Starship Asterisk*

Chris Peterson wrote:Keep in mind that photons are not really massless. They have energy, and therefore a mass equivalent. They also have momentum, which means a mass equivalent. The confusion likely comes from the understanding that a photon has zero rest mass. But that's a theoretical concept, given that a photon is never at rest.

While Art is correct that the kinetic energy of CMB photons is decreasing (because of redshift), this does not violate the law of conservation of energy, because kinetic energy is an observer dependent property- the kinetic energy we measure depends upon our frame of reference.

Guest wrote:

Chris Peterson wrote:Keep in mind that photons are not really massless. They have energy, and therefore a mass equivalent. They also have momentum, which means a mass equivalent. The confusion likely comes from the understanding that a photon has zero rest mass. But that's a theoretical concept, given that a photon is never at rest.

While Art is correct that the kinetic energy of CMB photons is decreasing (because of redshift), this does not violate the law of conservation of energy, because kinetic energy is an observer dependent property- the kinetic energy we measure depends upon our frame of reference.

Can you blame us non scientists for thinking that conservation of energy is a law when scientists commonly call it such? (Just an amusing irony :wink: )

The frame of reference energy dependence is interesting. Would it be correct to then assume that the energy of these CMB photons hasn't really decreased, it's just that they look that way due to the expansion of the universe?

Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

Chris Peterson wrote:

Bruce wrote:Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

I don't think it's normally suggested that the overall energy of the Universe is changing.

BDanielMayfield wrote:

Chris Peterson wrote:

Bruce wrote:Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

I don't think it's normally suggested that the overall energy of the Universe is changing.

My worldview is substantially less rocked, so thanks. But that's not counting Dark Energy, right?

RJN wrote:
After midnight when the APOD appeared, I showed a person I know hoping to get a "Cool!" response. But this APOD made that person sad. The dead cartoon cat made them remember a recently deceased real cat of which they were fond. Apparently the thought of a dead cat -- and possibly killing a cat -- even a virtual cat -- weighs heavily on people's minds. Oops. And so maybe the tie-in to Schrodinger was less appealing than I thought.

https://en.wikipedia.org/wiki/Wigner%27s_friend wrote:
<<Wigner's friend is a thought experiment proposed by the physicist Eugene Wigner; it is an extension of the Schrödinger's cat experiment. The thought experiment posits a friend of Wigner who performs the Schrödinger's cat experiment after Wigner leaves the laboratory. Only when he returns does Wigner learn the result of the experiment from his friend, that is, whether the cat is alive or dead. The question is raised: was the state of the system a superposition of "dead cat/sad friend" and "live cat/happy friend," only determined when Wigner learned the result of the experiment, or was it determined at some previous point?>>

https://www.mersenne.org/primes/?press=M74207281 wrote:
GIMPS Project Discovers
Largest Known Prime Number: 2^74,207,281-1

<<On January 7, 2016, at 22:30 UTC, the Great Internet Mersenne Prime Search (GIMPS) celebrated its 20th anniversary with the math discovery of the new largest known prime number, 2^74,207,281-1, having 22,338,618 digits, on a university computer volunteered by Curtis Cooper for the project. The primality proof took 31 days of non-stop computing on a PC with an Intel I7-4790 CPU. This is the fourth record GIMPS project prime for Dr. Cooper and the University of Central Missouri. Dr. Cooper's computer reported the prime in GIMPS on September 17, 2015 but it remained unnoticed until routine maintenance data-mined it. The official discovery date is the day a human took note of the result. This is in keeping with tradition as M₄₂₅₃ is considered never to have been the largest known prime number because Hurwitz in 1961 read his computer printout backwards and saw M4423 was prime seconds before seeing that M₄₂₅₃ was also prime.>>