Here is a hand picked list of some of the most interesting and highest quality discussions on Physics Forums. New discussions are added weekly. Also check the Hot Threads page for discussions choosen algorithmically.

#### Decarbonizing an Engine with Water: Myth or Fact?

I ran across several YouTubes that claim, and apparently demonstrate, that you can clean carbon deposits from a car engine by simply spraying or pouring (very slowly) water into the intake when the engine is fully warmed and running. I've never heard this from a reliable source and my reaction is... What is the real story on this?
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#### SI meter definition changed?

The wording of the definition of the meter has apparently changed recently. I'm wondering about the motivation for the change. The current definition is...

#### Is there an alternate fuel that de-emphasizes carbon?

Is there a potential fuel, for some kind of engine, not necessarily similar to existing car or diesel engines, that does not have carbon in it? That is, that you might manufacture from water and air without having to concentrate the carbon dioxide somehow. And that's liquid at ordinary temperatures and pressures one might find walking around in an ordinary town...

#### Challenges to QED from hyperfine measurements

Does this really challenge QED? Or does this mean that we miss certain contributions to QED calculations? The authors are speculating "new effects might appear in the interaction of the electron with itself, the vacuum or the nuclear fields in this regime, that is, the hyperfine interaction might be affected by the existence of new particles not included yet in the current standard model and therefore not considered in state-of-the-art...

#### Double field theory: Where is the extra space?

Double field theory [1] is an attempt to realize T-duality of string theory at the level of field theory. For instance, if a field in ordinary field theory lives in 4 non-compact space-time dimensions, then a field in double field theory lives in 8 non-compact space-time dimensions. I don't understand it from the physical point of view...

#### Vacuum or pressure to move spaghetti through a hole

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...

#### New Organ (mesentery) found!

"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."

#### Metallic hydrogen created in the lab

"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..."

#### Relativity using the Bondi k-Calculus

In the early 1960s, Hermann Bondi advocated a presentation of special relativity (called the k-calculus [which involves no calculus–just simple algebra]) that is advertised to be a “simple logical extension of well-known Newtonian ideas, without any of its mathematical trappings.”

#### Fitting circles inside other circles

I'm working on the following fun problem. I have a circle of a given radius, R0. (Green circle in the image). I want to be able to supply a radius of the first circle that is to fit into this large circle. Lets say R1 is 0.75 * R0. Following this I find the best position of R2 (to maximize its radius), is on top of the smaller circle. This is the largest circle that can fit inside the green circle without...

#### Math Challenges - November 2018

Rules: In order for a solution to count, a full derivation or proof must be given. Answers with no proof will be ignored. Solutions will be posted around 15th of the following month. It is fine to use nontrivial results without proof as long as you cite them and as long as it is "common knowledge to all mathematicians". Whether the latter is satisfied will be decided on a case-by-case basis...

#### For those who do their own automotive work

For those of you who work on their own vehicles, what has been your experience recently with aftermarket replacement parts? To me it seems the quality has gone down the tubes in general. Some of my experiences go back quite a few years (like 20) with solid state ignition modules. They might last several hundred miles...

#### Math Challenge - March 2019

1.) Using the notion of double integrals prove that $$B(m,n) = \frac{\Gamma (m) \Gamma (n)}{\Gamma (m + n)}\; \;(m \gt 0\,,\, n\gt 0)$$ where $B$ and $\Gamma$ are the Beta and Gamma functions respectively.

#### Niches for Publishable Undergraduate Research

Undergrads struggle with research ideas, because they often tend to assume their work needs to be within the domain of new fundamental science of the sort that would be suitable for the Physical Review, when often their skills and scientific maturity have not yet really empowered them for that level of contribution. As mentors of a lot of undergrad (and high school) research, we’ve found that...

#### Murray Gell-Mann on Entanglement

In this video Murray Gell-Mann discuses Quantum Mechanics and at 11:42 he discuses entanglement. At 14:45 he makes the following statement: "People say loosely, crudely, wrongly that when you measure one of the photons it does something to the other one. It doesn't." Do most physicists working in this field agree with the above statement?

#### Authorities in science

Highly respected journals and books are the authorities in science. But they can be challenged by arguments based on other facts. The latter are also taken from highly respected journals and books. Thus one cannot dispense with the authorities....

