Expert: Courtney Seligman - 9/25/2010

Question
Hi I've always understood that we cannot travel faster than light speed and in sence dont need to because once we reach light speed volocity times stands still for the traveller but time continues for the rest of the people?
  So say Alpha Centuri is 4.5 light years away and if i was to travel there at light speed its a case of the 6 seconds to press the button etc but for me only seconds would pass but for my family on earth 4.5 years have passed and for my return the same so a total of 9 years as passed on earth but for me mearly minutes have gone by?
  My question is I've been told galacxies or some galaxies can travel very close to the speed of light and if thats so does that mean then if our galaxies travelled at near light speed volocity then time for us should be at a near stand still but for other galaxies around us like Andromada time would pass more quickly. Or another way of putting in does that mean for those galaxies that are travelling at near light speed everything inside it from our point of view wouldnt age etc i just can get my head around it maybe im wrong with time slowing down as you reach light speed and stopping at light spead volocity  

Answer
You are correct in that when you travel at the speed of light, time "seems" to stand still for you, in the sense that while (in your example) 9 years passed for those you left behind and returned to, only a few seconds passed for you.

However, that does not mean that time did not seem to pass for you, in a completely normal way. No observer, no matter how they are moving, perceives time as flowing in any way other than the normal way. Every second, a second of time passes for the observer (this is called the observer's "proper" time). It is the outside observers, relative to whom you are traveling at the speed of light, who would see your time as slowing and stopping, as you neared that speed. So for them your time stops, but for you it does not. (There is a paradox here, which I will get back to at the end.)

Similarly, if we could observe a galaxy that is moving away from us at the speed of light, we would see it at the same moment in time, no matter when we looked at it. But for those in the galaxy, time would be passing at a completely normal rate.

The reason we see the galaxy at the same moment in time is that all the light that reaches us was emitted at that same moment, not because its time is passing any differently from ours. As an example, suppose the galaxy was approaching us at the speed of light. Since it would be keeping up with the light it emitted at the moment it reached light speed, throughout its approach we would see it at only that moment. Unfortunately, this simple example doesn't easily lend itself to explaining what happens when the galaxy passes us. Also, to return to the paradox I noted above, it doesn't explain how, once you return home, you have only aged a few seconds, while those who stayed there are 9 years older.

The problem is, dealing with the apparent slowing of time during the journey is much simpler than dealing with the change in relative time observed when the traveler and the stay-at-homes get back together. In my experience, that can be confusing even when well explained; and I happen to be getting over a bad bug, and am feeling very fuzzy-headed, so I'm afraid that if I try to go over that right now, all I may do is increase your state of confusion. So I'm going to stop here, let you digest this, and have you send me a follow-up in a day or two, when I will hopefully be feeling much better, and better able to give you a proper answer.