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Seems to me as being mutually exclusive, photons not interacting due to their gauge properties and interacting in GR. All we know what GR does is bend spacetime, wich obviously affects light aswell.

I am far from an expert on stuff like that though.

What topic did you do your thesis on and was it Masters or PhD?

So, two questions.

First: Why do (gravity) fields not get affected by the doppler effect like waves, even though they propogate at a fixed speed (of light.)

Second: If the gravitic doppler effect (something moving towards a gravity source) blueshifts some light, that light has had it's energy increased right? Where did this energy come from? What's the equal and opposite reaction here?

First, the doppler effect is a shift of frequency caused by light or soundwaves rapidly moving at or away from you.
Secondly, yes there is a "doppler effect" for grav. fields, but it obviously does not involve motion at all. Instead, all that matters is the difference of the grav. field between where you are and where the source of radiation is located. Clocks "run slower" in stronger grav. fields, therefore light leaving the surface of its emitter will gradually have its frequency extended, so the signal comes at you with a longer wavelength than it had been send. You can also think about it as photons losing energy while they work their way through the grav. field. Since they lose energy, their frequency must decrease because their speed is always the speed of light.

If we keep the "photons exchange energy with the grav. field" picture, a photon travelling towards a strong grav. attractor will gain energy as the grav. field pulls the photon towards it. Since a photon's speed always remains fixed, the frequency is increased.


The deeper the well, the more energy you need to get out of it, or the more you gain respectively if you fall into it.

Yeah but where does this energy come from to avoid violating conservation of energy?

Like, suppose I had a box that fired light at something and then converted that light back into electricty, and it could do this in either direction. And the box is being orbited by something strong enough to produce tidal forces (Lets say the moon.) So when the moon is directly overhead, light fired upwards gains energy and therefore frequency before being converted back to electricity. And when the moon is on the opposite side of the planet, it has the same effect when firing light in the opposite direction. So why does my troll science machine not break the first law of thermodynamics (and potentially generate infinite electricity if it were 100% efficient?)

This is old but I thought raw gave a bad answer (he basically said it was the anthropic principle). Well maybe so, but it's also just sphere geometry. It's only the formula for the surface area of a sphere (4*pi*r^2) that's generating those inverse square laws. The easiest example to understand is the inverse square for radiative intensity. If a point source is radiating spherically (in vacuum), its intensity at a given distance is inversely proportional to the surface area of the imaginary sphere enclosing the point source with that radius. So like a familiar example would be a flashlight, which illuminates an arc of a sphere - if you compare the intensity of the light it casts on an object 5 feet away versus 10 feet away, it will be falling off like as an inverse square (although you'd need to use a photometer to measure because your eyes don't have a linear response to light intensity).

The idea with gravitational and electrical fields has the same reasoning.

Edit - fuck me that's from 2012, I thought the thread was a month old or so.... whatever

light->electricity conversation is called the photoelectric effect. to generate a current you just need enough energy to punch an electron out of whatever potential it is sitting in - energy beyond that threshold carried by the photon has no effect on the current. einstein observed exactly that behavior in his 1905 paper and he correctly reasoned that energy is transmitted in "packets". that paper kickstarted quantumphysics and won einstein the nobelprize in 1921.

I've never managed to 'get' time dilation. I get the normal examples, but when ever I try to make up my own scenarios it seems to fall apart. Explain, please.

If I, in my ship, circle the Earth(or another spaceship) for 10 days at 99% c, 10 years will have passed for observers on Earth. However, from my own reference point it's the Earth that is circling me since I can consider myself at relative rest. Objects at relative rest will see relatively accelerated objects as moving more slowly. The Earth, as seen by me, would be relatively accelerated and clocks on Earth should appear to be moving more slowly than on my ship. While the clock on my ship does 10*24 hours, the clocks on Earth, from my vantage point, should then have done less than 10*24 hours. That leaves about 9 years and 355+ days of catching up to do. First, does all of this even apply given that I'm circling then Earth rather than flying towards or away from it? And secondly, if so, when does the catching up happen? And how can I, on the 10'th day, see something that happened 9 years and 355+ days ago when the photons that I'm receiving are merely seconds old?

Pretty sure theacceleration required for rotational movement takes you out of Special Relativity territory and into General Relativity... And I no get GR/

Has nothing to do with orbiting or photons. Time is measured relative to the observer. I could put down some fancy symbols and formulas but that's all fairly irrelevant. It's simply how the universe works.
To see this effect at work you don't even need to go as far and construct a hypothetical spaceship moving at 0,99999c. You can observe the effects of TD with GPS satellites. fast and low = less time, slow and high = more time. So the clocks on those satellites gain about 1.7s p.a.

I'm not questioning the effects though. I don't doubt the validity of the theory as I know it has been proven as well as such things can be. What I'm questioning is how those effects would present themselves in a variety of scenarios.

You'd need to mathematically model each scenario and then solve it.

That's just for precision though, I'm talking more generally. I.e., in my example it's clear there's a discrepancy between the two time lines, so how is it solved? If the oribiting/circling space ship starts to decelerate, will his perception of the planet( which is currently 9+ years out of date) suddenly start to speed up? So if the deceleration process take e.g. 10 seconds, then the planet and everyone on it would suddenly age almost ten years in those 10 seconds while he watched?

From my very limited knowledge about relativity I think I can at least say that your example is a pretty complicated one.
The thing is relatively simple if we consider 2 observers that move with constant speed relative to each other, but acceleration will already make it a bit more complicated, as it will mean that the two observers are no longer equivalent.
In your case we now also have two rotating systems, so in principle both are constantly accelerated. Both are also moving at different angular velocities, so both should constantly see the other one accelerating/decelerating...

no, the time runs at normal speed in both systems for the respective observer within that system. it's only for the observer outside of each system that the respective clock runs faster/slower. There is no "catching up" neccessary.

