# I am confused about the passage of time for an outside observer versus an observer moving at the speed of light [duplicate]

Specifically, why would some say that the time experienced for those on Earth would be far greater than the time for light to travel to the destination and back. Example, I have heard it said that if you travel to Alpha Centauri and back at the speed of light, far more than 10 years will have passed on Earth than what it takes for light to travel there and back.

Here is my thinking on this when considering the speed of light time dilation versus actual distance traveled. The speed at which I am traveling at means that I experience time far slower. So if I were to adjust my space ship's experienced travel time to compensate for this time dilation, then I could experience travel to say Alpha Centauri in seconds for me. Even though the actual time it took, relative to Earth, would be about 4.5 years.

If that is the case, then all of these concerns about travelling at relativistic speeds should theoretically be moot. Yes, that means in a round trip to and from Alpha Centauri for the study of the stellar system it would take almost 10 years to those on Earth. But for those on the flight, it should take what I would think is seconds plus time spent in the system doing the study.

All the arguments I see center around the idea of traveling those distances over the duration of time as experienced by the traveler. Thus, when I hear people such as Neal DeGrasse Tyson talking about traveling 4.5 light-years, it always sounds like he is saying that the traveler experiences the passage of time as 4.5 years while moving at relativistic speeds. Thus the problem for the traveler is that going there and coming back while experiencing those years means far more time has passed here on Earth.

But if you are traveling at those speeds and those speeds slow the passage of time from your perspective, then you shouldn't be traveling for 4.5 years from your perspective because you would severely overshoot your target. Instead, you should calculate how much time would pass from your perspective in order to travel 4.5 light-years, which I imagine would be experienced in seconds or possibly minutes. Then travel for that long from your perspective to reach your target area.

Is that correct or am I fundamentally missing something about time dilation and traveling at relativistic speeds?

• Does this answer your question? How long would it take me to travel to a distant star? May 8 '21 at 16:08
• @JohnRennie, no it does not. My question isn't about how long it would take me to travel to the distant star, but instead on the practical experience of interstellar travel from the perspective of both the traveler and the outside observer. May 8 '21 at 18:46
• This post (v3) appears to be non-mainstream as one cannot travel at the speed of light in vacuum. May 8 '21 at 19:37

As pointed out in another answer: in the case of macroscopic objects there is the practical problem that a force large enough to cause close-to-instantaneous acceleration to the speed of light will disintegrate that object.

Sub-atomic particles such as Muons are not subject to such a limitation.

There is the 1963 'Measurement of the Relativistic Time Dilation using $$\mu$$-Mesons' by Frisch and Smith.

Cosmic rays impacting atoms of the atmosphere have such energy that the Muons that are created (in the overall shower of particle creation) have a velocity close to the speed of light. Instant acceleration!

For the muons the amount of proper time that elapses during their journey from the upper atmosphere to where the scientists are is shorter than their own half-life; thus special relativity accounts for the fact that a percentage of the muons created in the upper atmosphere makes it all the way down.

Incidentally, I notice that you are cautiously phrasing your question in terms of 'experiencing time slower' and 'from your perspective'. Presumably that is the kind of phrasing that is used in the sources you read.

I believe it is superior to think of it as a difference in amount of proper time that elapses.

There are setups in particle physics where muons are circling around in a storage ring, close to the speed of light. Because of that velocity there is more time available to make use of the muons for the intended collision experiment. Because of the velocity of the muons in the storage ring: the amount of proper time that elapses for the muons is less than the amount of proper time that elapses for the experimental setup as a whole. So the muons last longer. You still lose muons to spontaneous decay, but the half-life is longer than if they would be moving slower.

There is nothing subjective there. It's not that it is something that only the muons are experiencing. The experimenters make good use of it; per unit of laboratory time the rate of loss of muons to spontaneous decay is less.

• I think I should have phrased my question better. Because I am less concerned with how long it would take to get there than the travel time as experienced by the traveler versus the outside observer. I do understand that there are many physics limitations on fuel, time to get up to speed, and practicality. May 8 '21 at 18:49
• Instead, assume we have solved for all of that, we have gotten up to speed (possibly instantly), and we have a destination in mind. To travel X distance, why do some say it would take far longer to the outside observer than the time it takes light to travel the same distance there and back? If I go 5 light years out at the speed of light, I know time dilation means that I would not experience 5 years of life, it would be less. But I don't see why an outside observer should experience far more time than 10 years (round trip) plus time lingering at the destination. May 8 '21 at 18:52

Yes you can get there in a short time as far as your own aging goes, but maybe not seconds because your body can only withstand some modest amount of acceleration. If one could somehow accelerate the whole body at once, without compressing or altering the blood pressure, or otherwise allow it to withstand large accelerations, then the time overall could be as short as some seconds.

• Thanks, and I did a poor job of wording my question. My interest was more related to why so many say that the time passed on Earth would be far greater than the time it takes for light to travel to the destination and back. May 8 '21 at 18:53