# Traveling at the Speed of Light and time travel **URL:** \ **Category:** Factual Questions\ **Created:** [August 10, 2009, 12:57am UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895 "2009-08-10T00:57:33Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [August 10, 2009, 3:52pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/21 "2009-08-10T15:52:37Z")
> [@AK84](#): > > One thing I have read in various popular science magazines is that time goes slower as you approach the speed of light. My question is lets say in a craft moving away from the eath at the speed of light to a point 1 light year away, and the returning to earth, would the occupants of this craft discover that more time than two years has elapsed on earth when the return? No. At a speed close to the speed of light, time inside the spaceship slows down. If you took a 2 light-year round trip near the speed of light, then 2 years would pass on Earth, but only a few months would pass inside the ship. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [August 10, 2009, 3:59pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/22 "2009-08-10T15:59:11Z")
> [@CookingWithGas](#): > > And technical. As mentioned upthread, as velocity increases, so does mass, so you have to continue to increase thrust to maintain the same acceleration, and at some point it’s impossible to provide enough thrust. So those speeds we’re talking about are infeasible given today’s technology (and maybe infeasible due to the laws of physics). Well of course technical - but age impacts on society (or the desire to expanded into space) as as show stopper was the OP’s point. At this current time, unless we actually find a neat way to fold space that doesn’t require unobtainium, interstellar expansion will at best result in a loose knit confederacy of isolated islands in the dark. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [August 10, 2009, 4:01pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/23 "2009-08-10T16:01:16Z")
> [@Peter\_Morris](#): > > No. At a speed close to the speed of light, time inside the spaceship slows down. If you took a 2 light-year round trip near the speed of light, then 2 years would pass on Earth, but only a few months would pass inside the ship. Which means I think, that from your perspective, you travelled a distance of 1 light year in only a month, and thus subjectively seem to have passed the speed of light. (Not with respect to your origin or destination, though.) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [August 10, 2009, 4:04pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/24 "2009-08-10T16:04:24Z")
I just can’t stand it. Two posters have stated that mass increases with velocity. THIS IS NOT TRUE! If it were true, one subatomic particle traveling fast enough would collapse all the objects in the universe, including the Earth, into black holes. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [August 10, 2009, 4:09pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/25 "2009-08-10T16:09:19Z")
Yes, but the OP asks about time passing on Earth. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 10, 2009, 4:55pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/26 "2009-08-10T16:55:26Z")
> [@chrisk](#): > > Which means I think, that from your perspective, you travelled a distance of 1 light year in only a month, and thus subjectively seem to have passed the speed of light. (Not with respect to your origin or destination, though.) Nope. Distance contracts along the path of your travel such that it will never appear to you that you are exceeding light speed. Your destination will seem closer than when you started. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [August 10, 2009, 6:20pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/27 "2009-08-10T18:20:43Z")
> [@Peter\_Morris](#): > > No. At a speed close to the speed of light, time inside the spaceship slows down. If you took a 2 light-year round trip near the speed of light, then 2 years would pass on Earth, but only a few months would pass inside the ship. 😕Huh? I am now even more confused. Physics generally goes over my head. When you mean “time slows down”…actually I can’t come up with an example here, simply what do you mean? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 10, 2009, 6:40pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/28 "2009-08-10T18:40:32Z")
> [@AK84](#): > > 😕Huh? I am now even more confused. Physics generally goes over my head. When you mean “time slows down”…actually I can’t come up with an example here, simply what do you mean? Has to do with frames of reference. If you are on a spaceship moving towards another star the clock on your spaceship will run more slowly than my clock back here on earth. To me looking at you it would seem you are moving in super slo-mo (if you are going very nearly light speed). Thing is, from _your_ perspective on the spaceship, all will seem normal. You will perceive one second as passing per second. If you look back to earth (or communicate with me) you will find my clock running faster than yours. This effect has been tested numerous times. Heck, even living at a lower altitude (say in New York versus in Denver) will see your clock run more slowly than those at a higher altitude because you are deeper in the Earth’s gravity well. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [August 10, 2009, 6:50pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/29 "2009-08-10T18:50:58Z")
