How does one measure the absolute velocity of an object?

[Sydney Self]

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?

 

[phyzguy]

You seem to be missing the point. There is no such thing as "absolute velocity". As you said, velocity can only be determined relative to another object. Why do you think the fact that the speed of light is constant in all frames of reference requires there to be an absolute velocity?

 

[Sydney Self]

The reason I ask is because, as I understand it, Einstein's Special Theory states that an object cannot move faster than the speed of light. To me, that implies that somehow its speed is being controlled so that it cannot exceed the speed of light and, if so, its speed must somehow be known.

 

[phyzguy]

The point is that nothing can go faster than the speed of light, as measured by any observer in any frame of reference. You might try reading

http://en.wikipedia.org/wiki/Special_relativity_for_beginners

 

[Sydney Self]

Yes, I know that, you know that, my question is: How is the velocity of the object determined - how is its velocity dermined if it can't be measured?

 

[DocZaius]

First of all, I'm not sure this belongs in Quantum Physics... (edit: it's since been moved to relativity)

Special relativity doesn't state that there exists a speed limit in relation to some preferred, central, or absolute frame of reference (which your questions imply you believe must necessarily exist). Rather, it states that there exists a speed limit in relation to all possible inertial frames of reference.

The rule is, where speed of light is 'c':

1) Pick a non-accelerating frame of reference
2) Nothing will be moving faster than 'c' in relation to it

The consequences of this rule are what lead to the funny business which you observe at speeds near c. I encourage you to create scenarios involving 3 or more frames of reference (say, 3 spaceships) and see if you can come up with logical impossibilities as a result of this definition of a speed limit based on a non-absolute frame of reference.

 

[phinds]

Yes, I know that, you know that, my question is: How is the velocity of the object determined - how is its velocity dermined if it can't be measured?

HUH ? Who says you can't measure it? You DO have to measure it relative to something, which just gets us back to what you've already been told. You seem to think that "measure it" / "determine it" means measure it against some absolute frame of reference even while you seem to understand that there is no such thing.

 

[russ_watters]

Yes, I know that, you know that, my question is: How is the velocity of the object determined - how is its velocity dermined if it can't be measured?

The first sentence of your first post contains the answer.

 

[ghwellsjr]

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?

The whole point of Einstein's Special Relativity is that you are free to select any Inertial Reference Frame (IRF) as the absolute frame of reference against which all velocities, including the speed of light are determined. So instead of thinking "that the velocity of an object can only be determined in relation to the velocity of another object," think of the velocities of both those two objects being determined in relation to the coordinates of your arbitrarily selected IRF.

I think your concern is how can the velocity of one object be dependent on the velocity of another object far removed from the first one, is that right? But if you realize that an IRF extends out infinitely in all directions and covers all time, then any object is intimately associated with coordinates local to it no matter where it is. This allows you to consider just one object and analyze everything about it without regard to any other object.

Does that help you?

 

[phyti]

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?

An object cannot know how fast it is moving. This is another case of the meaningless questions involving personification of objects.
If your intended question is: How do you measure the absolute speed of an object?
There is no known method of doing this. All you can measure is the difference in speeds.

 

[phinds]

An object cannot know how fast it is moving. This is another case of the meaningless questions involving personification of objects.
If your intended question is: How do you measure the absolute speed of an object?
There is no known method of doing this. All you can measure is the difference in speeds.

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.

 

[Sydney Self]

Thanks George,
My problem is that Brian Greene states ". . . the combined speed of any object's motion through space and its motion throught time is always precisely equal to the speed of light". He also says, "An object's velocity can be specified only in relation to that of another object." and ". . . special relativity says that nothing can travel faster that the speed of light. . ." and " . . . according to special relativity, absolute spacetime does exist."

Given the above, if all velocities are relative, how can we know know what the speed of anything (except light) is? Also, if the velocity of an object can't be determined, how can it be demonstrated that an object isn't traveling faster than light?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.)

 

[A.T.]

It is NOT that there is no known way to measure it, it is that it does not exist,

As far physics is concerned, there is no known way to measure it. Existence is philosophy.

