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**Ask a monkey a physics question thread**

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  1. #2101
    CoccoBill's Avatar
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    Quote Originally Posted by MadMojoMonkey View Post
    Gov'ts limit the spread of technology all the time.

    Just one example
    Confidential / Top Secret tech is in the nose of nearly all of our military aircraft, and the detection and avoidance hardware and software we sell to our allies is many years behind the current tech we're using ourselves.
    That means they're using it though, not delaying it. I think that "this is too dangerous, humankind is not ready yet" stuff only happens in movies.
    Our brains have just one scale, and we resize our experiences to fit.

  2. #2102
    Quote Originally Posted by CoccoBill View Post
    "this is too dangerous, humankind is not ready yet"
    You mean like my biceps?
  3. #2103
    MadMojoMonkey's Avatar
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    Quote Originally Posted by CoccoBill View Post
    That means they're using it though, not delaying it. I think that "this is too dangerous, humankind is not ready yet" stuff only happens in movies.
    Did you catch the most recent John Oliver about facial recognition software?

    Turns out it was developed like 3 different times and then halted because too dangerous for humans until some unscrupulous person went ahead with it. It was inevitable.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  4. #2104
    CoccoBill's Avatar
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    Quote Originally Posted by wufwugy View Post
    You mean like my biceps?
    No, the world is definitely ready for those, you manly man manthing.
    Our brains have just one scale, and we resize our experiences to fit.

  5. #2105
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    Quote Originally Posted by MadMojoMonkey View Post
    Did you catch the most recent John Oliver about facial recognition software?

    Turns out it was developed like 3 different times and then halted because too dangerous for humans until some unscrupulous person went ahead with it. It was inevitable.
    Hm ok, maybe there are some instances, but like the quote from Google's chairman said "this is the one technology where we've held back". I'm not optimistic about anyone holding back AGI or quantum computers or death stars too much.
    Our brains have just one scale, and we resize our experiences to fit.

  6. #2106
    https://www.forbes.com/sites/waynera.../#38bd4a7e5939

    “Our machine performed the target computation in 200 seconds,” the announcement said, “and from measurements in our experiment we determined that it would take the world’s fastest supercomputer 10,000 years to produce a similar output.”

    But while it was wowing scientists, computer security experts became worried. Very worried.
    If someone has a quantum computer, and the world still relies on classical encryption, then kiss goodbye to your savings. And banking is just the tip of the iceberg. Imagine if you could bypass India's nuclear security and gain access to their systems.

    What Google have done is impressive, but not yet useful. But quantum computers aren't that far away, and the first nation to successfully create a useful quantum computer will likely have a very serious advantage over other nations.
    Quote Originally Posted by wufwugy View Post
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  7. #2107
    Point being, quantum computers will not be allowed into the general market until they have quantum security.
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  8. #2108
    CoccoBill's Avatar
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    Well sure, but that doesn't mean they're delayed, it just means the ones with the technology are gonna do their best no one else gets it. Restricted would be a better word.

    I thought you said you were pedantic about the meaning of words.

    But yeah, quantum computing probably isn't going to be the next breakthrough in gaming.
    Our brains have just one scale, and we resize our experiences to fit.

  9. #2109
    Well I mean "delayed" in the context of us, the general public, getting it. So it means exactly the same a "restricted" in that context. I'm not quite defining words myself.

    Governments won't delay their efforts to get these things, quite the opposite, so yeah if you took "delay" in that context you're right.
    Quote Originally Posted by wufwugy View Post
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  10. #2110
    So according to General Relativity, all reference frames are equally as valid as one another. So if I observe someone travelling at 0.9c, and take ten minutes or whatever to get to the sun, and the distance he traveled is around 150 million km, that's a valid observation. Likewise, to the lunatic travelling to the sun, he gets there in like 20 seconds or whatever (didn't calculate that but it's obviously much faster than ten minutes). The distance he thinks he traveled is much less than 150 m km, let's say a million km (again can't be bothered to calculate). This too is a valid observation. This is fine, I've got no problems with this.

    However, let's look at this from the FoR of a photon. It gets there in zero time, and travels zero distance. Also a valid observation. The implication of this is that, from the photon's pov, the sun and the Earth occupy the same region of spacetime.

    How is this not a violation of physics? How does the photon not observe a much denser object that is a black hole? How can two objects occupy the same location in spacetime from one FoR, but not another?
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  11. #2111
    MadMojoMonkey's Avatar
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    Quote Originally Posted by OngBonga View Post
    So according to General Relativity, all reference frames are equally as valid as one another.
    I'm pretty sure you mean all inertial reference frames are equally valid.
    Inertial reference frames (non-accelerating frames) are probably what you mean. If not, just say so.

    GR can handle accelerating frames just fine, too, but that wont really help us in the case of photons or any massless particles.

    Quote Originally Posted by OngBonga View Post
    However, let's look at this from the FoR of a photon. It gets there in zero time, and travels zero distance. Also a valid observation. The implication of this is that, from the photon's pov, the sun and the Earth occupy the same region of spacetime.
    I just had a conversation about this last night that ran until 2:30 in the morning.

    My issue is trying to visualize how it is that a photon which does not experience time can be a wave in the E-M fields.
    My issue is visualizing how the 2D planar timeless universe of a photon can stretch out into a regular wave pulse that travels through 4d spacetime.

    Quote Originally Posted by OngBonga View Post
    How is this not a violation of physics? How does the photon not observe a much denser object that is a black hole? How can two objects occupy the same location in spacetime from one FoR, but not another?
    The short answer is
    A reference frame moving at c is neither an inertial reference frame, nor is it an accelerating reference frame.
    The equations we're using to try to understand it do not actually tell us about that special case.

    It is a special case that we can only interpret the nature of through the use of mathematical limits. There are infinities in there, and we can't say what happens at c. We can only say the limit as something approaches c.
    This is problematic because no object with mass can cross from less than c to c, and no object without mass can cross from c to less than c.

    When we take the limit as v -> c, we are implicitly assuming that any object with mass will behave the same if it reaches c, but that's a false assumption. All we can really say is what happens as the difference between v and c becomes very small to an outside observer.
    The speed of light is c in all inertial reference frames. No matter how fast you're going relative to another reference frame, light still moves at c according to you. So the idea that you've gotten closer to moving at c is bad language. No matter what you do, light moves at c in your reference frame.
    The statement "as you approach c" is problematic. In your reference frame, you're not approaching c. The speed of light is unchanging, so you're not approaching it.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  12. #2112
    GR can handle accelerating frames just fine, too, but that wont really help us in the case of photons or any massless particles.
    Yeah and I'm assuming the guy going on a trip to the sun is moving at constant velocity, so yes, inertial frame of reference. I'm sure GR can handle non-inertial FoR, but my brain can't!

