# Can someone explain time slowing down

So I watched a youtube documentary about future rockets and space travel etc and the narrator said if you were moving at 99% the speed of light, 1 day onboard the ship would be an entire year on earth.

Now what I don't quite understand is the word time. Do they mean if someone on earth set their stop watch for 24 hours and someone on the ship set their stopwatch for 24 hours also, the stopwatch in the ship would move much much slower?

Do they mean that the person in the ship ages slower or what? Lets just assume the ship was orbiting the earth ar 99% the speed of light and did so for 1 day and then landed again on earth, would everybody be 1 year older? As in 365 days have passed on earth but the persons stopwatch on the ship has only counted 24 hours?

I'm not sure what they mean by the word time in this instance.

Related Special and General Relativity News on Phys.org
You pretty much have it right. It doesn't matter if you measure it with a atomic clock, mechanical clock, or biological clock (as in someone aging). Time is defined as that which is measured by clocks. Any of these types of clocks will measure the same quantity of time.

So why does someone physically age slower the faster they travel to the speed of light? What happens to the body and the watch on your wrist when you travel this fast? A mechanical watch should work just the same no matter how fast or slow you move.

See if this helps...

The clocks they talk about at the end can be generalized to any interaction. If you could watch the nerve signals moving around in my brain as I fly past you, you would see them taking longer paths, just like the light pulses, and therefore you would see my thoughts slowing down, as well as all other body functions.

Mechanical clocks rely on electromagnetic forces at the atomic and molecular level to hold the parts together and to transfer motion from one part to another. For this reason they behave the same way.

Last edited by a moderator:
Ok that kind of explained it but just to clarify if I was on a spaceship orbiting the earth at 99% the speed of light and did so for 365 days. My physical body would only be 1 year older but everyone on earth would be effectively 365 years older... IE everyone I knew of is now dead and the world for me now seems in the future when I land after 1 day of orbiting?

If you were measuring days using a clock on your ship then yes.

If you were measuring days by watching how the earth below you revolved reletive to the surrounding stars then when you came back everyone else would be 1 year older and you would have aged only a few minutes.

Ken G
Gold Member
Ok that kind of explained it but just to clarify if I was on a spaceship orbiting the earth at 99% the speed of light and did so for 365 days. My physical body would only be 1 year older but everyone on earth would be effectively 365 years older... IE everyone I knew of is now dead and the world for me now seems in the future when I land after 1 day of orbiting?
There are several problems with this, though you have the basic idea. First of all, if you go 99% of the speed of light, the people you leave behind would appear to age about 7 times faster than you, not 365 times faster (perhaps the program said a day maps into a week, not a year?). Second of all, you cannot "orbit" Earth at 99% of the speed of light, you'd just fly off into space. But you could force your ship to follow a circular path, at 99% of the speed of light-- we just wouldn't call that an "orbit" in the conventional sense. But these are minor details-- your basic point is correct.

As to "why" this holds, there doesn't seem to be any advantage in imagining the speed you are moving at is doing something physical to your body, because you are not "moving at a speed", speed is just relative-- as far as you are concerned, you are always stationary. So instead, think of it as two different paths in spacetime that begin and end at the same event, but don't correspond to the same elapsed time. We are very used to two different paths between the same two points not corresponding to the same distance travelled, so it's not necessarily anything all that new to have it not correspond to the same elapsed time either, it's just not what we are used to because we never move at relative speeds near c.

One more quick thing. The observer on the spaceship, unless they undergo acceleration, will see the observers off the spaceship also appear to have "slower" clocks. So there's no point in saying that observer B's clock moves slower than observer A's without noting by whose observations you mean.

Before you can understand this concept, you have to understand why time dilates and space contracts for observers in different intertial reference frames (i.e., one observer moving at some constant velocity relative to the other.) I suggest looking at the videos for The Mechanical Universe, put out by CalTech in the 80's (do a Google search ;)), episodes 42-44.

Everything can be explained by doing Einstein's famous thought experiment about a pair of observers, one on the ground, and the other on an open boxcar on a superfast train, each with a "light clock" - i.e., a pair of mirrors in which light reflects off of each back and forth, such that since the speed of light in a known constant for all observers, the measuring of each reflection is like the tick of a clock. Each observer sees the light in his light clock go up and down, but sees the light in the other's light clock travel diagonally, which because there is more distance that is measured (but with the same speed of light), there must be more time measured relative to what the other observer measured.

