Re: What is a photon

Am Samstag000014, 14.06.2025 um 11:33 schrieb Paul.B.Andersen:

Den 13.06.2025 09:55, skrev Thomas Heger:

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Velocity is meant as change of distance per unit of time.
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Since 'distance' is meant as a length of a vector between two points, we need a point 'at rest' in the first place, if we like to measure velocity.
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Since we cannot use empty space itself as reference, we need to define, what we mean by 'at rest'.
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This is actually a little difficult, because 'the universe' or 'the background stars' do not provide any 'natural' anchor for position vectors.
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So, we need to take what we have and that is actually the observer himself, because observers do not move in respect to themselves.
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But: all inertial observers are of equal rights!
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That's why we could use any observer, even hypothetical observers.
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Now this would include, that we measure velocity of something from one point, which rests in respect to the object in question and from another point, which moves with velocity v.
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This would mean: that object is at rest and moving with velocity c at the same time!
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 When I drive in my car the speed of the car in front of me
relative to me is v(t) = dl(t)/dt where l(t) is the distance
between our cars. Right?
 Does that mean that I am at rest and moving with velocity c
at the same time?
 

When driving your car you usually don't use other cars as reference, but (most likely) the surface of planet Earth.
But that surface does in fact move (actually very fast) and you simply ignore that movement.
You could, however, consider also that movement, which is caused by rotation of Earth around its own axis.
But that axis moves, too, but around the sun. And also the sun moves around the center of our home galaxy.
And even that moves.
Therefore your odometer needs to show a simplified version of your velocity and ignores cosmology entirely.
Relative velocities in respect to other cars is mostly not very useful, but is used in certain cases, especially when collisions occur or at car races.
TH