between3and26characterslon

So if A remains at constant v with respect to K and B remains at constant v with respect to K albeit 1 second time t and vt(I think??) distance behind A then A will be younger than B by more than 1 second of time t'

If they both start of at rest with respect to coordinate system K and A takes off at v with respect to K and continues with velocity v with respect to K then B, who leaves one second later, will have to travel faster than v to catch up with A, their clocks will be synchronous. If A accelerates to v with respect to K and then decelerates with respect to K eventually coming to rest again with respect to K and B does the same thing, but one second behind, their clocks will be synchronous.

Ok my understanding (and I could be wrong here) is that distant galaxies are accelerating away from us, that this acceleration is due to a force and that this force comes from dark energy. If space is expanding then galaxies are not accelerating and therefore there is no force and no need for dark energy. The problem I have is that local galaxies are not receding from us due to the expansion of space because they are grvitionaly bound. If you were to transport to a galaxy 10bn lightyears away you would see that the local galaxies are not receding form each other because they are gravitionaly bound. If you went anywhere in the universe you would see local galaxies exhibiting the same behaviour. So how can it be at all points in the universe local objects are not receding but at great distances they are receding. Ok lets say you were at 1cm on a steel rule, when you look out you see 2cm is not receding, 3cm is not receding 4cm is not etc etc... but cm 90cm is receding. The same must be true at 90cm however, when you look out you see 89cm is not receding, 88cm is not etc etc... Wherever you are on the steel rule you see nearby cm's remaining where they are but the most distant ones receding.

I recommend you do try it, remember the faster you go the more chance you have of getting through

I was thinking something similar the other day and something hit me. If there are infinite universes, or in the sense you're asking, infinite futures are there also an infinite number of histories/pasts? I arrived at the conclusion that if there is only one past there can only be one future. I don't like the idea that there is only one future, it means I have no choice in anything as everything is predetermined. I much prefer the idea there are infinite futures but this means there are infinite pasts as well. I find this notion bizarre as I only remember one past. Here's my logic. There is more than one possibility what might happen in 5 mins time There is aslo more than one possibility what might happen in 10 mins time If in 10 mins time I look back 5 mins ago there must have been more than one possible future but as it is now in the past there must be more than one possible past. It's a strange world (universe) we live in.

Ok to get back to your question I will try to explain it simply. First of all we need to go back to Galileo who suggested 'The Principle of Relativity'. The principle of relativity says that whatever laws of nature are true for one observer they must also be true and the same for another observer. The laws in one system of coordinates must be no less simple in another system of coordinates. It also said that all observers are in motion relative to something, that is to say NOTHING is absolutely at rest. If something was absolutley at rest then every observer would be in motion relative to it. That means an observer would have to take into account their motion when formulating their laws of physics. An example; when I drop a ball I see it fall straight to the ground. If I was on a moving train and I dropped a ball I would also see it fall straight to the ground (floor of the train) If the principle of relativity was not true I would have to take into account the motion of the train when describing how the ball falls. This means the laws of physics on the train would be different to those on the ground outside. to clarify, the principle of relativity is 1) Objects or observers can be at rest with respect to each other but nothing is absolutely at rest. 2) All objects or observers are in motion with respect to something. 3) The laws of physics for one observer must be the same and no less simple for another observer. What did Einstien do? He realised that if the laws of physics state that the speed of light in vacuo is constant then it must be constant for every observer regardless of how they are moving. But how can that be? If I am standing on a platform at a train station and a train shoots past at some enormous speed and a beam of light is fired in the direction the train is travelling then I on the platform measure the speed of light to be c (3x10^8 meters per second). Also the person on the train measures the speed of this light beam to be c, he does not add or subtract his speed from the speed of light because the speed of light in vacuo must be constant for all observers regardless of how they are moving. If you accept Galileo's principle of relativity is true and that both observers measure the speed of light to be the same something doesn't make sense. So how can it be that if I'm stationary relative to the source of light and another observer is moving with incredible speed in the same direction as the beam of light we both measure the light to be travelling at the same speed relative to us. The theory (Einstein's) is that on the train time has slowed down relative to the observer on the platform and distance (in the direction the train is moving) has got less relative the the observer on the platform. This means when the light gets to the observer on the train it has less distance to travel because the train is shorter and has more time to do it in because time has slowed down. The observer on the train does not notice that his time is slower or his length is less, to him nothing has changed. Exactly the same laws of physics apply to him moving at speed as apply to the observer on the platform who is stationary (relative to the light source) The principle of relativity is preserved and the speed of light is preserved for both observers but there is a transformation form one observer to the other. The laws of physics that describe the transformation of time and length of one observer as viewed by the other observer are the same for both observers viewing each other and are no less simple as per the principle of relativity. Hope that makes sense.

