ELIF: how is time relative?
In: Physics
As far as theoretical physics goes, time doesn’t really exist. It is just how our brains process cause and event. But, our perception of time flowing is also based on where we are and how fast we are moving. If you look at other thing, you are always “stationary.”. So, if you are in a car driving at 60 mph, someone standing on the side of the road is actually moving past you, from your perspective.
This means, for you, time is constant, but that guy outside appears to move much slower since your relative speeds are different. Now, will you see a difference? Absolutely not.
However, experiments have shown that time dialation is real and measurable.
You measure time by seeing it fly.
Suppose there is a light in your living room. It is off. You turn it on, and you suddenly travel away from it at the speed of light. Just after you leave, someone shuts the light off.
That someone will see the light was on only for a couple seconds.
For you, the light will always be on (the image of when the light was on is traveling at speed of light, so are you).
Time is relative!
Let us you stayed on Earth and your identical twin took a spaceship travelling at a very high speed on a long voyage of 20 years. Suppose both of you had synchronized identical watches that runs forever without any time loss.
When your twin gets back your watch will show 20 years have passed. Your twin’s watch will show it has been less than 20 years.
The place to start is here: the universe has some kind of underlying set of fundamental rules. We know about 30% of those rules. In the past when I have posted a statement like that I received a huge number of replies saying that’s absurd and we know so much less, or that there is no way to know how much we know, etc. etc. etc. That’s why I like to do it. If we knew 90%+ of the rules we would feel pretty confident about things. If we knew <10% we wouldn’t even be able to make a sensible guess. We do OK. There is a ton we don’t understand, probably we mostly don’t understand, but we have definitely identified a bunch of basic particles, we definitely understand at least a few fundamental forces, and we have enough understanding that often our guesses and deductions about how things should work based on what we have seen prove to be true.
That’s a lot of setup for the actual answer.
The answer is we don’t really know. Time is slippery at the moment. Some people think it doesn’t even really exist. I have a very hard time accepting that proposition. What we do know is that the structure of the universe can be bent by gravity and time bends with both gravity and speed.
Lets take two examples. First. If I built a huge rocketship that could travel at just under the speed of light, I could point it at a distant galaxy a billion light years away, climb in, step on the gas, and I would be able to get to that other galaxy and still be alive. To me it would take a few months. Anyone watching me do it from Earth it would look like I was frozen in time inside the rocketship. The ship would be moving ahead at near the speed of light, but I would be frozen in time.
Second. Lets turn it around, if I could watch the people on Earth running around it would look as though they had been sped up. Is time going slower for me, or faster for them? Its harder to say than you might think.
The point is that time is really relative to the observer.
Gravity affects time. The way we experience time on Earth, 24 hours in a day & 365 days a year, is not a universal constant. That is to say that another star system experience it differently. They age different, too.
In fact, your feet, being closer to the Earth, are younger than your head, which is farther from Earth and is affected by gravity (and time) differently.
Haha well, time is still moving forward. So it will slow it down relative to the rest of your body.
Lets say you are on Earth, but you have a friend that lives on Mars. For some reason, suddenly the sun goes out/disappears. It takes 8 minutes for the light the sun was emitting to stop reaching you. You just happen to have a magic phone that can talk to your friend on Mars instantly. You call him up and tell him the sun has vanished. He is confused and has no idea what you are talking about because he can still see the sun so he hangs up on you. 4 more minutes later, he calls you back on his magic phone and apologizes, the sun has just disappeared for him too!
Four years later, your distant relative that lives on a planet orbiting Proxima Centauri calls you up on his magic phone asking why the hell the sun just disappeared. Of course this is old news to you by now.
For each of the three people in this story, you, your friend on Mars, and your distant relative on Proxima Centauri, they all saw the same event occur at a different time. Time is relative because things do not travel instantly. It takes time for changes in one area of the universe to propagate to others. This is called the speed of causality and just so happens to be the speed light (and other electromagnetic/gravitational waves) travels as well.
When you move fast (and by fast we talk about significant fractions of the speed of light — 100mph isn’t “fast” here), there are 2 things that happen:
– for you, you experience time moving at the same rate you always experience time. The second hand on your watch would still tick once a second.
– for someone else who is standing still watching you, they see your time as going much slower than their time. If they could see your watch, the second hand would be moving much slower.
The faster you go, the slower your time appears to an observer looking at you.
Interestingly, when you look at the person who is standing still, you will see their time as moving much slower too — if you could see their watch, the second hand would also be going slow. This is because, from your perspective, you are completely still and they are moving very fast. (This is relativity)
Time, speed, and relativity are interesting, but very strange, phenomena.
One consequence of this is that anything that travels at the speed of light (a photon, for example) basically experiences no time passing. So a photon that leaves a star 100 light years away would take 100 years to get here, as we would observe that photon. From the photon’s perspective, no time passed at all!
The speed of light is constant (in a constant medium).
If we were able to be an observer on an object traveling at half the speed of light and we were to turn on a flashlight in the direction we were going, it would appear that the light traveled away from us at the speed of light. We might assume that to a stationary observer the light leaving the flashlight would appear to travel at 1.5 times the speed of light. But this would be impossible because the speed of light is constant and can’t be exceeded.
The thing that has to alter to keep the speed of that light constant for all observers is time. Our time would pass differently than the time of the stationary observer.
Does it make sense to say that, if you moved at the fastest speeds possible (exaggerated example: speed of light or even much faster), then you’d be doing all sorts of stuff and time would barely have passed? For example, do a years worth of stuff (in our current standard of time) in a millisecond. Then that could mean that you would live almost forever or a really, really long time?
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