> [@AK84](#): > > When you mean “time slows down”…actually I can’t come up with an example here, simply what do you mean? Imagine I’m on Earth and you’re traveling in a spaceship at near light speed. We both have telescopes so we can see each other. And we both have big clocks that are keeping time. I can can see your clock and you can see mine. When I look at you through my telescope, I see that your clock is ticking more slowly than mine is. For every one tick your clock, my clock ticks twice! Furthermore you look like you’re moving in slow motion. But from your perspective, everything is moving at normal speed. You don’t feel like you’re moving in slow motion and your clock appears to be ticking completely normally. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [August 10, 2009, 6:54pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/30 "2009-08-10T18:54:48Z")
> [@Whack-a-Mole](#): > > If you look back to earth (or communicate with me) you will find my clock running faster than yours. Actually, this is not true. Assuming that the spaceship is cruising along at a constant velocity when you look back at Earth you’ll see the clocks there running more _slowly_, not faster. From the perspective of Earth, the clock on the spaceship is running slow. And from the perspective of the spaceship, the clock on Earth is running slow. Isn’t relativity fun! ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 10, 2009, 7:02pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/31 "2009-08-10T19:02:56Z")
> [@The\_Hamster\_King](#): > > Actually, this is not true. Assuming that the spaceship is cruising along at a constant velocity when you look back at Earth you’ll see the clocks there running more _slowly_, not faster. > > From the perspective of Earth, the clock on the spaceship is running slow. And from the perspective of the spaceship, the clock on Earth is running slow. Isn’t relativity fun! True. Forgot about that. It sorts itself out though and the person in the spaceship, upon returning to earth, will have clocks/calendar behind that of people left behind on earth. ---
**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\ **Post date:** [August 10, 2009, 7:24pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/32 "2009-08-10T19:24:18Z")
This is how I tried to explain it to my kids. It’s over simplified and actually not entirely correct but it will help. You have to just believe one rule: that nothing can exceed the speed of light. Period. So, say you were sitting in a train station and a train came through at 100mph. The driver throws a rock out the front of the train at 50mph. From the driver’s perspective, the rock travels at 50mph away from him. From your perspective, it is travelling at 100+50mph or 150mph away from you. Now…let’s say we have a spaceship travelling at half the speed of light or 0.5c. You are sitting in a spaceship station and the ship comes through without stopping. The pilot shines his flashlight out through the front window. From the pilot’s persepctive, the light beam moves away from him at 1.0c, the speed of light but what you see in the station is not what you would expect. You also see the light beam moving away at the speed of light, not 1.5c as you would expect since this simply isn’t allowed. A quick and easy explanation is that to the observer, the spaceship’s length has contracted (which it has) and that the clock on the spaceship is going slower (which it is). With this in mind, we can see how the pilot (oblivious of these contractions) calculates his beam to be travelling away at c, the speed of light and the observer also calculates it to be travelling at c since they both have different frames of reference. I remind my kids that speed = distance/time although this newtonian equation can’t be applied in these situations. But it helps and since distance and time are different for the pilot and the observer, they both end up with the same answer. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [August 10, 2009, 7:57pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/33 "2009-08-10T19:57:30Z")
> [@zombywoof](#): > > I remember hearing of experiments where a highly accurate clock was taken on something like a long, fast plane ride and compared to an identical sort of clock upon return, and there was a measureable difference between the two consistent with relativistic principles (an extremely tiny fraction of a second, given the relatively low speed). If you’ve ever used a GPS, both special and general relativity equations are necessary to correctly determine your location. If you don’t account for both the speed of the satellite and gravitational time dilation, your GPS is useless. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [August 10, 2009, 8:13pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/34 "2009-08-10T20:13:43Z")
