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Today's Physics Question....?

#41
Tituscrow said:
Another thing worth thinking about that begins to link special relativity (the subject of this thread) with the much more gruelling general relativity, is this:

According to special relativity, as speeds approach c, mass approaches infinity. If, then, if we are moving away from other galaxies at speeds approaching c, why don't we 'feel' that extra gain in mass?
Click to expand...
Because we're not moving at an appreciable fraction of c relative to the things that enable us to "feel" our mass, such as the Earth, our muscles and other objects in our immediate environment, like the damn piece of furniture you always stub your toe on when going to the bathroom in the dark at 3AM. :D
 
#43
i_got_a_mohawk said:
The emitter will be, but the light will not keep the momentum of the source.

Also, physics side note, there will still be scatter. Scatter is what happens when light interacts with *stuff*, even a perfectly collimated beam would suffer scattering effects. Divergence is what you want to avoid (focusing would also be an issue, as beyond a focal point the beam will diverge). Collimation is where the photons are parallel along the beam path.
Click to expand...

I know, but at the outset, a particle-free space is one thing that should have been a given. Regardless of whether the light comes from a laser or is a point source, the light that's traveling in the opposite direction of the emitter's movement will stay on course unless something is in the way or a magnetic field acts on it.
 
#44
Bocete said:
Pretty sure that's not how special relativity works. By special relativity, if two lightbeams are moving away from eachother, in the same line and in vacuum and all that, they are getting away from eachother at the speed of light. That is, either beam is moving at the speed of light if we were to measure from the relative perspective of the other beam.

The numbers I gave correspond to this. Either beam is moving at 0.75c from a common point, so from the perspective of either beam, the other beam is moving away at about 0.96c. This is significantly faster than the speed at which the common point is getting away, but still less than 1c. To get to 1c both beams would need to accelerate to 1c.

It's weird and so fun. I in no way understand all of this, but speeds to 'bend' to be below 1c as you change the perspective, and in case of actual light beams in vacuum, things get sooooo unintuitive.
Click to expand...

The original question had to do with two objects moving in opposite directions, not light beams and only one is emitting light.

If the light is from an instantaneous burst, the speed/velocity of the source shouldn't matter because it could be argued that at that brief moment, the emitting object is effectively at rest by virtue of the extremely short duration of the emission. The objects may be moving at 3/4 of the speed of light, but the light emitted is moving AT the speed of light, so it should require .75 seconds to reach the receiving object.

Right?

If the light source is emitting continuously and we assume it's happening in free space, one analogy would be that it's like a tube of toothpaste that squirts out at the speed of light while the object moves away from the second object at 3/4 the speed of light. In that case, the leading end of the toothpaste would be moving away from the emitting object at 1/4 the speed of light, relative to the point where it started squirting out. If the speed of the toothpaste remains constant at the speed of light and we look at this as if it's happening along a number line (or the ordinate/abscissa of a coordinate system) where c=1 unit/second and the reference point is where emission begins, the object moving in the negative direction moves to -.75, the other object moves to .75 and the light moves to .25 in that first second. In the next second, the emitter moves to -1.5, the receiver moves to 1.5 and the light moves to 1.25. in the third period, the emitter moves to -2.25, the receiver moves to 2.25 and the light also moves to 2.25, catching the receiver.

What a bunch of propeller heads!
 
#46
therefore, shouldn't the receiving object's speed cause the light waves to be received at a slightly greater distance from each other than if the object were stationary?

yeah but it gets there when it gets there. universal speed limit.
 
#47
1958Bassman said:
The original question had to do with two objects moving in opposite directions, not light beams and only one is emitting light.

If the light is from an instantaneous burst, the speed/velocity of the source shouldn't matter because it could be argued that at that brief moment, the emitting object is effectively at rest by virtue of the extremely short duration of the emission. The objects may be moving at 3/4 of the speed of light, but the light emitted is moving AT the speed of light, so it should require .75 seconds to reach the receiving object.

Right?

If the light source is emitting continuously and we assume it's happening in free space, one analogy would be that it's like a tube of toothpaste that squirts out at the speed of light while the object moves away from the second object at 3/4 the speed of light. In that case, the leading end of the toothpaste would be moving away from the emitting object at 1/4 the speed of light, relative to the point where it started squirting out. If the speed of the toothpaste remains constant at the speed of light and we look at this as if it's happening along a number line (or the ordinate/abscissa of a coordinate system) where c=1 unit/second and the reference point is where emission begins, the object moving in the negative direction moves to -.75, the other object moves to .75 and the light moves to .25 in that first second. In the next second, the emitter moves to -1.5, the receiver moves to 1.5 and the light moves to 1.25. in the third period, the emitter moves to -2.25, the receiver moves to 2.25 and the light also moves to 2.25, catching the receiver.

