So, I'm having another conceptual problem with Relativity. The faster you go through space, the slower time gets until time stops at the speed of light (issue part 1). If time stops at the speed of light, why does it still take time for light to travel from point 'A' to point 'B' (issue part 2)?

I might be wrong but does time get slower at all? Locally time tracks as it always has, its only to people external that it appears slower, right?

Time only stops in the reference frame of the photon, for all external observers c remains constant as predicted by both relativity (general and special) and Maxwells formulae describing the actions of electromagnetic fields. Hypothetically if you could survive the fall into a black hole time would stop for you as you reached the event horizon and all of the future would flash past you in an instant as the rest of the universe wouldn't share your reference frame.

The speed of light is constant in all inertial reference frames - this is one of the postulates of special relativity.

However if we imagine Person A watching Person B moving past him, person A can observe the interval between the beats of Person B's heart. As Person B moves faster and faster, this interval will appear (according to Person A) to increase - this is what you referred to as time going slower. If we now replace Person B by a light source, and his heartbeat by the crests of the electromagnetic wave, then according to Person B, the time interval between those crests will increase as the light source moves faster and faster. In other words, the moving source will be redshifted. As the source reaches the speed of light, this redshift will be infinite (the time between crests has gone to infinity). However, the postulate of relativity still stands - the speed of the light received from the source is still equal to the constant speed of light c.

I think. More or less. Haven't thought about this shit for 26 years !

The thing about Relativity is, well, it's all relative.

IIRC it has been verified insofaras unstable particles with known "life-spans" when accelerated appear to "live" much longer. They actually survive for the same time (from their perspective) but from our perspective they seem to last much longer.

There is an implicit assumption in the OP that a massive object can be accelerated to the speed of light, which is not possible under special relativity.

How things would actually appear in an inertial frame travelling at the speed of light is a bit of a non-question, since not only time but space are relative, the direction in which the IF moves contracts and would have 0 length at the speed of light. So it's hardly surprising that the journey would be instantaneous in that IF, since there would be no journey to make!

trubble76 wrote:The thing about Relativity is, well, it's all relative.

IIRC it has been verified insofaras unstable particles with known "life-spans" when accelerated appear to "live" much longer. They actually survive for the same time (from their perspective) but from our perspective they seem to last much longer.

Specifically about muons, that should not survive the trip down to the Earth's surface, because their life span is very small, but they do because for them time is dilated, with respect to our frame of reference.

You have to remember about frames of reference; the special theory of relativity deals with inertial frames of reference, that is frames of reference where Newton's first law is valid; with special relativity, it is shown that all physical laws are valid in an inertial frame of reference. Basically an inertial frame of reference is a frame of reference with constant velocity, in very simple terms.

Even though the Earth rotates, and thus is a non-inertial frame of reference, the velocity and acceleration are very small in reference with astronomical and physical phenomena, so we can make the assumption that the Earth is an inertial frame of reference.

Mr P wrote:Hypothetically if you could survive the fall into a black hole time would stop for you as you reached the event horizon and all of the future would flash past you in an instant as the rest of the universe wouldn't share your reference frame.

I admit it's a while since I studied this, but this doesn't sound right. I could be wrong though.

Edit: Never mind, it's just me misremembering.

I have some "armchair physics" type questions rolling around my head, and this seems as good a place as any to ask them. (I didn't study physics - which will be obvious - so if you can answer, please talk to me like I"m a 5th grader

1. I have heard that the speed of light is a limiting factor. I have also heard speed is relative. So if two objects are moving away from each other at near the speed of light, doesn't that mean they're both going almost twice the speed of light relative to each other? What am I missing?

2. Does this imply that components of light are not moving, or that it doesn't have components, since if they were in within something moving at the speed of light they would be at times moving faster than the speed of light. I'm thinking this has something to do with light not being physical at all times. Any hints?

