I just watched the movie Interstellar and the question was asked of why/how high-speed travel would slow time within an area.

Curious as I am, I watched several YouTube videos to quickly 'educate' myself. I believe I have the concept, but almost every example uses a light mirror as the core component.

I fully acknowledge that my issue with the theory probably either stems from the way it was explained (the internet being what it is) or my understanding, so I'll state my assumptions:

  1. Rays of light always move at the same speed
  2. I presume this means that regardless of the speed at which the object emitting the light is moving, the light travels the same speed (i.e. light does NOT inherit momentum).

So if an object containing a light mirror is moving at 99% the speed of light, it seems to me that the ray of light will not only move diagonally toward the opposite mirror. Rather, it will emit in all directions. Thus, some light will move straight toward where the opposite mirror WAS at the moment it was emitted/reflected. However, some light will also make it to where the opposite mirror will be. So it seems ridiculous to assume the light is traveling a further distance -- you're just measuring a different ray of light - one that happens to be going somewhere else entirely!

Simplified example?: Two men are in the back of a truck moving 100 mph north. the men are facing each other looking east and west. One man passes a ball to the other man. But let's assume this ball has the characteristic of light that does not allow it to inherit momentum. As soon as the ball leaves one man's hand, it will move 100 mph SOUTH relative to the men, so it never makes it to the second man. Let's call this "Ball A". But if the ball also has the characteristic of light that allows it to scatter in all directions, let's call the ball (of countless balls 'ejected') that will happen to make it to the other man some seconds later, further down the road (thrown to where the man WILL be, not where he IS) "Ball B".

My point is, it seems like the theory of special relativity (again, as explained) is comparing Ball A and Ball B as if they are the same thing. One does move further and does takes longer to get there, but ITS A DIFFERENT RAY OF LIGHT! It just shares the same origin!

This theory, as explained, seems to both utilize and ignore a critical characteristic of light - it's static speed (and thus, lack of inheriting momentum).

What am I missing or misunderstanding?

Until I'm more properly educated, it seems to me the only thing that changes is the time at which events are observed, not the actual occurrence of events. So yeah, if you could travel incredibly quickly, you could watch the dinosaurs roam the earth with an incredible telescope from ~65 million light years away, but you wouldn't be able to do a bunch of laps around the universe at 2000% the speed of light (assumed possible) and later walk WITH dinosaurs. You'd just be able to observe something in time, not alter/slow time.

  • $\begingroup$ Your question seems to be missing "an observer looking at the experiment relative to...". Depends on where you are looking (sounds like quantum entanglement lol but it's not) If I'm stationary relative to you and you are traveling close to speed of light, I'll see that you are thinner that's a complement for you and you clock will seems to slow down. Space can be bend just like the black hole, time dilation is more noticeable near to the black hole. Just remember the only things we know that can exceed speed of light is space so try not to tell light what to do cheers drinks on me. $\endgroup$
    – user6760
    Commented Apr 6, 2015 at 4:45
  • $\begingroup$ Light inherits momentum. Sunlight emitted by sun in one second has momentum: velocity of sun * 4.69 million tons. $\endgroup$
    – stuffu
    Commented Apr 6, 2015 at 8:48

2 Answers 2


Special Relativity is based on two assumptions: any reference frame (viewpoint) is valid and light moves at the same speed in all reference frames. enter image description here

In the first picture picture, light is trapped between two mirrors. If the distance is 300,000m then it takes 2 seconds to go up and down.

enter image description here

As you can see, when the mirrors moves (with the light) it makes a longer diagonal path. I will save you the calculations and say that this has a scenario. If you are an a rocket ship with this system, you will see the first image. If, however, you are outside the rocket, watching it zoom by, you will see the second picture. Therefore, the 2 seconds (the time for the light to bounce up and down) seems to take longer when you are longing into the rocket, but inside, nothing changes. This is why the crew-member in Interstellar didn't age as fast as the people on Earth. To the people on Earth, time passed more slowly for the rocket ship.

About, the dinosaur thing, we cannot go faster than light. If you want me to explain why, just ask. And they are the same light beams because they have the same photons. If we accept that they are different, we accept alternate realities for each observer, with different existing objects.

  • $\begingroup$ This being a reasonable explanation of special relativistic time dilation, Interstellar concerns gravitational time dilation, which is a different phenomenon. $\endgroup$
    – Muphrid
    Commented Apr 6, 2015 at 4:52
  • $\begingroup$ @Muphrid it doesn't concern gravitational time dilation until the very end (and they messed up that physics anyway). Their ship, travelling at high speeds, caused the captain to see his daughter aging faster than he was. $\endgroup$
    – Jimmy360
    Commented Apr 6, 2015 at 4:54
  • $\begingroup$ That had far more to do with them going deep inside a black hole's gravity well than anything to do with their speeds. $\endgroup$
    – Muphrid
    Commented Apr 6, 2015 at 5:10
  • $\begingroup$ @Muphrid they experience time dilation before they even get close to the black hole $\endgroup$
    – Jimmy360
    Commented Apr 6, 2015 at 5:26
  • $\begingroup$ I suggest perusing some of Thorne's consulting for the movie, then. They specifically remark on how fast the black hole must be spinning to produce the amount of time dilation experienced, in terms of gravitational time dilation on a planet (the water planet) at the innermost stable circular orbit of a kerr black hole. The time dilation experienced in the middle act of the movie is almost entirely from this phenomenon--and incidentally, such a black hole must be spinning at almost maximal spin parameter to produce such a pronounced effect. $\endgroup$
    – Muphrid
    Commented Apr 6, 2015 at 5:29

See,the philosophy of relativity is that all the observers will agree with the natural phenomena, or what is happening; like, if one observer sees two particles to collide or a lightning causing damage then all the observers will agree with the collision and lightning causing damage. But there will be disagreement in positioning and timing (and in interval) of the events.

Therefore, the observer moving with the mirror-light clock sees the light ray reflect between the mirrors. So the observer outside, at rest, will also see the same light ray reflect between two mirrors.So, this is not actually inheriting momentum by light, it is actually seeing the same event(s) to occur. This is true for balls also, if you replace the light with a single ball bouncing inside two mirrors (or, walls) with respect to the observer moving with that system, then the observer outside the system at rest also has to see the same event(s): collision of the ball inside two walls. So the rest observer will see the ball to move in a zigzag path as the whole system is moving. But this is not surely inheriting of momentum by the ball as you see in “Ball and wall game”.

Now, as you are saying that light will radiate in all direction; that's correct. But you'll arrive at the same conclusion considering any of those light rays. See the image below:

enter image description here

As you can see, the apex of both of the rays lie on a circle (sphere, if you take rays out of the plane) of expanding radius. The length of the path traversed by each ray (D) equals the radius of this sphere, and hence the conclusion will be the same no matter which ray you take. But this problem will not appear, if you use focused directional beam like Laser.


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