• Aug. 23, 2019, 3:11 p.m.

    Hey, I've been a long time subscriber to the channel and after many of the topics discussed, I wonder most about why gravity has an effect on time. I'm hoping that folks explain or share links that can explain (to a layman) why gravity causes time dilation. It's something that has baffled me to no end and Google searches have only left me more baffled. I do grant that this may just be outside my scope of understanding. Any effort on this request will be appreciated.

  • Aug. 27, 2019, 12:54 p.m.

    I've been wondering about these things myself for a long time too.

    I have only ever read about the gravity-time relationship as a simple assertion, and I suspect that's all anyone else knows about it too. That is, if you had the absolute, true answer to this question you would know something that no other physicist on Earth knows right now. I don't think scientists would be shy in admitting if this is the case.

    That said, I've had some impressions (don't confuse my opinions with facts!):

    Maybe time dilation causes what we see as gravity and not the other way around. This is at least half true. Light bends in a gravitational field in part due to bent space, but also because the speed of light is slower the deeper you are in a gravity well. As a result, light is 'refracted' or bent around objects like the Sun (in part) because the wave front propagates more slowly on one side than the other. Before the 1919 solar eclipse measurements, Einstein got his math wrong by only expecting time dilation to bend the starlight. Later he corrected this by also including space curvature, which as far as I understand doubled the amount of bending expected. So half of the 'falling' of light is really time dilation.

    Anyway if you chose to model all matter as little knots of light, then this same refraction (my term) would cause all matter to drift inward to wherever time is slowest. This does nothing to answer your question, but it does turn it inside out!

    Another, contradictory possibility is that time dilation is caused by space being more timelike and vice versa near a large mass. That is, visualize the classic funnel of curved space we always see in magazines and note that its surface is more and more slanted near the center. If you assume the future is 'up' in this model, then for someone close to the center, their progression in time will be at more acute of an angle to their space, so what would have been time progression instead becomes more spatial acceleration and less time.

    I may be completely wrong but I have been reading and thinking about these same questions for decades and this is the best I can offer.

    BTW FTLdrummer is a great handle. FTLguitar would be thrash?

  • Oct. 27, 2019, 5:16 a.m.

    I do agree that the concepts of special and general relativity are hard to grasp. To answer your question we need to explore:

    • The nature of ‘speed’ (which relates distance with time). It is as simple as it sounds - how fast something goes over time.
    • The nature of 'gravity' - not in that it is a force, but it's effect of accelerating objects together
    • The concept of relativity and how it relates to gravity

    So let's look at the concepts of relativity first. There was substantial confusion century-before-last as to how observational data was not lining up with theories at the time in relation to magnetic fields and classical mechanics. This distilled down essentially to two observable, seemingly (note) contradictory facts:
    - light was measured as being a constant speed no matter where measured, or how fast measurements were 'made'
    - otherwise 'classical' motion is indistinguishable no matter where, or how fast you are. In other words it makes sense doesn't it that a ball thrown while in a rocket is expected to behave predictably the same as if I was on the ground and threw a ball.

    So how can both be true, if the speed of light from a torch on a rocket is measured the same if I am on a rocket, yet when I observe that same light looking at the rocket it should also be the same speed? Previous logic stated that it should be different speeds when measured from different velocities, but they were the same speed. This was the conundrum (simply put - there was a lot of stuff about electric fields and so on, but don't need to get into detail here).

    Einstein reconciled these in special relativity, by saying both are true, because in essence observations were that that they were.
    He simply then started to think: if they were both true, what else would be true? The difference is really in concepts of time, velocity and distance. So if the speed is of light is the same, it follows on that time must change to accommodate this truth. The rocket must be experiencing slower time to ensure that the speed of the light from the torch is the right speed when measured by someone on the ground. Hence, time and distance dilation occurs when objects are accelerating or decelerating.

    Now: general relativity. Einstein had the same truths about gravity. He was likely wondering what gravity is, as concepts of ‘ground’ is also not really defined. Newton said gravity was just a ‘force’ between two masses, however Einstein was dissatisfied with this. If motion and acceleration is relativistic, why not acceleration due to gravity? Surely there shouldn’t be two forms of acceleration out there in the universe…

    Hence two postulates for general relativity:
    - no matter where you are or how fast you are, special relativity holds true
    - including on a planet - where you are experiencing gravity. Ie. there is no difference between gravity and accelerating. Eg. if you are in a rocket, it is the same as if you were on a planet, you are experiencing the same special relativity effects.

    So given these two factors - it holds that when you experience gravity it is the same as if you were in a rocket, you experience time and distance dilation as per the previous explanation of special relativity.

