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B Why do we need the concept of "event" in SR

  1. Jun 15, 2017 #1
    Event is anything which can be located by (x,y,z,t).
    We don't need this concept in Galilean transformation.
    I was first introduced to the concept in the beginning of Special relativity.
    I haven't understood the significance of this concept till now.
    Can anyone please illustrate me using an example that if we had not defined the concept "event", where we would face difficulty in our studies?
  2. jcsd
  3. Jun 15, 2017 #2
    I'm not sure why do you say that we don't need the concept of Event in Galilean transformations. An event is just a point in space-time, a point with specific space and time coordinates (without the concept of Event, there wouldn't even be Galilean transformations). With Galilean transformations we can be sure of absolute simultaneity of events, so we don't need to know the space coordinates of some event in some inertial FoF to know when it will happen in another inertial FoF. This is not so in SR.
    Frankly, I don't even know how we could start studying SR without the concept of Event (at least, implicit).
  4. Jun 15, 2017 #3


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    Sure it existed before, it was just not emphasized so much. The Galilean transform involves events - to get coordinates in the new frame you need coordinates and a time for the old. If we want to specify a historic event, we specify location and time. The new thing about the Lorentz transform is that different frames attach different times to the same event, making it somewhat more important to think in terms of the event as a physical happening to distinguish it from different ways of labeling it in different frames.
  5. Jun 15, 2017 #4
    This is what I,too ,wanted to say.
    Thanks for it.
  6. Jun 16, 2017 #5

    Mister T

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    The notion of an event is essential to the understanding of motion, regardless of whether the theory is Einstein's, Newton's, or Galileo's.

    Introductory physics usually starts with the study of motion, and very soon equations like $$v=v_o+at$$ are introduced. To understand them the notion of events is essential, for ##v## is the instantaneous velocity, that is, the velocity at a particular instant, and it's important to understand that that instant has a time duration of zero. For example, tossing a ball upward, it's speed reaches zero at the apex of its trajectory, but its acceleration is not zero at that instant. This is an example of an event. The ball is there, but it spends no time there!

    There is nothing special about this particular event. At any point in the ball's trajectory it occupies a specific position at a specific time, but does so for no time at all.

    One of Zeno's paradoxes was to imagine an arrow flying through the air. At any point in its trajectory it occupies a specific position, so it must spend some time at that position. While it's spending time there it's not moving. Since this is true of every location along the arrow's path, the arrow can't be in motion anywhere!

    The resolution of this paradox came with the invention of calculus, and one of its central tenets was that a particle has a specific position at a specific clock-reading, but for no time. This is the notion of an event.
  7. Jun 20, 2017 #6
    4D spacetime speaking the arrow is not in motion. Because the arrow is not a 3D entity that moves, but a collection of co-existing 3D arrow events forming a 4D entity that simply is there, part of 4D spacetime. Motion only occurs when an observer introduces a specific reference frame, reading the successive (but co-existing, 4D speaking) arrow events. Relative moving 'observers' and their reference frames read a different event of the full 4D arrow existence. Remember Einstein's quotes about 4D existence...
  8. Jun 20, 2017 #7
    Brilliant analysis, Ebeb. That makes it so simple and so clear. Just one question: If the observer reads successive arrow events, and if the observer himself is a 4-D object, then how do the successive observations come about? In what sense is the observer moving in order to make those observations?
  9. Jun 22, 2017 #8
    Brilliant question, Tophat ;-)
    Your question is about consciousness. We don't know (yet) how the relationschip consciousness/time experience etc works. We will need another Einstein genius to sort that out ...

    But not only time is a mystery. We don't even know whether the concept of a 3D space (i.e. of simultaneous events) 'really' exists... Depends what your paradigm for 'reality' is... But I guess this takes us too far off topic ...
    Last edited: Jun 22, 2017
  10. Jun 22, 2017 #9
    I think we reached the end of the rope here. Let's don't stray from the physics.
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