This never made any sense to me whatsoever.
I’ve see all the physicists (Michio Kaku, Stephen Hawking, Neil DeGrasse Tyson, etc.) explain this principle but it doesn’t make sense. They say that if you were to go to the moon and back at a certain speed near the speed of light, you might return to Earth a thousand years into the future like what happened in Planet of the Apes. But if you were going at the speed of light, you would arrive at the time light takes to arrive there. Why the dip? What is being missed?
The light takes the same amount of time to get there from an external point of view. It’s more like time slows down for you the faster you go, which from an external perspective would look like you moving and acting slower than normal. So in the time it would take light to travel 1 light year, it always takes one year. However, you would be slowed down so much that it would appear to you that much less time had passed, maybe only a few days. If you travel at the speed of light you slow down so much that no time passes for you at all at that speed. So you instantly arrive, from your point of view. However, from the point of view of an external observer, it still took one year.
Essentially, it slows down the amount of time you experience, but the amount of time that actually passes externally doesn’t change. If you go to the moon, it will take only 1 second at light speed, so you wouldn’t really notice whether it felt instant or to take a second. However, if you go somewhere further like Proxima Centauri, which is 4 light years away, you will arrive back on Earth at least 8 years later (there and back). If you go at light speed, it would appear to be instant, suddenly you’re at Proxima Centauri 4 years later, suddenly you’re back at Earth 8 years later. If you go just below light speed, you’ll see the world outside go past like it’s being fast forwarded, and when you return, 8 years will have been compressed into something that seems much shorter to you.
Great explanation!
Perhaps a stupid follow up but what would this mean for things necessary for survival like food and water? Would I theoretically starve on that 4 year trip before I even realized I needed water?
No. Let’s say you and I start each one a stopwatch at the same time here on earth. Then you get in your spaceship and travel the 8 years at light speed, get back to earth and land your ship. When you get out and we put the watches side by side mine shows 70000+ hours while yours the couple minutes it took you to get on and off of the ship.
Just want to add that what the person on the ship observes is length contraction. When their ship is at near light speed, the distance to their destination contracts to nearly zero (because it is moving at near light speed relative to them); this is why the trip seems short to them.
It’s not just your perception - your actual atoms slow down in “time”. This is because any movement they make, from their vibration alll the way up through chemical reactions and physical movements, from your metabolism to your thoughts being transmitted through your neurons, will require acceleration towards light speed from their already high speed.
The faster you move through space, time literally moves slower- you sacrifice one for the other, like a ratio.
No, you would physiologically age at the same fast-forwarded time rate. Essentially, for you, less time would pass at the same time that more time passes for the outside world.
Small correction, but this bit isn’t quite correct:
If you go just below light speed, you’ll see the world outside go past like it’s being fast forwarded, and when you return, 8 years will have been compressed into something that seems much shorter to you.
During the time that you are just below light speed at a constant velocity, clocks that are “stationary” will appear to be moving slow to you. And clocks moving with you will appear to be running slow for a “stationary” observer. As I mentioned in a comment in another reply, the trip would feel short to you because the distance to your destination would contract to nearly zero. “Fast forwarding” (ie, having both you and a stationary observer agree that more time has passed on the stationary observer’s clock) would happen during the periods of acceleration/deceleration at the beginning and end of the trip.
Great explanation! Thanks!
Also if you’re looking for actual practical applications of this that have happened, Cosmonaut Sergei Avdeyev is actual .02 seconds younger than everyone else born at the same moment he was on Earth. He’s an honest to goodness time traveler that has traveled .02 seconds into the future (relative to the rest of us on Earth). He did this by staying in space for a sum total of 748 days on a space station traveling 27,700 km/h around Earth.