    My issue is trying to visualize how it is that a photon which does not experience time can be a wave in the E-M fields.
    That's a good question, not sure if it's essentially the same question, gonna mull this over for a bit.

    and no object without mass can cross from c to less than c.
    Interesting side note - light appears to travel at <c through a non-vacuum due to refraction. Apparently this is due to the photon exciting electrons, which in turn create EM waves, which interact with the photon. The sum of all these waves result in a lower observed value for c. But the photon still moves at c, it just appears not to. I'd be curious if you can explain this in language I can understand, because I'm not really getting it.

    The statement "as you approach c" is problematic. In your reference frame, you're not approaching c. The speed of light is unchanging, so you're not approaching it.
    I do get this, but it doesn't really answer the question of how the photon observes two objects clearly separated by space to be occupying the same location. More than that, it observes ALL objects in the direction of travel, in both directions (behind and in front of) infinitely, to occupy the same location in space.

    I mean the best I can do is to conclude that space emerges when something moves at <c, the photon doesn't "observe" anything because from its FoR it's stationary in a timeless spaceless universe, it occupies a singularity with everything in the universe. But that's akin to saying the photon's FoR is not valid. Not sure if that's a problem for GR.
    Quote Originally Posted by wufwugy View Post
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  13. #2113
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    Quote Originally Posted by OngBonga View Post
    Yeah and I'm assuming the guy going on a trip to the sun is moving at constant velocity, so yes, inertial frame of reference. I'm sure GR can handle non-inertial FoR, but my brain can't!
    I think you mean "free-falling in a gravitational field," rather than "moving at constant velocity."

    But maybe I'm forcing an assumption.

    Quote Originally Posted by OngBonga View Post
    That's a good question, not sure if it's essentially the same question, gonna mull this over for a bit.
    I can settle on an answer that is both good and bad.
    If the photon's existence in the plane bears some angular momentum (nevermind that without time, momentum cannot be defined, take it as an intrinsic value of photons)
    then we can describe that angular momentum with a vector, a 2D object.
    When that photon is viewed in 4D spacetime, it is still described by a 2D vector - it's Poynting vector (which is pointing where the photon is headed, conveniently enough).
    So I can find a way to visualize some aspect of this conundrum.

    The big problem is that not all photons have rotational polarization. Some have planar polarization.

    Quote Originally Posted by OngBonga View Post
    Interesting side note - light appears to travel at <c through a non-vacuum due to refraction. Apparently this is due to the photon exciting electrons, which in turn create EM waves, which interact with the photon. The sum of all these waves result in a lower observed value for c. But the photon still moves at c, it just appears not to. I'd be curious if you can explain this in language I can understand, because I'm not really getting it.
    It comes down to phase lag and interference. The accelerated electrons (and other charged particles to a lesser extent) are accelerated by the photon, not moved by the photon. The phase lag between the acceleration and velocity of the charged particles results in new wave, with equal magnitude and direction, but a new phase.

    As the phase is continually being shifted, the photon's location is moved "backward" a little bit... in a continuous manner.

    It's important to note that phase velocity and group velocity are different. The phase change isn't changing the phase velocity, but it's changing the group velocity. So the E-M fields are still propagating at c, but the group velocity of the wave packet appears to move at less than c due to the interference.

    (This one is difficult to explain, so don't count these answers as an expert opinion, but a good metaphor.)

    Quote Originally Posted by OngBonga View Post
    I do get this, but it doesn't really answer the question of how the photon observes two objects clearly separated by space to be occupying the same location. More than that, it observes ALL objects in the direction of travel, in both directions (behind and in front of) infinitely, to occupy the same location in space.
    It's more a matter of physics not describing the photon's "rest" frame in a satisfying, intuitive way.

    We have an easy time thinking of something with mass moving faster and faster, but a hard time accepting that no matter how fast it gets, it's still effectively infinitely slower than c, as no amount of acceleration could ever get an object with mass up to c. We have a hard time understanding that no matter how fast you're moving, you still measure c as much faster than you.
    Moving at c is a totally different beast.

    It just has to be that way, though, IMO, no matter how hard it is to visualize.
    If light didn't move at c, which is the upper limit of all speeds in the universe, then causality is broken, and a cause no longer has to precede its effect.

    Quote Originally Posted by OngBonga View Post
    I mean the best I can do is to conclude that space emerges when something moves at <c, the photon doesn't "observe" anything because from its FoR it's stationary in a timeless spaceless universe, it occupies a singularity with everything in the universe. But that's akin to saying the photon's FoR is not valid. Not sure if that's a problem for GR.
    It's not a problem for GR. It's a consequence of SR.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  14. #2114
    Quote Originally Posted by CoccoBill View Post
    No, the world is definitely ready for those, you manly man manthing.
    brofist
  15. #2115
    I think you mean "free-falling in a gravitational field," rather than "moving at constant velocity."
    High quality pedantry.

    I'll rephrase it into language I like... in motion along a geodesic.

    They all mean the same thing though. If something is not moving along a geodesic, it is accelerating, ie not moving at constant velocity.

    I can settle on an answer that is both good and bad.
    If the photon's existence in the plane bears some angular momentum (nevermind that without time, momentum cannot be defined, take it as an intrinsic value of photons)
    then we can describe that angular momentum with a vector, a 2D object.
    When that photon is viewed in 4D spacetime, it is still described by a 2D vector - it's Poynting vector (which is pointing where the photon is headed, conveniently enough).
    So I can find a way to visualize some aspect of this conundrum.


    The big problem is that not all photons have rotational polarization. Some have planar polarization.
    Not gonna lie, I don't understand this. What makes some photons different to others though?

    It comes down to phase lag and interference. The accelerated electrons (and other charged particles to a lesser extent) are accelerated by the photon, not moved by the photon. The phase lag between the acceleration and velocity of the charged particles results in new wave, with equal magnitude and direction, but a new phase.


    As the phase is continually being shifted, the photon's location is moved "backward" a little bit... in a continuous manner.


    It's important to note that phase velocity and group velocity are different. The phase change isn't changing the phase velocity, but it's changing the group velocity. So the E-M fields are still propagating at c, but the group velocity of the wave packet appears to move at less than c due to the interference.


