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How to measure absolute velocity

  1. Mar 31, 2008 #1
    you are in a plane flying east with a head wind of 10 kph. The speed of the plane is measured by measuring the wind speed outside the plane and then adjusting for any head wind. Lets say you get a result (after allowing for the head wind) of of 200 kph.

    Unbeknownst to you the arm of our galaxy called the milky way is heading west at 400 kph so really your speed is 200 kph backwards!

    but THE universe is heading east at 400000 kph so your speed is now ....... ummm ..... a lot AND YOUR EYES START TO BURN :)

    As a photon has a constant velocity in all frames of refernce and assuming technology advances to the point that the speed of a photon can be measured accurately. By measuring the speed of the plane relative to the speed of photons (measured simutaneously in all three dimensions) you should be able to derive the true speed of the plane regardless of the frame of ref? true or false :)

    You will need to measure the speed of the photons simutaneously in 3 dimesnsions (east, west,south,north,up,down) as it may turn out the plane could be moving in any direction.

    this would be similar to measuring the planes velocity realtive to the michelson morely experiment.

    photons are completely diconnected from space
  2. jcsd
  3. Mar 31, 2008 #2


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    I just wanted to point out that the speed of a photon is measured accurately, it is 299,792,458 m/s (in vacuum, by definition of meter). What you could do it emit a photon at the back of the plane and time how long it will take the photon to reach the front. This is a useful experiment, as:
    • someone inside the plane will be able to measure the length of the plane. (he cannot measure the velocity however, because the speed of the photon is the same at any speed. So whether the plane is stationary (OK, make it a helicopter) or moving at 99.999999% the speed of light, he will always measure the same time.
    • someone outside the plane will be able measure the velocity of the plane (with respect to the reference system in which he is himself at rest).

    The rest of your post doesn't really make sense to me because a) you assume there exists an inertial reference system with respect to which we can measure absolute velocities, while the whole point of relativity is to show that there is no such system and b) I have no idea what to think of when you say something like "the universe is heading east" (you mean we are moving through the universe in the direction that we call west? and if not, with respect to which reference point do you define "east"?).
  4. Mar 31, 2008 #3


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    False. A photon has the same constant velocity in every frame of reference--if I am an inertial observer and I see you fly by me at 0.99c, and then you shoot a photon out the front of your rocket, then I will measure the photon to be moving at 1c and thus I'll only see the distance between the rocket and the photon increasing at a rate of 0.01c (so every 100 seconds I measure the distance between the photon and the rocket to increase by 1 light-second), but in the rocket's own rest frame the photon will also be measured to move at 1c, so you'll measure the distance between the rocket and the photon increasing at a rate of 1c (so every 100 seconds you measure the distance between the photon and the rocket to increase by 100 light-seconds). This difference of opinion about the rate the photon is getting ahead of the rocket is a consequence of the fact that each of us uses rulers and clocks which are at rest in our own frame (with the clocks also synchronized in our own frame) to measure distance and time and get speed, but each of us also sees the other guy's rulers to be shrunk due to length contraction, and the other guy's clocks to be ticking slow due to time dilation, with the clocks also being out-of-sync due to the relativity of simultaneity.
  5. Apr 1, 2008 #4
    the point of ref for east is east as it exists on earth so point east and that is the direction the entire universe is heading, hypothetically of course.

    Its astounding that the speed of a photon,measured accurately, is an integer! are you sure you dont want to throw a decimal in there at least somewhere ?

    I beleive nothing is measured accurately it is only measured relative to something else as all measurements take place inside a moving frame of ref and no one knows how fast, what direction or what acceleration that moving frame of ref is experiencing, so much for accuracy.
  6. Apr 1, 2008 #5


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    The symbol "c" represents the speed of light, so by definition the speed of light is exactly 1c. But the speed of light is also an integer if you represent it in meters/second, namely 299792458 meters/second, because a meter has now been defined as the distance light travels in precisely 1/299792458 seconds (obviously the meter was defined differently in the past).
    But why do you assume there is any objective truth about whether a frame is "moving" or "not moving"? If the laws of physics don't distinguish between frames, then it's plausible that there is no truth about which of two objects is "moving" and which is "at rest" in any absolute sense (relativity is usually seen as removing the need to imagine absolute time and space).
  7. Apr 1, 2008 #6
    so a meter in our galaxy isnt the same as a meter in a different galaxy as the other galaxy is in a different frame of reference and as such time will pass at a different rate ?

    Galaxy one phones galaxy two to order a meter of cloth but when it is delivered there is a wardrobe malfunction now thats pretty serious!
  8. Apr 1, 2008 #7
    theroretically a meter will also vary depending on where on the planet you do the measurement. Lets say for arguements sake there is an absolute frame of ref and lets say it is represented by a single vector which points west on 1 April 2008 at 1.26 pm wst australia.

    now we have one hundred sicentists measuring a meter using your technique at 100 different locations all over the planet. So the earth is travelling around the sun, it is spinning on its axis etc etc. Most of them would get a diffreernt answer as all thier clocks would be travelling at different speeds and thus different rates of time passing.

    Of cousre some of the scientists due to a coincidence of their location would be travelling
    in the same direction at the same speed and so time would pass at the same rate for them.

    who knows how long a meter is
  9. Apr 1, 2008 #8
    the rate of time is a function of velocity

    man 1 measures a meter in his car travelling east at 100 kph
    man 2 measures his meter in a car travelling west at 100 kph
    the absolute frame of ref is travelling east at 100 kph
    so man 1 is actually travelling at 200 kph, but he doesnt know it and man 2 is in fact stationary but he doesnt know it either. What is the rate of time for each?
  10. Apr 1, 2008 #9
    Let's say for the sake of argument the absolute reference frame is exactly where you say it is. Then scientists in other parts of the world, or in jets or wherever, will have their meter sticks length contracted and their clocks slowed down in such a way, that they measure the speed of light to be exactly c wherever they are and however they are moving. Any one of them could claim to be in an absolute reference frame, and you could not tell anyone of them from any other.
  11. Apr 1, 2008 #10
    I find that the more I think about things like absolute position, velocity or time, the less comfortable I become with them. It is simply too much of a leap for us, with only our poor experience of local position, velocity and time, to make sweeping declarations about the proper nature of the entire universe.

    The topology of the universe is not subject to our demands.
  12. Apr 1, 2008 #11


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    The only meaningful measure of velocity is relative velocity between observers. The only time you will see time dilation is when you measure the clock rate in another frame, moving relative to you. Your own clocks ( and your rate of ageing) will always appear the same to you in your frame.

    There is no absolute motion, and no absolute velocity.
  13. Apr 1, 2008 #12
    In general relativity there is no such thing as an absolute velocity, absolute distance or absolute time in the existing universe.
    Last edited: Apr 1, 2008
  14. Apr 1, 2008 #13


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    Man 1's speed relative to this "absolute frame" is only approximately 200kph, even if Man 1's velocity (relative to the Earth) is exactly 100 kph due east, and the velocity of this "absolute frame" (also relative to Earth) is also exactly 100 kph, but due west.

    You can not simply add or subtract the speeds to find relative velocities - that's wrong. And that's the same mistake you are making when you try to do the experiment with light, where the error will be bigger.

    If you wish to understand where you are wrong, please show that this is your intent. If you wish instead, to show everyone where Einstein is wrong, you will find yourself in trouble very soon.
  15. Apr 1, 2008 #14


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    Well stated! IMO, that is the single biggest insight of modern physics.
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