On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
I'm not incredibly knowledgeable on the subject, but isn't whether photons have mass or not still an experimental question? Also, this would only mean that c is a constant for maximum possible speed, rather than the speed of a photon per se, right?
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
only when traveling through a medium, not in a vacuum.
On April 25 2011 01:18 micronesia wrote: Here is a video from that link:
Can you tell what is wrong with this video? XD
Is this simply false because he's not going nearly as fast (or for as long) as he needs to be going for any actual time dilation to occur?
Quite obviously just false as I see it.
What confuses me about this subject / way of testing it as a whole though is that this movement is only relative to our planet - not space. Imagine the Earth is right this second traveling (around the sun) through space to the direction you would call "west". Then imagine you started driving east. Wouldn't you then essentially slow down your own speed i space? I have no idea how this works, but that would seem logical to me. At least if our speed can at all be considered independent from that of the Earth's.
Please correct me.
There is no universal reference frame that we know of. Everything about relativity is relative (surprise surprise). It's common to use the sun or the Earth as a 'fixed' reference point.
Actually, I would argue against that point. General relativity covers space-time, and although I advise to avoid general relativity in a forum thread, this part is relative (no pun intended, damn it Einstein!)
Space-time, is almost by definition the universal frame of reference. Space-time is the plane upon which we move, and consists of all 4 dimensions. (Don't start string/M theory with me, I just wont listen to a theory that has no chance of ever being proven, you can keep your 10/24 dimensions!). Although often portrayed as a taught sheet of rubber (or the like) that still misses time, which I'm afraid, you're just not going to get your head round. But, the effect of this is that it is an absolute frame of reference, and was constructed to be so. Space time was made using the same postulates that Einstein used for his theories of relativity. Essentially, the outcome is that everyone and everything travels through space-time at the same rate.
Here is a time-space graph depicting movement through space-time. Dont ask me why they do a time-space graph instead of a space-time graph, I never understood.
The red arrow depicts every day objects, that move at non-relativistic speeds. Because everything moves through space-time at the same rate, and they aren't moving through space very quickly, the majority of their movement is through time. And this is the time you measure on your watch whilst going about your life.
The green arrow shows what would happen as we accelerate. Because the object is moving (much) faster through space, and has to move the identical amount through space-time, it moves less along the time axis. I.e. It experiences less time (relative to space-time) than the red arrow.
The yellow arrow is for photons, and anything travelling at the speed of light. Here, all of its movement through space-time is going through space (you can prove this by putting in v=c in the equation for gamma/time dilation). Therefore, it experiences no time at all.
If there wasn't this absolute frame of reference, then the twin paradox would be all together more confusing. If one twin is off in a spaceship travelling at near the speed of light, relative to them it will seem like their sibling is travelling at near the speed of light (or at least earth is). So they would both observe the other one travelling slower through time, so it would cancel out yes? No. Because the twin in the spaceship has travelled further through space (ie, is the green arrow to the earthbound twins red arrow)
I'm very tired at the moment so may have not been very clear, or in fact may have been wrong, so please do point out anything I've missed/you don't understand.
P.S. With my last point, I am infact missing out parts. It is also due to the acceleration of the space ship twin, and how they decelerate through gravitational fields. But the explanation I gave suits. And it avoids nasty explanations .
P.P.S. Also, when I say everything has to go through space-time at the same rate, that is also a lie. In fact everyone must observe everything else going through space-time at the same rate. Individual paths may differ. But they are *almost* the same thing. It only makes small changes in this area.
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
only when traveling through a medium, not in a vacuum.
Irrespective of whether that's the case - what part of "only when travelling in a vacuum" amounts to, and I quote, "ALWAYS"? :/
On April 25 2011 01:18 micronesia wrote: Can you tell what is wrong with this video? XD
To lose 2s in 24 minutes you'd need a gamma = 1442/1440, so a speed of .002772c, 831600m/s, 3million miles per hour. Luckily if your car can go that fast they usually don't pull you over for filming yourself while driving.
Ok, so he has a really fast car. The problem is that he accelerates massively (~120g) during the trip and so this should be treated as a general relativity problem, not special.
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
I'm not incredibly knowledgeable on the subject, but isn't whether photons have mass or not still an experimental question? Also, this would only mean that c is a constant for maximum possible speed, rather than the speed of a photon per se, right?
Its one of those things which agrees so overwhelmingly with all theories and tests, that it is nigh on impossible for it not to be true.
But if through some bizarre circumstance it turns out photons have a minuscule amount of mass, then yes. c would cease to be the speed of light, and return to being a cosmological speed limit (of sorts).
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
only when traveling through a medium, not in a vacuum.
