# Doesn't the slowing of time when travelling at almost light speed create a paradox with an observer? [duplicate]

Disclaimer: I am extremely new to this and have no proper knowledge of this subject at all, this is just an idea that I had which I want to properly understand. I don't have any knowledge of necessary equations, proper terminology, etc.

In Sci-Fi movies that involve travelling at almost the speed of light, there is sometimes a plot where it involves a character travelling almost at the speed of light for a few minutes or hours, and when they return to earth, they discover that a very long period of time has passed. I know these are Sci-Fi movies, but obviously, this part is not made up.

Now, take this scenario for example. A person aboard some sort of spaceship is travelling close to the speed of light. They travel for a small period of time, maybe a few minutes. For this person, time is moving slower than for a person that is standing still. When this person was to stop, he would find that a long time had passed for everyone else.

Hypothetically, If someone was to look at this person travelling almost at the speed of light, wouldn't they see him travelling much slower than the speed of light? Because if time if passing slower to him, wouldn't he be moving in... slow motion...?

So if it took him a few minutes to travel a very large distance, wouldn't the observer, for whom the time is passing slower, have time to observe the passage of the person moving at the speed of light?

What I don't understand is in this case, it would mean that to the observer, the person is not travelling almost at the speed of light, but much slower. But to the person travelling almost at the speed of light... well, they're travelling almost at the speed of light.

Any help with this is appreciated. If someone can help me, please try to explain the answer in simple terms. If I have gotten one key idea of this wrong, please don't hesitate to correct me.

## marked as duplicate by StephenG, Jon Custer, Aaron Stevens, sammy gerbil, Bill NNov 19 '18 at 15:12

• Possible duplicate of What is the proper way to explain the twin paradox? – StephenG Nov 15 '18 at 11:40
• I don’t think that this question is actually a duplicate of that one. Here they are specifically confused about the reciprocity of the relative velocity which is not addressed there. – Dale Nov 15 '18 at 12:06
• Good point @Dale but the linked page, and its links, are probably still a good resource for this OP. – PM 2Ring Nov 15 '18 at 13:01
• The first thing to understand in relativity is that velocity is always relative. There's no absolute velocity, so there's no absolute slow observer and absolute fast observer. If I'm not accelerating, then I'm at rest in my inertial reference frame, no matter how fast you think I'm moving. – PM 2Ring Nov 15 '18 at 13:06
• Agreed, I think they are a very good source. I just hope the question doesn’t get locked as a duplicate of that question. – Dale Nov 15 '18 at 13:34

Hypothetically, If someone was to look at this person travelling almost at the speed of light, wouldn't they see him travelling much slower than the speed of light? Because if time if passing slower to him, wouldn't he be moving in... slow motion...?

Don’t forget that to the fast moving observer he is at rest. So it is you that is moving fast to him. His time dilation obviously does not affect your speed. The relative velocity is the same in each frame. He sees you moving as fast as you see him moving.

Time dilation refers to something different. It refers to how long a physical process takes as described in different frames. Suppose that he is in his spaceship’s kitchen and wants to cook a soft boiled egg. His ship is moving at 0.866 c (time dilation factor = 2), and the water is already boiling. He drops the egg in as he passes the sun and takes it out as he passes the earth. In your frame it takes 9.2 minutes for him to move from the sun to the earth at 0.866 c, far past the optimum 6 minutes. In his frame it takes the earth 4.6 minutes (after passing the sun) to pass him at 0.866 c, substantially less than the optimum. So is the egg over cooked or under cooked? It turns out that it is undercooked. This is what is meant by time dilation.

His time dilation does not slow down his velocity, it slows down physical processes like cooking an egg (or radioactive decay, or clock ticking, or heart beats, or ...)

• So if the observer gets older than the person aboard the spacecraft, because their physical processes run faster because the spacecraft is travelling close to light speed... So technically, since his physical process are slower, wouldn't that mean that to him, point A to B at almost light speed is perceived faster than it is for the observer? Thanks very much for the help, by the way, it's helping me wrap my head around this! – skillz21 Nov 15 '18 at 20:52
• Yes, that is correct. All physical processes, including physiological and mental processes, will slow by the same time dilation factor – Dale Nov 15 '18 at 23:18

The speed of the ship does not change depending on your frame reference. If an observer sees a ship going at speed v, a person on the ship will see the observer moving at the same speed in the opposite direction.

In your analysis you have only taken into account time dilation, but space is also affected. In the frame of the space ship, space is actually contracted. If you want, from the perspective of the observer, the time of the observer of the ship is slower. Hence the ship can travel a greater distance before the inhabitants die. But from the perspective of someone on the ship, the reason the ship travel a greater distance is because that space itself is contracted.

Now if you really want to analyse this problem properly, you need Lorentz transformation, because the argument Up to this point is totally symmetric. The ship also sees the time of someone on Earth to be going slower...

You might ask, if the observer on the ship and the one on Earth both see each other time go slower, then why is the observer on the ship younger when the ship comes at rest in the frame of the Earth.

The reason is acceleration. Acceleration breaks the symmetry of the argument and in this case it is the ship who accelerate and decelerate, so this is why the observers on board is younger at the end.

You should look up atmospheric muon decay, it's a perfect example of the duality between time dilation and lenght contraction.

• Acceleration is a red herring. The symmetry is broken by the traveler using two (or more) reference frames. Acceleration is just the means of changing frames. You can do a version of the twin paradox without any acceleration. You just need an extra traveler who makes the return trip after synchronizing his clock with the outbound traveler as they pass each other. – PM 2Ring Nov 15 '18 at 13:12