    NOW THE IMPLICATIONS: Space becomes very weird (or rather just more complicated). Time dilation and distance dilation occur in relation to mass, not just if they were accelerating on rockets. This means light speed from a torch on a rocket is the same speed as measured from a stationary space station (meaning the rocket experiences time dilation) is the same effect as a torch on a planet also (meaning the location on the planet is experiencing time dilation).

    This has matched observations, mainly Mercury, which Einstein proved was experiencing slower time close to the sun than when it was further from it, effectively proving the theory of General Relativity.

    There is a lot of maths here that is omitted, and this is ‘thought experiment’ realm only, but is the basic principle - which is acceleration causes time dilation (because light is the same speed no matter where measured from), and gravitational acceleration is the same as normal linear acceleration.

    Now another aspect of your question of 'why' relativity is true is more philosophical - and calls into question 'why' behaviours we observe are like this and hence the investigations of black holes and so on which push this theory to the limit - and there are still many mysteries to be solved...

  • Oct. 27, 2019, 6:07 a.m.

    When you are in a gravity well, you are falling. Suppose get into a balloon, rise to the edge of space, and jump off. You are not in orbit. You are going to fall for all practical purposes straight down. As your speed increases while you plummet to the earth time slows down.

    When you are in orbit around a black hole, your falling down but also moving to the side. To stay in orbit close to a gravity well you have to be moving very fast and time will slow down. Suppose you are not in orbit and just using rocket thrusters to keep from falling into a black hole. Well you might be stationary relative to the singularity. Relative to the space itself that is rushing by you as it falls into the black hole, you are moving quit fast through that space and time will slow down. Think of a speed boat at the edge of a water fall using its motor to keep itself from being carried by the water down the water fall. From the shore the boat may appear stationary, but it is actually moving through water at a very high rate of speed.

    Hope that helps.

  • Nov. 6, 2019, 4:45 p.m.

    Maybe you can clear up something for me too...

    Speed itself causes time dilation, but so does acceleration, which is just the change of speed.

    If you accelerate for a long time and then stop accelerating, you are still coasting at a great speed.

    Then what happens to your time dilation? Does it decrease?

  • Nov. 13, 2019, 12:48 a.m.

    Yes well they are actually related, although in different ways. Special Relativity relates to 'object in comparison to...', whereas General Relativity relates to 'object is not at rest as it is accelerating, when in a gravity field'.

    So imagine a 'photonic clock', ie, 2 mirrors locked in place with a photon bouncing between them. By moving the clock, the photon will need to 'travel' a longer distance and the effect is:
    - Under Special Relativity: if you are inside the clock, you notice no difference. If you are outside the clock moving at a different speed, looking at it, it will tick at a dilated rate because the photon will still travel from your point of view at the same speed, so the 'ticking' will be slower.
    - If it accelerates, Special Relativity still occurs but at the same rate as the new speed. The photon will begin it's journey as normal in the clock from one of the mirrors, but by the time it gets to the other mirror, the clock has accelerated and thus the distance is larger. So, in regards to Special Relativity, there is no difference between velocity and acceleration. In your example above, if you stop accelerating, your dilation will now remain the same from then on, relative to someone else at a different speed.

    Now consider the clock also is affected by General Relativity, if it is in a gravitational field (which all objects in the universe actually is). This is an additional effect, so you could say both forms of time dilation exist at the same time on the clock. But there are some differences:
    - The clock is inside a gravitation field, and time elapses differently in the space around mass. This is not a function of the clock's velocity, but rather its location close to mass. Yes, it is experiencing 'acceleration' if it is close to a mass, however to clarify my simplistic post above it is not actually due to acceleration (the acceleration is the manifestation) but more accurately its 'gravitational potential'. So if the clock is on a planet, it looks like it is at rest, however it has 'gravitational potential' so if a hole suddenly appears under it and it starts to fall, this only manifests as acceleration.
    - It is this 'potential' that causes the time dilation, different to special relativity's 'velocity'.
    - I have omitted the obvious next step here, which is to do with 'spacetime', worthy perhaps of another thread. This explains why dilation would occur in the middle of the planet too, which theoretically there is no 'acceleration' because mass is all around you, yet the dilation would be the strongest here because that is where the field is the strongest.

    SO, there are some key differences between SR and GR dilation:
    - SR dilation is dependant on someone's viewpoint traveling at a different speed to yours.
    - GR dilation is dependant on if someone is closer to mass.
    - The difference means that if we all had clocks, in SR, our clocks would be seen as slower by each other, as we are moving away 'from each other'.
    - Whereas the GR effect (which is in addition to SR), our clocks are moving at different rates by each if we are in different areas of Gravitational Potential (distance from mass) so this means a clock in orbit, as seen by a clock on the planet, could be faster, and a clock on the planet, as seen by a clock out in orbit, would be slower. This is a different result to SR where both would be slower as seen by each other.