Relativity

The twin paradox...once they start a conversation about the age of the universe, will they agree on the years?

There is literally nothing in the universe but the string. Logic and illogic rule the two realities. "Two" sides to the same string. I hate using numbers to explain this. They are not real. But neither are letters so i dunno how it could possibly conveyed otherwise

They say the Universe is expanding since the big bang. However, relatively thinking, is it possible that we and our scale are shrinking?

I have been reliably informed that ,if we could listen in directly to an object like a spacecraft that was approaching us at a relativistic speed then any ongoing conversations would be heard slowed down rather than sped up as my intuition have supposed. Since we also have the relativistic doppler effect (ie in addition to time dilation) am I to think of a conversation that would be both high pitched and slowed down? And what about the same situation but purely with sound? If we had a plane approaching us at just below the speed of sound could we reflect a sonic beam against the cabin window and eavesdrop on…

If a space craft is approaching the Earth with a speed of v and the Earth is rotating around its axes are there simple transformations btw a frame of reference in the craft and a frame with its origin on the surface of the (completely spherical earth)? Or are we just talking about an approximation and does the Earth drag its frame of reference ,even if unnoticeably so?

What is meant by "coming apart from" in this context? An alternative rule would be to require that all clocks, everywhere on Earth, keep time with Greenwich Mean Time. We don’t usually use that alternative rule, because it’s inconvenient for noon to occur in the middle of the night in some parts of the world, but that doesn’t make it wrong, just less useful. (Indeed, in some circumstances—military operations, for instance—it turns out to be more useful, with the benefits of a shared time outweighing the costs of that time coming apart from Sun and Moon.) Source: Philosophy of Physics by David Wallace

It seems there is still an argument on whether acceleration is needed or not. Two opposing viewpoints. Pro against Who's right for the general layman ? I was thinking about this on the dog walk and it did indeed make it go quicker. Though neither of us were any younger sadly.

I was pondering this when I read the aliens and FBI thread, and didn't want to steer it that far of topic. I mean supposing an alien had an FLT spaceship, could they go home for reinforcements and get back in time for the battle?

We have an intelligent community living in an environment where the speed of light is slowed by the medium they live and communicate in . Then they discover a method to create channels through the medium so that they can now pass messages at the speed of light. Will it seem to them that " causality has been broken"? Is this scenario an accurate analogy to us discovering a faster than light process of signaling?

I note that both these seem to give rise to spacetime curvature What might be the apparent connection between them? Is mass a measure of some sort of concentration of the various fields? (If that means anything )

Sure, no problem. Even theoretically a living being. The event horizon is not a solid sphere, it is simply a region of space, technically the surface of a sphere where the sphere coincides with the place where the escape velocity equals that of light. Depending on the size of the hole, that surface may be a great distance from the center, so much so that tidal forces do not have a serious effect on anything that crosses it. A different issue is that many black holes, especially those that we can detect, have an environment of very high radiation outside of them due to the gamma rays emitted by the matter that falls inside due to the high temperatures that they r…

As one who meditates on cosmology, relativity, and quantum (you know, why are we here), as a layperson, I don't know that I've seen an answer to this question. If gravity is the curvature of spacetime by mass, why is the standard model "incomplete" without a graviton and a quantum theory of gravity. Why wouldn't we postulate that gravity doesn't apply in quantum because there is effectively no mass, just energy, that only becomes a particle when somehow the wave function collapses? Why does there need to be any relationship between quantum and spacetime? Can't it just be said that one exists inside the other? Apologies in advance if this is naïve and n…

I discovered on google that the speed of light and the speed of gravity are the same. Does that mean that gravity fails to have any effect on light, because gravity and light travel at the same speed? i.e is that why light has no mass?

(Or maybe "What are the smallest objects with mass?") I think I may have seen that mass may be defined as a body's resistance to being accelerated. How would that apply to that smallest massive object? What would apply the force ? Another massive object? Does mass require another massive object to define itself?