    (This one is difficult to explain, so don't count these answers as an expert opinion, but a good metaphor.)

    It's really difficult to grasp this, I'll probably have to research it a little to see if it's something I can get my head around. I'm happy with the word "interference", that does make me think I can get it, but it's not immediately clear to me.

    We have an easy time thinking of something with mass moving faster and faster, but a hard time accepting that no matter how fast it gets, it's still effectively infinitely slower than c, as no amount of acceleration could ever get an object with mass up to c. We have a hard time understanding that no matter how fast you're moving, you still measure c as much faster than you.

    I'm actually ok with this, at least I think I am. If the guy going to the sun decides to shine a torch towards the sun, both he and the observer at home (with the help of an amazing telescope) see the light moving at the same speed. What the two observers don't agree on is the distance the light has traveled, and in how much time. Observer A will say it moved x distance in x time, while observer B will say it moved y distance in y time. The ratios of these two figures will always be the same... c. Space and time changes, but velocity does not.

    The guy moving at 0.9c doesn't think he's going that fast. He didn't take 20 seconds to travel 150 million km, he took 20 seconds to move 1 million km (these numbers are clearly arbitrary and incorrect). He doesn't think "woah I'm going fast", he thinks "woah the sun is closer than I thought".

    I'm still not close to understanding this from the pov of something actually moving at c though. No space, no time, the universe is a singularity. But clearly it isn't. There is time and space. I'm struggling here.

    It's not a problem for GR. It's a consequence of SR.

    This might be a dumb thing to say, but isn't SR basically GR in flat spacetime (no gravity)? My understanding of SR is that it's a special case of GR where gravity is negligible.
    Quote Originally Posted by wufwugy View Post
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  16. #2116
    Not gonna lie, I don't understand this. What makes some photons different to others though?

    *other than frequency
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  17. #2117
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    Quote Originally Posted by MadMojoMonkey View Post
    We have a hard time understanding that no matter how fast you're moving, you still measure c as much faster than you.
    Wait much faster? Doesn't that difference get infinitely small?
    Our brains have just one scale, and we resize our experiences to fit.

  18. #2118
    Quote Originally Posted by CoccoBill View Post
    Wait much faster? Doesn't that difference get infinitely small?
    No. The speed of light is constant for all observers. So if I observe you moving at 0.99999999999999999999999999999999999999 c, you still observe light to move away from you at c, not at 0.00000000000000000000000000000000000001 c.
    Quote Originally Posted by wufwugy View Post
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  19. #2119
    It's space and time that are not constant for all observers.
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  20. #2120
    CoccoBill's Avatar
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    Ok yeah I've heard that, my mind had just blocked that silliness away.
    Our brains have just one scale, and we resize our experiences to fit.

  21. #2121
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    What I mean is that none of that is any more intuitive than QM.
    Our brains have just one scale, and we resize our experiences to fit.

  22. #2122
    Quote Originally Posted by CoccoBill View Post
    What I mean is that none of that is any more intuitive than QM.
    I would respectfully disagree. I mean "intuitive" is subjective, of course, but I can get my head around time dilation (which is what this is), at least for objects not moving at c.

    QM is another beast altogether. Nobody understands it!
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  23. #2123
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    Yeah but things moving around at a constant speed related to everyone else moving at various speeds is just bizarre. Or cheating. I'm guessing it's a crude software hack in our simulation.
  24. #2124
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    Quote Originally Posted by OngBonga View Post
    High quality pedantry.
    TY, sir. I do my best.

    Quote Originally Posted by OngBonga View Post
    Not gonna lie, I don't understand this. What makes some photons different to others though?
    Some photons travel with circular polarization and others travel with planar polarization.
    Circularly polarized photons' E and M fields rotate about the axis defined by the photon's line of travel.
    Linearly polarized photons' E and M fields oscillate in planes.
    At all times, in both cases, the E and M fields of the photon are orthogonal (at right angles).

    Quote Originally Posted by OngBonga View Post
    It's really difficult to grasp this, I'll probably have to research it a little to see if it's something I can get my head around. I'm happy with the word "interference", that does make me think I can get it, but it's not immediately clear to me.
    Start by learning the difference between phase velocity and group velocity.
    A photon is a wave packet, and as such it has an "internal" waviness that is bounded by an "external" envelope. These words in quotes are only referring to a plot of the wave. I'm not saying a particle has any internal waviness... all the waviness taken together is the photon.

    The phase velocity is not necessarily limited by c. The group velocity is. It's a tricky distinction.

    Quote Originally Posted by OngBonga View Post
    I'm actually ok with this, at least I think I am. [...]

    I'm still not close to understanding this from the pov of something actually moving at c though. No space, no time, the universe is a singularity. But clearly it isn't. There is time and space. I'm struggling here.
    I don't really get it, either.

    Quote Originally Posted by OngBonga View Post
    This might be a dumb thing to say, but isn't SR basically GR in flat spacetime (no gravity)? My understanding of SR is that it's a special case of GR where gravity is negligible.
    I think that's a good way to put it. My point is that a photon's frame of reference is "not valid" as you say, since it's neither inertial nor accelerating.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  25. #2125
    MadMojoMonkey's Avatar
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    The best answers I can find for the reasons light appears to move more slowly in a dielectric medium rely heavily on field theories of matter and light. It's hard to explain them without a rigorous mathematical background without losing all the richness of conveying meaning without hand waving.

    My above statements about phase velocity and group velocity are probably as close as we're going to get without talking about how electric fields interact with charged particles, and by extension, atoms and molecules, in the language of field theory.


    The photon transfers some energy to the electrons, which the electrons, being massive particles, respond to with a time lag. The velocity lags behind the acceleration like the derivative of sine is cosine - same shape, but behind in phase.
    This is understood by citing F = ma = m dv/dt.
    Note that F = qE, the origin of the force on the electron is the electron's charge, q, interacting with the instantaneous value of the E field (not the derivative of the E field).
    Now we have qE = m dv/dt and note the velocity responds like a time derivative of the E field, and that should motivate us to see the phase change in the way a photon accelerates an electron with the motion of the electron. The photon oscillates in a sinusoid, so the electron responds like a sinusoid with offset phase.

    Then we have to understand that part of Maxwell's Equations which says that accelerating charges create changing electric fields. In this case the electrons are being accelerated in a sinusoid with the same frequency as the photon, so create a changing electric field which is of the same frequency, but with a phase lag.


