How does one measure the absolute velocity of an object?

[ghwellsjr]

so then is it possible that if i view one ship moving at the 99.999999% of the speed of light say (in reference to my observations), that the speed of light for them is then much faster than the speed I am viewing, while observing them?

Instead of thinking about how your viewing of a ship can have any effect on that ship, why don't you think about a single ship and a single reference frame? You don't have to attached that reference frame to yourself. Just think about a ship moving at 99.999999% of the speed of light in a reference frame. Then as a ship emits a flash of light in the forward direction, it will travel away from the ship at a very slow speed. If the ship emits a flash of light in the reverse direction, it will travel away from the ship at almost twice the speed of light.

But the ship will have no knowledge or awareness of this. It's simply the coordinates that are used in the frame of reference to describe what is happening.

If the ship tried to measure the speed of light, it would have to set up a reflector some measured distance away and time how long it takes for the light to make a round trip. The calculated value would turn out to be exactly c because its clock would be running slow and its ruler would be contracted along the direction of travel.

Now if we transform this scenario into the rest frame of the ship, as phinds suggests, then the speed of light in that reference frame will be c.

 

[phyti]

I disagree w/ the bolded part. It is NOT that there is no known way to measure it, it is that it does not exist, thus the question of measuring it is meaningless.

Referring to Einsteins 1905 paper, or any of his later sources:
The 'invariant interval' X, between two events is fixed. The events do not move. He can therefore state X-ct = 0. He knows that for a moving observer, the outbound and return trips for a reflected signal in the direction of motion have different space and time values, since he uses c±v. He knows an observer can't be at the emission and detection of the same photon. He proceeds to define the path lengths as equal to preserve a constant c, and provide values where measurement cannot provide them.

The photon and its path must exist, since it is detected!

As of 1800, all particles included in the standard model of quantum physics were unknown because there was no experiment capable of their detection. Looking back, it would be foolish to say they didn't exist.

 

[Nugatory]

The photon and its path must exist, since it is detected!

As of 1800, all particles included in the standard model of quantum physics were unknown because there was no experiment capable of their detection. Looking back, it would be foolish to say they didn't exist.

You're misunderstanding phinds here - he was not saying that the photon and/or its path don't exist, he was saying that "absolute velocity of an object" (the topic of this thread) doesn't exist.

 

[HomogenousCow]

Yes, and most of them having nothing to do with science.

comment of the year

 

[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]

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".

Great analogy. Well said.

 

[mkarger]

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".

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]

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".

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]

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."

Fair enough.

 

[stu dent]

It is well-known that the velocity of an object can only be determined in relation to the velocity of another object (the two trains in a station). Einstein's relativity theory limits the velocity of an object to the speed of light; it also been demonstrated that no matter what the velocity of an object is, the speed of light remains constant. Given the above, how does an object 'know' how fast it is travelling?

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]

...
I've read quite extensively - Brian Greene, Paul Davies, Gary Zukav, Roger Penrose, Stephen Hawkings, and no one has explained how velocity through absolute spacetime can be measured.

(I've got my own answer to the problem, but I hesitate to describe it because I don't have a PhD.)

Is this your own answer to the problem:

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.

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.