# Help me understand this: Einstein & time. **URL:** \ **Category:** Factual Questions\ **Created:** [May 2, 2003, 5:51am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577 "2003-05-02T05:51:12Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![BrentLumkin](https://avatars.discourse-cdn.com/v4/letter/b/9d8465/32.png) [@BrentLumkin](https://boards.straightdope.com/u/BrentLumkin)\ **Post date:** [May 2, 2003, 5:51am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/1 "2003-05-02T05:51:12Z")
Don’t ask me why, because I don’t even really know (it’s just my nature), but I was sitting here researching various different things which don’t really have an exact answer, like ending the aging process, and time travel. Anyway, I was researching Einstein, and I came to an example dealing with time. If you’re on a train that’s moving forward at 50 mph. You throw a ball in the direction that the train is moving. Relative to you and the train, the ball leaves your hand travelling at 20 mph. From the point of view of someone standing alongside the tracks the ball is moving 70 mph. Ok, I get that. Again, you’re on a train. This time, though, the train is moving much faster – at half the speed of light, or 93,000 mps (miles per second). And instead of throwing a ball, you turn on a flashlight. The light travelling relative to the observer standing alongside the tracks is 186,000 mps, the speed of light. Ok, I get this also. The speed of light is always a constant 186,000 mps, got it. Again, you’re on a train moving at 93,000 mps, and again, you turn on your flashlight. Relative to you, on the train, the light travels 186,000 mps, again because light is CONSTANT, never changing basicly. All of this brought me to the equation v=d/t, velocity (speed) equals distance travelled divided by time. For the first scenario this equation works out fine, but for the ones dealing with light it doesn’t work out because light is always a constant 186,000 mps. This obviously leads one to see that time and distance is not the same for all observers. Ok, I understand all of the above, accept it, and all of that. I get it, and I’m ok so far. However, I then came to the next bit of text, which I AM having a problem with understanding. Here we go… > [@](#): > > In our second and third train examples, the speed of light turns out to be exactly the same for both you and the observer standing along the tracks because time, as measured by your watch, ticked along at a slower pace than time measured by the observer. Not only that, distance changed, too. For the observer, a one-foot ruler whizzing by on the train would have measured less than a foot. > > The weird thing is that, for you on the train, time wouldn’t seem to be moving slower and your ruler wouldn’t be shorter – all would appear normal. However, time on the rest of the Earth would appear to be ticking along slower and its rulers would be shorter. > > Now let’s say you want to do some time travelling. You board a spaceship and take off for deep space. > > The ship approaches the speed of light. Time for you seems to pass as it always has. It takes you about five seconds to tie your shoe. But to an observer on Earth (assuming he or she could watch you), you are moving at a snail’s pace. It takes hours to tie your shoe. > > Anyway, you continue on your journey. You slow down, stop, and accelerate back to Earth. You arrive home. You have aged two years during your flight. Two hundred years have passed on Earth. You have successfully travelled forward through time. …I don’t understand this at all, even with my understanding of everything else. Is it that this is just taking everything to an entire new level, or is it that I don’t truly understand the other basic ideas? Why would time seem normal to me if I tie my shoes while going the speed of light, and to other observers it would seem like hours for them? I don’t understand the ruler bit, or any of this. Can someone here explain this to me? ---
**Author:** ![js\_africanus](https://avatars.discourse-cdn.com/v4/letter/j/87869e/32.png) [@js\_africanus](https://boards.straightdope.com/u/js_africanus)\ **Post date:** [May 2, 2003, 6:37am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/2 "2003-05-02T06:37:19Z")
> [@](#): > > \*Originally posted by BrentLumkin \* > \*\*…I don’t understand this at all, even with my understanding of everything else. Is it that this is just taking everything to an entire new level, or is it that I don’t truly understand the other basic ideas? > > Why would time seem normal to me if I tie my shoes while going the speed of light, and to other observers it would seem like hours for them? I don’t understand the ruler bit, or any of this. Can someone here explain this to me? \*\* I’m probably not qualified, but I’m bored. So think of this: I have a clock that measures time by shooting a laser beam against a mirror and back. So shoot-bounce-back is one tick on the clock. Now imagine that I’m travelling on a train at high speed and you’re not. You and I have identical clocks. Our clocks are oriented perpendicular to my train’s direction of travel. For your clock, it ticks off at the normal shoot-bounce-back covering, let’s say, one foot to the mirror and one foot back. For me it does the same thing. However, relative to you, my clock shoots it’s laser beam at one point in space, then it travells to another point during the time that it takes for the beam to travel that one foot, then a third point as the beam travels back. So you see your clock just shoot a beam back and forth along a straight line. You see _my_ clock shoot a beam that goes along two diagonals since the clock is travelling forward! The diagonals that my beam travels are longer than the perpendicular lines that your’s travels. So you see my clock ticking off more slowly than yours! However, since I’m not allowed to determine that I’m in motion through any experiment that I can conduct without looking outside of the train, the clock looks normal to me. For my clock to seem normal to me time must literally slow down relative to your time. I’m not sure I recall why distance shortens. Things get shorter as they go faster. Anyway, here is a website that may help: [http://www.glenbrook.k12.il.us/gbssci/phys/Class/relativity/U7l2b.html](http://www.glenbrook.k12.il.us/gbssci/phys/Class/relativity/U7l2b.html) Check it out. ---
