Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 25

Thread: Cosmic Speed Limit

  1. Top | #11
    Veteran Member
    Join Date
    Sep 2004
    Location
    California
    Posts
    3,716
    Archived
    4,797
    Total Posts
    8,513
    Rep Power
    60
    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by steve_bank View Post
    I had electromagnetics and I understand Maxwell’s Equations.

    Given C is not relative across all frames and motion is..

    A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.
    A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.
    Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

    CMBR dipole anisotropy

    "From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."

  2. Top | #12
    Contributor skepticalbip's Avatar
    Join Date
    Apr 2004
    Location
    Searching for reality along the long and winding road
    Posts
    5,156
    Archived
    12,976
    Total Posts
    18,132
    Rep Power
    64
    ^^^
    Indeed, that sorta works if we only make the assumption of a universally uniform temperature as a proxy for Newton's universal inertial reference frame. The near uniformity does seem to be real with only a bit of granularity.

  3. Top | #13
    Contributor
    Join Date
    Nov 2017
    Location
    seattle
    Posts
    5,053
    Rep Power
    13
    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by steve_bank View Post
    I had electromagnetics and I understand Maxwell’s Equations.

    Given C is not relative across all frames and motion is..

    A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.
    A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.

    A spaceship in orbit around Earth and deaprts out into space. On board I can measure acceeration and calculate change in velocity. I turn off the engine and the ship is traveling at a constant speed.

    I have a number for velocity but it is relative to the starting point. I get the same change in velocity regradless of the velocity of starting point.

    From relativity there is no absolute frame, so it is impossible to know an absolute velocity of a starting point. Given that, on the ship how do I know what percent of C the ship is traveling at?
    There is no such thing as absolute velocity in relativity. That is a Newtonian concept. Newtonian imagined a fixed universal reference frame. Relativity assumes that measurements in all reference frames are equally valid regardless of their relative motions with respect to each other. In relativity, the second, meter, kilogram, etc. measured in a reference frame in motion with respect to the observer are all dependent on the relative velocity between those two reference frames.
    A Mach meter is relative to the ground. The aircraft has a velocity km/hour. Velocity is in an inertial frame. The Erath is moving around the sun and sun around the galaxy. I do not see how an avsolute velocity can be assigned.

    The only solution I can see is that C as a limit applies only within an inertial frame. But that does not make sense either. Just thinking out loud.

  4. Top | #14
    Contributor
    Join Date
    Nov 2017
    Location
    seattle
    Posts
    5,053
    Rep Power
    13
    Quote Originally Posted by Bomb#20 View Post
    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by steve_bank View Post
    I had electromagnetics and I understand Maxwell’s Equations.

    Given C is not relative across all frames and motion is..

    A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.
    A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.
    Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

    CMBR dipole anisotropy

    "From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."
    But relativity says there can be no absolute frame.

    This out of my depth, how can the CMBR be eater moving or stationary in terms of position.

  5. Top | #15
    the baby-eater
    Join Date
    May 2011
    Location
    Straya
    Posts
    3,794
    Archived
    1,750
    Total Posts
    5,544
    Rep Power
    37
    Quote Originally Posted by steve_bank View Post
    Quote Originally Posted by Bomb#20 View Post
    Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

    CMBR dipole anisotropy

    "From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."
    But relativity says there can be no absolute frame.

    This out of my depth, how can the CMBR be eater moving or stationary in terms of position.
    It's not an absolute frame. It's just a useful reference frame for measuring the speeds of things at intergalactic scales.

    When you look at the CMB, you're looking at light that's been travelling for over 13 billion years from points in space that are moving away from us very quickly.

    Those distant points in space are moving away from us faster on one side than the other. Add all of those velocities up and you get a rest frame that differs from Earth's rest frame.

  6. Top | #16
    Super Moderator
    Join Date
    Sep 2000
    Location
    Nevada
    Posts
    25,122
    Archived
    96,752
    Total Posts
    121,874
    Rep Power
    97
    I must nitpick: Mr. One Mug effectively says you can't accelerate anything to superluminal velocity. That does not, however, preclude the existence of particles whose only existence is at superluminal velocity. Useless for travel, not useless for communications.

