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Nick666
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What is the minimum speed in the universe ?
How can everyone agree that the value of the speed of light is relative to them and not agree what is zero speed or what it is relative to? I think it would be clearer if you said the speed of light is defined to be the same value in any inertial reference frame but the speed of any massive object can be different in different inertial reference frames.Ibix said:Zero. But not everyone will agree what is zero speed.
The point about the speed of light is that it is invariant. Everyone agrees its value relative to them, so you don't need to specify relative to what. That's not true for any other speed.
HallsofIvy said:the "minimum speed" possible is obviously 0.
As I said in post #5, an object at rest in an inertial reference frame has a speed of zero in that reference frame. Having a speed of zero according to an inertial reference frame and being at rest in that reference frame mean the same thing. But just because it has zero speed in one inertial reference frame doesn't mean or imply that it is at absolute rest because we can transform to another reference frame moving at some speed with respect to the first reference frame and then the object has that same speed in the new reference frame. No reference frame is any better than any other. That's what relativity says.rede96 said:Relatively speaking, is it really possible for something to have zero speed? Wouldn't that imply that if an object has zero speed it can not change position, which would mean it is at absolute rest?
ghwellsjr said:As I said in post #5, an object at rest in an inertial reference frame has a speed of zero in that reference frame. Having a speed of zero according to an inertial reference frame and being at rest in that reference frame mean the same thing. But just because it has zero speed in one inertial reference frame doesn't mean or imply that it is at absolute rest because we can transform to another reference frame moving at some speed with respect to the first reference frame and then the object has that same speed in the new reference frame. No reference frame is any better than any other. That's what relativity says.
If it's not possible to talk about absolute rest or speeds, then why are you thinking in those terms?rede96 said:Yes of course, no argument there. But I was thinking of the OP's question in 'absolute' terms, if you will. As it is not possible to say an object can be at absolute rest, then by the same logic it can not be possible to say an object has an absolute speed of zero, or any absolute speed for that matter.
All speeds require an inertial reference frame according to Special Relativity, not just a minimum speed.rede96 said:So the minimum speed 'in the universe' can not be determined. Only the minimum speed in an inertial reference frame.
ghwellsjr said:If it's not possible to talk about absolute rest or speeds, then why are you thinking in those terms?
Yes hence why I saidghwellsjr said:All speeds require an inertial reference frame according to Special Relativity, not just a minimum speed.
rede96 said:or any absolute speed for that matter.
I don't know how I feel about that word, defined. We defined the speed of light to be the same? I thought we just noticed that it was.ghwellsjr said:I think it would be clearer if you said the speed of light is defined to be the same value in any inertial reference frame but the speed of any massive object can be different in different inertial reference frames.
A light pulse will pass me and you at c by our own measurements whether we are at rest with respect to each other or not. However an object traveling at any other speed (possibly stationary by my measurements) will not in general have the same speed by yours.ghwellsjr said:How can everyone agree that the value of the speed of light is relative to them and not agree what is zero speed or what it is relative to? I think it would be clearer if you said the speed of light is defined to be the same value in any inertial reference frame but the speed of any massive object can be different in different inertial reference frames.
BiGyElLoWhAt said:I don't know how I feel about that word, defined. We defined the speed of light to be the same? I thought we just noticed that it was.
Why ? Doesn't the standard method use light because it is already known experimentally to travel at the invariant speed?Cruz Martinez said:Well, you need a synchronization convention to measure one-way speeds, and the standard synchronization method uses the one-way speed of light, this speed is necessarily a defInition.
Nick666 said:What is the minimum speed in the universe ?
But the mere definition of velocity is distance traveled/time employed, how would you now the time employed without sychronizing? This time employed is a difference between the readings of two distant clocks.wabbit said:Why ? Doesn't the standard method use light because it is already known experimentally to travel at the invariant speed?
But actually I don't see where you need synchronization to measure a velocity. One observer can do that alone, he doesn't need to talk to anyone else. And he can also compare relative (to him) velocity of waves emitting by two other sources without ever synchronizing with those sources - and see whether that velocity is independent of the motion of those sources relative to him.
wabbit said:But actually I don't see where you need synchronization to measure a velocity.
Sure you can see the wave passing without the need of synchronization, but seeing the wave passing is not a measurement of velocity.wabbit said:I don't think so, you are measuring the wave as it passes near you, using your own clock in your own lab.
Or for a moving object you can use radar ranging, again involving only your own clock.
I can't think of a velocity measurement done using a distant clock(*). The velocity of an object relative to me does not depend on that object having a clock of its own.
(*) well yes redshift of light emitted by that object does that, but in GR this measures a combination of velocity and gravity - in SR it does measure velocity but you can also use reflection (radar) redshift, which does not depend on the object internal clock I think. The measurements you do are defined by your (local) apparatus, not by someone else's.