> [@Ring](#): > > I just can’t stand it. Two posters have stated that mass increases with velocity. THIS IS NOT TRUE! > > If it were true, one subatomic particle traveling fast enough would collapse all the objects in the universe, including the Earth, into black holes. Photons are massless particles so we don’t need to worry about them. Massed particles do have an increase in mass to go along with an increase in energy. Remember the famous [Oh-my-god particle](http://www.fourmilab.ch/documents/OhMyGodParticle/)? The author calculates the speed of the particle at: v = 0.9999999999999999999999951 c Even at that speed, the mass was about equivalent to a single bacterium. Nothing could push a subatomic particle fast enough to make the rest of universe care much about it. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [August 10, 2009, 9:03pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/35 "2009-08-10T21:03:00Z")
> [@Exapno\_Mapcase](#): > > Photons are massless particles so we don’t need to worry about them. > > Massed particles do have an increase in mass to go along with an increase in energy. Remember the famous [Oh-my-god particle](http://www.fourmilab.ch/documents/OhMyGodParticle/)? > > The author calculates the speed of the particle at: > v = 0.9999999999999999999999951 c > > Even at that speed, the mass was about equivalent to a single bacterium. Nothing could push a subatomic particle fast enough to make the rest of universe care much about it. You’re missing the point. If you transform to the particle’s frame how fast would the Earth, for instance, be moving? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [August 10, 2009, 9:08pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/36 "2009-08-10T21:08:10Z")
> [@Ring](#): > > You’re missing the point. If you transform to the particle’s frame how fast would the Earth, for instance, be moving? The same speed in the opposite direction. So, from the particle’s perspective the Earth is tremendously massive, exerting a huge gravitational pull. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 10, 2009, 9:24pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/37 "2009-08-10T21:24:08Z")
> [@](#): > > The same speed in the opposite direction. So, from the particle’s perspective the Earth is tremendously massive, exerting a huge gravitational pull. If this were true, then from the particle’s perspective, the Earth would be so massive and have such a huge gravitational pull that it would be a black hole. But whether something is or is not a black hole isn’t the sort of thing that different observers can disagree about. So, since we see that the Earth isn’t a black hole, the Oh-My-God particle would also have seen that the Earth wasn’t a black hole. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [August 10, 2009, 10:30pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/38 "2009-08-10T22:30:35Z")
> [@Chronos](#): > > If this were true, then from the particle’s perspective, the Earth would be so massive and have such a huge gravitational pull that it would be a black hole. But whether something is or is not a black hole isn’t the sort of thing that different observers can disagree about. So, since we see that the Earth isn’t a black hole, the Oh-My-God particle would also have seen that the Earth wasn’t a black hole. According to Wikipedia black holes don’t form around high-velocity objects because aberration prevents the formation of an event horizon. The high relativistic mass still produces an extreme gravitational field from the perspective of a stationary observer, but the high velocity of the object allows light to escape. [Mass in general relativity - Wikipedia](http://en.wikipedia.org/wiki/Mass_in_general_relativity) (See the section titled “Can an object move so fast it turns into a black hole?”) From the perspective of the Oh-My-God particle the Earth is massive enough to form a black hole, but it is still visible. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [August 10, 2009, 10:42pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/39 "2009-08-10T22:42:52Z")
> [@Exapno\_Mapcase](#): > > Even at that speed, the mass was about equivalent to a single bacterium. Nothing could push a subatomic particle fast enough to make the rest of universe care much about it. Maybe nearly all the energy in the universe accelerating it would. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [August 10, 2009, 11:59pm UTC](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895/40 "2009-08-10T23:59:21Z")
> [@Voyager](#): > > Maybe nearly all the energy in the universe accelerating it would. Outside of a really bad movie, how would this be possible? [Previous page](https://boards.straightdope.com/t/traveling-at-the-speed-of-light-and-time-travel/505895.md?page=1)