Having read all the replies I can say it has nothing to do with dopler effect, the problem is you are not playing by the rules, you have false axioms in your question. You are assuming that as Brian moves away from Andy that this is no different to Andy moving away from Brian or both moving away from each other. This is wrong. This leads you to conclude that they will both see each other's clocks running slower and therefore they will both be younger than each other, hence the paradox. This is also wrong. Let us reset the experiment using the correct axioms. 1) Andy and Brian must find themselves an inertial frame. That is, as I'm sure you know, Where a body is sufficiently far removed from any force so as to not be subject to any force. 2) We will now refer to this inertial frame as a system of coordinates and, using Einstein's notation, call it coordinate system K made up of coordinates (x, y, z) 3) In this inertial frame (coordinate system K) Newton's law of inertia is true. That is a body at rest with respect to K will remain at rest with respect to K until acted upon by a force and a body traveling with uniform translation (i.e. same speed in a straight line, i.e. constant velocity) with respect to K will continue with constant velocity with respect to K until acted upon by a force. 4) Both Andy and Brian are at rest (motionless) with respect to K. 5) Andy and Brian both have clocks which are both precisely accurate and synchronous whith each other. They both have measuring rods s which are identical to each other (you'll see why later). 6) Brian is then subjected to a force and therefore accelerated in the direction of x of coordinate system K. He has no velocity or acceleration in the direction of y or z. 7) It is now true for coordinate system K that Brian is in motion (i.e in reality) and Andy is at rest (i.e in reality) with respect to K. 8) Brian is now in a coordinate system K' which is moving with respect to K. 9) In K time=t and distance=s. 10) in K' time=t' and distance=s'. 11) The Lorentz transformation will tell you what t' and s' are in K' when measured from K. 12) It is important to understand that the axes (x, y, z) of K are parralell to and positive in the same direction as (x', y', z') of K'. 13) When Andy, who is in K, looks at Brian he sees Brians clock running slower and his measuring rod will be shorter. 14) This means that when viewed from K, t'> t (seconds t' are longer than seconds t) and s'< s (distance s' is less than distance s) 15) You're now thinking "what's the difference between K' moving relative to K or K moving relative to K' ?" Here's the bit I struggled with, I am not an expert (physicist or mathematician) and I'm sure people will disagree but, it must be true within coordinate system K, the values of t, x and s in K are negative with respect to K' . 16) See 12, If (x, y, z) coincide with and are positive in the same direction as (x', y', z') then K' is moving with (+)velocity with respect to K and K is moving with (-)velocity with respect to K' 17) Therefore when viewed from K', t< t' (seconds t are shorter than seconds t') and s> s' (distance s is longer than distance s') 18) Andy will see Brian's clock go slower and his measuring rod get shorter, Brian will see Andy's clock go faster and his measuring rod get longer (lucky ol' Andy). 19) However much time and distance change in one system of coordinates they change equally and oppositely in the other. 20) I hope that all makes sense and more than that I hope it's correct but it makes sense to me.

My original question made an assumption which may have been wrong so to clarify here are two statements. A) A photon can only interferre constructively and destructively with itself. B) Two photons can interferre constructively and destructively with each other. Which one is correct or are they both correct? Statement 'A' makes sense in Young's slits experiment and I understand the conservation of energy in this case. However the peeks on the track of a CD are a height that is 1/4 the wavelength of the light used to read it therefore there is a phase difference of 1/2 a wavelength between peeks and troughs which cancels out the light at this point. So in this case what happens to the cancled light? Is it just bad wording and the light is only cancled in the direction it would otherwise have been read and is actually defracted in other directions? And is statement 'B' how lasers work or is constructive interferrence not the same as coherent light?

If energy can not be created or destroyed then what happens to the energy of 2 photons when they destructively intefere with each other?

This is getting quite complicated so lets go back to the beggining. I get the impression that what the OP saying is something along the lines of; Let's say there is a planet 1 lightyear from Earth and this planet has intelligent life on it and we want to communicate with them. When we see this planet we see it as it was 1 year ago, if we send a message to them it will take exactly 1 year to get there. Therefore they recieve this message in the present. When they look at us they see us one year in their past, when they reply to our message it takes exactly 1 year to reach us so gets here in the present. What your suggesting is the speed of light is therefore infinite and this would mean we could communicate with our alien friends in real time which we know can't happen. I think the confusion is that from light's perspective it is travelling at infinite speed because t=0 but from our perspective the speed of light is finite. Does this make sense or am I not understanding your point?

Hi Everyone, this is my first post so hopefully the topic title got your attention. I've been thinking about this for some time and I'm getting confused. Hubble's arguement is that the further away an object is the faster it is moving. I understand that type 1a supernovae are used to determine this because thay are standard candles. So it goes if you know how bright something is you can calculate its distance and then by measuring its red shift you can calculate its speed. The results are that the further away something is the faster it is moving so if its twice as far away its moving twice as fast. This would suggest that as an object it moves away it gets faster, it accelerates. Thus the conclusion the Universe is expanding at an accelerating rate. To my knowledge no single object has been shown to have a greater red shift year on year (correct me if I'm wrong). It's more a case of object 'a' is 'x' distance away and moving at speed 'v' and object 'b' is '2x' distance and moving at speed '2v' So my problem is this: Object 'b' is twice as far away but we're also seeing it as it was twice as long ago. This suggests that the further back in time you go the faster objects were travelling. Or to put it another way the nearer to the present you get the slower things are travelling. Does this not suggest the expansion of the Universe is slowing down with time?

Hi my name is between3and 26characterslon(g) because that's what it said it should be when I registered. Only I can't count so missed the g off the end. I'm a thinker but not an expert.