**Author:** ![Scarf-Ace](https://avatars.discourse-cdn.com/v4/letter/s/bc79bd/32.png) [@Scarf-Ace](https://boards.straightdope.com/u/Scarf-Ace)\ **Post date:** [May 2, 2003, 11:32am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/3 "2003-05-02T11:32:22Z")
Top explanation js! I have been struggling with understanding relativity for ages! I see now that its simply due to the fact that time IS speed, ie the measure of a body moving through space. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [May 2, 2003, 11:38am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/4 "2003-05-02T11:38:51Z")
> [@](#): > > \*Originally posted by BrentLumkin \* > If you’re on a train that’s moving forward at 50 mph. You throw a ball in the direction that the train is moving. Relative to you and the train, the ball leaves your hand travelling at 20 mph. From the point of view of someone standing alongside the tracks the ball is moving 70 mph. Ok, I get that. You may realize that even this isn’t exactly true. It’s actually more like 69.99999999999985 MPH. Relativistic effects are theoretically there at the smaller scales, but they’re too small to notice. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [May 2, 2003, 12:11pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/5 "2003-05-02T12:11:14Z")
> [@](#): > > Relativistic effects are theoretically there at the smaller scales, but they’re too small to notice. Technology has now been able to directly produce measurable relativistic effects. Two incredibly accurate clocks were synchronized and one was put on a jet and flown around at high speeds. After several hours, the moving clock and the stationary one had different times. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [May 2, 2003, 12:14pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/6 "2003-05-02T12:14:25Z")
Yes, I agree. Notice though that you said, “at high speeds”. 😃 ---
**Author:** ![Marley23](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Marley23](https://boards.straightdope.com/u/Marley23)\ **Post date:** [May 2, 2003, 1:58pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/7 "2003-05-02T13:58:11Z")
He did say high speeds, but no jet goes anywhere near the speed of light. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [May 2, 2003, 2:09pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/8 "2003-05-02T14:09:16Z")
> [@](#): > > \*Originally posted by Marley23 \* > \*\*…no jet goes anywhere near the speed of light. \*\* Relative to? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 2, 2003, 3:42pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/9 "2003-05-02T15:42:30Z")
In spacetime the velocity vector has four components proportional to; x, y, z, and t. Now this may seem a little strange but the four-velocity vector has a constant magnitude equal to c, the speed of light. This means that we are _always_ traveling through spacetime at c. So when we’re sitting perfectly still we’re moving through time at the normal speed of c. However, when we also start moving in three dimensional space we can’t continue to move through time at c so time slows down. 🙂 ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [May 2, 2003, 5:22pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/10 "2003-05-02T17:22:53Z")
I’m not sure if this was a problem for the OP, but this part : > [@](#): > > Anyway, you continue on your journey. You slow down, stop, and accelerate back to Earth. You arrive home. You have aged two years during your flight. Two hundred years have passed on Earth. You have successfully travelled forward through time. needs to be addressed. I really hope that the text went on to explain after that, because if it just left it there, it’s horribly misleading. While you’re on the spaceship, the Earth observer sees you take hours to tie your shoe. But if you (the space traveller) look back at an Earthling tying his shoe, you see the same thing – it takes him hours to do the task that for him only takes a few seconds. This is because while you’re moving along you can’t tell if it’s him or you moving (as pointed out by the text in the railroad example). And yet it is true that when you come back, two years for you equalled 200 years for Earth (depending on how fast you went). This is traditionally known as the “twin paradox”, though it’s easily resolved (the name comes from the idea of taking newborn twins, put one on a spaceship, and leave the other on Earth). The resolution is that you have to both speed up to leave and slow down and stop when you come back (you have to turn around, too). During these times you _can_ tell the difference between you and the Earth, and you will be able to know that the spaceship is the one moving. More technically, you are no longer in an “inertial reference frame” whenever you’re accelerating. Also, “travelling forward through time” is relatively easy. I do it all the time. ---
**Author:** ![Jayrot](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@Jayrot](https://boards.straightdope.com/u/Jayrot)\ **Post date:** [May 2, 2003, 5:54pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/11 "2003-05-02T17:54:28Z")
[related hi-jack] Is our sense of time slightly warped because we are, and always have been, moving through _space_ at however many thousands of miles per hour? (i.e. the earth revolving around the sun) I guess this really just stems back to the idea of relativity and frames of reference. Earth is spinning on its axis, it is also revolving around the sun, our solar system is revolving in our spiral galaxy, and I presume our galaxy is moving as well (cool pictures of galaxies crashing into each other). Is it possible that even sitting on my ass here in my chair, I’m already moving through space at a good fraction of c? [/related hi-jack] ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [May 2, 2003, 5:59pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/12 "2003-05-02T17:59:46Z")
