
#1
Nov2312, 01:02 PM

P: 9

It is wellknown 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
Nov2312, 02:50 PM

P: 2,080

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
Nov2312, 03:43 PM

P: 9

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
Nov2312, 04:04 PM

P: 2,080

How does one measure the absolute velocity of an object?
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..._for_beginners 



#5
Nov2312, 09:57 PM

P: 9

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
Nov2312, 10:14 PM

P: 290

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 nonaccelerating 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 nonabsolute frame of reference. 



#7
Nov2312, 10:51 PM

PF Gold
P: 5,720





#8
Nov2412, 12:35 AM

Mentor
P: 22,010





#9
Nov2412, 01:12 AM

PF Gold
P: 4,542

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? 



#10
Nov2412, 11:15 AM

P: 411

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
Nov2412, 11:23 AM

PF Gold
P: 5,720





#12
Nov2412, 11:24 AM

P: 9

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
Nov2412, 11:36 AM

P: 3,554





#14
Nov2412, 11:39 AM

Sci Advisor
Thanks
P: 2,973

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



#15
Nov2412, 11:42 AM

P: 3,554





#16
Nov2412, 12:24 PM

P: 242

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 4vector: an operation that can be done independently in any reference frame with any observer's measurements, even though different observers measure different quantities. 



#17
Nov2412, 12:35 PM

P: 9

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 4vector 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. 



#18
Nov2412, 12:51 PM

P: 9

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. 


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