It doesn’t help that media often portrays this incorrectly. Take the movie Lightyear, for example. Sure, it’s a sci fi movie for kids so it doesn’t have to be scientifically accurate, but the way they portray it is completely nonsensical. They show Buzz Lightyear trying to reach light speed in his space ship. Each time he flies around the sun, he goes faster, but more time has passed on the planet he left by the time he gets back. In reality, the faster you go, the less time it takes you to get somewhere (from some external reference of time). It’s just your experience of time that changes.
Interstellar is at least a little bit closer (ignoring the whole time travel part)
It doesn’t? If you travel at the speed of light (which isn’t possible since you have mass) you don’t experience any time or distance. To the perspective of a photon, it is released and absorbed at the exact same moment in time, and it has traveled no distance at all.
That’s some major title gore.
Another important thing to mention is that you can’t exceed the speed of light. going to the moon and back at 99.99% the speed of light would maybe feel like a millisecond to you but like a few seconds to people on earth, but you wouldn’t go far into the future.
I think you’re probably misremebering what you read, so let’s first set it right.
The time always depends on the observer. If you’re going at the speed close to the speed of light (let’s say 0.99c, that is 99% of the speed of light), the time will slow down for you from the point of view of others who aren’t moving. If you go this fast, it will look like you were slowed down to the others. You yourself will feel normal and everyone else will look sped up. This effect is the larger the closer to the speed of light you go. We’ll get to the speed of light and faster in a minute.
If you went to the moon and back at 0.99c, to everyone on earth, it will look like it took you just a tiny bit less timw then it would take light, I think 2.5 seconds or so. To you, it would feel much faster, I didn’t calculate it but let’s say 0.1 second. There’s no way you would come back to distant future of Earth after a single trip to the moon.
You could come back to the distant future if you went much further. If you spent a year at 0.99c, much more time would pass on earth. If you kept looking, you would spend a year watching earth at fast forward. In the meantime, earthlings will spend many years wathing you slowed down from their point of view. For you it would feel like a year, for Earth, it would feel like many years, because the flow of time depends on your speed and there is no universal reference time.
Now let’s get to the speed of light. First of all, it is impossible for anything with mass to reach the speed of light. As you approach speed of light, the amount of energy needed to accelerate increases and you need infinite amount of energy to reach the speed of light, which you obviously can’t have. Massless particles, like photons, that is light, move at the speed of light. Hence speed of light :-) It is said that they don’t percieve time, they are sort of simultaneously everywhere along their path from their point of view. Easy to say, hard to imagine. But no dip is happening. From the point of view of someone not moving, just standing on Earth, if you are speeding up, you appear to be moving slower and slower and if you reached the speed of light, you would appear to stop moving altogether. From your point of view, you would feel normal, but Earth would be more and more sped up and then I guess you would be everywhere at once and time would stop to have a meaning? BTW, observing stuff would in fact be problematic, since you need light for that and getting light at the speed of light doesn’t quite work and there’s a bunch of interesting other complications even before that.
To get to speeds faster than speed of light is even more impossible. But there is a theoretical framework for particles faster than speed of light called tachyons. In a way, they are an opposite. They have to be faster, never can reach the speed of light and the closer to the speed of light they are (and therefore the slower they are), the more energy they need to slow down further. They are said to move backwards in time. They have not been showed to exist (once it looked like they might, but it was a technical issue with measurements). I know very little about them.
I saw a brilliant explanation some time ago that I’m about to butcher back into a terrible one, bear with me:
Think about 2 particles traveling together. When one gets tugged, it in turns tugs the other one with it. This tug takes some time, since one particle essentially “tells” the other particle to come with it, meaning there’s some level of information exchange happening between these two particles, and that exchange happens at the speed of light. Think about the travel distance between these two particles, it would be pretty linear, and pretty short, so you essentially do not notice this effect since it’s so fast.
