I can't just put angles and numbers on a triangle and solve anything. I have to tell myself "SOH CAH TOA." Every time...

One of the things I read and hear about is how poorly the American educational system teach math or science. I would like to pose the opposite question: what does the American educational system (K-12) teach well? What, to your knowledge or experience, do students graduating from the American educational system in general come away knowing best?

Humans, other animals, plants, fungi and almost all other forms of complex, multi-cellular life are known as eukaryotes. How eukaryotes evolved from simpler prokaryotic organisms is a major question in evolutionary biology. The current view is that eukaryotes evolved from the fusion between a bacterium (which would eventually become the mitochondrion) and an archaeal host through a process called endosymbiosis.

I watched the 6-episode series called Mars this week. Elon Musk kept emphasizing that humans must spread out to at least one other planet to ensure human survival in the event of some extinction event on Earth. Wouldn't colonization of the Moon achieve the same purpose? Seems like that would be a more viable option.

The missing element mystery in the earths core may have been solved. Lead researcher Eiji Ohtani from the University of Tokyo told BBC News: "We believe that silicon is a major element - about 5% [of the Earth's inner core] by weight could be silicon dissolved into the iron-nickel alloys."

I was zooming around the NE region of the South Island of New Zealand in Google Earth today and discovered a classic example of offset stream channels across a major fault, the Awatere Fault. The Awatere Fault which is one of the major faults that the Alpine fault splinters into in the upper South Island. It's companions are the Wairau Fault...

"The development of quantum mechanics in the first decades of the twentieth century came as a shock to many physicists. Today, despite the great successes of quantum mechanics, arguments continue about its meaning, and its future."

"Researchers have classified a brand-new organ inside our bodies, one that's been hiding in plain sight in our digestive system this whole time. Although we now know about the structure of this new organ, its function is still poorly understood, and studying it could be the key to better understanding and treatment of abdominal and digestive disease."

I can create a small vacuum in my mouth to move that "string" of spagetti into my mouth. I know it is possible, but I do not understand how spaghetti is possible to suck in if the shape is even and round like a "piston". I have learned that pressure is acting angular to any surface. With the spaghetti, that surface will cause the pressure to act 90° to it, and (in my thoughts) not be able to create a force...

The Nobel prize in chemistry for 2016 has been awarded "for the design and synthesis of molecular machines." These are fascinating constructions made up of just a few molecules, so they are the smallest machines possible. It is far from trivial to get these things working, especially since thermal noise is important at that scale (although in some cases, thermal noise can be rectified and serve as a source of energy)...

The situation of course is that I tell somebody that I am studying math, and if I mention some specifics, like mention Topology or Algebra, (which I have to sort of explain is not "college algebra"), or whatever. Then comes the question "So what's this used for in..you know, real life?" As I see it there are two extremes to answer this question...

You know a book which is rarely cited, mentioned or recommended, quite unknown even to the experts, and yet you have discovered that this book is really great? Please share it with us!

The problem I have is if the human body was able to mutate, without any design at all, completely mindless mutations, why is it that our arms don't have big lumps all over them or, our rib cage have random points sticking out of each one, or an extra toe poking out the side of our foot. What I cant understand is why did only the useful organised mutations...

Any pointers about where to get information and perhaps pictured of the different whiteboards shown in Big Bang Theory? I am particularly interested on the season where Sheldon abandons string theory; the boards for this chapter (155, season 7 ep 20, aired 10th April 2014) seem to show some M-theory + D-branes generic content, and the next day seem to be some model building with GUT groups but not the common presentation....

I've often read that beam-powered propulsion is the only basic interstellar propulsion concept without physics problems. To me, that doesn't seem far from the truth. However, for a long time I've felt that the biggest obstacle to overcome is the beam divergence problem. Most of the concepts I've read about use a single laser...

I know that rain is charged and the lightening is a discharge, and both are relative to the ground, I think, but I don't know the relative polarities... is there some way that the charged rain drops are being momentarily attracted upwards by the lightening or its regional effects? Could it be that the metal car itself is altering its charge and somehow repelling the rain charges for a moment?

We hear so often of the great advances of medicine. Every day one reads of breakthroughs in research in the news. But then I never hear of it again. I'm not a doctor or biologist, but am wondering what are the important discoveries ***and which had practical applications*** of the last 20 years?

A popular video I just watched described Fred Hoyle's discovery that the elements of the universe are created in stars. Key to his theorizing was the prediction that fusion would produce of a new state of carbon that had never been observed and which theory predicted would be unstable. Hoyle believed that under the physical conditions present in stars this form of carbon could exist. What was this new type of carbon?

This is the problem: A person blows through a straw between two empty soda cans. Do the cans move closer together or away from each other? Explain why they move the way they do. Partial answer: The cans will move closer together. Given that information, we can conclude that the air pressure in the region between the soda cans must be lower than normal...

Here is an interesting item I would like to share. It seems that China has drastically pushed the boundaries of power transmission grid operations. They boldly go where no man has gone before...

So I'm about to be done with my first class in programming where we learned C programming. Unfortunately, I don't actually know what you can do with it. All of our programs in my class have involved us programming and running everything using Visual Studios, not developing standalone executables or something...