    All of this is missing a huge amount of important information... if we just keep moving the phase back a little bit, and adding it to the incident wave, then we can easily visualize a phase shift of 180 degrees, which exactly destructively interferes with the incident wave and exactly cancels it out. That doesn't happen. The reason (I think) is that I've only talked about the effects in the electric field and not the effects in the magnetic field.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  26. #2126
    Struggling for time today but will revisit this tomorrow. I'm not ignoring you! I appreciate your replies very much.
    Quote Originally Posted by wufwugy View Post
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  27. #2127
    Ok so I've had a busy couple of days, but while thinking about what we've been talking about, I do have another related question...

    Gravity depends on mass, and, crucially, distance between masses. The problem here is that on the one hand, distance is relative, depending on your frame of reference. But on the other hand, the gravitational attraction between two objects is absolute, which heavily implies the distance between them is absolute.

    How is this paradox resolved?

    I've got some time tomorrow, so I'm going to get round to my homework of learning about phase and group velocity.
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  28. #2128
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    Quote Originally Posted by OngBonga View Post
    Gravity depends on mass, and, crucially, distance between masses. The problem here is that on the one hand, distance is relative, depending on your frame of reference. But on the other hand, the gravitational attraction between two objects is absolute, which heavily implies the distance between them is absolute.

    How is this paradox resolved?
    4-D spacetime curvature


    You described Newton's law of gravity in a GR universe.
    The classical law has to give way to the full GR treatment when dealing with relativistic effects.


    [EDIT] or maybe if you just take into account relativistic masses, it balances out the space contraction and time dilation to work. Just a thought. [/EDIT]
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  29. #2129
    I think I get it actually. Let's imagine the orbit of Earth, as observed by a traveler going to the sun at 0.9c. It will be an extremely eccentric ellipse, with a perihelion much smaller than observed from earth, but the aphelion will be identical (assuming direct motion to the sun). The lateral velocity of the orbit will be the same, but the observed longitudinal velocity will be much slower for the traveler than the person back home. So balance is restored, the orbital path and velocity will make sense, regardless of the frame of reference.

    The observed orbits change to balance out the difference in observed distance.

    Paradox resolved.
    Quote Originally Posted by wufwugy View Post
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  30. #2130
    It's probably not resolved. Because the perihelion is much less, the traveler should expect to see a much faster lateral orbital velocity. Hmm this is headbending, especially for 7.30am.
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  31. #2131
    You described Newton's law of gravity in a GR universe.

    This is clearly the problem I'm having.
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  32. #2132
    MadMojoMonkey's Avatar
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    If you want a mind-bending thought experiment, consider a relativistic disk.

    A disk is spinning such that the edge of the disk is moving at relativistic speeds. (Never mind that no known material could handle the centripetal forces.)

    What does an observer on the edge of the disk see?
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  33. #2133
    That's interesting, I'll think that through.

    I just watched a youtube vid about phase and group velocity. I now know the difference between the two. There's some interesting questions that arise though.

    For a wave that describes a particle, a higher frequency (shorter wavelength) means a faster phase velocity, but the opposite is true of classical waves (water or sound). Why is this?

    I at least appreciate now why a photon can move slower than c though. The phase velocity is c, but if several interfering waves have different wavelengths, the group velocity can be faster or slower. But this raises another question... if the sum of phase velocities can result in a slower photon, why not a faster photon?
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  34. #2134
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    Quote Originally Posted by OngBonga View Post
    For a wave that describes a particle, a higher frequency (shorter wavelength) means a faster phase velocity, but the opposite is true of classical waves (water or sound). Why is this?
    I don't there's any dispersion in empty space. The change in speed for different frequencies is a matter of the dielectric properties of a medium through which the photon travels.

    For classical waves the phase velocity may be faster or slower than the group velocity. It depends on the viscous properties of the material though which the wave travels.

    For waves on a string the phase velocity is equal to the group velocity.
    For water waves, the phase velocity is 2x the group velocity.
    Unless I'm missing something, for quantum wave functions of free particles (not bound states), the phase velocity is 1/2 the group velocity.

    [EDIT]The group velocity is 0 for bound states, which are standing waves.[/EDIT]

    Quote Originally Posted by OngBonga View Post
    I at least appreciate now why a photon can move slower than c though. The phase velocity is c, but if several interfering waves have different wavelengths, the group velocity can be faster or slower. But this raises another question... if the sum of phase velocities can result in a slower photon, why not a faster photon?
    Good fucking question. Wow. I never thought of that.

    IDK. I'm asking around.
    Last edited by MadMojoMonkey; 06-21-2020 at 01:19 PM.
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  35. #2135
    So this disc thought experiment... I'm visualising a massive disc, basically the size of the observable universe. It's rotating very slowly, however it is so big that the velocity at the edge is massive, relativistic even. Assuming the disc is strong enough (lol), and assuming constant rotational velocity, oh and also assuming the observer is fixed into position and is an infinitesimal point (fuck you tidal forces), the observer wouldn't notice how fast he was going. In fact from his pov, he is stationary and the centre is rotating around him at relativistic velocity. From his pov, time is normal, while the centre is dilated and in a slower time reference frame.

    One of two things is going to happen... either the disc appears to be stationary, or an observe at both the edge and the centre would observe extreme spacetime warping, resulting in an ever-tightening spiral. I'd guess the latter.

    Definitely headbending.
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  36. #2136
    Good fucking question. Wow. I never thought of that.
    I'm wasting my life.
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  37. #2137
    What's the scoop on older generation stars? What I'm getting at is that AFAIK over the life of the universe, there have been different generations of stars. Like, say, the stars that popped up 10bn years ago were fundamentally different than the ones that pop up now.

    If this is true, I'd like to know more, because of my interest in the Great Filter and Fermi's Paradox. If, for example, complex life could be supported in 10bn year old star systems, it means there is probably a Great Filter that is killing off virtually every advanced species. Otherwise, one or more would have expanded galactically by now.

    However, if it is thought that complex life might only be able to exist in the kind of star generation we have -- and if that system didn't exist long enough in the past -- then it could make sense why the galaxy is not populated by a species that had billions of years of technological advancement before our star system even came about.
  38. #2138
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    The early universe was almost entirely composed of Hydrogen, with a bit of Helium and very small amounts of Lithium. Those are the smallest 3 atoms.

    The earliest planets, if there were planets around 1st generation stars in the early universe, would have been gas giants, sometimes called "failed stars" if they're particularly large, but not large enough for fusion. There's no strong reason to think there was any solid land on those planets, as Hydrogen does not form a solid, and Helium only does so under extreme pressures and low temperatures. Lithium was only present in trace amounts, so not likely to significantly contribute.