If gravity is just the effect of warping space, then why does it cause acceleration? For example, an object is dropped from a height and moves towards the ground. When the object reaches height/2 another object is released from that location. Since they both exist in close proximity relative to the earth, shouldn't they both be in the same warped space? Hence, they should be moving at the same speed because the warping of the space for each object would be the same. I could see that the closer to the earth, the more the warping of space and consequently some "apparent" acceleration would occur. But an object released should immediately move at the speed of the w…

I drew a diagram to illustrate Einstein's moving light clock thought experiment but it doesn't seem to work out right with the time dilation factor for the velocity involved. The light clock is 0.5 m in height so the two-way light beam trip would be 1 m. The clock completes its up/down cycle in the time it takes for it to move to the right 0.866025403 m, which means the clock's velocity is 0.866025403 c. The time dilation factor for that velocity is 2. How do you get the zig-path down to 1 m, like it is inside the clock when it's not moving, by dividing it by 2? Even if I contract space and say that the clock's path is only 0.43301270189221932338186158537647 c, the time d…

l wrote: and you can't deny that 😄 You are very attached to the current understanding/definition of time, the one used in GR, but you really think that this is the final/ultimate theory that we can have in order to explain gravity with all its aspects? There are already many complaints about it since dark "stuff" appeared. Some even say that dark matter is not real and is used to maintain GR valid (I don't agree but it is possible). There are MOND theories proposed. There are other attempts also, including my theory, based on dark matter. Moreover, if the GR definition/notion of time is "the one and only", please explain how is this particular def…

I’ve seen this claim before, but it’s actually erroneous. It’s true that if the GPS clocks were not adjusted they would accumulate a time difference of ~38 microseconds a day as compared to ground clocks (and ct would be around 10 km), but this would not show up as a positioning error in GPS, since the GPS clocks nominally run at the same rate, and the trilateration uses timing differences between signals from the GPS clocks. These clocks would random walk away from each other, and accumulate differences from orbital variations, if not synched up. But this would be on the scale of nanoseconds, not microseconds, per day. One of the reasons the clocks are adjusted is s…

I think I have heard it said( a few times) that fhor an object entering (the EH?) of a black hole that space and time are reversed. Is that correct? If that is indeed what is said then (here we go again?) is this just saying that this is what the model does and the actual body notices no change?

Imagine three ideal clocks. Clock A is on the surface of the Earth. The other two, B and C, are side-by-side in space, remotely distant from the Earth's gravitional well. As I understand it, clock A on the Earth's surface will be running slower than the two out in space, which will both be running at the same rate. Now imagine that the two in space become separated. Clock B is tethered so stays the same distance from Earth. Clock C is not, and falls very slowly, picking up speed faster and faster, down to the Earth, where it is momentarily side-by-side with the original Earth clock A. It's now going to be falling at Earth's escape velocity, about 11.186 …

A comment in the ongoing "Nature of Time" thread made me wonder what gravity would look like if the Galilean version of Relativity was correct I have seen the animations depicting Galilean spacetime diagrams and wonder whether or not they can be extrapolated to a model of gravity in a similar way to how AE moved on to GR from Special Relativity. Just to say,I am not attempting to attach any credence to Galilean Relativity. I just wonder if that model might be stretched to incorporate a kind of Galilean General Relativity and what "Galiliean Gravity" might look like.

I came across this by chance the other day, and felt that this is a particularly good explanation of the meaning of “metric” in the context of GR, so I thought I’d share it here for whoever is interested: Some nice visualisations in there. Also, the other GR-related videos on this particular channel are quite good. No particular point for discussion, I’m simply sharing this for all those who wish to learn more about this subject without having to dive too deeply into the abstract maths. Particular emphasis on comments from time stamp 19:40 onwards (on coordinate vs proper measurements), as that’s were all those many common misconceptions arise. ED…

The question is, if time passes by slower close to heavy masses, what effect does that have on light passing by a heavy object (black hole). According to Einstein, all processes take place slower and, although light keeps travelling at the speed of light, we (from our point of view) should see that this light should pass by slower. In the intergalactic space, however, the total opposite should happen. As time goes by faster and the light there also passes by at the speed of light, from our point of view we should see this light travelling faster than the speed of light as we know it. If this is real, the galaxies we see, should actually be slightly farther away from us, b…

Hey folks, i am an S1 student in Scotland ( it should be 7th grade for the usa ). I've been asked to write a 6 pages essay about General Relativity and it's implications. First, i looked some documentaries on Youtube to understand better how space is connected to time and how both are not absolute as newton described. I think i understood what Albert is trying to say pretty good now. What i don't get is how is it possible that gravity bends space-time, is it the matter that bends it ? or the relation of the mass and the space-time, like in the classic view of gravity ? or does spacetime alone "do the job"?. One last thing i don't really understand in …

If any point in the spacetime model is specified wrt any reference point is it inevitably the case that that point can only ever be an approximation to any physical activity that actually takes place there? If so is this because of theories like the Uncertainty Principle or does it simply follow from the spacetime model itself, because it models both position and time ,as well as (I imagine) that at the most detailed level that all things are in relative motion no matter how we try to set up any scenario that might illustrate processes at rest to each other?** **Not completely sure if that is completely accurate and I think Studiot recently disa…