 

[Nugatory]

Thanks George,
My problem is that Brian Greene states ". . . the combined speed of any object's motion through space and its motion throught time is always precisely equal to the speed of light". He also says, "An object's velocity can be specified only in relation to that of another object." and ". . . special relativity says that nothing can travel faster that the speed of light. . ." and " . . . according to special relativity, absolute spacetime does exist."

That's an example of the misunderstandings that can arise when someone (even an expert someone) uses less-than-precise words to describe a mathematical concept. The "speed" that he's talking about here is the magnitude of the velocity 4-vector, which is a very useful mathematical concept in general relativity and the modern formulation of special relativity; but it has very little to do with speed and motion as we use these terms to describe the motion of objects in space. Einstein himself didn't use 4-vectors to develop SR; that came later.

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

Don't let that stop you - this stuff is more accessible than that. The relative/absolute velocity question this thread started with, approached in the more old-fashioned formalism, the way Einstein originally worked it out, is within the reach of anyone who knows basic algebra. Even the 4-vector formalism is standard fare by the second year of an undergraduate physics program.

This is a long-winded way of encouraging you to learn the math instead of (as well as?) listening to the pop-sci crowd. It's not as hard as you think, and it is amazingly much more fun when you get it.

 

[A.T.]

if all velocities are relative ...if the velocity of an object can't be determined

You confuse "is relative" and "cannot be determined",

 

[ZikZak]

Thanks George,
My problem is that Brian Greene states ". . . the combined speed of any object's motion through space and its motion throught time is always precisely equal to the speed of light". He also says, "An object's velocity can be specified only in relation to that of another object." and ". . . special relativity says that nothing can travel faster that the speed of light. . ." and " . . . according to special relativity, absolute spacetime does exist."

Brian Greene's ideas of how to teach this concept are rather unique to him. I don't think many other Relativists would agree that "the combined speed of any object's motion through space and its motion throught time is always precisely equal to the speed of light." This is his idiosyncratic way of saying that the magnitude of the velocity 4-vector is unity. But that is not a speed in any sense anyone would actually use the word "speed." It's more like it's a unit vector pointing out the direction the body travels through space-time. All unit vectors have magnitude 1.

The body does not have an absolute velocity through space, or an absolute velocity though time, because different observers define space and time differently. His "combined speed" is a 4-vector: an operation that can be done independently in any reference frame with any observer's measurements, even though different observers measure different quantities.

Given the above, if all velocities are relative, how can we know know what the speed of anything (except light) is?

You use your meter sticks and clocks to measure the distance a body travels in a certain time, and then divide one by the other. You then know the speed of the body in your reference frame.

Also, if the velocity of an object can't be determined, how can it be demonstrated that an object isn't traveling faster than light?

The velocity is easily determined by measuring the distance it travels in a certain amount of time. That velocity will always be less than c.

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.

That's because there is no such thing. It's called "Relativity" precisely for that reason.

 

[Sydney Self]

Nugatory
The math I got that accompanied my second year undergraduate physics ended with differential equations and I'm afraid that I've long forgotten what I learned about matrixes. I had to look up 4-vector math in Wikipedia and although I know what a Minkowski diagram is, I don't know how to approach it mathematically.
I've been approaching the concept of time from the perspective of philosopy, and since much of time is not subject to experimentation, philosophy has some relevance.

A.T.
I mean't to say 'the absolute velocity'. Also, I've given a great deal of thought about the concept of reality.

 

[Sydney Self]

ZikZak
I have a basic problem with relativity. Einstein, and all the other individuals I've read who discuss it, base everything in terms of observers, as if everything that occurs in the universe is, and needs to be, observed. I have no quarrel with what reality deals with, my problem is with what it doesn't.
There is no direct, defined relationship between what we perceive and what physically exists. I have come to recognize that if one deliberately tries to distinguish between physical reality and subjective reality that one arrives at some interesting ideas.

 

[phyzguy]

It sounds like this discussion is not about science, which is concerned with the objective reality that we perceive and can measure, but is instead a discussion about mysticism. I suggest it be moved to the philosophy section or discontinued altogether.