Yeah, you are moving at a few hundred km/s with respect to the Big Bang frame. But it’s meaningless to talk about how fast you’re moving period. You realize that, right? ---
**Author:** ![BrentLumkin](https://avatars.discourse-cdn.com/v4/letter/b/9d8465/32.png) [@BrentLumkin](https://boards.straightdope.com/u/BrentLumkin)\ **Post date:** [May 2, 2003, 6:48pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/13 "2003-05-02T18:48:20Z")
Is the concept presented about the tying of the shoes the same concept as watching a plane overhead? To them they are going hundreds of mph, but to me on the ground it looks as if they are going so slowly. Is this the same TYPE of thing? ---
**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:** [May 2, 2003, 7:20pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/14 "2003-05-02T19:20:18Z")
> [@](#): > > \*Originally posted by BrentLumkin \* > \*\*Is the concept presented about the tying of the shoes the same concept as watching a plane overhead? To them they are going hundreds of mph, but to me on the ground it looks as if they are going so slowly. Is this the same TYPE of thing? \*\* Nope…this example is just a trick of perception due to distance. Relativity is not at play here (at least not in significant enough of a fashion to be noticeable by you just looking). > [@](#): > > \*Originally posted by Little Nemo \* > Technology has now been able to directly produce measurable relativistic effects. Two incredibly accurate clocks were synchronized and one was put on a jet and flown around at high speeds. After several hours, the moving clock and the stationary one had different times. You need not even get as fancy as this. They did a similar experiment with two highly accurate clocks and a water tower. One clock at the base and one on the top of the water tower. As light leaves the gravity well of the earth it speeds up (the further from the center of the earth you are the less gravity pulls on you). Since the speed of light always remains constant you need to change the time variable. Sure enough the two clocks showed different times after sitting there awhile. So, living on the top floor of a skyscraper will see you moving through time at a different rate than those on the ground. Of course the effect is amazingly small so nothing you are going to notice without atomic clocks at your disposal. They also determined that cosmonauts aboard the Mir Space Station were about three seconds ‘behind’ those they left on earth upon their return after six months in orbit. ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [May 2, 2003, 7:31pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/15 "2003-05-02T19:31:46Z")
[/hijack] Here’s an interesting thought. Is the curvature of light in response to gravity caused by the gravity, or by the time dialation? Consider a photon travelling through a gravity well. Time will be dialated on a gradient, so the portion of the photon that is closest to the mass causing the time dialation will experience a slower time scale than the portion that is furthest away. Of course, since the speed of light is constant, and since speed is distance/time, the distance travelled by the bottom portion of the photon will have to decrease to keep the speed constant. The end result would be that the photon’s path will curve, since the “top” of the photon is moving in a time frame that is slightly faster than the “bottom”. [/hijack] ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 2, 2003, 7:50pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/16 "2003-05-02T19:50:09Z")
> [@](#): > > \*Originally posted by Joe Random \* > \*\*[/hijack] > to keep the speed constant. > > The end result would be that the photon’s path will curve, since the “top” of the photon is moving in a time frame that is slightly faster than the “bottom”. > [/hijack] \*\* What you say is correct but it’s much easier to see this if you think in terms of a plane wave propagating through the field. On the other hand this only accounts for one half the curvature observed ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 2, 2003, 8:00pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/17 "2003-05-02T20:00:44Z")
> [@](#): > > Here’s an interesting thought. Is the curvature of light in response to gravity caused by the gravity, or by the time dialation? Missed this part. Gravity _causes_ the time dilation. ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [May 2, 2003, 8:06pm UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/18 "2003-05-02T20:06:05Z")
> [@](#): > > \*Originally posted by Ring \* > \*\*Missed this part. Gravity _causes_ the time dilation. \*\* Would it be possible for time dialation to cause gravity, instead? Actually, now that I think about it, I kind of doubt it. Gravity is indistinguishable from acceleration, and, while acceleration always causes time dialation, time dialation certainly does not cause acceleration. Still, it was an interesting line of thought. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [May 3, 2003, 2:35am UTC](https://boards.straightdope.com/t/help-me-understand-this-einstein-time/172577/19 "2003-05-03T02:35:49Z")
> [@](#): > > He did say high speeds, but no jet goes anywhere near the speed of light. That was my point. Relativity affects all moving objects, regardless of their speed. It’s just that the effects only become noticable at very high speeds.