3. A analogy commonly used to explain relativity is conductors at different parts of a moving train flashing lights. To different observers, the same light appear to be flashed simultaneously or not. What I don't get is the conclusion that the "Simultaneity is relative to a frame of reference; it is not absolute." What does the appearance of the lights have to do with when they actually were flashed? For instance, I can see a cricket bat hit a ball from the other side of a field, but the sound doesn't reach me until way after, and that's the same event! I don't conclude the sound and the action happened at different times, simply that the effects took different times to reach me. What am I missing, or is it really just an analogy for something I'm not getting, and has nothing to do with the speed of light reaching observers at different times to arrive at the conclusion "simultaneity is relative". Hmm.. Hard to explain my question - I hope that makes sense.

4. Does gravity propagate at a known speed? If an object moves, does its gravity move with it instantly, or take it take time to catch up?

5. Why is the speed of light a limiting factor? Is it that some things just go as fast as is physically possible, and light happens to be one of them, or is it the other way around; does the fact that light goes that fast play any part in determining its speed is the limit for everything?

6. Does "quantum entanglement" really imply that physical objects can affect each other instantaneously no matter how separated they are, or is that a just load of old Deepak Chopra?

Thanks in advance, and please paste me in a more appropriate place if appropriate

CookieJon wrote:I have some "armchair physics" type questions rolling around my head, and this seems as good a place as any to ask them. (I didn't study physics - which will be obvious - so if you can answer, please talk to me like I"m a 5th grader

Give me that spliff. IMMEDIATELY !

CookieJon wrote:
1. I have heard that the speed of light is a limiting factor. I have also heard speed is relative. So if two objects are moving away from each other at near the speed of light, doesn't that mean they're both going almost twice the speed of light relative to each other? What am I missing?

There is a formula in special relativity for addition velocities. It turns out that the composite of V1 and V2 is not V1+V2. Google relativistic velocity addition.

CookieJon wrote:
2. Does this imply that components of light are not moving, or that it doesn't have components, since if they were in within something moving at the speed of light they would be at times moving faster than the speed of light. I'm thinking this has something to do with light not being physical at all times. Any hints?

Thinking of light as an electromagnetic wave, no, the wavefront moves at c in vacuo. Light is always physical.

CookieJon wrote:
3. A analogy commonly used to explain relativity is conductors at different parts of a moving train flashing lights. To different observers, the same light appear to be flashed simultaneously or not. What I don't get is the conclusion that the "Simultaneity is relative to a frame of reference; it is not absolute." What does the appearance of the lights have to do with when they actually were flashed? For instance, I can see a cricket bat hit a ball from the other side of a field, but the sound doesn't reach me until way after, and that's the same event! I don't conclude the sound and the action happened at different times, simply that the effects took different times to reach me. What am I missing, or is it really just an analogy for something I'm not getting, and has nothing to do with the speed of light reaching observers at different times to arrive at the conclusion "simultaneity is relative". Hmm.. Hard to explain my question - I hope that makes sense.

The problem is with the concept of when they "actually" were flashed. There is no "actually" on which all observers agree. That is the crux of SR.

CookieJon wrote:
4. Does gravity propagate at a known speed? If an object moves, does its gravity move with it instantly, or take it take time to catch up?

According to GR, gravitational radiation propagates at the velocity of light (locally, at any instant for any pedants).

CookieJon wrote:
5. Why is the speed of light a limiting factor? Is it that some things just go as fast as is physically possible, and light happens to be one of them, or is it the other way around; does the fact that light goes that fast play any part in determining its speed is the limit for everything?

Well, the whole special relativity framework, built upon the relativity of simultenaity is constrained by the fastest rate at which information can be transferred. This is c.

CookieJon wrote:
6. Does "quantum entanglement" really imply that physical objects can affect each other instantaneously no matter how separated they are, or is that a just load of old Deepak Chopra?

Affect, yes. Affect in a way that can be used to transmit information from one to another, no.

CookieJon wrote:I have some "armchair physics" type questions rolling around my head, and this seems as good a place as any to ask them. (I didn't study physics - which will be obvious - so if you can answer, please talk to me like I"m a 5th grader

1. I have heard that the speed of light is a limiting factor. I have also heard speed is relative. So if two objects are moving away from each other at near the speed of light, doesn't that mean they're both going almost twice the speed of light relative to each other? What am I missing?