 

[ZikZak]

By definition, science is the description of that which is observed. If you are trying to talk about something that is unobservable, then you're in the wrong place.

 

[phinds]

... I have come to recognize that if one deliberately tries to distinguish between physical reality and subjective reality that one arrives at some interesting ideas.

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

 

[zonde]

I have a basic problem with relativity. Einstein, and all the other individuals I've read who discuss it, base everything in terms of observers, as if everything that occurs in the universe is, and needs to be, observed. I have no quarrel with what reality deals with, my problem is with what it doesn't.
There is no direct, defined relationship between what we perceive and what physically exists. I have come to recognize that if one deliberately tries to distinguish between physical reality and subjective reality that one arrives at some interesting ideas.

In science your ideas should be testable. There are little questions about testability of observable things. Testability of things that are not directly observable is more tricky. You have to relate unobservable things to observable things in order to talk about science.

I would recommend to read about Scientific method.

 

[Nugatory]

ZikZak
I have a basic problem with relativity. Einstein, and all the other individuals I've read who discuss it, base everything in terms of observers, as if everything that occurs in the universe is, and needs to be, observed. I have no quarrel with what reality deals with, my problem is with what it doesn't.

That criticism is more fairly applied to quantum mechanics than to relativity, I think.

The "observer" in relativity isn't an active participant in the process of "observation", no act of perception is involved, and there is no question that the phenomena being studied exist whether they are observed or not. (It's an amusing irony that even as I write this, another thread on black holes is busily demonstrating that just because something cannot be observed that doesn't mean it's not real).

It is true that many explanations of relativistic phenomena are described in terms of what a human observer would see: Beams of light bounce between mirrors on their way to someone's eyes, I observe the readings on a clock moving relative to me and compare them with a clock at rest relative to me, I measure the length of a moving rod, and so forth. But that's just a particular style of description, one that comes naturally to a practicing scientist working with the results of observations.

Approaching the problem from a more philosophical stance, you may be more comfortable with a different model of what "observer" and "frame of reference" mean in relativity. So try this one, which I first encountered in Taylor and Wheeler's "Spacetime Physics":

Imagine that we fill a large volume of space with a three-dimensional grid of meter sticks, all at rest relative to one another, fixed at right angles to one another where their ends meet. At every intersection, we place a machine containing a recording device and a clock; all the clocks are synchronized. (It may be not be practical to construct such a ensemble across an interestingly large volume, but it is clear that I can do this is a small volume and that there is no theoretical objection to expanding it to an arbitrary size).

Now each recording device can generate an independent record of events at its location: At 3:00 PM a spaceship flew through this spot at .5c; at 3:38 PM a bomb exploded right here; at 4:07 PM a nuclear decay occured; and so forth.

Collectively, these recordings amount to a description of everything that happened within that volume of space, both when and where, to a resolution of one meter (and if we aren't happy with that resolution we could have chosen to build a more closely spaced grid).

Now our intrepid scientist can gather all these recordings at his leisure, maybe centuries after the events in question happened, and use them to piece together a complete history of what went on across the volume of space. Or he can choose to burn them... but most of us would agree that the events in question "really" happened, independent of any act of observation, no matter what he does with the recordings.

(BTW - don't be fooled into thinking that my lattice of rods and clocks will allow you to define an absolute velocity. I specified that all the rods were at rest relative to one another, but there's nothing to prevent another researcher from building his own lattice of rods and clocks, also at rest with respect to itself, but moving relative to my lattice).

 

[Nugatory]

Nugatory
The math I got that accompanied my second year undergraduate physics ended with differential equations and I'm afraid that I've long forgotten what I learned about matrixes. I had to look up 4-vector math in Wikipedia and although I know what a Minkowski diagram is, I don't know how to approach it mathematically.

Again... Don't let that stop you. You don't need the 4-vector formalism, calculus, differential equations, and matrix algebra for your purposes. Dig up a copy of Einstein's "Relativty: The Special and General Theory" and read it. Elementary algebra is all you'll need, and it will be a far better starting point than any amount of Brian Greene lectures

 

[Dale]

no one has explained how velocity through absolute spacetime can be measured.

