# Everyday applications of relativity

1. Dec 12, 2006

### resurgance2001

I have heard about the use of SR in GPS systems. A friend of mine asked whyh it is necessary for GPS systems to be able to measure time so accurately? What has time got to do with finding one's location?

Also, I've heard about the application of SR by physicists doing high energy particle experiments. Are there any other every day applications of SR?

2. Dec 12, 2006

### Staff: Mentor

Last edited: Dec 12, 2006
3. Dec 12, 2006

### resurgance2001

cool - thanks

4. Dec 12, 2006

### marlon

Spintronics is one of the most promising technological applications of special relativity (together with QM ofcourse).

To make GPS systems more accurate, the principles of SR are also used. More specifically : In Special Relativity, Einstein came to the conclusion that time will evolve slower when objects are moving very fast. So the faster you move, the slower you will grow older. This is the famous twin paradox. Another effect is that time will evolve faster when the strength of the gravitational interaction is lower. Here on earth we will never feel these effects because we all feel the same gravity and we never move that fast with respect to one and other, so that we will see these time-differences.

However the satellite of GPS moves at about 14.000 km/hour and Special Relativity can prove that a clock on this satellite will be 7 microseconds, per day, behind with respect to a clock here on earth. Given the fact that these satellites are at about 20.000 km above the earth’s surface, they will feel only 25% of the gravity that we feel here on earth. This results in the fact that the clock on the satellite will be 45 microseconds ahead with respect to a clock here on earth. If we add up these relativistic effects we need to incorporate a time difference of 45 – 7 = 38 microseconds for each day that has passed. If we did not incorporate these corrections, the satellites would exhibit a deviation of 11 kilometers, per day, in their measurements. So they would give us an error of 11 km per day !!! Thanks to these corrections we can position someone via GPS with an accuracy of about 11 meters !!!!

greets
marlon

5. Dec 13, 2006

### grant9076

I am not a scientist but I know that the Ring Laser Gyros in inertial navigation systems are based on the speed of light being the same from all inertial frames of reference.

6. Dec 13, 2006

### Jheriko

I thought that GPS corrections were mostly necessary due to General Relativity. Since the satellites are further out in the Earth's gravity well and there is a time dilation/contraction effect to counterbalance when the satellites communicate with the ground. The special relativistic effect from the satellite's velocity is much smaller...

7. Dec 13, 2006

### Staff: Mentor

That is true (the link tells the proportions), but both are still needed.

8. Dec 13, 2006

### ben griffiths

How many decimal places is 30 nano seconds compared to a second?

9. Dec 13, 2006

### Staff: Mentor

10. Dec 19, 2006

### Chris Hillman

Just thought I'd point out that a good expository paper on relativistic effects in GPS is http://www.arxiv.org/abs/gr-qc/0507121 (don't be confused by last sentence of the abstract; the author is not implying that he thinks relativity theory is not needed, a common claim among the anti-Einstein crowd; quite the opposite, since in fact he is working on a next generation navigation system which will be fully relativistic from the outset, rather than adding "relativistic corrections" to a Newtonian model.)

11. Apr 6, 2011

### JohnN

If we're after everday aspects or effects of special relativity, how about magnetism?

Seems to be a field which arises (or at least can be explained and quantified) as a change which occurs in an electrostatic field when the charges are moving relative to the observer, and which is a result of the relativistic contraction of the volume containing the moving charges.

I can't think of any other relativistic effect which is so apparent in everyday life... so apparent that we've given it a separate name 'magnetic field'. And it occurs at speeds (few cm/s?) which are so far below c...

J.

12. Apr 6, 2011

### ghwellsjr

There are a great many GPS products that are used only to display the correct time to better than a microsecond. These would not work without the location also being known because it takes light one microsecond to travel a thousand feet. Prior time distribution methods (WWV for example) require the user to manually take into account the time it takes for the radio transmission to arrive and on a moving platform, this can be very difficult.

13. Dec 30, 2013

### Thinkor

GPS is not really an application of SR, but a convenient means for testing SR.
Any regular clock disagreement would be adjusted for automatically based on experience.

14. Dec 30, 2013