Irrespective of whether that's the case - what part of "only when travelling in a vacuum" amounts to, and I quote, "ALWAYS"? :/
Its still interesting that it's slowed down in a medium. I seem to remember that you could slow down light to 24 miles an hour in sodium crystals. But, yes. All things assume travelling in a vacuum, unless stated otherwise. Which I guess should have been stated earlier
Relativity is interesting, but there are many factors and variables in theory that are not true or doesn't reflect the way things behave in the normal universe and is also incomplete.
As for the first one, light doesn't behave like that and doesn't apply in real world, only in theory. We shouldn't even use light as an example of relativity to sight, because we are dependent on light as sight. Or light = sight
You can't see the photon of light you are chasing. It's like taking a picture in completely dark vacuum space with a flash of light and running after it. You will never see the light again if you don't have enough speed to get in sync with it in the same direction. Theoretically, if you travel faster than that flash of light, you can't even see as well since it's too slow to hit your eyes. In order for you to see the same light source twice, you would have to see it first, move faster than it in the same direction, then stop and wait for it to hit you again, or move in the speed to stay in sync with it for it to hit your eyes constantly.
For the stationary person, all he will see is a flash at most of both the photon and the person chasing it, depending on whether his brain can comprehend the little amount of time to see it.
So the question is, what are we using to see sight, if light or something to the speed of light is used as a reference in relativity?
On April 25 2011 04:14 Pleiades wrote: Relativity is interesting, but there are many factors and variables in theory that are not true or doesn't reflect the way things behave in the normal universe and is also incomplete.
As for the first one, light doesn't behave like that and doesn't apply in real world, only in theory. We shouldn't even use light as an example of relativity to sight, because we are dependent on light as sight. Or light = sight
You can't see the photon of light you are chasing. It's like taking a picture in completely dark space with a flash of light and running after it. You will never see the light again if you don't have enough speed to get in sync with it in the same direction. Theoretically, if you travel faster than that flash of light, you can't even see as well since it's too slow to hit your eyes. In order for you to see the same light source twice, you would have to see it first, move faster than it in the same direction, then stop and wait for it to hit you again, or move in the speed to stay in sync with it for it to hit your eyes constantly.
For the stationary person, all he will see is a flash at most of both the photon and the person chasing it, depending on whether his brain can comprehend the little amount of time to see it.
So the question is, what are we using to see sight, if light or something to the speed of light is used as a reference in relativity?
I'll get back to the second one later.
Ahh, you're ruining all the fun of though experiments. One of the major advantages of thought experiments is that the don't have to feel the restrictions that real experiments do. It is true that you cannot effectively chase a photon, because you cannot observe it. But you can certainly use the situation in a thought experiment. In fact I defy you to try to imagine the real life circumstance. I doubt that anyone could realistically imagine some of the situations that come up when you talk about these topics.
for some reason, I was usually fine doing simple time dilation / length contraction, but could never get the hang of Lorentz transforms (at least the way I was taught). Maybe I'd be better at them now that I know linear algebra... lol.
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
I'm not incredibly knowledgeable on the subject, but isn't whether photons have mass or not still an experimental question? Also, this would only mean that c is a constant for maximum possible speed, rather than the speed of a photon per se, right?
Its one of those things which agrees so overwhelmingly with all theories and tests, that it is nigh on impossible for it not to be true.
But if through some bizarre circumstance it turns out photons have a minuscule amount of mass, then yes. c would cease to be the speed of light, and return to being a cosmological speed limit (of sorts).
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
only when traveling through a medium, not in a vacuum.
Irrespective of whether that's the case - what part of "only when travelling in a vacuum" amounts to, and I quote, "ALWAYS"? :/
Its still interesting that it's slowed down in a medium. I seem to remember that you could slow down light to 24 miles an hour in sodium crystals. But, yes. All things assume travelling in a vacuum, unless stated otherwise. Which I guess should have been stated earlier
I'm pretty sure that the speed of the photon doesn't slow down, but it only appears to. This is because of the mass about of molecules in a medium, meaning that the photons will hit those molecules. They, in turn, produce another photon after the collision, and it continues on. So the speed of the photon doesn't really change but it looks like it because of the multiple collisions.
On April 25 2011 04:14 Pleiades wrote: Relativity is interesting, but there are many factors and variables in theory that are not true or doesn't reflect the way things behave in the normal universe and is also incomplete.