Similarly, the observer on the ground can measure (i.e., using his own clock & yardstick) the speed of the train and the distance between 2 posts along the train track - which of course are stationary in his inertial reference frame - while the observer on the train can also measure (i.e., also using his own clock & yardstick) the distance between the posts, and the speed with which they are hurtling toward him, concluding that the distance between the posts must be the product of that speed and the elapsed time on his clock. But since the speed that the observer on the ground measures the train moving is exactly the same as the speed that the observer on the train measures the posts hurtling toward him, since the observer on the train measured a certain fraction of the time that the observer on the ground measured, the observer on the train must have measured the distance between the posts at exactly the same fraction of the distance that the observer on the ground measured, so the yardstick of the observer on the train must be perceived by the observer on the ground as being contracted relative to his own. (And of course, all this time, the observer on the train is looking at the clock and yardstick of the observer on the ground and perceiving that to be dilated and contracted by that amount as well.)

So the yardstick of the observer on Earth that measured, e.g., Sirius at 8 light years away, would be, as measured by the yardstick on the rocketship to be about 1/365th of the length of his own yardstick, so therefore using his own yardstick, Sirius is only about 8 light-days away, and is hurtling toward him at virtually the speed of light (i.e., as measured by his clock & yardstick), so it ends up reaching him in ... 8 days. But that 8 light-days measured by the observer in the rocketship is still 8 light years as measured by the observer on Earth - who BTW sees Sirius as being stationary, and the rocketship hurtling toward Sirius at virtually the speed of light, and thus sees him actually reach Sirius in ... 8 years.

Last edited:
Their time only slows down from your perspective; and yours from their perspective. It's like I see a "smaller" image of you from a distance; and you see a "smaller" image of me. Analogously, once we meet up/synchronize speed, the images/times will match up again as well.

phinds
Gold Member
2019 Award
Their time only slows down from your perspective; and yours from their perspective. It's like I see a "smaller" image of you from a distance; and you see a "smaller" image of me. Analogously, once we meet up/synchronize speed, the images/times will match up again as well.
So you think that in the "twin paradox" everyone is wrong when they say the the traveling twin will be younger when they meet back up?

Ken G
Gold Member
To be fair, dammay would be correct if the motion of the two was symmetric, like a person and their mirror image. But not in the general case, no.

So you think that in the "twin paradox" everyone is wrong when they say the the traveling twin will be younger when they meet back up?
Usually in the twin paradox acceleration is not symmetric, hence the difference in elapsed time upon re-union.

The image analogy is not completely the same as time dilation, but you can think of the size of the image as the rate of the passage of time. Even when you meet up and the rate of the passage of time becomes the same (say 1 click per second), a difference in the elapsed time has occurred (say 10 clicks on my clock versus 20 clicks on yours). This is the amount independent of how far away the two now-stationary clocks are/how long the delay in communication is.

Now you have several ways to proceed, namely (1) undo the difference by having one observer travel back in time; (2) move yourself in order to incur the same amount difference. Going forward with option (2), in terms of acceleration, it's like having the rocket stop (now time passes at the same rate but there is still a difference in elapsed time) and the earth move to meet up with the rocket. When they meet up again, they will have the same times exactly.

It can be a difficult situation to grasp because while we can travel in opposite directions in distance, we are still traveling forward in time. Therefore to even things up, something has to be done by the "other" party. It also reflects how time does not lie in motion (laws of physics) per se but in the degrees of freedom (decoupling of motion).

Last edited:
Wow, Thanks very much. This is so much clearer now. Thanks again.

Earlier today I was driving along the highway to test something. I wanted to see if I could slow time down, all to find that I only had more time. As I went along the road I purposely set my rate of speed at an average to other motorist. All the while I safely cut every corner on this double lane highway. From my estimation I was catching up to the vehicales in front of me and moving futher away from the ones behind. This would stand to reason because I'm traveling the same speed but a shorter distance over the whole 15min trip. Did I slow 'time' down, no not really. I was simply able to get more done since I arrived at my destination in a shorter time span. At least that's here on Earth. Now all motorists look at their watches and its all the same time. Go figure.

Last edited:
Earlier today I was driving along the highway to test something. I wanted to see if I could slow time down, all to find that I only had more time. As I went along the road I purposely set my rate of speed at an average to other motorist. All the while I safely cut every corner on this double lane highway. From my estimation I was catching up to the vehicales in front of me and moving futher away from the ones behind. This would stand to reason because I'm traveling the same speed but a shorter distance over the whole 15min trip. Did I slow 'time' down, no not really. I was simply able to get more done since I arrived at my destination in a shorter time span. At least that's here on Earth. Now all motorists look at their watches and its all the same time. Go figure.
Another thing to muse upon is, can we have any change in space without any change in time, or vice versa? No; unless you have something totally static, which by definition changes neither in space nor in time. And you cannot have any event happening without change. How much (or little) space we have & how much (or little) time we have depend on each other. Trippy eh?