2. Does this imply that components of light are not moving, or that it doesn't have components, since if they were in within something moving at the speed of light they would be at times moving faster than the speed of light. I'm thinking this has something to do with light not being physical at all times. Any hints?

3. A analogy commonly used to explain relativity is conductors at different parts of a moving train flashing lights. To different observers, the same light appear to be flashed simultaneously or not. What I don't get is the conclusion that the "Simultaneity is relative to a frame of reference; it is not absolute." What does the appearance of the lights have to do with when they actually were flashed? For instance, I can see a cricket bat hit a ball from the other side of a field, but the sound doesn't reach me until way after, and that's the same event! I don't conclude the sound and the action happened at different times, simply that the effects took different times to reach me. What am I missing, or is it really just an analogy for something I'm not getting, and has nothing to do with the speed of light reaching observers at different times to arrive at the conclusion "simultaneity is relative". Hmm.. Hard to explain my question - I hope that makes sense.

4. Does gravity propagate at a known speed? If an object moves, does its gravity move with it instantly, or take it take time to catch up?

5. Why is the speed of light a limiting factor? Is it that some things just go as fast as is physically possible, and light happens to be one of them, or is it the other way around; does the fact that light goes that fast play any part in determining its speed is the limit for everything?

6. Does "quantum entanglement" really imply that physical objects can affect each other instantaneously no matter how separated they are, or is that a just load of old Deepak Chopra?

Thanks in advance, and please paste me in a more appropriate place if appropriate

I'm an armchair physicist, but i'll have a go (while we wait for someone that actually knows what they are talking about)

1)This is where the link with time becomes important. Imagine a train travelling at c - 2mph, if you start jogging at 3mph you don't break the speed of light by 1mph, your experience of time slows by just enough to limit you to c. It's this relativity of time that preserves the cosmic speed limit. This is where Hubble made his mark, reading a little about him should explain a bit.

2)As far as i know, light has only one componant, photons, which are virtual particles. Photons in a vacuum always travel at c (i think, gravity may slow them a little)

3)Sorry, I don't really follow you on 3)

4)Don't know this one.

5)If you think about the interlink of time and space (space-time), objects like trains move through both time and space. If the train has expended all it's energy to reach c, it has no energy to move through time. Time slows the closer you get to c, the more energy the train put into reaching c, the less it has to move through time. This is over-simplified and most likely ultimately wrong, but it's how I understand it.

6)It is a theory that has been abused by the chopra kind. Imagine you have two buckets filled with balls, one bucket full of red, one full of blue. You take out one ball from each bucket and wrap them up so neither of us knows which is which. Then if I travel to the other side of the universeand unwrap my ball, the instant i see what colour it is, i also know what colour your ball is. I am across the universe, but I instantly have "gained information" about your ball, seemingly faster than the speed of light.

I await correction.

twistor59 wrote:Give me that spliff. IMMEDIATELY !

No it's mine, get your own!

twistor59 wrote:1. There is a formula in special relativity for addition velocities. It turns out that the composite of V1 and V2 is not V1+V2. Google relativistic velocity addition.

Ta. Will do. (No, I really will!)

twistor59 wrote:2. Thinking of light as an electromagnetic wave, no, the wavefront moves at c in vacuo. Light is always physical.

Hmm.

twistor59 wrote:3. The problem is with the concept of when they "actually" were flashed. There is no "actually" on which all observers agree. That is the crux of SR.

Right - this is what I don't get. So what if the observers don't agree on the perceived order of the flashes? This is just the result of the time taken for the light to reach them, right? There still was an "actually" when the lights were flashed. What has the time taken for the effects to be visible by an observer got to do with anything? Close my eyes and change reality? Yes? No? Going back to the cricket bat, *I* can't even agree with myself that the sound & vision "actually" happened at the same time, but the problem is mine, not with the cricket bat being out of sync with itself. Still confused.

twistor59 wrote:4. According to GR, gravitational radiation propagates at the velocity of light (locally, at any instant for any pedants).

