Relativity

The worst myth ever about relativity is that a spaceship can never accelerate to light speed because it is steadily getting heavier, making it ever harder to accelerate. That is rubbish, and I'll convince you right here and now. Under relativity, the speeding space craft can rightfully consider itself to be stock still in space, and so cannot attribute causal distortions of any kind to itself... it's other folks who are in motion, duh. However, in a collision, then there is relativistic mass increase ascribed to the speeding projectile, and it becomes a factor in how much of an impact will result. In that case, it is the collidee's perspective that what impacts it is…

in another thread Jacques (a new poster) asked and JaKiri who has been at SFN for a couple of years and posted around 2500 messages replied IMHO Jakiri was right, and it didnt "take" in that thread. So I am going to amplify his point here: Acceleration is the key to resolving the apparent discrepancy between the clocks, but it was really too noisy in the other thread for anyone to discuss it JaKiri and Swansont were saying the right things but they were getting drowned out. So maybe JaKiri would like to explain this. Or someone else discuss it who knows what they are talking about. It has to be able to happen that the traveling brother Bob whizz…

Imagine X and Y moving at a steady non-zero velocity relative to each other. X might claim that he is stationary and that Y is moving. Y could reverse the claim. We scratch our heads and say that provided we stick to steady motion we do not know any way of distinguishing between moving and stationary. But this raises a problem. Although a huge number of different claims about the velocities of X and Y can be made (none, apparently, provable) it can't be the case that both X and Y are stationary. The steady non-zero velocity they experience relative to each other would vanish. It has to be true that at least one of them is moving. Is there a solution?

as you approach c (the speed of light) your mass decreases... but where does the mass go?

I have a question on Einstein's theory of special ralativity. They say that if two baby brother borned at the same day, but when one of them been put on a spaceship that travels at near the speed of light to another star. After earth's time of 10 years, he returns to earth, but his own borther who satyed on earth is 10 years oler than him. My questions is the brother stayed on earth growing for ten years, then wouldn't the brother in the sapceship also grows for ten years due to the metabolism? If this is not too much to read, can anyone please give a answer.

I am not very knowledgable on this subject, but from what I understand, a clock in motion at a certain velocity compared to a synchronized clock on the ground that is not in motion will "slow down" and be out of sync with the clock on the ground after the motion, and this was something predicted by special relativity. Now my question is regarding how time is actually measured, and I guess relates to the nature of time itself. Now a clock itself does not measure time by somehow tapping into whatever we call "time," it is just an arbitrary measure of intervals, using a quartz crystal, or an atom or whatever other measure you want to use. If you were to measure the v…

Let us assume that time travel has just been made within our grasp. That the entire technology has been perfected and taht we can go anywhere in time. Also, let us assume that a close loved one has suddenly expired. So, driven by rage, you go back in time using the time machine and stop the death, and change time itself. However, when time itself continued from the very point from which you changed time, when if got to the point that you eventually went back in time to change the past, you have no reason to go back, hence you never do go back, and your loved one is never saved from death because you never went back to change it. Hence, time collapses upon itself. Th…

well i know the photon has zero rest mass... but i was wondering about the other mass's, like relativistic mass and inertial mass etc. i mean, i know for a photon you use: e2 = p2c2 + m2p4 and not: e = mc2 ( ^ where all those numbers are the previous letter to the power of the number ^ ) but can someone just like do a 3 line post like: photon mass: rest mass: 0 g relativistic mass: x g inertial mass: y g where g = grams, i assume that's what its measure in, or you can use micro-grams, or nano-grams or whatever is needed, i mean, there's no point in saying 0.00000000x KG you may as well put it in nano-grams. and maybe you could include aft…

What constraints set C at 186,000 mps? Why not 196,000, or 176,000, or for that matter 1,860? aguy2

Maybe we could use a thread to collect people's various ideas about the "permittivity of free space" Is it a real physical quantity or just an artifact of the way MKS units are defined? In some units systems---like some version of the CGS---there isnt any "permittivity of free space", or so we are told: http://www.newton.dep.anl.gov/newton/askasci/1993/physics/PHY48.HTM this is a link from the Wikipedia article on the subject, they say "for further discussion see..." and give this. So maybe it is controversial, or maybe permittivity of free space has only a semantic existence---and is an artifact of whatever (possibly arbitrary) convention is used to defi…

How do you measure a distance between two points, if one of them is static (has 0 velocity relative to you), and the other one moves with near c velocity relative to you? Is there length conraction in this case?

I know it's blasphemy, but what are the reasons (other than Einsteins assumption) that lightspeed must be constant. It's always bugged me that this one quantity is "nailed down" and it screws up everything else...time, space, mass... What if it were not constant but actually did vary. Couldn't we look at something else being the "universal constant" and see what would happen with all the equations? I'm sure there is more to it than I know, but someone please tell my why lightspeed has to be a constant. KAC

Does gravity travel at the speed of light? (Meaning is there a lag between when an object moves and another object 1000km’s away is affected by a different level of gravitational pull) or… Is gravity instantaneous? If so, doesn’t this defy Einstein’s Theory of Special Relativity?

