So time is relative??

Question

I am having an insanely difficult time understanding how time is relative. If anybody with a knack for explaining tricky issues like this could let me know, I would really appreciate it. Just to be safe though, when you do explain it, pretend that I'm really REALLY thick and stupid, that way you dont assume that I know anything that I don't. :) Thank you!!!!!

Answer 1

so ... yeah ...

I think you might get stuck on a senario like: "Person A is standing on a corner for 1 minute. How much time has passed for Person B?" This is not where time is realative. A minute is a minute is a minute.

Where relativity of time becomes an issue is when you start getting into Einstein stuff. We took about 2 months of AP Physics going over Einstein's Theory of Realtive Motion and I understood it enough to understand it, but not enough to explain it very well.

Times realtivity has to do with out perception of light. Our perception is the thing that is really realtive and you can get into a Philosopical discussion of perception and reality and you may understand it a little better.

For instance take this senario. You have Car A parked five feet from you and you have Car B parked 1 mile away form you. If Car A and Car B both start at noon and travel the 1 mile at the 60 miles/hour which car will you see get to the goal first. Both cars will get to their goal in one minute, it is a fact that no one would dispute, but you will see Car A get to the goal before Car B becuase the light has to reflect off Car B and go a longer distance to reach you. Thus the one minute for Car B seems longer than the one minute from Car A.

It is all about planes of existance and you would really need a full semester on realtivity to really understand exactly how E = mc^2 actaully works.

It is all about how light travles to the person. IF (and this is a HUGE if) We could travel faster than light -- c in the equation -- If we moved fast enough we could see ourselves disappear if we ran behind where we were and turned around. The light would hit us standin still, bounce off, we'd run past the light, turn around, have the light hit our eyes and we'd see us disappear. ... just something to think about and if you don't understant that don't feel bad and ignore it, It was just a fun weird fact.

Answer 2

Time is relative and will depend on the relative velocities between the observers. An observer will always feel that his own watch moves at the same speed (always, he feels his own time is absoult), but while observing others watches, he will see them move slower than his if they are traveling very fast in relation to him. Because movement is relative, (if A think he is still and sees B moving , B sees A moving in the opposite direction, and B can say that he is still and not A) , every observer sees the watch of the other going slower than his.
It is not so difficult to understand, but takes a bit more than a couple of paragraphs to explain it well. All this relativeness of time comes from the fact that the speed of light is absolute, not depending on the speed of the observer. There are many books where they explain this in a simple way, but more pages are needed.

Answer 3

Doug explained pretty well I think. Perhaps I can add to it by explaining some experimental evidence:

This difference in the "flow" of time has been measured with atomic clocks. We've done the following experiment: Take 3 atomic clocks set to exactly the same time. Fly one E to W around the world (using a very fast jet), fly the other W to E around the world, and leave one at the airport. When they all land back at the airport, all three clocks have a different time by precisely the amount that Einstein predicted (within experimental error). Flying E to W is not the same as flying W to E due to the Earth's rotation - the relative speeds is therefore different. This means that the effect is more than just in our perception as one answerer seemed to suggest.

We can see time dilation effects in other experiments as well. Take a unstable particle, say a muon. A muon is a particle that is in the same 'family' as an electron, but it's unstable. It decays into an electron and two neutrinos (an muon-antineutrino and an electron-neutrino for those who want the details), has a charge equal to that of the electron, a mass 207 times that of the electron, and most importantly for this discussion, the average lifetime is 2.2microseconds.

Now, particle decay is random, but on average you can expect a particle to decay within a certain lifetime, which is determined through QM, and is independent of outside influences. In other words, it depends on the properties of the particle, NOT on external stimuli. So it's a perfect measure of time dilation. So when you measure the average lifetime of muons that are traveling at relativistic (near-light) speeds, you find that in our "lab" frame, the muons got a much longer lifetime due to time dilation just as would be predicted by relativity.

Also, time dilation occurs due to gravity curving spacetime according to General Relativity. That has also been measured.

Answer 4

This may be a bit lengthy, but hang in there with me. It seems a lot of people are confused by this.

Imaging there is a rocket traveling very fast (near the speed of light, about 300,000,000 meters/second) and that on the rocket is an experiment. The experiment consists of a beam of light 'bouncing' back and forth between two mirrors.

To someone on the rocket, the distance that the beam of light travels making the trip from one mirror to the other and back (to the first mirror) is 2*d (where d is the distance between mirrors) and it takes time t to make the trip (according to this observers watch).

Now imagine another observer standing on the ground and watching the experiment as the rocket goes past them. They will see the beam of light follow a sort of 'triangular' path since it will (to them) be moving forwards as well as up and down. As a result, the observer on the ground will see the beam of light travel a much greater distance than the observer on the rocket.

You might want to draw out a rough sketch of this:

--------------------
/\/\/\/\/\/\/\/\/\/\/\/\
--------------------
is about what the observer on the ground would 'see' for the path of the light beam. And that's quite a bit longer than
--
|
--
which is what the observer on the rocket would 'see'.

Now here's the difficult part. The distance between the mirrors hasn't changed, and the speed of light hasn't changed, but the total distance travelled *has* changed. Which means that, since the speed of light has stayed the same, the observer on the ground will measure a larger t on their watch since the light has travelled a further distance. And the *only* way for that to happen is if time moves 'faster' for the observer on the ground, or more slowly for the observer on the rocket.

This (and a closely related thing called the 'Lorentz-Fitzgerald Contraction') is one of the more difficult things to wrap your head around when you first get exposed to Special Relativity.

The exact magnitude of this 'time dilation' (as it's called) is given by
√(1-v²/c²)
where v is the speed of the rocket and c is the speed of light. As you can see, it's really almost nothing at speeds much less than that of light. But it *has* been measured using atomic clocks on satelites and deep space probes.

Hope that helps.


Doug

Answer 5

Yes, time is relative - basically, the faster you go (towards the speed of light) the slower time is for you. So, a photon (a particle of light) traveling at the speed of light, or very close to it, time is virtually stopped. This why if you get on a spaceship and travel near light speed and come back to Earth, people on the Earth will have aged more than you have (hypothetically).

Answer 6

Time is a measurement devised by humans to explain why everything doesent happen all at once. So, if you think about how you measure what is happening now as you read this response compared to how time is for me now as I write this reply, you have to compare the written times. That means how you understand my time is relative to your time now. ie right now its Sat. 8.27 PM, is this time how much in the past compared to your time? Relative.