Ok.

twistor59 wrote:5. Well, the whole special relativity framework, built upon the relativity of simultenaity is constrained by the fastest rate at which information can be transferred. This is c.

twistor59 wrote:6. Affect, yes. Affect in a way that can be used to transmit information from one to another, no.

I see.

Thanks for your time!

twistor59 wrote:3. The problem is with the concept of when they "actually" were flashed. There is no "actually" on which all observers agree. That is the crux of SR.

CookieJon wrote:
Right - this is what I don't get. So what if the observers don't agree on the perceived order of the flashes? This is just the result of the time taken for the light to reach them, right? There still was an "actually" when the lights were flashed. What has the time taken for the effects to be visible by an observer got to do with anything? Close my eyes and change reality? Yes? No? Going back to the cricket bat, *I* can't even agree with myself that the sound & vision "actually" happened at the same time, but the problem is mine, not with the cricket bat being out of sync with itself. Still confused.

There still was an "actually" in the sense of there being a point in spacetime at which it happened. However, the time coordinate assigned to it depends on the observer. Since the length of, say, the train is the difference in position of the front and the back measured at the same time, you can see that when "at the same time" is different for a different observer, the length will be different. I could have a ferret around to see if there are any good explanations on the internet. It's not easy to explain without diagrams. I'm sure someone has done some good pictures somewhere....

Edtted for quote tags

CookieJon wrote:

twistor59 wrote:3. The problem is with the concept of when they "actually" were flashed. There is no "actually" on which all observers agree. That is the crux of SR.

Right - this is what I don't get. So what if the observers don't agree on the perceived order of the flashes? This is just the result of the time taken for the light to reach them, right? There still was an "actually" when the lights were flashed. What has the time taken for the effects to be visible by an observer got to do with anything? Close my eyes and change reality? Yes? No? Going back to the cricket bat, *I* can't even agree with myself that the sound & vision "actually" happened at the same time, but the problem is mine, not with the cricket bat being out of sync with itself. Still confused

As long as it does not violate causality, it's fine. Let's just say that you flash a red flashlight now, and a green one after some time (not important). Although observers outside this frame of reference will disagree on when the flashes occurred, they will always agree on one thing: that the red flash was first and the green second. So, causality cannot be violated at any point in space and time, exactly because of the constancy of the speed of light and the principle of relativity.

This link has the usual SR explanation in terms of diagrams. There may well be better ones, but I'm supposed to be working now, so don't really have time to search. Maybe some others know of good links...

twistor59 wrote:This link has the usual SR explanation in terms of diagrams. There may well be better ones, but I'm supposed to be working now, so don't really have time to search. Maybe some others know of good links...

thanks

Sovereign wrote:So, I'm having another conceptual problem with Relativity. The faster you go through space, the slower time gets until time stops at the speed of light (issue part 1). If time stops at the speed of light, why does it still take time for light to travel from point 'A' to point 'B' (issue part 2)?

part 1 - Correct
part 2 - It doesn't. You're missing the part about reference frames. On the reference frame of light, there is no passage of time, so if light was a sentient entity, it would not experience the passage of time while traveling from A to B. However from the reference frame of someone standing on earth, light would appear to take time to travel from A to B.

CookieJon wrote:

twistor59 wrote:This link has the usual SR explanation in terms of diagrams. There may well be better ones, but I'm supposed to be working now, so don't really have time to search. Maybe some others know of good links...

thanks

If you want an SR refresher with an animated diagram on simultaneity and mind bender to boot then this tachyonic anti-telephone stuff might help.

Mr P wrote:Time only stops in the reference frame of the photon, for all external observers c remains constant as predicted by both relativity (general and special) and Maxwells formulae describing the actions of electromagnetic fields. Hypothetically if you could survive the fall into a black hole time would stop for you as you reached the event horizon and all of the future would flash past you in an instant as the rest of the universe wouldn't share your reference frame.

Thanks for once again explaining physics to me man.