    At any rate, there was certainly no carbon, the atom most associated with life.


    H, He, and Li composed the earliest stars. Heavier elements are made through stellar fusion up to Iron and Nickel. The fusion of Iron and Nickel initiates a death spiral for a star, as the fusion of Iron and Nickel is not exothermic. The fusion processes for smaller elements creates energy to push back against the gravitational collapse. The fusion of Iron and Nickel does not supply any external pressure, so the gravitational collapse is not countered, and the star falls into its core. The massive increase in density and pressure causes all the heavier elements to be produced.

    A lot of different outcomes can happen at the end of a star's fusion, but for massive stars, a supernova happens.

    That ensuing supernova disperses those heavier elements out back into the stars galaxy, creating a planetary nebula. That nebula is now full of "heavy elements" (in this context meaning all elements larger than Lithium). As the galaxy ages, and other stars supernova, and shock waves pass through those planetary nebula, new stars may form.

    The presence of heavy elements includes Carbon, so it's the dust of a supernova that supplies the carbon to make life.


    There's no reason to say that all the early stars have burned out. The smaller the star, the longer its life. So there are still primordial stars burning in the universe. The composition of those early stars is still the most prevalent elements in the universe, so stars that are basically chemically identical to the primordial stars can still form today.
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  39. #2139
    Thanks. Found this:





    Seems like the conditions for life existed long before Earth's formation. Really does bring into question why we don't see billions-of-years life older than us everywhere. Some say Great Filter. Others say The Simulation.
  40. #2140
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    It's especially strange because there's not only life on Earth, but it's just everywhere. There's life in places we never thought it could exist, in highly toxic environments, where sunlight doesn't reach. In deep sea near volcanic vents. In secluded caves that were cut off from the outside world for millennia. In the antarctic... it's just everywhere on Earth.

    The Fermi Paradox doesn't really have an answer. The Great Filter hypothesis has strong proponents, but I don't think it really solves the paradox. It just adds one more small number to multiply into the Drake Equation.
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  41. #2141
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    I'm spamming this one website left and right, but again, this guy has my favorite piece on this:

    https://waitbutwhy.com/2014/05/fermi-paradox.html
    Our brains have just one scale, and we resize our experiences to fit.

  42. #2142
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    "Our night sky consists of a small selection of the very brightest and nearest stars in the red circle."

    Not only is that not the Milky Way galaxy, but also, it sure looks like a Hubble picture of a galaxy that is not the Milky Way.

    Even if it's not an actual Hubble photo, the fact remains that we have photographs of other galaxies, that are obviously not inside the Milky Way.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  43. #2143
    Quote Originally Posted by CoccoBill View Post
    I'm spamming this one website left and right, but again, this guy has my favorite piece on this:

    https://waitbutwhy.com/2014/05/fermi-paradox.html
    Great read.
  44. #2144
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    Quote Originally Posted by MadMojoMonkey View Post
    "Our night sky consists of a small selection of the very brightest and nearest stars in the red circle."

    Not only is that not the Milky Way galaxy, but also, it sure looks like a Hubble picture of a galaxy that is not the Milky Way.

    Even if it's not an actual Hubble photo, the fact remains that we have photographs of other galaxies, that are obviously not inside the Milky Way.
    I'd say it's this one:
    https://commons.wikimedia.org/wiki/F...Way_Galaxy.jpg

    Don't know whether it's a photo of some other galaxy or cgi, but yeah, obviously not a photo of the milky way.
    Our brains have just one scale, and we resize our experiences to fit.

  45. #2145
    That article brought up some more Great Filters that I hadn't thought of before.

    A counter (which I'm not sure what to think about): we would still see the old technology of advancing civilizations spread everywhere before some Great Filters like the secret empire civilization that got big first and wipes everything else out. Although, if that old technology is old enough, we probably wouldn't.

    Then there's the Simulation. We don't see anything because there is nothing, because this is a simulation. The only thing I've come up with that counters the simulation hypothesis is that (I think) it assumes math exists outside this reality. I can't say that's a good assumption or not.
  46. #2146
    Rather, I should say The (3 condition) Simulation Hypothesis assumes math exists outside this reality because it relies on the exponential function to say under which conditions the probability we're in a simulation converges to 1.
  47. #2147
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    @cocco: Yeah, there's simply no photo of the Milky Way from that perspective. The furthest man-made object from earth is Voyager, and it's barely away from the sun on those scales.

    It's just that we do see the Milky Way band across the night sky, which is brightness from stars, just too many and too far away to make out as individual stars with just our eyes. We can see distant galaxies with our eyes, though. They look like blurry stars - nebulae - as in nebulous or blurry.

    We have been observing the galactic center for long enough to plot the orbits of things, and to show that there must be a supermassive black hole at the center.

    Their deeper point that the stars we observe with our eyes are really not that far away from us on a galactic scale is fine.

    ***
    One conundrum with Great Filter is that the further out into space we look, the further back in time we look. It could be that civilizations have cropped up in distant places from us, but the evidence has not yet reached us.


    I mean... if there are multiple universes, then numbers exist - and therefore math exists. QED.
    If there's an "outside" of this reality, then the phrase "all is one" is not the only useful description of the universe, and therefore numbers exist.
    (not sure if serious)
    Last edited by MadMojoMonkey; 06-27-2020 at 11:37 AM.
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  48. #2148
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    Quote Originally Posted by OngBonga View Post
    I at least appreciate now why a photon can move slower than c though. The phase velocity is c, but if several interfering waves have different wavelengths, the group velocity can be faster or slower. But this raises another question... if the sum of phase velocities can result in a slower photon, why not a faster photon?
    FYI, I've asked my colleagues in the intro physics program, and 2 of them responded that they don't know the answer to this.
    None of them are experts in QM. It's crazy how much you forget when you're not using it for a few years.

    I'll ask some of the professors who teach QM when I can.

    It'll probably have to wait until August, though, as it's frowned upon for me to use the university email while I'm furloughed.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  49. #2149
    It's awesome that I've asked a question that three physics guys can't answer.

    I'm mean I can answer the question myself in a wholly unsatisfying way... because causality would be broken. But obviously that's as hand wavy as it gets.

    I'll see if I can beat you to it. I'll try to learn the answer to this before August. Homework for July!
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  50. #2150
    A wonderful demonstration of the incredible weakness of gravity...