What a bunch of propeller heads!
Click to expand...

Nope.

In relativity you cannot add and subtract speeds like that. Time flows differently for an observer on the origin point and for an observer on one of the objects, and speed is a function of time.

If you shoot two objects in opposite directions, in the same line, both at 0.75c from the origin point as the OP suggested, then from the perspective of either object, the other object is moving away at 0.96c. You calculate the red shift using that speed.
 
#48
chuck norriss said:
therefore, shouldn't the receiving object's speed cause the light waves to be received at a slightly greater distance from each other than if the object were stationary?

yeah but it gets there when it gets there. universal speed limit.
Click to expand...

BINGO....good ol' Chuck....this is exactly the question i was trying to ask....(in addition to how much the light would be stretched)

since the receiving object is moving away at a substantial fraction of the speed of light

and since as mohawk said, each object is actually moving along a line away from the other.....at least that is what i extrapolated from one of his posts

THEREFOR....should the receiving object's motion cause as much red shift ( or whatever shift; obviously far beyond red) as does the emitting object?

davesignatureII-1.png
 
#50
alright, i am sorry, but let me hijack my own thread for a minute to ask a related question (or questions) about objects traveling near light speed

let's say you contacted the planet Magrathea from Hitchhiker's Guide.....and let's say you woke them up to do a large contract for you....build a perfectly straight 10 trillion mile long track, mark it in the exact middle with a platform for you and assistant to stand (or a cage rather, since you will ultimately be using this in inter-galactic space....also, build two rocket ships to ride the track and capable of 3/4 light speed

and let's say you had them tow the track, rockets and you with some assistants out to inter-galactic space and set this perfectly straight track up, with the two rockets strapped on and facing in opposite directions in the middle of the track.

and let's say that on a given signal your assistants touch off both rockets (which use a new, exotic, inexhaustible fuel) going in opposite directions.....your assistants have telescopes which can see infinitely and measure frequency shift and both are staring at one clock in the exact middle of the track....so they both see the same time.

let's call the directions the rockets go east & west, for lack of anything better. let's say that after two seconds they each make a measurement and the east rocket watcher sees that the rocket going east is @ 139,000 X 2 miles away and the west-watcher observes the rocket going west is 139,000 X 2 miles away

now you stand in the middle of the track, so the rockets' motions are clearly relative to you, but each one has clearly moved, while, relatively speaking, you have not

am i wrong in any of my analysis?

now, back to the subject, suppose the east-bound rocket started emitting a steady stream of ordinary white visible light, and suppose the other rocket received it at some point down the track where the was a large, stationary reflector of some type, so you could get a good reflection off a non-moving surface at the same time you got one off the rocket .

let's say both rocket and reflector beams reflected back to you and were each received by one of your assistants

i believe the beam coming back from the rocket will have considerably longer wavelength than that coming back from a sort of stationary reflector

am i right in this? just thinkin'....always dangerous

davesignatureII-1.png
 
#51
Lonesomedave said:
BINGO....good ol' Chuck....this is exactly the question i was trying to ask....(in addition to how much the light would be stretched)

since the receiving object is moving away at a substantial fraction of the speed of light

and since as mohawk said, each object is actually moving along a line away from the other.....at least that is what i extrapolated from one of his posts

THEREFOR....should the receiving object's motion cause as much red shift ( or whatever shift; obviously far beyond red) as does the emitting object?

davesignatureII-1.png
Click to expand...
Yes - Doppler distortion / redshift can be caused by the emitter moving relative to an observer, or the receiver, or both.

It's still the case that it's wrong to think of either object as absolutely stationary or absolutely moving. Motion, time, position and even size and mass are all relative and depend on your frame of reference. The only thing that matters is that the objects are in motion relative to one another. Saying that one is still and the other moves at c is the same (from a Doppler viewpoint) as saying that one moves at 0.5c in one direction while the other moves at 0.5c in the opposite direction.

(HINT - none of us have ever been absolutely stationary in our lives. Or to put it another way - if there was nothing in the Universe but you, how would you know if you were stationary or moving?)

