Time dilation, observer vs traveler. Confusion!

[Daniel Foreman]

Hi,

 

I may have a complete misunderstanding of this topic, so I apologize in advance if I sound like a complete idiot misunderstanding the subject matter.

 

So here's my question.

 

The idea as I understand it is, that if a space ship moves at, or very near the speed of light for 1 light year then everyone on earth ages at a much faster rate. I found an online calculator at http://keisan.casio.com/exec/system/1224059993which states that (if I used it correctly) that if you travel for 1 year at the speed of light, then 57.2 years will have passed on earth.

 

This makes no sense to me because to achieve this effect you must be moving a person at the speed of light. We know that to travel 1 light year at the speed of light, from the observers point of view (earth) it takes 1 year. Why, when you add time dilation to the mix does it actually take 57.2 years from earths point of view. Surely you are no longer traveling at the speed of light then, but instead at 1/57th the speed of light?

 

This seems counter intuitive to me which probably means I don't understand the concept properly.

 

From my point of view, if you fly to Alpha Centauri at a distance of 4.22 light years then from earths point of view, you will arrive 4.22 years later. Yet with time dilation in the mix it will take from earths point of view 241.384 years. So therefore, from earths point of view you are not traveling at the speed of light, instead you are traveling much slower than the speed of light.

 

My question as well, is does the same amount of time 241.384 years pass for Alpha Centauri as well?

 

Sorry this all seems counter intuitive to me.

[Janus]

First off, you can't travel at the speed of light.

 

With the example given, the speed is more like 99.985% of the speed light.

 

Now here's the trick, not only does the person and the Earth measure the times differently, they also measure the distance differently. Thus if the person travels at 99.985% of c for 1 year by his clock, he will say that he has crossed 1 light year.

However, that same distance that the person measures as being 1 light year is measured as being ~57.2 light years by the Earth, and so it takes 57.2 years by the Earth clock for the person to cross the distance at near light speed.

 

Yes, it is counter-intuitive. But that just shows that our intuition, which was developed by dealing with velocities much, much smaller than light speed and at which these effects are all but immeasurable, fails under conditions where we are dealing with velocities that are a good fraction of the speed of light.

Edited October 21, 2015 by Janus

[swansont]

 

This makes no sense to me because to achieve this effect you must be moving a person at the speed of light. We know that to travel 1 light year at the speed of light, from the observers point of view (earth) it takes 1 year. Why, when you add time dilation to the mix does it actually take 57.2 years from earths point of view. Surely you are no longer traveling at the speed of light then, but instead at 1/57th the speed of light?

 

 

If you want to look at it from the point of view of the earth and have it take 1 year, then it takes 1/57th of a year on the space ship because they are moving close to the speed of light and their clock runs slow, relative to the earth.

[Daniel Foreman]

So, if my understanding is correct now. It is personal time for the traveler that is affected, that is time as it appears to the traveler. So no more than 4.22 years can pass in the real universe, while at the same time from the travelers point of view it will take approximately 0.074 years (approx 27 days). Is that right?

[swansont]

So, if my understanding is correct now. It is personal time for the traveler that is affected, that is time as it appears to the traveler. So no more than 4.22 years can pass in the real universe, while at the same time from the travelers point of view it will take approximately 0.074 years (approx 27 days). Is that right?

 

Not quite. From earth the 4.22 LY trip looks like it takes just over 4.22 years, since v is close to but still below c — it can't take less time. From the traveler's perspective it can take 27 days, given the proper choice of speed. (It's also a proportionally shorter distance from the traveller's perspective)

[pzkpfw]

So, if my understanding is correct now. It is personal time for the traveler that is affected, that is time as it appears to the traveler. So no more than 4.22 years can pass in the real universe, while at the same time from the travelers point of view it will take approximately 0.074 years (approx 27 days). Is that right?

Try to avoid thinking "real universe".

 

That'll lead to thinking there's absolute time and absolute distance. It's all relative.

[Daniel Foreman]

Ok lets simplify this because frankly I'm not following either of you.

 

In practical terms, if we could (and I know we cant) accelerate a body of mass up to near light speed say 99% then it would take a little over 4.22 years for the mass to arrive at Alpha Centuari from the perspective of the depature point (earth) and the arrival point (alpha).

 

That body of mass however, would age at a slower rate, so from it's internal perspective it would be a matter of weeks or months old rather than a year older?

 

Does this accurately reflect the nature of time dilation?

[swansont]

Ok lets simplify this because frankly I'm not following either of you.

Not to worry. puppypower's post was entirely off-topic and wrong to boot. Best to ignore both it and my response.

 

In practical terms, if we could (and I know we cant) accelerate a body of mass up to near light speed say 99% then it would take a little over 4.22 years for the mass to arrive at Alpha Centuari from the perspective of the depature point (earth) and the arrival point (alpha).

 

That body of mass however, would age at a slower rate, so from it's internal perspective it would be a matter of weeks or months old rather than a year older?

 

Does this accurately reflect the nature of time dilation?

Yes.