As for the first one, light doesn't behave like that and doesn't apply in real world, only in theory. We shouldn't even use light as an example of relativity to sight, because we are dependent on light as sight. Or light = sight
You can't see the photon of light you are chasing. It's like taking a picture in completely dark space with a flash of light and running after it. You will never see the light again if you don't have enough speed to get in sync with it in the same direction. Theoretically, if you travel faster than that flash of light, you can't even see as well since it's too slow to hit your eyes. In order for you to see the same light source twice, you would have to see it first, move faster than it in the same direction, then stop and wait for it to hit you again, or move in the speed to stay in sync with it for it to hit your eyes constantly.
For the stationary person, all he will see is a flash at most of both the photon and the person chasing it, depending on whether his brain can comprehend the little amount of time to see it.
So the question is, what are we using to see sight, if light or something to the speed of light is used as a reference in relativity?
I'll get back to the second one later.
Ahh, you're ruining all the fun of though experiments. One of the major advantages of thought experiments is that the don't have to feel the restrictions that real experiments do. It is true that you cannot effectively chase a photon, because you cannot observe it. But you can certainly use the situation in a thought experiment. In fact I defy you to try to imagine the real life circumstance. I doubt that anyone could realistically imagine some of the situations that come up when you talk about these topics.
Oh I have some ideas... like jet planes and Mach numbers or lightning and thunder. Just that I haven't had the time to do a scenario yet
On April 25 2011 04:06 QuAnTuM314 wrote: Here is a time-space graph depicting movement through space-time. Dont ask me why they do a time-space graph instead of a space-time graph, I never understood.
The red arrow depicts every day objects, that move at non-relativistic speeds. Because everything moves through space-time at the same rate, and they aren't moving through space very quickly, the majority of their movement is through time. And this is the time you measure on your watch whilst going about your life.
The green arrow shows what would happen as we accelerate. Because the object is moving (much) faster through space, and has to move the identical amount through space-time, it moves less along the time axis. I.e. It experiences less time (relative to space-time) than the red arrow.
The yellow arrow is for photons, and anything travelling at the speed of light. Here, all of its movement through space-time is going through space (you can prove this by putting in v=c in the equation for gamma/time dilation). Therefore, it experiences no time at all.
I'm not entirely sure what exactly you're trying to say in your post. I think you're saying that there is an absolute frame and I'm pretty sure that's not true. It sounds like you're describing the concept of an aether. But like I said, I don't really get what you're saying so I will just comment on what I quoted above.
I'm not really sure if there's a "real" reason why spacetime diagrams always have time on the vertical axis. It really does just make sense that way, especially in general relativity. Horizontal cones are just awkward to look at.
Your diagram is also not correct. Something traveling at the speed of light is not the same as something traveling at infinite speed (your yellow line). Light still takes a certain amount of time (relative to a stationary observer) to travel a certain distance. For instance, light takes about 8 mins to get from the sun to earth. The green arrow is what is actually traveling at the speed of light. The usual point of that diagram is that if you are an observer at (0,0) then you can't affect anything outside of the light cone (the lines x=t and x=-t (c=1)). Likewise anything outside your light cone won't affect you until their light cone cross your light cone.
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
I'm not incredibly knowledgeable on the subject, but isn't whether photons have mass or not still an experimental question? Also, this would only mean that c is a constant for maximum possible speed, rather than the speed of a photon per se, right?
Its one of those things which agrees so overwhelmingly with all theories and tests, that it is nigh on impossible for it not to be true.
But if through some bizarre circumstance it turns out photons have a minuscule amount of mass, then yes. c would cease to be the speed of light, and return to being a cosmological speed limit (of sorts).
On April 25 2011 04:07 Dagobert wrote:
On April 25 2011 03:58 Luddite wrote:
On April 25 2011 03:35 Dagobert wrote:
On April 25 2011 03:00 Duka08 wrote: Photons, the way we treat them as particles, have NO mass. Ever. They carry momentum and energy, but they have no mass. And they ALWAYS travel at the speed of light.
Nope, they can be slowed down (Akopian et al., 2011).
only when traveling through a medium, not in a vacuum.
Irrespective of whether that's the case - what part of "only when travelling in a vacuum" amounts to, and I quote, "ALWAYS"? :/
Its still interesting that it's slowed down in a medium. I seem to remember that you could slow down light to 24 miles an hour in sodium crystals. But, yes. All things assume travelling in a vacuum, unless stated otherwise. Which I guess should have been stated earlier
I'm pretty sure that the speed of the photon doesn't slow down, but it only appears to. This is because of the mass about of molecules in a medium, meaning that the photons will hit those molecules. They, in turn, produce another photon after the collision, and it continues on. So the speed of the photon doesn't really change but it looks like it because of the multiple collisions.
On April 25 2011 04:46 rubio91 wrote: I'm pretty sure that the light produces in the nucleus of the stars takes years to get on the surface, this is pretty much impressing isn't it?
