# How does one measure the absolute velocity of an object?

1. Nov 23, 2012

### 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?

2. Nov 23, 2012

### 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?

3. Nov 23, 2012

### 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.

4. Nov 23, 2012

### phyzguy

5. Nov 23, 2012

### 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?

6. Nov 23, 2012

### 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.

Last edited: Nov 23, 2012
7. Nov 23, 2012

### phinds

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.

8. Nov 24, 2012

### Staff: Mentor

9. Nov 24, 2012

### ghwellsjr

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.

10. Nov 24, 2012

### phyti

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.

11. Nov 24, 2012

### phinds

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.

12. Nov 24, 2012

### 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 travelling 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.)

13. Nov 24, 2012

### A.T.

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

14. Nov 24, 2012

### Staff: Mentor

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.

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.

Last edited: Nov 24, 2012
15. Nov 24, 2012

### A.T.

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

16. Nov 24, 2012

### ZikZak

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.

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.

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.

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

17. Nov 24, 2012

### 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.

Last edited by a moderator: Nov 24, 2012
18. Nov 24, 2012

### 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.

19. Nov 24, 2012

### 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.

20. Nov 24, 2012

### 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.

21. Nov 24, 2012

### phinds

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

22. Nov 24, 2012

### zonde

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.

23. Nov 24, 2012

### Staff: Mentor

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).

Last edited: Nov 25, 2012
24. Nov 24, 2012

### Staff: Mentor

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

25. Nov 24, 2012

### Staff: Mentor

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.