    Quote Originally Posted by Holly Krieger
    Gravity: so pitifully weak that the entire Earth can’t overcome this little droplet’s surface tension
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  51. #2151
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    What's the deal with temperature? I've watched a couple lectures now and it seems like whoever you ask you get a different answer. Isn't it always relatable to pressure of a gas in a closed system, and if so, what is all this noise:

    The kelvin is now defined by fixing the numerical value of the Boltzmann constant k to 1.380 649×10−23 J⋅K−1. This unit is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition respectively.
    They're just fucking around, aren't they?

    I think I kind of get the gist of enthalpy and entropy, but i have no idea what temperature is.
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  52. #2152
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    Quote Originally Posted by oskar View Post
    What's the deal with temperature? I've watched a couple lectures now and it seems like whoever you ask you get a different answer. Isn't it always relatable to pressure of a gas in a closed system, and if so, what is all this noise:



    They're just fucking around, aren't they?

    I think I kind of get the gist of enthalpy and entropy, but i have no idea what temperature is.
    Temperature is a measure of kinetic energy on the particle scale.

    Quantum mechanically, temperature is expressed in the rotational and vibrational states of atoms and molecules.


    ***
    They're trying to pin down all the physical constants to a minimum number of constants deemed the "most fundamental" which is ultimately a silly task, IMO, but standardizing units and definitions is important.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  53. #2153


    More jiggle = hotter
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  54. #2154
    I think I kind of get the gist of enthalpy and entropy, but i have no idea what temperature is.
    Heat is an excellent example of entropy. Heat is the flow of thermal energy, it is the spreading out of thermal energy. Thermal energy is just kinetic energy on the atomic scale. If you imagine two atoms colliding, one fast and the other slow, the faster on loses energy to the slower one. Now imagine trillions of atoms constantly colliding. The faster ones are gradually slowing down as they collide with slower atoms, and the slower atoms are getting faster, until we reach thermal equilibrium, all atoms moving at the same speed. At this point, heat (thermal flow) ceases to happen.

    I actually don't know what enthalpy is, but I note it's measured in joules, so I assume it's a form of heat.

    What's enthalpy, mojo?
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  55. #2155
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    Quote Originally Posted by OngBonga View Post
    What's enthalpy, mojo?



    It's not cool, not fun to say, and no one is happy about it, frankly.
    Well, I guess... chemists think it's kinda cool, but really... who trusts a chemist? All of us? Shit. You mean I can't just blow this off 'cause it's hard? Fine.
    ...
    Fine.

    FINE! I'm doing it, OK?!




    Gets to the point around 3:00 minutes in and makes a pretty good and simple argument to understand it over the next 2 minutes.
    Whole video is pretty good, and better than I could explain on my own, and has cool animations and stuff ... and things.

    I learned about it in Thermodynamics in college, but it hasn't come up since then in my work.
    My actual definition without research would have been:
    It's one of about 30-ish forms energy can take and sneak around in, and it's one of the less intuitive ones.
    Turn out I would've been kinda right and kinda wrong because it's a sum of other energies, so not technically one of the forms energy can take, but more just a convenient package of a couple forms of energy to make math easier.
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  56. #2156
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    Quote Originally Posted by OngBonga View Post
    If you imagine two atoms colliding, one fast and the other slow, the faster on loses energy to the slower one. Now imagine trillions of atoms constantly colliding. The faster ones are gradually slowing down as they collide with slower atoms, and the slower atoms are getting faster, until we reach thermal equilibrium, all atoms moving at the same speed. At this point, heat (thermal flow) ceases to happen.
    Slight correction that at thermal equilibrium, it's not that everything is moving at the same speed, but that the distribution of speeds has settled to a specific bell-curve distribution (the Boltzmann distribution).

    All of which is to assume we are talking about a gas of 1 type of molecule or at least entirely of molecules of the same mass.
    What actually settles to the Boltzmann distribution is kinetic energy, or 1/2 mv^2 of each particle, if I remember correctly.

    So heavier molecules will be moving (translating, vibrating, rotating) more slowly than lighter molecules.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  57. #2157
    Nice correction, I have no idea what you just said! I guess I'm imagining this is a classical sense, like there's lots of tiny little balls flying around, colliding. In this instance, everything would settle to the same speed, as eventually all the balls' speeds would average out as the fast ones continually lose energy to the slow ones. Of course, atoms are not tiny little balls, so it's no real surprise that my analogy is flawed.

    Gonna watch that vid now, cheers.
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  58. #2158
    ^ also yes I'm assuming all these little balls are identical.

    I'm none the wiser after watching that vid, though my intuition that it's heat related seem correct.

    I guess a better question is... what's the difference between heat and enthalpy? You can answer that if you like, but I'm gonna do my own digging because I'm curious.
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  59. #2159
    Enthalpy is basically the potential heat stored in chemical bonds... right?
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  60. #2160
    ^ hmm that's not quite right. I'm neglecting work, which is an important factor. Plus, we should know how much "potential heat" is stored in chemical bonds, yet it seems it's impossible to know how much enthalpy a system has.

    It's further complicated by things like your comment above...
    "It's one of about 30-ish forms energy can take..."

    There's lots of forms of energy, but ultimately they fall into one of two categories... kinetic and potential. And even then, whether you observe kinetic or potential energy is frame of reference dependent... it's relative. Energy is complicated as fuck.

    I'm going to bed.
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  61. #2161
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    Quote Originally Posted by OngBonga View Post
    Nice correction, I have no idea what you just said! I guess I'm imagining this is a classical sense, like there's lots of tiny little balls flying around, colliding. In this instance, everything would settle to the same speed, as eventually all the balls' speeds would average out as the fast ones continually lose energy to the slow ones. Of course, atoms are not tiny little balls, so it's no real surprise that my analogy is flawed.
    I'm fairly certain the Boltzmann distribution is known from classical physics, meaning known prior to quantum mechanics, so it's about tiny balls flying around, not wave functions.

    I'm actually having trouble visualizing why it's not a stable equilibrium if everything's moving the same speed, and I'm not sure which of my assumptions is giving me grief.

    A) we can't assume perfectly elastic collisions, because in that case, the final speeds are equal to the initial speeds, just swapped, so there is no tendency toward the mean. Like a Newton's cradle desk toy. The falling ball hits the other ball and stops, and another ball swings away. The final picture is just a mirror image of the initial picture.