#### Rigged Hilbert Spaces In Quantum Mechanics

In discussing stuff in another thread I used the standard Dirac notion expanding a state in position eigenvectors namely |u> = ∫f(x) |x>. By definition f(x) is the wave-function. I omitted the dx which is my bad but the following question was posed which I think deserved a complete answer. It was also off at a tangent to the main threads topic so really required a separate thread...

#### Nobel Prize Winning Molecular Machines

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)...

#### How is it that mathematics describes reality so well?

Humans created that tool, that language, that consists of axioms and their implications. Mathematics does a good job of communicating the behavior of physical phenomena. How is it that mathematics and the physical reality agree with each other ?

#### Using CRISPR to cut HIV out of infected cells

A major impediment to curing patients with HIV is the fact that the virus integrates its DNA into the genome of the cells it infects. While antiretroviral drugs and therapies can prevent infection of new cells and kill cells that are actively producing virus, a "latent reservoir" remains that can be cause an active infection to re-emerge after drug treatment has stopped. Therefore, current treatments for HIV-infected individuals requires...

#### A terrestrial, temperate planet around Proxima Centauri

Published today in Nature: "At a distance of 1.295 parsecs1, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun’s closest stellar neighbor and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun2 and a mass of about 12 per cent of the mass of the Sun."...

#### How is physics knowledge used in everyday life?

What I am struggling to come to grips with is how knowing physics, the topics typically taught in a high school intro course, can be useful in everyday life. I am not looking for everyday examples of physics concepts. I mean real actual examples of using Newton’s laws or momentum in real life purposefully. Can you think of a time when you actually used your physics knowledge in everyday life?

#### Interval between double sonic booms

Yesterday afternoon I heard (and so did every bird in the district) a very loud, low frequency, double boom which was very vigorous and I suspected all sorts of things but then I thought "double boom!!" and (engaging smartarse mode) I informed my wife that it was only a sonic boom. I was later proved right...

#### Nobel laureate perspective on the history, status and future of GR

This paper offers a Nobel laureate perspective on the history, status and future of GR in the astrophysical regime..., General Relativity and Cosmology: Unsolved Questions and Future Directions.

#### Is it possible to detect a single phonon?

Is it possible to detect a single phonon? If yes, can it be detected at a well-defined position?

#### Can modern physics be understood qualitatively?

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...

#### If Maxwell's equations are linear...

In the course of my career, (in the years 1986-1990), on two occasions I discussed with two very intelligent physics PhD.'s a problem that surfaces in regards to interference patterns. The first PhD. knew very little Optics and (around the year 1986) was asking me, "how can you possibly get an interference pattern? Are you telling me...

#### Why does formation of snow flakes not violate entropy law?

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?

#### 2017 Nobel Prize in Physiology or Medicine

This morning, the 2017 Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for their discoveries of molecular mechanisms controlling the circadian rhythm. "Their discoveries explain how plants, animals and humans adapt their biological rhythm so that it is synchronized with the Earth's revolutions."

#### The state of carbon in stars

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?

#### Boeing 737 Max MCAS System

Is it not a huge error in the flight laws and the MCAS software to execute a nose down maneuver at any altitude? Should the system not have a rule to prohibit such a maneuver below a minimum altitude threshold?

#### What's the feasibility of space mining companies?

Recently, I've been reading about those space mining companies (like Deep Space Mining and Planetary Resources), and from an amateur point of view, I thought that the overall idea was pretty interesting. But what's the feasibility of such a project? Are there any chances that we could develop this technology within the next decade or so?

#### Cancer causes: circa 67% are Replication error

"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."

#### Math Challenge (with prizes!) - June 2019

Test your skills on 15 math challenges! We have a prize this month donated by one of our most valued members, and that's what the points are for. The first who achieves 6 points, will win a Gold Membership!

#### Should Algebra Be Required At Community Colleges?

The chancellor of the California Community Colleges system believes that students who are not majoring in math or science should not have to take intermediate algebra to earn an associate degree. California has the largest community college system in the US, and what goes in California sometimes spreads across the country. What do you think?

Recently, I came across an individual who claimed to research AI professionally and who expressed to me the following view after some discussion of various technologies and the rate of advancement of such tech... I would very much love to read what anyone has to say regarding this, and would also greatly appreciate where you think machine intelligence will be in the next 10 year span...

