Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Today I learned

  1. Jul 24, 2016 #1621

    DrGreg

    User Avatar
    Science Advisor
    Gold Member

  2. Jul 24, 2016 #1622

    jim hardy

    User Avatar
    Science Advisor
    Gold Member


    I'm still kinda stunned by that thought.
    Are photons affected by gravity?
    If so, do they arrive here with a wavelength different than they started out with ?

    Have a laugh at that question i don't mind
    i'm still struggling with modern physics
    as Leonard Cohen says, 'waiting for the miracle to come.... '


    old jim

    ps i kept my Whitworth wrenches .
     
  3. Jul 24, 2016 #1623

    OCR

    User Avatar

    Yeah, it is an amazing story...[COLOR=#black].[/COLOR]

    Here is the video of The Gimli Glider incident ... Air Canada Flight 143 .

    Fuel Pressure.jpg
     
    Last edited: Jul 24, 2016
  4. Jul 24, 2016 #1624

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    TIL Russia is doing simulated nuclear attacks on Sweden, and has apparently been violating Swedish airspace. Sweden is a neutral country.
    http://nationalinterest.org/blog/th...raft-killer-missile-now-service-its-not-17100

     
  5. Jul 25, 2016 #1625
    Yep, you're right. My mistake. I'm off by 000 :smile:.
     
  6. Jul 25, 2016 #1626
    Talking about Brexit...
    http://www.dailymail.co.uk/news/art...ridlock-Indonesian-junction-named-BREXIT.html
    I live in Indonesia, Brebes is some 200 km east of me. Every year in Idul Fitri celebration, millions people from Jakarta are coming to their hometown. And most of them pass the north route. The jam in Brebes Exit Tol is so severe. Cars can get stuck for days in 40 km long traffic jam.
    Brebes.jpg
     
  7. Jul 25, 2016 #1627

    Ibix

    User Avatar
    Science Advisor

    There isn't any interesting physics lurking in this one - it's just a change of units. A light year is a distance and a year is a time, and a distance divided by a time divided by a time is a unit of acceleration. However this particular choice of units is very handy for thinking about relativistic rockets for a number of reasons. A light year is an appropriate scale for interstellar distances; a year is an appropriate time scale, and useful for "how old will I be when I get there"; the speed of light is exactly 1 by definition so all the annoying factors of c in relativistic equations drop out; the acceleration due to gravity at Earth's surface is very close to 1, and most people want to accelerate their rockets at 1g so it feels like home inside.

    This is unrelated to the above. But yes, light is most certainly affected by gravity. Sometimes astronomers see multiple images of a galaxy because a nearer galaxy pulls the light out of a straight line path. https://en.m.wikipedia.org/wiki/Gravitational_lens. Also, black holes wouldn't be black if their gravity couldn't trap light.

    Yes, light can arrive with a different wavelength compared to when it was emitted if the emitter and receiver are at different heights (more formally, different gravitational potentials). Pound and Rebka were the first to verify this experimentally. https://en.m.wikipedia.org/wiki/Pound%25E2%2580%2593Rebka_experiment [Broken]

    I'm drifting pretty far off topic.
     
    Last edited by a moderator: May 8, 2017
  8. Jul 25, 2016 #1628
    It stuns me, too. Even the units match. Gravity = Distance/time square.
    Light year per year square = distance per time square.
    [Edit: the unit and the number matches]
     
    Last edited: Jul 25, 2016
  9. Jul 25, 2016 #1629

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Thanks ibix ! I saw the convenience of the units ,

    to make things so simple is the mark of genius.

    The pound rebka link returns a "bad title " error,
    is this it ?
    https://en.m.wikipedia.org/wiki/Pound–Rebka_experiment

    Fascinating, thank you,
    my old mentor said that would happen , but we were not aware of the experiment.

    I think your post is right on topic for "today i learned"
     
  10. Jul 25, 2016 #1630
    Far off?? I'd say deep into.
     
  11. Jul 25, 2016 #1631

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    Yes. That's why black holes are black.

    I'm sure you really knew that. :oldbiggrin:
     
  12. Jul 25, 2016 #1632

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    yes,
    you know how something rattles around in the back of your mind and you can't even phrase the question ?
    I'm still trying to weigh that with respect to Hubble's Red Shift

    plodder that i am a lot of things just never have clicked together.

    old jim
     
  13. Jul 25, 2016 #1633
    Hubble red shift??
    Don't tell me.
    Today I learned that constant can change??
     
  14. Jul 25, 2016 #1634

    jim hardy

    User Avatar
    Science Advisor
    Gold Member


    Tell ?? i'm only in a position to ask......
     
  15. Jul 25, 2016 #1635

    fresh_42

    User Avatar
    2017 Award

    Staff: Mentor

    Sokrates?
     
  16. Jul 25, 2016 #1636
    No, Jim. It's just that you guys are discussing about Hubble, and just several hours ago I received that reply, that can't help my hand to comment. :smile: Frankly, I didn't read all your and Mr. epenguin's comment. It's just that red shift comes up and I recall the answer I had this afternoon (my time). :smile:
     
  17. Jul 25, 2016 #1637

    NascentOxygen

    User Avatar

    Staff: Mentor

    Browsers don't like that URL.
     
    Last edited by a moderator: May 8, 2017
  18. Jul 25, 2016 #1638

    mfb

    User Avatar
    2017 Award

    Staff: Mentor

    Hubble red shift probably means "the redshift due to the Hubble constant".

    But the Hubble constant is actually changing, it decreases slowly. It is constant in terms of the distance of objects (if they are not too far away), but not constant in time.
     
  19. Jul 25, 2016 #1639

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    TIL that a third of claims for lightning strikes on boats in Florida!
    http://www.boatus.com/seaworthy/swlightning.asp
     
  20. Jul 26, 2016 #1640

    Cdz

    User Avatar

    Today I learned that I registered on PF more than one year ago!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Today I learned
  1. I graduated today! (Replies: 22)

  2. I was hit by a car today (Replies: 40)

  3. I had a colonoscopy today (Replies: 104)

  4. Today I Accomplished (Replies: 20)

Loading...