    After all, the same equation says you can't accelerate anything to lightspeed, yet we talk by radio all the time.

    (Now, the paradoxes are another matter.)

  7. Top | #17
    Contributor skepticalbip's Avatar
    Join Date
    Apr 2004
    Location
    Searching for reality along the long and winding road
    Posts
    5,156
    Archived
    12,976
    Total Posts
    18,132
    Rep Power
    64
    Quote Originally Posted by Loren Pechtel View Post
    I must nitpick: Mr. One Mug effectively says you can't accelerate anything to superluminal velocity. That does not, however, preclude the existence of particles whose only existence is at superluminal velocity. Useless for travel, not useless for communications.

    After all, the same equation says you can't accelerate anything to lightspeed, yet we talk by radio all the time.

    (Now, the paradoxes are another matter.)
    Not exactly. It is anything with rest mass (fermions) that can't be accelerated to lightspeed. We talk by radio using photons (EM radiation) which have no rest mass and can only exist at c... at least according to uncle Al and cousin Max.

    ETA:
    Now if you come up with a way of using tachyons (which may or may not exist) for communication it would earn you Nobel prizes in a couple different fields.
    Last edited by skepticalbip; 06-22-2019 at 12:03 AM.

  8. Top | #18
    Fair dinkum thinkum bilby's Avatar
    Join Date
    Mar 2007
    Location
    The Sunshine State: The one with Crocs, not Gators
    Posts
    21,726
    Archived
    10,477
    Total Posts
    32,203
    Rep Power
    82
    Quote Originally Posted by skepticalbip View Post
    ^^^
    Indeed, that sorta works if we only make the assumption of a universally uniform temperature as a proxy for Newton's universal inertial reference frame. The near uniformity does seem to be real with only a bit of granularity.
    But the CMB is nevertheless an arbitrary choice of reference frame. Why should we give it special status, while we don't give the same status to my personal reference frame? I am, after all, according to my observations, at the centre of the universe. I am the standard of normality against which everything is measured, and the fact that the CMB is moving at 368km.s-1 is only of passing interest to me.

  9. Top | #19
    Fair dinkum thinkum bilby's Avatar
    Join Date
    Mar 2007
    Location
    The Sunshine State: The one with Crocs, not Gators
    Posts
    21,726
    Archived
    10,477
    Total Posts
    32,203
    Rep Power
    82
    Quote Originally Posted by steve_bank View Post
    Quote Originally Posted by skepticalbip View Post
    A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.


    There is no such thing as absolute velocity in relativity. That is a Newtonian concept. Newtonian imagined a fixed universal reference frame. Relativity assumes that measurements in all reference frames are equally valid regardless of their relative motions with respect to each other. In relativity, the second, meter, kilogram, etc. measured in a reference frame in motion with respect to the observer are all dependent on the relative velocity between those two reference frames.
    A Mach meter is relative to the ground. The aircraft has a velocity km/hour. Velocity is in an inertial frame. The Erath is moving around the sun and sun around the galaxy. I do not see how an avsolute velocity can be assigned.

    The only solution I can see is that C as a limit applies only within an inertial frame. But that does not make sense either. Just thinking out loud.
    c as a limit applies equally in EVERY inertial frame. That's the great and extraordinary finding that made Einstein famous.

    Regardless of how you are moving, c is always a constant. No frames are preferred. It's bizarre; But it's the only way of thinking about reality that works.

    Oh, and by the way, a Mach meter is relative to the air, not to the ground.

  10. Top | #20
    Contributor skepticalbip's Avatar
    Join Date
    Apr 2004
    Location
    Searching for reality along the long and winding road
    Posts
    5,156
    Archived
    12,976
    Total Posts
    18,132
    Rep Power
    64
    Quote Originally Posted by bilby View Post

    c as a limit applies equally in EVERY inertial frame. That's the great and extraordinary finding that made Einstein famous.
    Uncle Albert's unique step here is that he dared to accept the result of the many, many measurements that showed c was constant for any observer. No other scientist at the time dared to risk their reputation by contradicting Newton... that is if they even considered that Newton could have been wrong. The common interpretation was that either the measurements or set ups were wrong.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •