# If Time and Space Are Relative Concepts, Then

1. Aug 24, 2004

### Curious6

If time and space are relative concepts than how can you qualify both of them in terms such as for example

(a) the Big Bang happened 13.7 billion years ago,
(b) he took 10 seconds to run 100 meters
(c)the shortest distance to that place is 7 km

Wouldn't all these claims be stating time and spaces in absolute terms? By this I mean, wouldn't for example 100 meters be different as observed from different reference frames?

Just a thougt.

2. Aug 24, 2004

### Prometheus

Of course.

3. Aug 24, 2004

### Curious6

But if they are expressed in absolute terms, how it can be made to agree with relativity? I don't really understand this.

4. Aug 24, 2004

### Prometheus

The fact that the terms are relative to a particular frame of reference must be implicit if not explicit.

I am not exactly sure what your question is, or if this is an appropriate or a satisfactory response.

5. Aug 24, 2004

### Curious6

OK, I'll attempt to ask my question in a different way. How can we know that the 100 metres of a 100 metre race are actually 100 metres (if space is just a relative quantity) or that it took a person 10 seconds to do something (if time is just a relative quantity)? Thanks for helping me.

6. Aug 24, 2004

### Curious6

I think that my question has been answered though by reading an FAQ on relativity on this website: http://users.pandora.be/nicvroom/index_en.htm (section 1: absolute and relative) where it says that true physical reality is absolute but as how we observe phenomena is not absolute but relative. That is the answer to my question I think. It means that the concepts of time and space are absolute in the true physical reality, but as they appear to us are not and that is when relativity comes into action to explain observed effects of time dilation and length contraction.

7. Aug 24, 2004

### Prometheus

I believe that this is easy to answer.

When someone says that a race of 100 meters was run in 10 seconds, relativity is typically not considered at all. It is taken for granted that our reference frame is considered; in other words, it can be ignored even by people who recognize such a concept.

The meter and the second have definitions. These defintions are what are being used.

Although people who consider Einsteinian physics and the idea that relativity enables these values to be subject to further analysis, people who talk about a 100 meter race being run in 10 seconds typically do not. Such people do not consider that time or space is relative, and they have no need to do so.

8. Aug 24, 2004

### pervect

Staff Emeritus
You have to be a bit careful about stuff you read about relativity on the www, unfortunately.

This sounds like one of those webpages that isn't representative of the mainstream from the quote you gave.

I believe astronomers usually report distances to each other by quoting the 'z' factor, the doppler shift they measure. This is usually converted into distance for the general public by making some assumptions about cosmology.

http://www.astro.ucla.edu/~wright/cosmo_02.htm

has some information on this, there may be a better source.

9. Aug 24, 2004

### Prometheus

I caution you against thinking that there is someting meaningful in the phrase "true physical reality".

10. Aug 24, 2004

### pervect

Staff Emeritus
Yes - there may or may not be a true physical reality, but invoking it to explain measured results is a "red flag" to me that something is wrong with the explanation. "True physical reality" is a philosophical concept, not a scientific one, so it generally has nothing to say about measured results.

11. Aug 25, 2004

### Curious6

OK, but I'm still a bit in doubt though: how can we know the Big Bang happened 13.7 billion years ago if time is relative? Wouldn't it appeared to have happened more than 13.7 billion years or less than 13.7 billion years ago if you are measuring it from different places in the Universe?

12. Aug 25, 2004

### Staff: Mentor

Of course to which part? The second part was right, the first wrong.

The issue here is simple: unless otherwise specified, all measurements are with respect to Earth's reference frame.
Be careful with that. It sounds like you are saying our eyes are playing tricks on us. They aren't. Time dilation and length contraction are physically real.
Yes, it would - but why (how) would a scientist go about measuring it from somewhere else besides the surface of the earth? It is understood that all measurements are taken from earth and as such are expressed in our terms.

13. Aug 25, 2004

### Curious6

OK, thanks again for the answers. Russ, does this mean then that there is a possibility for example that the Big Bang actually only happened 10 billion years ago(for example)?

14. Aug 25, 2004

### neoweb

For someone in another reference frame... yes...

15. Aug 25, 2004

### Prometheus

Excellent observation.

The number 13.7 billion is definitely based on our frame of reference here. It is quite reasonable to assume, in my opinion, that this number has no meaning outside of the context of our frame of reference.

You use the phrase "appeared to have happened", from which I assume that you are refering to observation only. Why? There is no requirement that time move at the same rate throughout the universe. The age could well be different in different parts of the universe.

16. Aug 25, 2004

### LURCH

Because for all practical applications, these units of measure are going to be correct. When an announcer says that the runners are going to run 100 meters, he means 100 meters from his frame of reference, which is your frame and mine. To an observer travelling at near-light speed, the distance would be shorter, but observers travelling at near-ligth speed are not from this planet and probably don't get ESPN, so he isn't talking to them.

17. Aug 25, 2004

### pervect

Staff Emeritus
We use the earth as an (almost!) inertial frame to determine the distances when we set up the race-course.

We again use the earth as a reference frame when setting up our clocks, which should ideally be trackable back to the NIST standard. The clocks we use are stationary on the earth.

We should ideally use the Einstein convention to synchronize the clocks at the start and end of the race - this is to set off a beam of light at the exact center of the course, and use it to start both clocks, and signal the runners to start.

I suspect that in practice race organizers cheat here. Done incorrectly, the runners times could be wrong as much as .3 microseconds, even more if sound is used rather than electronics. I don't think we time the runners to the microsecond anyway, so it's not really a problem. If we were timing faster "runners", like elementry particles, it could be an important source of error not to do this.

18. Aug 31, 2004

### Alkatran

For the 13.7 billion years: It's measured from our frame (as has been stated)

For the runner:
It is assumed that all calculations are done from the runner's frame of reference. If you're going by a .5c, you'll convert from your frame to his before assuming he did very poorly or very well.