There is no known way to do that, and there is no need to do so either. Simply choose a reference frame and measure the velocity with respect to that arbitrarily chosen reference frame. The nice thing about physics is that there is no right or wrong choice of reference frame, you can use any that is convenient.

 

[Dale]

ZikZak
I have a basic problem with relativity. Einstein, and all the other individuals I've read who discuss it, base everything in terms of observers, as if everything that occurs in the universe is, and needs to be, observed. I have no quarrel with what reality deals with, my problem is with what it doesn't.
There is no direct, defined relationship between what we perceive and what physically exists. I have come to recognize that if one deliberately tries to distinguish between physical reality and subjective reality that one arrives at some interesting ideas.

Unfortunately, that is philosophy and not science. Science deals with trying to model and predict the outcome of experiments. That is as far into "reality" as science delves.

Regarding the relativistic obsession with observers. Generally it is simply a short hand way of establishing a reference frame. Don't read too much into it.

 

[ghwellsjr]

Thanks George,
My problem is that Brian Greene states ". . . the combined speed of any object's motion through space and its motion throught time is always precisely equal to the speed of light". He also says, "An object's velocity can be specified only in relation to that of another object." and ". . . special relativity says that nothing can travel faster that the speed of light. . ." and " . . . according to special relativity, absolute spacetime does exist."

Given the above, if all velocities are relative, how can we know know what the speed of anything (except light) is? Also, if the velocity of an object can't be determined, how can it be demonstrated that an object isn't traveling faster than light?


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.)

Let me see if I can explain what velocity through absolute spacetime means in a simple way. Normally, when we talk about velocity we mean a distance divided by a time, correct? So when we talk about velocity through spacetime, we have to come up with a definition for our "distance" that combines both space and time in such a way that it won't be dependent on any arbitrary assumptions that we make. In other words, we want a definition that always comes out the same no matter what assumptions we make. So the definition that we use is what is called the "Spacetime Interval". We use the coordinates from our arbitrarily selected Inertia Reference Frame to measure time period over which an object has "traveled" some spatial distance. Then we square the time period and the spatial distance and subtract them and take the square root. The answer will be the same no matter what IRF we use. Then we use a clock that is carried with the object to measure the Proper Time period (or calculate the Proper Time period based on the IRF's coordinate and the speed of the object). We divide the Spacetime Interval by the Proper Time period to calculate the velocity through spacetime.

So let's do an example to see what we get. I like to use units where c=1 to make the calculations simpler. Let's say an object has traveled x=6 light-seconds in t=10 seconds. First off, we can see that its velocity through space is 0.6c. This is the same as saying beta, β=0.6, We can see that its spacetime interval is √(10²-6²) = √(100-36) = √64 = 8. Now we want to calculate the Proper Time, τ. It is τ = t/γ. Gamma, γ is equal to 1/√(1-β²) = 1/√(1-0.6²) = 1/√(1-0.36) = 1/√0.64 = 1/.8 = 1.25. So τ = 10/1.25 = 8. Now we can calculate the velocity of through spacetime as 8/8 = 1 or in my selected units, c.

Now it is no coincidence that we ended up with a spacetime velocity of 1 or c. We can do the above calculation symbolically as follows:

The spacetime interval is √(t²-x²)

The Proper Time is τ = t/γ = t√(1-β²) = √(t²-(tβ)²)

Now we note that since β=x/t, then tβ=x so we can substitute this in the above equation and get:

τ = √(t²-x²)

Since the spacetime interval equals the proper time, the spacetime interval will always equal 1 or c.

It's nothing more than a mathematical curiosity based on the definitions.

Knowing that, I take you back to my first post #9 to answer your original question.

 

[ghwellsjr]

Since the spacetime interval equals the proper time, the spacetime interval will always equal 1 or c.

The third paragraph from the end of my previous post should read:

Since the spacetime interval equals the proper time, the spacetime velocity will always equal 1 or c.

 

[broncorvette]

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?

 

[phinds]

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?

Emphatically no. The speed of light is the same in all reference frames. They see themselves as standing still, you as moving at 99.99999% of c, and you both see light moving at c.

 

[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".