Huh? Well yes, of course everyone sees their reference system running at normal speed, but the guys on the planet will see the clocks on the spaceship moving slower than their own clocks, while the guys on the spaceship will see clocks on Earth moving slower than their clocks. Each party sees the other party as moving slower.

So when ten days have passed on the space ship, as seen by the people and clocks on the space ship, the clocks on Earth, as seen by those on the space ship, will have incremented *less* than ten days. But we know that once the spaceship has decelerated back to a relative stand-still with Earth, the actual time passed on Earth will be 10 years. But the guys on the spaceship saw <10 days passing on Earth. Thus my point about 9 years and 355+ days missing from the space travelers observation of the planet which would need to be caught up with somehow.

No.

Thinking about it...
Let's adjust the thought experiment a bit.
Say the spaceship has a distance that is large enough that the angular velocity of both observers is the same, so that the spaceship seemingly "hovers" over the same spot, while still traveling at 99.9% c (or whatever fraction of c you want) tangential to the line earth-ship.
To reach the necessary distance, the spaceship accelerates away from earth, slowly increasing the tangential component of the velocity (and changing the radial component along the way).
In that case, once the ship has reached its position/"orbit" (which is pretty far out, actually), both should see their clocks running at the same speed again - say earth sends out time signals at certain intervals, then those signals should reach the ship with the same frequency as they get send out on earth. But, due to the travel to the orbit, the clocks will show different absolute times.
So in summary for that special case, the whole time difference is caused indeed by the ship traveling radially away from/towards earth, not by the ship traveling parallel.

To be honest, I have no idea if that is true OR if it helps with the initial problem - thinking to long about that kind of stuff makes my brian hurt.

Ye, same. But the problem could be made 'easier' by simply allowing for the ship to travel either away from or towards the planet. The same issue comes up though. A trip that takes 10 years when watched from Earth is experienced as 1 year aboard the spaceship. It's known that the people on Earth will see the people on the spaceship moving very slowly( 10% of their normal speed), while the people on the space ship will see the people on Earth moving very slowly( 10% of their normal speed). Both will see people in their own reference point moving normally.

To the people on the space ship, the trip takes 1 year. When they look at Earth, they see the people on Earth( and effectively, time ) moving 10% as fast as normal. In other words, in the 1 year they travel, they see Earth 'aging' 0.1 year. My confusion comes from the fact that, when the space ship has arrived at its destination and decelerated, 10 years have passed on Earth. But they were looking at the planet the entire time, and they only saw 0.1 year pass. So while the planet should have done 10 full rotations around the sun, it had yet to do a single one from what they had seen. And thus my question, when/how does it catch up with the remaining years?

I'm pretty sure that the people on the ship will see the people on Earth moving ten times faster, not slower.

They'll be going slower, that's part of what makes the whole relativity stuff a mindfuck. It makes some logical sense though, because given two objects with different speed and direction, you don't know which is moving away from which. Nor does it actually matter; all that matters is the relative speed between the objects. But of course, the relative speed for two objects has to be the same on both ends. And thus both experience that the other's clock is moving slower.

Sorry, I was getting confused with the usual formulation of the twin paradox where I believe there is an apparent speedup during turnover.
Fuck no, I was thinking of time dilation caused by gravity not relative velocity.

Note to self: check references before typing crap.

Yeah, I don't get it. Time dilation is a mess. The solution is most probably somewhere in the acceleration/deceleration phase, but the only thing I can see happening is that, seen from the space ship, movement on Earth becomes much faster than normal to fit the remaining 9 years into the time required to bring the relative velocity to a stop. And that doesn't make sense either, since if the deceleration was quick enough it would make objects move, or at least appear to move, faster than light. I.e. given a deceleration time of 10 seconds, the Earth would need to rotate around the sun 9.9 times in those 10 seconds.

If you had a magic mirror that transmitted information instantaneously (thus breaking the laws of physics), the people on the spaceship (travelling in one direction, then stopping, and travelling back) would literally see time "skip forward" on Earth. As in, two events separated by a certain amount of time on Earth would be perceived as simulatenous on the spaceship, which is perfectly in line with special relativity. The only reason this is weird is because of the magic mirror.

If we have no magic mirror, and communicate clock readings back and forth using radio waves, what happens is that on the journey away each side sees the other side's clock tick X times slower, and on the journey back, each side sees the other side's clock tick X times faster than their own - this is due to the Doppler effect. If you factor that in, everything works out allright - both sides conclude that the spaceship aged slower.

As to why the asymmetry exists, the canonical answer is that the spaceship has to accelerate/decelerate, which causes it to switch intertial reference frames and fuck everything up.

Take into account that I'm not an actual physicist or physics student, so take the fine details of this explanation with a grain of salt.

Hmm, if both parties really see the other's clocks going faster than their own when the distance is closing, doesn't that mess up some stuff too? E.g. I'm flying towards Earth and you're watching me the entire time( using radio waves), ten years passes for you but one year passes for me. But you were looking at my ship, and the clocks there appeared to move faster than your own clock...That means you saw *more* than ten years worth of clock rotations on the ship, while I experienced only 1 year's worth? Wat?

Eh, never mind. I must be tired. Obviously, the radio waves from the return trip will only reach Earth a little while before the actual ship, since it's already on the way back before you even see it arriving at the destination.