When the emitter and the receiver have relative velocity high compared to c, then things don't add up in a linear way. Imagine you had two guns capable of firing pellets at 0.75c. You hold one in each hand, stretch out your arms either side and fire. The two pellets leave you at 0.75c, to your left and to your right. Their relative velocity would not be 1.5c - it would be 0.96c. If they were flashing pellets, light from one could still reach the other and any light travelling in either direction would be equally redshifted when it reached the other pellet.
 
#52
Tituscrow said:
Avoiding the maths, it's also worth considering the Cosmic Microwave Background Radiation.

This is evidence of the wavelength of photons from the most distant reaches of the universe being 'stretched' by the Doppler Effect way beyond red-shift, through infra-red and into microwave.

I think some of the confusion arises from not being clear about where any proposed measurements c would be taken from.

As an external observer, it would seem that if two galaxies receded from each other at a relative speed >c, then yes, there's your 'faster than light' argument. But despite the maths saying this is intuitively possible - in fact it does happen at much slower speeds - at light speed that maths doesn't work. You need to consider the Lorentz factor and the idea of inertial frames of reference.

In order to determine their relative speeds, you'd have to travel within one of the galaxies. Even then, you wouldn't be able to record the movement of a photon from any point A to any point B and arrive at a result >c.

Another thing worth thinking about that begins to link special relativity (the subject of this thread) with the much more gruelling general relativity, is this:

According to special relativity, as speeds approach c, mass approaches infinity. If, then, if we are moving away from other galaxies at speeds approaching c, why don't we 'feel' that extra gain in mass?
Click to expand...

At constant velocity approaching c, we may feel ourselves collapsing due to the increase in mass- you're not referring to feeling heavy while walking or lifting an arm or leg, are you?
 
#54
bassybill said:
.....When the emitter and the receiver have relative velocity high compared to c, then things don't add up in a linear way. Imagine you had two guns capable of firing pellets at 0.75c. You hold one in each hand, stretch out your arms either side and fire. The two pellets leave you at 0.75c, to your left and to your right. Their relative velocity would not be 1.5c - it would be 0.96c. If they were flashing pellets, light from one could still reach the other and any light travelling in either direction would be equally redshifted when it reached the other pellet.
Click to expand...

their relative velocities might not be 1.5 c, but their behavior suggests otherwise....

see my last post above....one to the east, the other to the west....after 2 seconds they are 278,000 X 2 miles apart

right?

davesignatureII-1.png
 
#55
Lonesomedave said:
their relative velocities might not be 1.5 c, but their behavior suggests otherwise....

see my last post above....one to the east, the other to the west....after 2 seconds they are 278,000 X 2 miles apart

right?

davesignatureII-1.png
Click to expand...

Wrong, at relativistic speeds, perceived time, distance and speed are all modified by the Lorentz transformation, specifically the squareroot of 1-v^2/c^2. See this link which explains why velocities don't simply add at relativistic speeds. Fundamentally no matter how fast I am moving relative to you, we both perceive the speed of light in free space as the same because because our real sense of time and space is modified by our relative motion.

http://en.wikipedia.org/wiki/Velocity-addition_formula

Unfortunately, they dig into the matrix math without providing a plain English explanation but you might get a sense for how this all works. Much of this is driven by the fact that the speed of light in free space is a fundamental constant.

There's a section about 2/3 down that deals with doppler shift at relativistic speeds.

As I said earlier - take two twins and send one off into space at close to the speed of light. When the speedy twin returns to Earth, he will be much younger than the twin who remained on the planet. Here's link explaining that:

http://en.wikipedia.org/wiki/Twin_paradox
 
#56
Lonesomedave said:
their relative velocities might not be 1.5 c, but their behavior suggests otherwise....

see my last post above....one to the east, the other to the west....after 2 seconds they are 278,000 X 2 miles apart

right?

davesignatureII-1.png
Click to expand...
They would be 2 x 279,000 = 558, 000 miles apart after two seconds, as measured by you. An observer positioned on either one would measure their separation as 2 x 0.96 x 186,000 = 357,120 miles. Neither answer is absolutely right everywhere in the universe, or absolutely wrong. Their separation is not an absolute quantity, it depends on how it's measured.

Space and time are not absolute - they are relative and the figures you get for things like distance and speed depend on who is doing the measuring and their motion relative to what is being measured. The effects become very noticeable when objects are travelling at relative velocity close to that of light and the effects are also very often at odds with our everyday "common sense" experience of how things appear to behave.
 

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