# How come we say space and time when it should be space=time. You can

1. Jun 12, 2010

### binbots

How come we say space and time when it should be space=time. You can eliminate measuring distances and measure them in time. Two objects can always be measured apart by time as long as there is a universal speed (light). I am 0.000008sec (at light speed) from my mailbox for example. The universe expanding could be time. There is no distance at all. Like the big bang all matter is one thing only seperated by time. The closer to people come together the closer in time they are. Have I gone to far with relative time?

2. Jun 12, 2010

### atyy

Re: Space=time

Yes, space=time in terms of units of measurement. Properly, we say spacetime to indicate that it is a 4 dimensional entity, needing 4 coordinates to specify the location of one point of it. For convenience, we often work with one coordinate system, in which case we arbitrarily choose one way of dividing spacetime into space and time, in which case we say space and time.

3. Jun 12, 2010

### Staff: Mentor

This is not quite correct. Even in units where c=1 space does not equal time as you can easily see from the Minkowski metric:
$$ds^2=dt^2-dx^2-dy^2-dz^2$$

They are certainly related, but not the same.

4. Jun 12, 2010

### Passionflower

Re: Space=time

Well actually ds is time in this equation, dt is coordinate time, which only equals proper time in the special case where dx, dy and dz are all 0.

5. Jun 13, 2010

### Staff: Mentor

Only for timelike intervals, not spacelike or null intervals. In which case time and space still differ in the sign of the interval squared.

6. Jun 15, 2010

### binbots

Re: Space=time

So it is crazy to say that there is no such thing as space or distance. Only time seperates all mass.

7. Jun 15, 2010

### nismaratwork

Re: Space=time

No, there can be space-like separations too... unless you mean within a specific coordinate system where time acts for distance.

8. Jun 16, 2010

### valavel

Re: Space=time

I will not attempt to answer this question, rather ask a related one: Is this the right thread to discuss the possibility of an Lorentzian relativity where it is not space and time that vary but the dimensions of moving masses that contract, and the rate of the clock mechanism that slows down - in a classical world? Other theories propose that space and time do not exist as separate dimensions and that c varies according to the density of the gravitational potential.

9. Jun 16, 2010

### nismaratwork

Re: Space=time

There are such theories, are you promoting them?

10. Jun 16, 2010

### binbots

Re: Space=time

What I am trying to say that distance is a man made thing. There is no universal measurment for distance. Time on the other hand has light, the universal constant.

11. Jun 16, 2010

### Mentz114

Re: Space=time

You will probably find it ironic then that the standard metre is defined in multiples of the wavelength of certain light emissions. See here

http://en.wikipedia.org/wiki/Metre#Standard_wavelength_of_krypton-86_emission

12. Jun 16, 2010

### bapowell

Re: Space=time

Why does time have light while distance does not have light?

13. Jun 18, 2010

### valavel

Re: Space=time

The premises I mentioned seem very reasonable and I would like to know if others have worked out their full implications - i.e. can they rigorously lead to Einstein's results? In chapter VI of his book Space, Time & Gravitation, Eddington likens a gravitational field to a dense medium with a refractive index n where light slows down. He explains it is the 'coordinate' velocity of light he is talking about... but is this the right forum to discuss these things? Thanks.