Relativity

After reading around the subjects of Special and General relativity (with admittedly patchy results) for quite a few years now, I have begun to tell myself that perhaps the most important result in Relativity might be the first : the way moving frames are connected. Never mind the speed of light , the lack of need for an ether , the equivalence or not between gravity and acceleration as well as the Lorentz Transformation perhaps it is this simple correlation (the way moving frames are connected.) that is the cornerstone of Relativity? I have seen the geometric illustration of this as it comes at the very beginning of the subject and is easily learnable by s…

To clarify the title, How far away from an object, and how big must the object be, for you to see it travel at the speed of light for at least 30 seconds? As an example, when you are standing on the ground, you can easily see an airplane traveling faster than the speed of sound for a long while as it goes across the sky. But, for an object traveling the speed of light.... if you were in a space ship relatively staying still, what's the closest you can be in order to see an object traveling at the speed of light for 20-30 seconds, before it gets too far away (and tiny) to see? And, how big would the object need to be for you to clearly see it, as in the airpla…

Hi everyone hopefully i wont come across to daft ,i,m not even sure if this is the correct section to post this question. i'm certainly by no means the most educated of people and really have very little understanding of the sciences, but i do find a'lot of things fascinating and want to learn more. My knowledge of these subjects comes from watching documentary's on the subjects basically , so if the questions i ask sound very silly i appolagize. Any way on to my question , i was watching something on planets going supernova and the creation of black holes. Is it possible that we are watching a big bang and the creation of a new universe in these events , is it possi…

This one has perplexed me for some time. Scenario: Travelling close to light speed towards say the Andromeda galaxy. If we were to look directly towards our destination, would we be totally frazzled by intensely blue-shifted radiation? (Assuming we didn't have a really good pair of Ray-Bans)

Question: does the velocity of light remain an invariant when it goes through a material? Explaining the question: It is known that C is an invariant when C is speed of light in vacuum. It is also known that C has other values when it goes through materials (water, glass): is it still a constant regardless of the velocity of the observer / material or is it "linked" to the material?

does a velocity of zero exits in the universe, or any velocity apart from the speed of light, since everything is relative to something else, how can i say i am moving faster or slower than another object with confidence?

The twin paradox is created by both the twin on the ship and the twin on the earth seeing the other persons clock change by the same amount, therefore there doesnt seem to be a way to account for the age difference when they meet up after the voyage. Edit to add: The paradox is not just the fact that the twins are different ages at the end. Its that they have symmetrical observations about each others passage of time and still have a difference in time experienced. Read post # 11 for a better explanation of the specific paradox I'm talking about. First of all, I will use the example from the Wikipedia article about the twin paradox so that I can highlight the fact…

I think they are. A "light clock" (a system that counts the rebounds of a light signal between two mirrors) will evidently slow when it is in movement. But all clocks are sorts of light clocks. A "light ruler" (a system than determines the distance between two points by mean of a light signal) will evidently shrink when it is in movement. But all objects are sorts of light rulers. The "twin paradox" is easily solved when one realizes that the symmetry of the situation is merely theoretical. The two frames are not physically equal, although all laws of physics are the same and light speed = c in both frames, and no "proper" movement can be determined. I wonder why thes…

If we send a clock up into space or take it on a high speed journey then bring it back it will not be synchronised with a clock that was in the same position at the start. If we send a ruler out in the same type of experiment, has its length changed when it is returned to its starting position? If it hasn't changed why not, if a clock does? What's different about them that one is permanently changed but the other isn't?

I mentioned in another thread that we had a colloquium from one of the LIGO collaborators. (He was one of the mirror coating people, not one of the GR theorists). Afterwards, my colleagues and I were chatting, and one asked an excellent question: If the gravitational waves stretched space between the mirrors, doesn't that also stretch the light wave as well? One of the explanations about detecting GWs was that you couldn't just look at some object compared to a meter stick (conceptually, and magnified many times, obviously), because the meter stick would stretch just as the object did. You would never see anything. So if the light stretches as the distance between the t…

Suppose we have a body that is accelerated from point A to point B (and ,why not decelerated ,reaccellerated and returned to point A although this may not be necessary) My understanding is that the body as a whole will lose mass in order to maintain the acceleration and eventually if the acceleration is continued there will eventually be nothing left. I am wondering as to how this loss of mass is distributed through the body. Is is evenly distributed so long as every part of the body is accelerated? So ,if there is a timekeeping device on the body does this timekeeping device similarly lose mass and fade away to nothingness eventually?

Maybe this should be posted in cosmology. We know about the constancy of Speed Of Light. Whatever the relative velocity of 2 objects, light coming from one object and reaching the other will always be observed traveling at C. In cosmology, when objects are receding at velocity greater than C because of the expansion of space, we are observing light coming still arriving at velocity of C, but redshifted. And we say that the velocity grater than C is not a regular velocity (it is caused by expansion of space) because velocity greater than C is not physically possible. One of the reason is that (correct me if I am wrong) IF an object ever could go faster than SOL,…

The physicists I see in documentaries very much talk about Dark Matter and Dark Energy. Is there any model that can explain both alongside their relationships with the visible matter? Before raising the next point, let me say I am not a patient for whom you will call the nurse “he is trying to walk again." So please stay calm and reply on topic only. If someone wants to propose a theoretical model that somewhat can explain realities of subatomic particles, dark matter, dark energy, etc. then what’s the right place and right way to do it? Should it be in the shape as students get introduced to theory of relativity in academic books, or does it need to be in the shape of…

