# Time-negative quantity

1. Apr 7, 2009

### monty37

why cannot time be considered negative at times,the past can be represented using time as a negative entity.we have always considered time in accordance with the clock from
1 to 12.from the time earth evolved till today, do we know the current time in seconds,still
ticking?

2. Apr 7, 2009

### Ich

Yes, why not?
If you don't care about an uncertainty of several million years: yes, 4.54 Gy, 1.43*10^17 s.

3. Apr 9, 2009

### monty37

if it is possible ,why is it that in problems,time as a negative quantity is,usually discarded,that can be considered as an event that has already occured?

4. Apr 9, 2009

### Ich

i don't understand what you mean. Could you give an example?

5. Apr 9, 2009

### Naty1

monty:
Perhaps you are referring to the issue that while the equations of physics make no distinction between past and future, between negative time and positive time, in the real world it DOES appear to our subjective senses that past and future ARE different. It appears past is very different from future.

If you consider plus and minus distances, it doesn't seem there is so much difference relative to a zero reference point...in one you move a fixed distance and the other the opposite direction an equal distance....its a "phase difference" (direction) of 180 degrees...no big deal....but there is something fundamentally different in our experience of time that makes plus and minus time very distinct....classically the past SEEMS fixed, the future uncertain....

Einstein suggested our perceptions of time are wrong and the equations are right....it's really hard to understand. And using something like x =ict (where i is the square root of negative one) further complicates our intuitive understanding of real, imaginary and time and distance....If that were not enough, Einstein's use of Lorentz transformations confirmed that with observers in relative motion, time for one appears as as mix of time and space(distance) for the other....and vice versa.....all is not what our limited senses and intuition suggest!!!!

Last edited: Apr 9, 2009
6. Apr 9, 2009

### Staff: Mentor

For example, if you are working a projectile motion problem where you launch the projectile at t=0 then if you get an earlier root you know you can discard it since the free-fall equations don't apply before it was launched. Similarly if you were to launch the projectile at t=1000.

7. Apr 10, 2009

### monty37

speaking with respect to projectile problem ,say the time is found and we get two answers,
one positive and the other negative,we tend to discard the negative value saying time cannot be negative,why can't we consider that event to have already occured-if time is negative ,though our further calculations proceed only with the positive value?

8. Apr 10, 2009

### Staff: Mentor

Time can certainly be negative, that is not why the negative root is discarded. The key question is: Was the object a free-falling projectile at that time? If not, then it does not make sense to use the projectile equations to describe its motion at that time.

9. Apr 10, 2009

### Naty1

monty posted...
here you have touched on a central problem in broader physics....in some cases mathematical formulations reflect the world as we perceive it..in other cases not....and distinguishing "real" results is not easy....especially with the complex mathematical formulations in use today....

witness Schrodinger equation interpretations....people still argue about that....Einstein was a great physicst largely because he had an innate sense of what "smelled" right and followed his instincts...but he missed the expanding universe when his math TOLD him it was....and he even "proved" black holes could not exist...incorrectly.....

10. Apr 10, 2009

### Nick89

The equations you used to model the situation (projectile in the air) are simply not valid for your projectile before it was launched. Before the launch, the motion of your projectile is not described by the same equations as after the launch. Since you choose t = 0 for the launch time, anything less that t = 0 will not be valid because the model was not valid at that time; so we discard these results. This has nothing to do with past, present and future.