As an interested amateur, I recommend the book "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime" by Sean Carroll as a good overview of quantum fundamentals. The book discusses several interpretations of the reality of matter at the quantum level. Dr. Carroll himself believes that everything is waves/fields at the lowest level, and a many-worlds interpretation of why matter appears to be particles when we observe it, but also discusses de Broglie–Bohm pilot wave theory and spontaneous collapse theory.
Technically the formula for force is F = dp/dt, or the derivative of momentum with respect to time.
For particles with mass the momentum of such a particle is p = mv, and so you can use that to yield F = ma. However for a massless particle like a photon, its momentum is p = E / c. If you use that momentum to describe a beam of photons being absorbed by a material, then you get F = n * E / c, where E is the average energy of the photons, n is the rate of photons per second, and c is the speed of light.
Things that move at the speed of light carry momentum, even though they have zero mass. For something moving at the speed of light, the momentum is the energy divided by c. (You can't have something with mass moving at the speed of light.)
Since the photon has nonzero energy E and is moving at c, it has mass by virtue of E=mc^2. And since E=hv, the E part is determined by the photon's frequency v, so the equivalent amount of mass is as well. It shows up as radiation pressure when the photon hits an object, just as if something tangible had collided with it.
On a sunny day, I'd guess that sunlight exerts about the same force on an acre on the Earth's surface as a postage stamp lying on the ground.
E=mc^2 doesn't mean that a photon has mass. Photons do produce gravitational effects due to their energy, but not in the same way as a particle with mass m. See, for example, https://physics.stackexchange.com/a/6222
Objects in motion have kinetic energy, and Einstein says mass and energy are equivalent. This means in a very real sense objects in motion have additional "relativistic mass". When you annihilate that photon it's energy is transferred to whatever absorbed it.
Confused? You're not alone! Physicists are trying to move away from the terms "rest mass" and "relativistic mass" for reasons including one you've already identified: what does it even mean for a photon to be "at rest"?
Actually, the cmd/ctl-z advice is actually moderately useful. If you type it immediately after the closing formatting character it undoes the wysiwyg formatting. (I'm not arguing against a way to turn the wysiwyg input window off, however.)
Five and ten year US bonds have had lost less value than the three year bond in the past 8 months, so that doesn't seem sufficient to explain it (i.e. it makes the difference worse).
That said, I was very loose in my calculations. Without exact knowledge of their holdings and careful calculations, I'm not surprised that the numbers don't fully add up.
These books were created from Feynman's lectures to freshman and sophomore Caltech students. I have not looked at the books in detail, but I think that a dedicated reader who is comfortable with calculus should be able to understand them.
Yes, I agree, the premise that Netflix wants you to return the DVDs as quickly as possible so that they can send it to someone else is wrong. At any given time, you're going to have N DVDs out, and Netflix doesn't have any reason to particularly care whether they're the same N DVDs each day or N different DVDs.
When I return two DVDs to Netflix on the same day, I usually send them in one envelope in the hopes that the mailing costs will be slightly lower. (I figure that if their costs are lower that helps keep the subscription price down.) I've wondered why Netflix doesn't try to combine the discs that they send, but perhaps the cost savings aren't enough to be worth the extra handling that would be required on their end.
Interesting... I noticed a couple years ago that if I put the DVD in the return envelope backwards (and the bar code couldn't be seen thru the little window) it took longer to mark those movies as returned (I'd get the email at 3, 4, 5 PM instead of 7 or 8 AM) and it often led to an extra day of waiting. I now take care to always have them in the right.
You haven't noticed any extra delay by doubling them up?
Did you read the article about the Netflix processing center? There isn't any automation on the envelope side, rather there are teams of people opening them up and pulling the discs and the sleeves.
I think the barcode window is entirely for outgoing routing.
I did read the article, but I didn't see where it said that there was no automation on the receiving end. Maybe I missed it, it was a long article.
When I noticed this for myself, 2007 sometime, I had a common problem where every Nth disc would take an extra day to be received, and that would really throw off my pattern, which mostly was just to have a couple movies for the weekend.
So soon hypothesized that it'd happen on days the barcode was not visible. So to test, I put 1 of them wrong intentionally, mailed all 3 at the same time, and the 1 that was wrong did indeed take an extra day to be marked as returned.
So I didn't have any real facts, just an observation that seemed plausible to me.
No, I have not noticed an extra delay by doubling up two discs in one envelope.
Usually the mail delivery between the local Netflix center and my address is overnight both ways, but occasionally it takes two days for an envelope I return to be processed. I've attributed that delay to the post office, however, and not to having two DVDs in the same envelope.