These two points are very related. The reason photons produced in the centre of stars takes years to leave is because of the collisions. They collide, interact and then produce another photon. With the "random walk" statistical proof, its for every N "steps" a photon takes (at least, its a photon in this example) it travels sq.rt(N) in a straight line. So it takes so long, because it has to go through so much.Though I find it interesting that even though its random, it has a direction. I can't remember the reason exactly.
And the light is slowed down, at least I'm pretty certain. Though the collision/interaction is the reason it takes time to leave stars, it genuinely slows in medium. (Please do correct me if I'm wrong). I always thought of it as the material being more dense, so there is (relatively) more distance to go through. Speed=dist/time and all.
So the speed of the photon doesn't really change but it looks like it because of the multiple collisions.
Somewhat, the speed of light in a material is an average. The speed of light in a vacuum is a consequence of the permittivity and permeability of free space. In a material those values can change and so the speed of light will be lower even without collisions.
Photons travel at the speed of light, regardless of what that value is, because they are particles of light.
I've always been familiar with the concept (and it made sense), but, having never taken physics, I never knew the specifics.
What this suggests to me is that the idea of practical deep space travel is impossible, at least in the sense of simple travel between point A and point B. The speeds necessary to travel such a distance in a practical amount of time would mean that you are "accelerating" through hundreds of years, for a one way trip. If you were to return to your destination, everyone you knew and loved would be long gone.
This brings me to the idea that the only practical way to be able to travel to and from the deeper parts of the universe would require a sort of "fold" space itself (thus shortening the distance required to travel) and I don't even know if/how that would be possible.
You have to appreciate time dilation of the Relativity theory. What it actually means is that time is closely related to motion. Time is caused by motion in respect to something (Our cluster of galaxies/galaxy/cluster of stars/star/planet is constantly moving).
The real question is - motion with respect to what? And what will happen if you sufficiently slow down?
On April 25 2011 04:06 QuAnTuM314 wrote: Here is a time-space graph depicting movement through space-time. Dont ask me why they do a time-space graph instead of a space-time graph, I never understood.
The red arrow depicts every day objects, that move at non-relativistic speeds. Because everything moves through space-time at the same rate, and they aren't moving through space very quickly, the majority of their movement is through time. And this is the time you measure on your watch whilst going about your life.
The green arrow shows what would happen as we accelerate. Because the object is moving (much) faster through space, and has to move the identical amount through space-time, it moves less along the time axis. I.e. It experiences less time (relative to space-time) than the red arrow.
The yellow arrow is for photons, and anything travelling at the speed of light. Here, all of its movement through space-time is going through space (you can prove this by putting in v=c in the equation for gamma/time dilation). Therefore, it experiences no time at all.
I'm not entirely sure what exactly you're trying to say in your post. I think you're saying that there is an absolute frame and I'm pretty sure that's not true. It sounds like you're describing the concept of an aether. But like I said, I don't really get what you're saying so I will just comment on what I quoted above.
I'm not really sure if there's a "real" reason why spacetime diagrams always have time on the vertical axis. It really does just make sense that way, especially in general relativity. Horizontal cones are just awkward to look at.
Your diagram is also not correct. Something traveling at the speed of light is not the same as something traveling at infinite speed (your yellow line). Light still takes a certain amount of time (relative to a stationary observer) to travel a certain distance. For instance, light takes about 8 mins to get from the sun to earth. The green arrow is what is actually traveling at the speed of light. The usual point of that diagram is that if you are an observer at (0,0) then you can't affect anything outside of the light cone (the lines x=t and x=-t (c=1)). Likewise anything outside your light cone won't affect you until their light cone cross your light cone.
No. As I said at the end of my post, space-time *isnt* an absolute frame of reference, but the difference in these cases is not really worth noting (though in other circumstances, it would definitely have to be explored further).
I am also most definitely not describing an aether either.
And with my diagram? It isn't wrong, the time on the graph is time relative to space time, not time as we perceive it. In effect, it is the individual time for whatever is on the graph. If something is going at the speed of light, (eg, light), it will experience no time. This is what the graph is saying, and it doesnt because of poor wording on my part. If what you said is true (relative to my graph) then that would mean that the light would experience time, so therefore would not travel at the speed of light (gamma would have to be > 0, which would mean that the speed would be > c).
The graph you're thinking of is in reference to hyperbolic functions. Which although (in this case) have the same axis labels, they are subtlety different. Here they are not for an object, but for the properties of time and space themselves.
I believe the misunderstanding has been from my poor wording/expression. I'll return to this thread when I'm less tired.