Did Newtonian physics predict Black Holes? As I am belatedly understanding it was GR that predicted them but what would Newtonian physics have made of the situation ? Would it have envisaged explosions happening eventually as the mass increased ? How would it have viewed the scenario differently? Were Black Holes something that were theorized and discovered only after Newtonian Physics was "out dated" ? Coming back to GR ,might it be ,as I think I may have heard that we are not talking about a build up of matter but a change in spacetime curvature? Is there any theoretical way such a curvature can be reversed? Is that Hawking radiation? The BH …

Misleading Frame of Reference in Special Relativity. Suppose so we're on board of rocket R, traveling in cosmos from object B to object A. To simplify let's assume that A and B are stationary each other. Object A is sending red photons with wavelength 700 nm. Object B is sending violet photons with wavelength 400 nm. (you can choose any other pair) Rocket after acceleration can reach such velocity, that red photons from source A are blueshifted to the same energy as violet photons from source B are redshifted. Blue shift equation in Special Relativity: [math]f=f_0 \sqrt{\frac{1+v}{1-v}} = f_0 (1+v) \gamma[/math] Red shift equation in…

There seems to be a relationship between these two ...... (can't find the word that describes them both -"actors"? ) It is commonly said that the latter causes the former to bend and the former causes the latter to move. Are there any theories that describe( or aim to describe) both as one thing rather than two? (emergent properties ,would that be the term?) Is there any property of both Space and Energy (shorthand terms perhaps ) that is conserved,perhaps as the Universe evolves? That would imply that at the moment close to the Big Bang that this quantity (whatever it might be) be be the same as it is at this moment and at all other moments …

Suppose we are an eye in the sky and we look down to the ground and see two objects which are moving relative to the other . We can see that one is stationary (or more stationary ,perhaps?) wrt to the landscape and one is not.So clearly if the two objects are moving wrt each other it is the former that is moving. wrt to the latter and not vice versa. Is there anything at all to this argument? Does the "all motion is relative" position rely upon the background being entirely free of sources of acceleration? I am not espousing this position as it would break with my lifelong understanding but I would like to see it dismantled (if it is not too obvious to …

Assumption: The acceleration of objects in freefall in Earth’s field is 9.8m/s/s in a vacuum with grams, kilos or tonnes difference between them the rate is measured the same. Would the rate of an Earth-sized object within the gravitational field of this Earth ‘fall’ towards it at the same rate - or towards each other - to give a 9.8m/s/s figure or is there a difference at this larger scale because with much smaller objects the differences in mass are measurably negligible relative to the mass of the Earth , for computing purposes, and are thus considered to have the same rate of freefall? I put this in Relativity but a Newtonian explanation will do if it's…

I'm sure there's an easy answer to this, but it's bothered me for a while. It's about the Twin Paradox. So Allen stays on earth while twin brother Bob roars off in a space ship to some distant star at a significant fraction of the speed of light. Bob reached his destination and completes his business, then roars back to earth again at again, some near-light speed. When the brothers finally meet back up on earth again, Bob is significantly younger than Allen. Bob's near-c velocity has slowed time for him compared to what Allen experienced. That's how I understand the Twin Paradox. But-- All things being relative, couldn't we just as easily look at this as B…

I have heard that this is "notoriously difficult" and I think it must be done by computers (perhaps in the way that weather forecaster make millions of parallel calculations on ideally very small cells and ties them together) I also imagine (from what I have learned so far) that it may be that it is the spacetime tensors that could be summed in order to produce the resulting global field. Are there any alternative ways to do this process.?

Two particles moving very close to the speed of light in the north-south direction collide and end up moving in opposite directions due east and west. Each starts with gamma=22.37 and end up with exactly gamma=22.37. Total momentum is zero at all times. v = 0.999 gamma = 22.37 Now from the point of view of a rocket moving at 0.999c the two particles come together Collide and then one particle becomes stationary while the other flies away with all of the momentum. It should be possible from this thought experiment to determine the equation for relativistic momentum velocity of final nonstationary particle from the point of view the the rocket is cal…

What is the magnitude of the proper acceleration of an object moving in a circle at a given (unchanging) relativistic speed? For an object moving in a straight line proper acceleration = acceleration*gamma^3 My question has to do with the relationship between transverse mass and longitudinal Mass

Relative to what thing photon energy is finite?

Hello again. I have a question about why time slows down the faster your going, and why time will not let you go faster then light. Now my reason for this question is that Steven Hawkings says that if your on a vehicle, that's moving near the speed of light, and someone on board were to run towards the direction the vehicle is traveling, time will slow down EVEN more so that they aren't moving faster then light. My question is WHY does time slow down? I understand that speed is how far you travel in a period of time, so time slowing down prevents you from going faster then light. I also understand the reason you can't get something to go faster then light, is tha…

I thought I had organized my (basic) appreciation of GR in that the curvature of spacetime refers to the curvature of the GR model(I still don't understand the nuts and bolts of that but it seems to be bound up with tensors , I think) As for the reality and whether it is actually curved , I thought the idea was that we don't actually know . (the model includes a curvature and successfully predicts the dynamics of space time within its area of applicability) But my understanding is being shaken by the language employed by Prof. Brian Cox who is a popular science presenter on the BBC and as far as I can make out is actually a well respected scientist in this area…