#### How does green screening technology handle secondary reflection of green?

I get the basics of green screening. The processing software recognizes a small range of green and can substitute a different image where it occurs. Here's what I don't get: if a subject is standing in front of a green screen, they will have reflections of green...

#### Mathematicians Revive Abandoned Approach to the Riemann Hypothesis

Many ways to approach the Riemann Hypothesis have been proposed during the past 150 years, but none of them have led to conquering the most famous open problem in mathematics. A new paper in the Proceedings of the National Academy of Sciences (PNAS) suggests that one of these old approaches is more practical than previously realized...

#### Success! SpaceX Falcon Heavy Launch

"Years in the making, the commercial spaceflight company is preparing to launch its first Falcon Heavy rocket, which as its name implies, is a heavy-lift booster built from a core stage and two of SpaceX's Falcon 9 recoverable rockets. According to SpaceX, when the Falcon Heavy lifts off, it will be "the most powerful operational rocket in the world by a factor of two."

#### CLIC (CERN accelerator) proposal

CLIC is a proposed accelerator, potentially at CERN, that would collide electrons and positrons at energies far above what previous colliders achieved. Its overall energy would still be below the LHC but (unlike at the LHC) all the collision energy is available for the creation of new particles - it would combine the approximate energy reach of the LHC with the cleanliness of elementary particle collisions...

#### Submit your interview questions for Dr. James Gates

Met theoretical physicist James Gates at the WAPT meeting tonight. He said he'd be interested in doing an Insight Interview. Let's get some good questions rolling! Here are some links if you are not familiar with his history and work...

#### Inconsistency versus lack of knowledge

In traditional logic, a system is inconsistent if it can lead to a contradiction. Furthermore, if the inconsistency is non-explosive (not all consequences follow from the contradiction), then the system is para-consistent.
Both definitions fail to distinguish between the following two real-world scenarios...

#### What Should I Do If My Professors Don’t Teach?

I’m taking physics II and discrete math this semester. Long story short: I’m probably not going to be able to rely on the professors to teach me this semester. I’m really, really hoping that I can get all A’s this year. What do you guys recommend I do? I should note that I’m also working this semester (15-20 hours/week).

#### Easy ways you can support Physics Forums

We are all members of an amazing community of science and math passionate people from all over the globe. However, "amazing" doesn't just happen over night or by itself. "Amazing" happens when members take pride and responsibility for the health of the community...

#### Quantum mechanics is random in nature?

I heard from many sources that quantum mechanics is purely random in nature. Has this been demonstrated? Phenomena like the wave function collapse are considered 'purely random'. If we can establish that collapse is random, we have to base that on the property of being statistical in its nature, right?

#### A Poor Man's CMB Primer. Part 5: Quantum Seeds

The CMB establishes a record of ancient acoustic oscillations in the baryon-photon plasma. We’ve been studying how these primordial sound waves evolve, and how to analyze the last scattering surface to learn about them. Now it’s time to confront their origin: what process composed the cosmic symphony? A few different proposals have been advanced over the years to explain the origin of the primordial perturbations.

#### Mind boggling machine learning results from AlphaZero

I have always been on the skeptical, but not dismissive, end of judgments about achievemnts and rate of progress in this field. However, the following (please read through carefully) just completely blows my mind.
A self learning algorithm with no knowledge or base of games, starting only from the rules of chess, within 24 hours is much better than any other existing chess program despite running on hardware 900 times slower!

#### Is this a horribly ambiguous A Level Physics question?

I have an OCR A level "physics A" student, and we're working through past papers. Time and time again, we've come across questions which are astonishingly ambiguous and answer schemes which are in my view, astonishingly picky or just plane bizarre. I've taken some of these to OCR but got no progress...

#### Staged Blackouts

Again and again we hear of the consequences of loss of electrical power. Frequently, the authors call for more redundancies in the power grid so that blackouts will not happen. No complex system can ever be 100% reliable. However, as systems grow from very reliable to extremely reliable, people come to treat them as absolutely reliable. (A psychological quirk?) Lack of preparedness grows and the actual consequences become...

#### Black holes squishing Earth

Scares of the CERN accelerator creating a black hole that swallows Earth are in the news once again. I know it is not possible for an accelerator to produce such a black hole in practice. But surely it's impossible even in principle. Surely, a particle accelerator creating a tiny black hole that could grow to swallow the Earth would violate the law of conservation of energy...