How does one measure the absolute velocity of an object?

HomogenousCow

Well you don't, and in your frame you are not moving, to put it more precisely, the sentence "I am moving is meaningless for point particles".

HallsofIvy

The same way you measure the total weight of angels dancing on the point of a pin!

phinds

Going back to my original questioning of the existence of the absolute velocity of an object. I disagree that this is semantics or philosophy.

If you give me a yardstick and ask me to measure the length of the smell of the color purple, I am not going to say "I can't measure that", I'm going to say "There is no such thing ... it does not exist".

phyzguy

Great analogy. Well said.

mkarger

I'm not so certain what you said is inherently true.

But now I believe we are diving into the realm of religion. You can only ever make a claim about the physical limitations of our realm of existent. Nothing you say or do ever negates the possibility of something beyond.

But assuming a beyond implies something unattainable because if it was attainable, it would not be part of the beyond.

It's paradoxical. Which is why it is illogical and essentially why it is pointless to argue.

pervect

I would agree that absolute velocity doesn't exist with the context of SR, and that it's silly to talk about measuring it in such a context. Also, because this is the SR forum, I think it's reasonable to assume that's the context in which the statement is being made.

Since there are physical theories which do have an absolute velocity (the old outdated Ether theories, for example) I would argue that it's saved from such total sillyness, even though it's not terribly appropriate for this forum.

In a more modern context people do still look experimentally for effects which violate SR that could be described as "looking for effects of absolute motion".

So, in conclusion, I'd say that the question should be "Can we measure absolute motion", not "how do we measure....", which presumes a priori that it's possible. And the answer is along the lines of "According to SR it's not possible" and "To date, no experiment has succeeed in demonstrating the existence of any way to measure absolute velocity."

phinds

Fair enough.

stu dent

this is how i see that one.


in order to measure something correctly, you need two points of reference. you need this because this is how you create units.

for celcius, they took freezing point and boiling point of water at 1 atm, and split that into 100, i wish they would have split it into 200 though, but whatever.

i get what you mean, by there being a limit of the speed of light, and therefore we must be at some speed relative to it, but you need another stationary point for an absolute comparison.

i mean, you could get closer and closer and closer to the speed of light, but how close are you? time will move slower and slower, but how slow is it?

you can go faster forever and never reach the speed of light, you would go in smaller and smaller increments, but an increment is nothing.

you might talk of km/h let's say, but these are not constants. there is no other end in order to be able to make meaningful measures, in order to properly measure your speed to the speed of light.

in your given frame, you might have real seconds, and real distance, with which to measure other things from your frame of reference.

but outside your frame of reference there is the speed of light. that's it. and there's no way to say how close or how far you are to that. only how close or how far you are to that as compared to something else.

although, if they could figure out what causes mass precisely, and therefore what would be the exact actual rest mass of something, then you would have a second reference to go from, and you could measure an absolute speed based on your new mass, or how much energy is required to accelerate or decelerate you.

but this would likely require that the universe would actually be another absolute frame of reference, similar to an ether, i mean, it could stretch and move, but it could be a second universal reference that could be used.

but i'm not sure how possible that part is.

ghwellsjr

Is this your own answer to the problem:
If your concern is how do we establish a system of units for speed, it's very simple in our universe because everyone who tries to measure the speed of light gets the same answer. But you have to realize that the measurement has to be a round trip for the light and this removes the need to define or identify a coordinate system. All you have to do is have a unit of distance or length using a rigid measuring rod or stick and a unit or time using clock or timing device. Then you need a mirror and a light source. You measure how far away the mirror is using your measuring rod. You start your timing device when you turn on the light and you stop it when you see the reflection of the light after it makes its round trip to the mirror and back to you. You calculate the speed of light taking double the measured distance to the mirror divided by the measure time interval and this establishes one point on your scale of units of speed (just like the boiling point of water). Then you use a speed of zero as the other point on your scale of units of speed (just like the freezing point of water). Now we split that scale into 100 parts (just like the Celsius scale for temperature) and we have speed as a percentage of the speed of light. This is given its own symbol and name in relativity. It is called beta and uses the Greek letter β. Now we have an absolute speed scale with which we can measure or define the speed of any object.

ghwellsjr

Please ignore the first quote and my first short response in my previous post. I got Sydney Self and stu dent mixed up.