In an other thread Martin told I have difficulty to imagine the microwave background like a reference system. Microwave are radiations and radiations are not fixed in space... Can someone help me understand that one ? Thanks

I'm confused about something. I've been hearing that photons are massless, yet it is predicted that blackholes exist. The appearant contradiction here is that blackholes are predicted to exist because light is subject to gravity in the same way matter is. That is, light curves toward the source of gravity. They say that blackholes are objects with so much gravity that they do not let light escape (i.e. Light curves so much that it always ends up travelling directly towards the object's center of gravity). But how can light be influenced by gravity in this way if light, being composed of photons, is massless? Doesn't it require mass to be "heavy"?

A train is moving relative to earth at such a high speed that the clock on the train slows down to half rate compared to clocks on earth. A rocket is launched in the opposite direction train is moving to such a high speed that the clock on the rocket slows down to half rate compared to the clock on train. What will be the rate of clock time on rocket compared to that of clocks on earth ?

So, I understand that in a gravitational field, time slows down the deeper one goes into the field, but what difference does it make whether one is falling or on the ground? If I understand GR and SR correctly, then I'd assume when one is falling, you simply sum up the time dialation due to the process of falling (SR) and due to how deep one is at any point during the fall (GR). That is, when one is falling, time slows down because of the principles of SR. But in addition to that, when one falls, one ends up at deeper and deeper points in the gravitational field, and at each point, time slows down because of the principles of GR. When one is on the ground, SR would say ti…

I was wondering if there is something that can go faster than light. If an object, say a spaceship going the speed of light in between the sun and the earth would.nt the shadow on the earth go faster than the speed of light?

I read an interesting article about a month ago, and it made me think. Thus I will post the general outline of this idea for your reading pleasure. I am by no means an expert in this area, nor do I claim to be particularly knowledgeable. However I DO think that I have a adequate grasp of the basic principles. This topic has to do with the origins of the universe, from both a Big Bang and an Intelligent Design perspective. This is not intended to be a debate for or against EITHER side, so PLEASE do make it into one. I simply intend to post a working theoretical creation model that does a good job of accounting for some past discrepancies. I freely admit that all views on t…

Hi, I'm only 15 years old and new to relativity and I'm finding it a little bit confusing. Here are some of my questions. 1) Consider two spaceships. Each spaceship moves away from each other at a speed of 0.9c. Now, would light from either spaceship reach the other? If possible, provide some reasons and maybe a few interesting bits along with your answer. 2) As we all know, if a first observer was to view another second observer travelling at 0.9c relative to him/her, he/she would view the second observer to be contracted according to the direction of travel. But how would the second observer view the first observer? Would the first observer appear contracted to …

Hi, I'm new in here. I'm only 13 years old. I have a question : There are two car in a road. The first car run with speed 600 km/hour. And the second car run with speed 20 km/hour. How the first car look the second car ? How the second car look the first car ? Why ? Sorry if my english not good....

What is the cause of gravity. I know that it's caused by bent space-tme, but, every model we have shows space-time as a 2-D surface. If this is correct, then every boddy in the universe would be roughly the same highth, and (using the earth as an example, and assuming that the earth doesn't tilt) how could a north pole exist, if neither space nor time would exist there (although, on the geographical north pole, I'm not sure how you could set your watch [laghs]). So the innacuracy of this model is very clear, but I'm not sure how space-time could be bent any other way.

Hi there, I'm new here so I'll make a quick introduction for the few who care I'm a 19y old biology student from Belgium at university of Leuven. And in a few days I have my physics exam. There is this one thing that keeps bothering me because I dont know the answer. What if you were to travel at the speed of light with a car and you put on your lights. What would happen and can you explain this physical and quantumphysical. If I'd got this question I'd be shitting my pants. But this is what I came up so far. First of all, it's impossible to travel at the speed of light, see Lorentz-Einstein formula's, you could only travel at the fraction of light (a c…

Two inertial frames A and B are moving wrt to each other at a constant Va + Vb = Vab the realtive velocity of the A inertial frame with respect to the B frame. The Va fame launches a velocity probe, A', from the side of the A frame. At t < 0 Va'a = 0 and Va' + Vb = Vab. At t = 0 A' begins accelerating in the -X direction, the assumed direction of Vband opposite to that assumed for A. A is moving in the +X direction. A' is configured as a relative velocity probe and montors the Va'a and Va'b relative ve;ocities until the point at t = 1 when Va' = Vb at which time the Va' is noted as that velocity the B frame would have lost with respect to Vab, a constant, had…

Since GR says that space-time is curved in the vicinity of matter, does this not mean that there's something there is space? This would be quite different from the classical view of space which is that it's... well... "space" - a big fat field of nothingness. But space-time curvability (is that word?) must mean there's something there to be curved, like the idea of the ether. Is this what modern scientists believe? Gib