# Why measuring one-way trip impossible?

1. May 11, 2013

### ash64449

well,i have heard this in many threads. That we can only measure the speed of light by noting down when light completes its full round trip. Now why cannot we measure speed of light by considering the half-way trip?

2. May 11, 2013

### PAllen

Because you have to synchronize clocks at two different places. Then, the choice of clock synch is what determines how two way speed gets divided into two one way speeds rather than some property of light. The Einstein clock synch is defined as the choice which makes the two one way speeds the same.

3. May 11, 2013

### ash64449

let me know how the clocks are syncronised so that i can make sense to your statement.

4. May 11, 2013

### Bill_K

In Romer's original experiment, the moons of Jupiter serve as a distant clock. When Jupiter and Earth are at their greatest distance apart, the light coming from Jupiter has an additional 2 AU to travel, and we observe a 16 minute delay. This seems to me to be a one-way measurement of c with no synchronization required.

5. May 11, 2013

### ghwellsjr

You synchronize the clocks so that the one-way speed of light, as measured by those clocks, comes out to be c.

6. May 11, 2013

### ash64449

then should i again syncronise those two clocks again to measure the second return one-way trip to come out to be c?

7. May 11, 2013

### ghwellsjr

That is a slow transport of a clock--the same as Einstein's synchronization, assuming that the clock has not accumulated any offset during its trip.

8. May 11, 2013

### ash64449

how did scientists calculated the distance between jupter and earth?

9. May 11, 2013

### ash64449

what is Einstein's syncronization?

10. May 11, 2013

### Bill_K

Well-known story: the size of the astronomical unit was determined in ancient times by Aristarchus of Samos (or Eratosthenes, or Hipparchos), by comparing the sun's observed position in Alexandria and in Syene.

Last edited: May 11, 2013
11. May 11, 2013

### ash64449

do you know any current method used by scientists to measure the distance between them?

12. May 11, 2013

### ash64449

no need to reply.. I think it is unimportant for this discussion.

13. May 11, 2013

### Staff: Mentor

I thought this was how the circumference of the Earth was first determined (by Eratosthenes), not the distance of the Sun from the Earth. IIRC the assumption was that the Sun's rays in Alexandria and Syene were parallel, implying an infinite (i.e., too large to measure) distance to the Sun; the Sun was exactly overhead at noon on the summer solstice in Syene, so its angle from the vertical (as measured by the shadow cast by a vertical stick) in Alexandria allowed the circumference of the Earth to be determined. (This also assumes that Alexandria is due north of Syene, which I believe it is to a good approximation.)

14. May 11, 2013

### Bill_K

We're talking about what is possible in principle. So first measure the circumference of the Earth with a ruler, and then the Alexandria-Syene experiment will in addition tell you the solar parallax.

15. May 11, 2013

### glappkaeft

The relative distances of two planets and the sun can be computed using Kepler's laws (the reason for using the Astronomical Unit). Back in the day of Rømer's observations the best estimate of the AU was based on parallax measurements of Mars done just 4 years earlier by Richer and Cassini. Today radar ranging and space probe telemetry is used.

Last edited: May 11, 2013
16. May 11, 2013

### ash64449

let me imagine a train,i placed two clocks on extreme parts of train. I syncronized the clocks when i am in rest. Now i made the train move,in uniform motion. Why do clocks get unsyncronized?

17. May 11, 2013

### Staff: Mentor

Well, you did say it was an actual well-known story, not just something possible in principle. I agree that if the circumference of the Earth is already known (as well as the Alexandria-Syene distance, of course), then the difference between the Sun's actual angle from vertical at Alexandria, and the angle expected if the Sun's rays were exactly parallel, gives a measurement of the Sun's parallax. I'm not sure measurements in ancient times would have been accurate enough to measure the difference, though.

18. May 11, 2013

### ash64449

please,don't get diverted from the discussion. Please answer to my doubts.

19. May 12, 2013

### Bill_K

Given the value of the AU from Earth-based parallax measurements, why then doesn't the Romer experiment serve as a one-way determination of c?

20. May 12, 2013

### Staff: Mentor

By interpreting the time delay as a one-way measurement of the time it takes light to travel 2 AU, you are implicitly assuming that both time measurements are made with reference to a single frame. But they're not; the Earth is in different states of motion when the two time measurements are taken. So one has to correct at least one of the time measurements for the difference in frames; but applying that correction requires you to already know the speed of light.