Can I Send a Signal Faster than Light by Pushing a Rigid Rod?
One common proposal for achieving faster than light communication is to use a long perfectly rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. For...
View ArticleDo Photons Move Slower in a Solid Medium?
This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in a vacuum. This FAQ will strictly deal with that...
View ArticleDoes the Heisenberg Uncertainty Principle Imply Energy Nonconservation?
The short answer is: no, there’s no violation. The longer answer can be this: Given the probabilistic aspect of quantum theory, what do we mean now, by “conservation of energy”? In quantum theory, it...
View ArticleCan We Tell Whether the Universe Is Rotating?
If you believe wholeheartedly in Mach’s principle, then there is no way to test empirically for rotation of the universe as a whole, since there is nothing else for it to be rotating relative to....
View ArticleA Geometrical View of Time Dilation and the Twin Paradox
Based on the number of questions we receive on the topic at Physics Forums, there is a lot of confusion in the general public about how time dilation works and the resolution of the twin paradox. More...
View ArticleLearn About Energy Gained by Charge in an Electrostatic Field
I have seen this question being asked frequently on here. A lot of students have a bit of an issue in understanding why, if an electron is placed in a field with potential V, that the energy gain after...
View ArticleIs the Universe a Black Hole?
In the early universe, the matter was gathered together at very high density, so why wasn’t it a black hole? The first thing to understand is that the Big Bang was not an explosion that happened at one...
View ArticleHow Does an Airplane Wing Work? A Primer on Lift
Many people ask how an airplane wing works, and there is any number of answers that are commonly given. You might have even seen vigorous arguments between proponents of competing theories, such as...
View ArticleWhy Don’t Electrons Crash into the Nucleus in Atoms?
Atoms and Coulomb Forces If one describes atoms using only the Coulomb forces, the electron and the nucleus will attract each other and no stable atoms could exist. Obviously, this is not the case....
View ArticleWhat Is the Steady-state Model and Why It’s No Longer Viable
[Back in 2011, I wrote a FAQ entry for physicsforums on the steady state model and why it is no longer viable. I’ve learned more about the theory since then, and it turns out that Fred Hoyle was a lot...
View ArticleHow to Build a Definition for Heat
One of the most frustrating misconceptions in Thermodynamics centers on the actual definition of heat. Many science/engineering students often refer to a body as to be possessing heat, but as we shall...
View ArticleRadius of the Observable Universe in Light-years Is Greater than Its Age
The radius of the observable universe is about 46 billion light-years, which is considerably greater than its age of about 14 billion years. The radius of the observable universe is defined by the...
View ArticleDo Black Holes Really Exist?
The purpose of this article is to discuss the title question from several different viewpoints, in order to show that it isn’t as simple as it looks. We will look at some common misconceptions that...
View ArticleWas the Early Universe in a Disordered State?
No. The second law of thermodynamics says that entropy can only increase, so if the early universe had been in a state of maximum entropy, then the cosmos would have experienced its heat death...
View ArticleWhy Is the Speed of Light the Same in All Frames of Reference?
The first thing to worry about here is that when you ask someone for a satisfying answer to a “why” question, you have to define what you think would be satisfying. If you ask Euclid why the...
View ArticleCould Redshifts Be Intrinsic Rather than Cosmological?
How do we know that the redshifts of galaxies and quasars are cosmological and not “intrinsic?” Evidence for the cosmological interpretation The purpose of this FAQ entry is to explain why nonstandard...
View ArticleAt What Velocity Does the Universe Expand and Can It Be Faster than Light?
Neither of these questions actually makes sense in the form in which it was posed. To see why, let’s start by thinking about how we know the universe is expanding. The expansion of the universe was...
View ArticleWhy Won’t You Look at My New Theory?
In any forum where science is discussed, there will always be people who have a great new personal theory and can’t understand why no one else is interested in it. Here at PF we have rules about this,...
View ArticleWhere Did the Big Bang Happen?
According to standard cosmological models, which are based on general relativity and are found to agree well with observations, time and space did not exist before the Big Bang — or even at the time of...
View ArticleLenses and Pinholes: What Does “In Focus” Mean?
In introductory physics, the optics unit often teaches about virtual and real images, focal lengths, indexes of refraction, etc. Some questions that are sometimes glossed over in the rush to present...
View ArticleWhat are Virtual Particles? A 10 Minute Introduction
Definition/Summary Virtual particles are a mathematical device used in perturbation expansions of the S-operator (transition matrix) of interaction in quantum field theory. No virtual particle...
View ArticleWhat is the Double Slit? A 5 Minute Introduction
Definition/Summary The double-slit is a simple configuration used to demonstrate interference effects in waves. Equations At distances that are large compared to the spacing between the slits (i.e. the...
View ArticleWhat is the Moment of Inertia? A 10 Minute Introduction
Definition/Summary The moment of Inertia is a property of rigid bodies. It relates rotational force (torque) to rotational acceleration in the same way that mass relates ordinary (linear) force to...
View ArticleWhat is Potential Energy? A 5 Minute Introduction
Definition/Summary Potential energy is simply another name for (minus) the work done by a conservative force. Since the work-energy theorem states that change in energy minus work done is constant,...
View ArticleWhat is Mass Inflation? A 5 Minute Introduction
Definition/Summary Abstract from Poisson and Israel’s 1990 paper, ‘Internal structure of black holes’- ‘The gravitational effects associated with the radiative tail produced by a gravitational collapse...
View ArticleWhat is Impedance? A 5 Minute Introduction
Definition/Summary The impedance of a load (a combination of components) in an AC current is a complex number [itex]Z\ =\ R+jX[/itex] where [itex]R[/itex] is the resistance of the load and...
View ArticleWhat Are the Thermodynamics of Black Holes? A 5 Minute Introduction
Definition/Summary The four laws of black hole thermodynamics are as follows… The Zeroth Law Surface gravity [itex](\kappa)[/itex] is constant over a black holes event horizon. The First Law ‘This law...
View ArticleWhy We Don’t Discuss Perpetual Motion Machines (PMM)
Authorship Contributions From Anorlunda You may want to believe the article you read, or the video you saw, or you may be very proud of your PMM invention. It can be very difficult to analyze what is...
View ArticleIs there Evidence for the Big Bang?
Key Points Hubble‘s law (redshift of galaxies) and Doppler effect provides direct evidence for the universe expanding away from our reference point, implying the universe is expanding. Low temperatures...
View ArticleWhat Are Tensors and Why Are They Used in Relativity?
If you try learning general relativity, and sometimes special relativity, on your own, you will undoubtedly run into tensors. This article will outline what tensors are and why they are so important in...
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