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No, it doesn't force one to redefine relative velocity - it's covered in the theory of Special Relativity.
I grow tired of this circular reasoning.
"Exit, stage left even."
That's what the special theory of relativity does! This all stems from your misunderstanding my simple statement about the principle of relativity.
The principle requires physical laws to be the same for any body moving at constant velocity as they are for a body at rest. A consequence is that an observer in an inertial reference frame cannot determine an absolute speed or direction of travel in space, and may only speak of speed or direction relative to some other object.
Quote:
Originally Posted by jayrandom
You're right. Velocity only makes sense in reference to something else. There's no experiment you can do that will give you an absolute velocity. Without any reference one typically takes the velocity to be zero (it makes all the other problems easier), but you could assign yourself any velocity and the physics would all still work out.
Quote:
Originally Posted by Dirt Grinder
Measuring the speed of unmodified light through a vacuum.
Special relativity forces one to redifine relative velocity from a Galilean transformation to a Lorentz one. Before special relativity you would add relative velocities and now you don't (although it's usually a great approximation). I'm simply pointing out that special relativity isn't obvious (it wasn't discovered until the early 20th century). It's based on experimental measurements and isn't intuitive to people used to velocities much less than the speed of light. It wasn't intuitive to me or anyone up to and including Einstein, at least.
And I don't think you understand what circular reasoning is. This is just the historical progression.
Speed of light is always measured to be the same value, regardless of velocity of observer -> special relativity -> relative velocity law can't be Galilean, it must be Lorentzian.
In physics, a Galilean transformation is used to transform between the coordinates of two reference frames which differ only by constant relative motion within the constructs of Newtonian physics. This is the passive transformation point of view. The equations below, although apparently obvious, are untenable at speeds that approach the speed of light. In special relativity the Galilean transformations are replaced by Lorentz transformations; conversely, the c→∞ classical limit of Lorentz transformations yields Galilean transformations.
In physics, the Lorentz transformation (or transformations) is named after the Dutch physicist Hendrik Lorentz. It was the result of attempts by Lorentz and others to explain how the speed of light was observed to be independent of the reference frame, and to understand the symmetries of the laws of electromagnetism. The Lorentz transformation is in accordance with special relativity, but was derived before special relativity.
If I was alone in empty space and started accelerating (due to some undefined force) then what stops me from accelerating?
But, how would I know I was at the speed of light if I had no reference point in empty space?
Space is not truly empty, there are some hydrogen atoms per cubic meter. You would be moving relative to something regardless of selecting a reference.
What I don’t understand is what prevents us from traveling faster than the speed of light? If I was alone in empty space and started accelerating (due to some undefined force) then what stops me from accelerating? Supposedly, my mass would increase to the point that it would take infinite force to accelerate any more as I approached the speed of light. But, how would I know I was at the speed of light if I had no reference point in empty space? Speed only makes sense in reference to something. Doesn’t that imply there’s something out there that we don’t know about, some medium that establishes the reference and hence the limitation on velocity?
In reality we can travel faster than the speed of light, It would take infinite energy. But the universe is already traveling faster than light. With the aid of dark energy, and dark matter.
In reality we can travel faster than the speed of light, It would take infinite energy. But the universe is already traveling faster than light. With the aid of dark energy, and dark matter.
There's a little mistake on your sentence. There is no infinite amount of energy available on universe. All that exists within it has a finite total of energy, which is much less than required to do so, making it impossible to be true.
Also, your second point holds a misleading concept. We cannot measure expansion of universe only with velocity (which is change of position in space / time )because universe pretty much is space itself. When we say universe is expanding at ever-increasing speed, you have to think of time-space as a mere ingredient of a oven-rising bread.
Last edited by ricardobrazil; 07-05-2015 at 04:13 PM..
Basically the speed of universal expansion is not trivially related to local velocity.
The best explanation I have heard is very simple to understand. Suppose you are standing between two cars traveling in opposite directions with each car traveling at 51% of the speed of light away from you. Those cars are traveling away from each other at 102% the speed of light.
What's even more interesting is becsue the Universe is expanding in all directions equally the farther away the faster they are traveling. Let's suppose you have 5 balls on a bungy cord spaced evenly to represent galaxies. Expand that cord out to 100 feet in one second. From the middle ball to the first ball they traveled away from each other 25ft/sec. From the middle ball to one on the end it's 50ft./sec, the balls on the ends traveled away from each other at 100ft./sec.
The best explanation I have heard is very simple to understand. Suppose you are standing between two cars traveling in opposite directions with each car traveling at 51% of the speed of light away from you. Those cars are traveling away from each other at 102% the speed of light...
But that's relative velocity, neither is exceeding the speed of light. The expanding universe is evident in the redshift of galaxies, an indication that they are moving away from us.
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