Re: What is a photon

Am Sonntag000015, 15.06.2025 um 14:56 schrieb Paul.B.Andersen:

Den 15.06.2025 10:35, skrev Thomas Heger:

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?
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Does that mean that I am at rest and moving with velocity c
at the same time?
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When driving your car you usually don't use other cars as reference, but (most likely) the surface of planet Earth.

 If you are driving a car and want to measure the speed of the car
in front of you relative to you, your car is obviously the reference.

Sure, but usually you don't want to do that.
In almost all cases you are interesterested in your own velocity and not in the velocity of other cars.
Police would see this through other eyes and is usually interested more in your velocity than in their own.
But commpon people usually don't need to measure other cars velocity.
If you use radar systems and automatic driving, you certainly would want to adjust your speed to that of cars in front. But usual cars still don't have that kind of automation oboard.
...
TH