Now think about what happens when those 2 particles start going faster. The information exchange still happens, it still happens at the speed of light, but now that those particles are moving faster in some direction, the information exchange would seem to still go linearly from particle A to particle B, but in reality it would be traveling “diagonally”, since it would have to cover that extra distance being added by the particles moving in certain direction. This is the crucial part: what happens when those particles start getting closer to the speed of light? Well, the information exchange would have to cover the very small distance between these particles, plus the added distance from traveling closer to the speed of light. At first it’s pretty easy to cover this distance, but eventually you’re having to cover the entire distance light would take to travel in a given moment, PLUS the distance between the two particles, which…can’t happen since nothing can go faster than that speed.
That’s essentially why you can never reach the speed of light, and why the more massive an object, the less speed it can achieve: all those particles have to communicate with each other, and that takes longer and longer the closer to the speed of light the whole object moves.
See, this also perfectly explains what you’re asking: from the frame of reference of the particles, they’re seeing the information go in a straight line to them, so time is acting normally for them, but from an external perspective, that information is moving in a vector, taking a long time to reach the other particle since it’s having to cover the distance of near light speed in one direction, plus the distance between the two particles in another direction, for a total vector distance that is enormous rather than being negligible. At some point, you never see the information reach the other particle, or in other words, time for that whole object has slowed down to a near halt. This explains why time feels normal for the party traveling fast: they can’t know they’re slowed down since the information exchange is essentially the telling of time, but the external observer sees that slowdown happen, and in fact they get a compounded effect since those particles also communicate their state to the observer at the speed of light, and that distance between the observer and the particles keeps changing.
This also explains why the particles might be able to also see everything around them happening a lot faster than it should: not only is it taking them longer to get updates about themselves between themselves, but they’re also running into the information from everything around them pretty fast, essentially receiving information from external sources faster than they do from themselves, thus causing this effect of seeing everything happening faster and faster, until it seems to all happen at once at the speed of light.
Here’s the guy who made it all click for me, since I’m pretty sure I tangled more than one of you up with this long read: https://youtu.be/Vitf8YaVXhc
What’s described is Einstein’s theory of relativity. It states that time moves at different speeds depending on how fast things go. The faster you go, the slower you will age.
Say you go to the nearest star, which is about 4 light years way. If you go near speed of light this journey would feel like an instant to you. For anyone else on earth observing your journey, it will look like it has taken you 4 years to complete the journey. A journey that felt like an instant to you actually took many years to complete.
So, people seem to have explained how the time dilation works.
Now to the why:
We found out that the speed of light in a vacuum never changes. It is always the same. Usually in our everyday life speeds add up.
Let’s say you have a cannon that shoots out a ball at 100 km/h. It would hit a wall at 100 km/h.
If you were now driving a car at 100 km/h and shot the ball out of that it would hit the wall at 200 km/h.But the speed of light is different. If you had a light cannon to shoot some light at the wall it would hit at 1 c (the speed of light). But if you drove a car at 100 km/h and shot the cannon from there it would not hit the wall at 1c + 100 km/h. It would still be 1 c.
That’s pretty strange and counter intuitive. Albert Einstein calculated the results of that. What must really be happening is that time slows down and space contracts slightly the faster you drive. So that in the end the speed of light stays the same.
We usually don’t notice that effect at the speeds of your everday life. But we have satellites for GPS with atomic clocks in orbit where that effect becomes relevant. By now they are a few seconds out of sync with stationary earth clocks.
So GPS navigation proofs that time dilation is real.
Time for you is relative to your movement. You experience time because shit is moving inside of you. By shit I mean each particular atoms that build your body.
If you’d be close to a light speed, entirety of you would move in that speed. So for yourself the time is normal. One second is one second.
For someone observing you from earth, you’d blast the universe and after an hour you wouldn’t be visible any more.
For you tho, it’s not even a second.
Time is movement. No movement no time. If something moves then also time changes. If all your atoms would stop now you wouldn’t get older. Also you wouldn’t think any more and you wouldn’t notice any time.
Ps. I was at the same state as you some year ago. I’ve spent several hours absorbing this topic and eventually it clicked. Take your time.