    So what does that mean? Nothing shakes the foundation of Conservation of Momentum, but there's no Conservation of Kinetic Energy, only total energy. Kinetic energy is only conserved in purely elastic collisions.

    So where else can the energy go? In a molecule, there are modes of vibration and rotation that express energy, so a collision can result in lower translational kinetic energy, if there is higher vibrational or rotational energy.

    But what about single atoms, not in molecules? Like the noble gasses? This is where I get tripped up. They don't have any spacial assymetries to exploit for vibration or rotation, the only have their translational energy. I'm not talking about electrons changing energy levels, that's QM stuff. We're still classical, so forget that.

    I'm certain that the velocities tend toward a Boltzmann distribution at thermal equilibrium, but I'm not really sure how they're trading momentum but not energy in the collisions. I.e. I know they can't be perfectly elastic collisions, but I'm not sure what energy transformation is allowing them to fail to conserve kinetic energy.
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  62. #2162
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    Quote Originally Posted by OngBonga View Post
    ^ hmm that's not quite right. I'm neglecting work, which is an important factor. Plus, we should know how much "potential heat" is stored in chemical bonds, yet it seems it's impossible to know how much enthalpy a system has.

    It's further complicated by things like your comment above...
    "It's one of about 30-ish forms energy can take..."

    There's lots of forms of energy, but ultimately they fall into one of two categories... kinetic and potential. And even then, whether you observe kinetic or potential energy is frame of reference dependent... it's relative. Energy is complicated as fuck.

    I'm going to bed.
    Yeah. Energy is at the same time very simply and very mysterious, IMO.
    On the one hand it's just energy. You know when you have energy and when you don't. You understand it takes energy to "do stuff" which is basically the best definition of energy out there: the ability to do stuff.

    On that hand, it's very simple.
    On the other hand, it's so sneaky in how it transforms and hides from us that it becomes mysterious again, like... how can the change in potential energy correspond to a change in kinetic energy and why not in thermal energy? Why is it when I throw something up in the air, it slows down, rather than grows cold? How does energy know what form to change into and when and how much?
    Mysterious again.

    I don't mind the game of putting energy into categories. It's a physics tradition, after all.
    I find it more useful to think of the 2 categories as energies that result from conservative fields and energies that result from non-conservative fields.
    Like, gravitational potential energy and spring potential energy are conservative fields. They are path independent - meaning that if you start at one position and move through any simple or complicated path to another position, the change in energy is always the same. The path doesn't matter.
    Contrast to energy lost due to friction. Friction happens over distance. If you take a longer path to get to the same destination, you probably lost more energy to thermal heating caused by friction. The path matters.
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  63. #2163
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    Quote Originally Posted by OngBonga View Post
    ^ hmm that's not quite right. I'm neglecting work, which is an important factor. Plus, we should know how much "potential heat" is stored in chemical bonds, yet it seems it's impossible to know how much enthalpy a system has.
    It's the same issue in electronics where it's impossible to know the voltage at any point in space (or on a circuit board). We don't know where 0 is. We can wave our hands and say the Earth (ground) is 0 V, but we can't prove it. We can only prove the difference in voltage between any 2 points.

    It's not because of anything special about electronics or voltage. It's a property of conservative potential energy fields - path independent fields. (I think the word conservative is implied by the word potential, even.)

    It's even in gravitational potential, mgh. h is a free parameter, relative to our coordinate system. We're free to place our coordinate system wherever we like, so it's not even clear whether or not anything *has* potential energy without saying with respect to what.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  64. #2164
    Yeah, like I say, energy is complicated as fuck.

    Why is it when I throw something up in the air, it slows down, rather than grows cold?
    I think this is something I maybe have an idea about. When we throw a ball up, we observe it going up, then stopping, then falling again. But from the ball's FoR, it sees the thrower moving away from it, then stop, then return. The ball didn't move from its FoR, only ours. This is what I was referring to when I said kinetic and potential energy are the same thing observed from different frames of reference. There wasn't actually a conversion of energy, we just think there was. The potential+kinetic energy remains constant, and they are both the same thing. When something loses heat, energy is transferred because the molecules have lost vibrational energy. The ball doesn't get cold (unless we throw it high enough lol) because there is no change in ambient temperature.

    The ball moves in a straight line at a constant velocity from the instant we let go of it (ignoring air friction, naturally). It moves along a geodesic, until it hits the ground. The ball is essentially in freefall. So it is not losing energy, there is no acceleration. It gained energy when we threw it, but it doesn't keep gaining (or losing) it until it stops moving along a geodesic. This is the same principle as the moon moving in a straight line at constant velocity, despite our observations that seem to contradict this idea. Straight lines appear curved to us, because we're in a gravity field.

    So the observed conversion from kinetic to potential energy is entirely frame of reference dependent. It's better to realise they are the same thing, like electricity and magnetism, which is also FoR dependent. And once we accept that kinetic and potential energy are the same thing, we come to realise there is no conversion.

    When something cools, it is transferring kinetic energy from one system to another. This is a different case to the observed conversion from kinetic to potential energy within a single system.

    As for the rest of your comments, I think I'll need to read that again to try and absorb it.
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  65. #2165
    Another way to understand why kinetic and potential energy are the same thing is to fire a bullet at someone's head. The final thoughts of our unfortunate friend will be "holy fuck, this bullet has a lot of kinetic energy". But let's assume that as soon as we fire the bullet, we run at the same speed as the bullet in the same direction. As we look at the bullet, we think it has no kinetic energy. What makes our observation wrong? Nothing at all. We observe potential energy in the bullet and kinetic energy in the soon-to-be-dead friend, because our FoR is differently to the poor soul about to get hit by the bullet. Both of our observations are equally as valid, and therefore there is no difference between kinetic and potential energy, just a difference in velocity, and therefore different frames of reference.
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  66. #2166
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    Quote Originally Posted by OngBonga View Post
    Heat is an excellent example of entropy. Heat is the flow of thermal energy, it is the spreading out of thermal energy. Thermal energy is just kinetic energy on the atomic scale. If you imagine two atoms colliding, one fast and the other slow, the faster on loses energy to the slower one. Now imagine trillions of atoms constantly colliding. The faster ones are gradually slowing down as they collide with slower atoms, and the slower atoms are getting faster, until we reach thermal equilibrium, all atoms moving at the same speed. At this point, heat (thermal flow) ceases to happen.

    I actually don't know what enthalpy is, but I note it's measured in joules, so I assume it's a form of heat.

