# I Time travel thought experiment

1. Sep 24, 2016

### Shayne T

Not sure where the best place to post this is, but here it goes.

Imagine you were sent back in time, far enough back when a lot of the now known scientific laws, rules equations etc, havent been discovered yet. You're someone who is intelligible with many scientific fields, and you have brought a collection of scientific data such as equations and whatnot that were figured out by humanitys scientists up until the present, in an attempt to emulate their work, and become known as the greatest mind thats ever been. I realize in order to successfully emulate these established rules, such as demonstrating that the law of universal gravitation can actually describe real world phenomena, you would first need to determine the physical measurements of the units, such as length, volume, time, temperature, power etc, and they would have to be the same as they were in the future that you came from, or the scientific work youve claimed as your own would be useless and wouldnt accurately explain anything, which leads to my question.

Is there any possible way to determine or copy exactly what an established unit of measuremt is in the real world, if you were put in the situation above. For example, how would you determine exactly how much volume 1 litre is, or how long 1m is, without anything to compare with?

2. Sep 24, 2016

### andrewkirk

You could define time by the length of a day at a specific time of year, say the summer solstice. Divide that period up into a small number of equal amounts, called 'new seconds'. You could then define distance based on the speed of sound, defining a 'new meter' as being the distance to an event from which the sound arrives one 'new second' later than the light.

The relationship between length and time would vary with altitude and weather, but probably not enough to upset your calculations given that only crude instruments would be available.

Defining mass is not so easy. Even now I believe we still define it by reference to some lump of metal in Paris. You'd just have to pick a rock, keep it safe and agree that that's the 'new kilogram'.

3. Sep 24, 2016

### hsdrop

well to get a standard of measurable units that we use in the real world you will have to start with something that will not change through time, mass, density, temperature, exc... something that stays the same no matter where or when you are
so i would use water as a starting point as in a drop of water right before it freezes and you can base temp on when the water starts to freeze and work your way out from throws two starting points
it takes 480 drops to make a flooded ounce from there you can get your length and volume measurements started

4. Sep 24, 2016

### hsdrop

5. Sep 24, 2016

### jbriggs444

What makes you think that the laws of physics depend on the system of units that one uses?

6. Sep 24, 2016

### Orodruin

Staff Emeritus
This is a crucial point. The values of some of the natural constants you would measure would get different numerical values, but the laws themselves would not be different. It does not matter to a physical law what units you use for the measurements as long as you are consistent.

7. Sep 24, 2016

### mrspeedybob

Revive some of the original definitions of various units...
1 meter = 1/10000000 the distance from the north poll to the equator.
1000 Kg = weight of 1 cubic meter of water.
1 hour = 1/24 of a day
etc...

If you did go back with all the scientific knowledge, these original definitions should be familiar.

8. Sep 24, 2016

### sophiecentaur

That was an idea that took some while to catch on. I seem to remember that some ancient system involved the length of bits of the King (his toe?) as the standard for length. Not as daft as it sounds. perhaps as it would mean the reference was impossible to forge or change.

9. Oct 7, 2016

### Shayne T

They dont depend on them. But the laws of physics wouldn't be accurately measurable if one was using equations developed by a scientist who used a specific units to do so. Im just asking if theres any way to copy for certain, a unit of measurement such as the litre, kilo, metre etc..

10. Oct 7, 2016

### A.T.

Laws aren't measured, physical quantities are. Different units will just give different numerical values for some constants, but the laws stay the same.

11. Oct 8, 2016

### Staff: Mentor

The laws of physics do not have units in them. Some of them have constants where their numerical value depends on the unit system - but that does not influence the law itself. Choose different units and the constant has a different numerical value but the law is the same.

Concerning the most common SI units, see the previous posts.
- a second via the length of a day
- the length of a meter via the circumference of Earth or using g=9.81 m/s^2 (the approximate local gravitational acceleration due to Earth)
- the kilogram as the mass of water with a volume of 0.001 m3 (at the temperature of its highest density, if your measurements are that precise).
- temperature via freezing and boiling point of water
- Ampere has an ugly definition via the force between cables- Hard to measure, but not impossible
- candela is tricky, but not important
- Avogadro's number you should know, so the mole is easy to handle.
- all other units are derived from those

Those were old definitions of the units. With increasing precision we went to more suitable definitions, but those are very small corrections.

12. Oct 9, 2016

### David Lewis

F=ma works with any consistent set of units.

13. Oct 10, 2016

### Staff: Mentor

If you treat both F and a as 4-vectors, yes. As you should in special relativity.