Category: newton




Ask Ethan: How Do Massless Particles Experienc…

Ask Ethan: How Do Massless Particles Experience Gravity?

“Given the equation for gravity between two masses, and the fact that photons are massless, how is it possible for a mass (like a star or a black hole) to exert influence on said photon?”

You know the law of universal gravitation: you put in what any two masses are, how far apart they are from each other, and the gravitational constant of the Universe, and you can immediately know what the force is between any two objects. Set one of the masses to zero, and the force goes to zero. So why is it, then, that if you take the ultimate particle with no mass, a photon, and pass it close by a mass, its path does bend? Why do massless particles experience gravity?

To understand why, you should think about what happens if you and I start at the same place near a mass, but I’m stationary and you’re moving. How far away is that mass? What’s the “r” that goes into Newton’s equation? And who’s right: me or you?

The answer is that we both need to be right, and Newton won’t get us there. Come get the real story on gravity, and learn why, in the end, massless particles feel it, too!



The EmDrive, NASA’s ‘Impossible&rs…

The EmDrive, NASA’s ‘Impossible’ Space Engine, Really Is Impossible

“Tajmar’s results are exactly what you’d expect for the systematic error explanation: with a properly shielded apparatus, with no additional electromagnetic fields induced by the wires, there is no observed thrust at any power. They conclude that these induced fields by the electrical wires, visibly present in the other setups, are the likely culprit for the observed, unexplained thrust:

‘Our results show that the magnetic interaction from not sufficiently shielded cables or thrusters are a major factor that needs to be taken into account for proper µN thrust measurements for these type of devices.’

To the best of our knowledge, then, rockets will still require propellant.

The EmDrive isn’t a reactionless drive at all, and all the laws of physics should still work. In short, we fooled ourselves.”

For years, many tinkerers and inventors have been claiming that some sort of electromagnetic cavity, e.g., the EmDrive, can create a reactionless drive. That is, they claim they can change the momentum of a rocket without any sort of change-in-momentum of anything else, violating Newton’s action-reaction law. Needless to say, much like perpetual motion, physicists are largely skeptical. But until now, we hadn’t yet found why they were achieving the results that they did. However, a new source of error was just uncovered: magnetic fields originating from the cables that power the device. Properly set up the device, away from cables and loops of wires, as Martin Tajmar’s team did, and guess what: your ‘anomalous thrust’ disappears.

The EmDrive, billed as NASA’s impossible space engine, really was too good to be true.

Chaos Theory, The Butterfly Effect, And The Co…

Chaos Theory, The Butterfly Effect, And The Computer Glitch That Started It All

“In his 1814 treatise, “A philosophical essay on probabilities,” French mathematician Pierre Laplace speculated that Newtonian mechanics heralded a rigid determinism that would theoretical enable the successful prediction of the entire future of the universe, given absolute knowledge of its complete state at any given time. The only catch is that the prognosticator would somehow need to step outside of the universe and obtain a complete snapshot at once of all the particles in it and their instantaneous trajectories.”

It seems like a foregone conclusion: if you live in a deterministic Universe, one governed by deterministic equations like Newton’s Laws, all you need to do is know the positions and momenta of all the particles to arbitrary accuracy at any given time. You give me those, and I should be able to, with enough computational power, give you the positions and momenta of those particles arbitrarily far into the distant future. Only, that isn’t the way physics actually works! If you give me those for a complex enough system, I’ll only be able to predict their behavior for a short while; pretty soon, chaotic effects will take over. We may call this the “butterfly effect” today, but the truth is it was started by a computer glitch almost 60 years ago.

Come get the fascinating story of chaos theory, the butterfly effect, and how we don’t need quantum rules for an indeterminate Universe, courtesy of the one-and-only Paul Halpern!

What does it mean to say that energy is conserved ?

We will believe in the newton’s law of motion and for a particle whose force is dependent only on its position it states that:




Now let’s take a closer look at this:

On the LHS you have something that is dependent on x and t, but irrespective of all that dependency, this quantity  ( mx’’ – F(x) ) is ALWAYS 0.

Therefore we can define a quantity (or function) called Energy which is invariant with time i.e:


This implies that if you take any system doing any sort of crazy motion, there is always this quantity that will always remain constant with respect to time irrespective of all that craziness. That quantity is Energy !

Have a good one!

Ask Ethan: Why don’t we build a telescope without mirrors or…

Ask Ethan: Why don’t we build a telescope without mirrors or lenses?

“Why do we need a lens and a mirror to make a telescope now that we have CCD sensors? Instead of having a 10m mirror and lens that focus the light on a small sensor, why not have a 10m sensor instead?”

Every time you shine light through a lens or reflect it off of a mirror, no matter how good it is, a portion of your light gets lost. Today’s largest, most powerful telescopes don’t even simply have a primary mirror, but secondary, tertiary, even quaternary or higher mirrors, and each of those reflections means less light to derive your data from. As CCDs and other digital devices are far more efficient than anything else, why couldn’t we simply replace the primary mirror with a CCD array to collect and measure the light? It seems like a brilliant idea on the surface, and it would, in fact, gather significantly more light over the same collecting area. True, CCDs are more expensive, and there are technical challenges as far as applying filters and aligning the array properly. But there’s a fundamental problem if you don’t use a mirror or lens at all that may turn out to be a dealbreaker: CCDs without lenses or mirrors are incapable of measuring the direction light is coming from. A star or galaxy would appear equally on all portions of your CCD array at once, giving you just a bright, white-light image on every single CCD pixel.

It’s a remarkable idea, but there’s a good physical reason why it won’t pan out. For the foreseeable future, we still need optics to make a telescope! Find out why on this week’s Ask Ethan.