    What's enthalpy, mojo?
    This is all neat n' stuff, but the moment the entropy/temperature as well as the enthalpy/temperature relation shits the bed is when you reach a phase change. When water reaches 100°C and you keep adding heat, the entropy keeps increasing, the enthalpy keeps increasing, but the temperature stays the same while a considerable amount of energy is being used up for whatever it is a liquid is doing when it turns into vapor.

    So temperature is only transferable kinetic energy of molecules.
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  67. #2167
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    Quote Originally Posted by OngBonga View Post
    Enthalpy is basically the potential heat stored in chemical bonds... right?
    What I like about enthalpy is that if you put energy into something then its enthalpy increases linearly regardless of its state and it's nearly identical to the energy put in.

    As I understand it, Entropy does change when you fuck around with stuff, but it's not linearly related to either temperature or entropy and it's so little energy that as a person who rounds to the nearest whole number on principle find very hard to care about, but I acknowledge it exists.
    It's energy lost and cannot be used for work. So you're losing energy to entropy even in an ideal closed system when you're manipulating gasses - heating up, compressing, expanding... you always lose to entropy, but it's very little compared to the overall usable energy of the system.

    I get confusing information about this, so I'll just leave it for MMM to correct me.
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  68. #2168
    So temperature is only transferable kinetic energy of molecules.
    I don't think this is right. Temperature is a measure of an atom's vibrational energy. Heat is the transfer of thermal energy, temperature is just how much the atom is jiggling. Absolute zero is impossible, and since the universe seems to be quantum, it's probable that every atom can only reach a certain lower temperature. Let's say that value is 0.1 kelvin for hydrogen. That 0.1 degrees kelvin is non-transferable, as the atom cannot lose any more thermal energy.

    What happens when water gets to 100 degrees is that it starts to boil, and this phase change takes energy. That is, it literally cools the water. This is why sweat has a cooling effect, because it evaporates and takes heat with it. So the added heat you are putting into the water is replacing the heat that the evaporation is taking out.

    I still don't understand enthalpy, so I'm not sure how this fits into the picture. Entropy increases though because heat is being lost to the atmosphere. If literally all the heat you apply to the water is absorbed by the water, then perhaps during boiling entropy is constant, idk. But this is impossible, you're always going to lose heat to the surroundings. You'll never have a perfect vacuum, there is the object that applies the heat, there will always be loss.
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  69. #2169
    MadMojoMonkey's Avatar
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    Just to go back a couple posts.

    Enthalpy is the internal energy of a gas plus the energy required to move the stuff its volume displaces.

    So, if you imagine a balloon of some gas, there's the internal energy of the gas existing above absolute 0, but also the energy required to push the atmosphere out of the way of the balloon.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  70. #2170
    Does that energy not come from the temperature of the gas? That is, isn't this why gas cools as it expands? The gas takes up more volume for the same mass, so has lower average thermal energy. Eventually the gas stops expanding because it can lose no more thermal energy to the surroundings.

    I also can't help but think of buoyancy when we talk about displacement. I guess that's a red herring, an object immersed in water in space would neither sink nor float. But then again the object still experiences a force from the surface tension of the water. Is there an analogy here?
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  71. #2171
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    The energy of the temperature of the gas is calculated separately from the energy to "create" a volume to exist in.

    Basically, if you had a volume filled with some environment, and you want to add something to the environment, you have to make room for the thing you want to add, AND you add the thing. If the thing has its own energy, that energy is distinct from the energy required to move the environment out of the way.

    Does the energy to displace the environment come from the temperature? Not necessarily. It could, or it could have come from another source. Energy is tricky like that.

    At any rate, wherever it came from, the amount needed to create a volume to occupy is distinct from the amount contained within the thing that occupies said volume.


    The tangent to buoyancy is hard to analyze because things still float in water in space, its just that gravity is a relatively weak force and the surface tension is much stronger by comparison, if we're talking about a less-than-planet-sized body of water.

    To bring it back, whether the energy's origin is in a surface tension force or a gravitational force, it holds the water together, and if you were to put something into that water, you have to displace some water to make room for it. The volume of the blob plus your addition is greater than the original blob. So it had to work *against* surface tension to increase the volume, which required energy to do.

    The energy needed to create a vacuum in the water is needed to make room for whatever you want to add. Then whatever you add will increase the energy further, as it exists above absolute zero in temperature, so has positive energy due to thermal motions.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.
  72. #2172
    I think I get it. The surface tension analogy is helpful. I guess you just have to imagine empty space having a surface tension that needs to be overcome for an object to occupy space. Surface tension might not be accurate, but it's fairly intuitive, as it provides a constant inward pressure to an immersed object. If the object cannot overcome this pressure (it requires energy to do so) then the object will collapse into a singularity (lol).
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  73. #2173
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    I'm pretty sure that displacement term would just be 0 J in empty space. Assuming there's no environment to be displaced, the displacement term would vanish.
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  74. #2174
    So it's not a vacuum pressure, it's purely mechanical?
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  75. #2175
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    Quote Originally Posted by OngBonga View Post
    So it's not a vacuum pressure, it's purely mechanical?
    It's nearly mechanical, if not mechanical, but that's just semantics.

    Naming things mechanical vs not mechanical is a relic of the history of how humans learned physics over centuries. Mechanical energy is the forms of energy known the longest. Non-mechanical energy was discovered later as the fields of chemistry and thermodynamics came to fruition. Energy is energy, and it can change forms to other energy, so the label of "mechanical" vs "non-mechanical" isn't really bringing any insight into the nature of energy, rather it brings insight to the history of humans understanding energy.

    ***
    No, it's not a vacuum pressure term. It's the integral of P dot dV, where P is the pressure at the boundary of the system and dV is the change in volume of the system. The boundary may be an imaginary surface that we invent to do math on. It need not be a physical surface. The changing volume is due to the boundary moving through space over time. The integral of P dot dV is equal to the energy needed to move that boundary.

    The moving of the boundary is mathematically accomplished by starting with an infinitesimal volume that you integrate the expansion of through some path. Since the integral of P dot dV arises from a conserved force, the integral is path independent, and any path you choose to get from infinitesimal to your final volume will give the same result as all other paths that accomplish the same.

    The result of the integral is the amount of energy it takes to create a vacuum the size and shape of whatever you are inserting into that system. If the system is already a vacuum, then it takes no energy to create a vacuum.

    ***
    I really want to say this is mechanical energy, but I'm asking another group that contains some chemists for their thoughts.
    You can find any pattern you want to any level of precision you want, if you're prepared to ignore enough data.

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