I still don't like that word. You mean define c=c? Or less arbitrarily, c =2.99...x10^8m/s? I don't see how that is a definition, and not just an observation. When I think of a definition, I think of ##\Phi=B\cdot A##, the definition of magnetic flux. We made that quantity up, it did not exist before someone defined it. c, however existed always. The speed of light in a vacuum has always been the same, even before we took our first measurements of it.Cruz Martinez said:On the other hand, radar involves the two-way speed of light, so you have to define its one-way speed anyways.
We made up the quantity "speed" as well, that's not the problem.BiGyElLoWhAt said:I still don't like that word. You mean define c=c? Or less arbitrarily, c =2.99...x10^8m/s? I don't see how that is a definition, and not just an observation. When I think of a definition, I think of ##\Phi=B\cdot A##, the definition of magnetic flux. We made that quantity up, it did not exist before someone defined it. c, however existed always. The speed of light in a vacuum has always been the same, even before we took our first measurements of it.
Oh OK I missed the significance of that "one way" mention. For that you need a choice of coordinates (which implies a choice of simultaneity), I understand - but still your own clock is enough : you can measure the two events with your local equipment, and for instance assign them radar coordinates, and from this, differences in time and a distance.Nugatory said:You need synchronization to measure a one-way velocity. You calculate the one-way speed by dividing the distance traveled by the difference between the start and end times - and you need a synchronization convention to determine at least one of those time
Yes, the two way speed of light is observed, but the one-way speed is defined necessarily, it can't be other way. With this I mean the actual value has to be defined.BiGyElLoWhAt said:The quantity, not the value.
BiGyElLoWhAt said:I don't know how I feel about that word, defined. We defined the speed of light to be the same? I thought we just noticed that it was.
Yes, you can use just your own clock and radar to establish the time and distance to remote events but during that process, you must apply Einstein's second postulate that each radar signal takes the same amount of time to reach its target as it takes for the return echo to get back to you. This is where you define what simultaneity means between a local clock and a remote event.wabbit said:Oh OK I missed the significance of that "one way" mention. For that you need a choice of coordinates (which implies a choice of simultaneity), I understand - but still your own clock is enough : you can measure the two events with your local equipment, and for instance assign them radar coordinates, and from this, differences in time and a distance.
See the link in my previous post and you will see where it is "defined".nitsuj said:It's postulated to be invariant. The assertion of it being defined may have been referring the 1/2 a round trip (average speed) = c calculation. (i.e. c defined as 1/2 of a round trip, though have never seen it "defined" as being that)
Sure, You define coordinates, so you define simultaneity which means equal time coordinate.ghwellsjr said:Yes, you can use just your own clock and radar to establish the time and distance to remote events but during that process, you must apply Einstein's second postulate that each radar signal takes the same amount of time to reach its target as it takes for the return echo to get back to you. This is where you define what simultaneity means between a local clock and a remote event.
Look at the first article of Einstein's 1905 paper entitled "Definition of Simultaneity" where he uses the word "defined" or "definition" 13 times.
Does this mean you are ready to retract your statements from post #15?wabbit said:Sure, You define coordinates, so you define simultaneity which means equal time coordinate.
wabbit said:Why ? Doesn't the standard method use light because it is already known experimentally to travel at the invariant speed?Cruz Martinez said:Well, you need a synchronization convention to measure one-way speeds, and the standard synchronization method uses the one-way speed of light, this speed is necessarily a defInition.
But actually I don't see where you need synchronization to measure a velocity. One observer can do that alone, he doesn't need to talk to anyone else. And he can also compare relative (to him) velocity of waves emitting by two other sources without ever synchronizing with those sources - and see whether that velocity is independent of the motion of those sources relative to him.
pulled from the paper "We have not defined a common “time” for A and B, for the latter cannot be defined at all unless we establish by definition that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A."ghwellsjr said:See the link in my previous post and you will see where it is "defined".
ghwellsjr said:... the speed of light is defined to be the same value in any inertial reference frame ...
Cruz Martinez said:Well, you need a synchronization convention to measure one-way speeds, and the standard synchronization method uses the one-way speed of light, this speed is necessarily a defInition.
BiGyElLoWhAt said:I still don't like that word. You mean define c=c? Or less arbitrarily, c =2.99...x10^8m/s? I don't see how that is a definition, and not just an observation. When I think of a definition, I think of ##\Phi=B\cdot A##, the definition of magnetic flux. We made that quantity up, it did not exist before someone defined it. c, however existed always. The speed of light in a vacuum has always been the same, even before we took our first measurements of it.