Which way is our galaxy moving and how fast?

[Gozzer101]

I always under the impression we couldn't find out, we could only judge it in relation to another point, say we are moving at speed A to Galaxy 1 but at speed B to Galaxy 2

Consider a light beam (or a photon) being sent from Earth to a detector on the Moon.

 

Our galaxy has to be moving and in some direction through the universe (or extremely likely to be) as others are coming towards us (andromeda) and others are moving away (not including the expansion of the universe where all distance galaxies appear to be moving away)

 

I was thinking if we sent two light beams to a detector on the moon at opposite points on the earth then the time it would take to get their would change. Why? Because if we are moving in relation to the universe in the direction of X then if we sent the beam while the moon was parallel to Earth and directiom X then the distance the photon has to travel has increased: for example: the photon travels 100 m (for arguments sake we will say it takes 1 second to do this) towards the moon yet in relation to the universe the moon has travelled 5m further away (distance it travels in 1 second) from the original destination the photon was sent (not earth as this will be moving in the same direction and speed, so a point in space)

 

If we sent the beam from the opposite side of the earth then the distance it has to travel to the detector will decrease and so it should arrive quicker, even though light hasn't travelled any quicker it just appears to (for every 100m the photon travels, the moon gets 5m closer)

 

My question: Using many points to send a beam and using calculations could we then determine what direction and speed we are travelling through the universe?

 

Please don't confuse the beam being sent to the moon and us measuring the time it takes for it to appear on the moon (or for it to be reflected back to us) as this would cancel out the affect. Why? Because on the return journey, say use are moving in direction X then on the return journey (moon to earth) it would be quicker as the distance it has to travel has decreased.

 

If my reckoning is wrong then please tell me why?

Edited January 10, 2012 by Gozzer101

[questionposter]

Congratulations, you re-discovered relativity.

[Janus]

I always under the impression we couldn't find out, we could only judge it in relation to another point, say we are moving at speed A to Galaxy 1 but at speed B to Galaxy 2

Consider a light beam (or a photon) being sent from Earth to a detector on the Moon. 

 

Our galaxy has to be moving and in some direction through the universe (or extremely likely to be) as others are coming towards us (andromeda) and others are moving away (not including the expansion of the universe where all distance galaxies appear to be moving away) 

 

I was thinking if we sent two light beams to a detector on the moon at opposite points on the earth then the time it would take to get their would change. Why? Because if we are moving in relation to the universe in the direction of X then if we sent the beam while the moon was parallel to Earth and directiom X then the distance the photon has to travel has increased: for example: the photon travels 100 m (for arguments sake we will say it takes 1 second to do this) towards the moon yet in relation to the universe the moon has travelled 5m further away (distance it travels in 1 second) from the original destination the photon was sent (not earth as this will be moving in the same direction and speed, so a point in space)

 

If we sent the beam from the opposite side of the earth then the distance it has to travel to the detector will decrease and so it should arrive quicker, even though light hasn't travelled any quicker it just appears to.

 

My question: Using many points to send a beam and using calculations could we then determine what direction and speed we are travelling through the universe?

 

Such an experiment was already done in the 19th century. It didn't use the Moon, but a set of mirrors. It got a null result. (it could not detect any motion of the Earth with respect to the universe. This same experiment has been repeated with increasing accuracy since then, with the same results. The conclusion this result led to was that there is no such thing as a reference frame that can be said to be "at rest with respect to the Universe". All Motion is Relative. Einstein's theory of Relativity uses this as one of its cornerstones.

 

Please don't confuse the beam being sent to the moon and us measuring the time it takes for it to appear on the moon (or for it to be reflected back to us) as this would cancel out the affect. Why? Because on the return journey, say use are moving in direction X then on the return journey (moon to earth) it would be quicker as the distance it has to travel has decreased.

 

 

 

Actually, this would not "cancel out the effect".

Assume you can measure your speed with respect to the universe with such a method. If you are at rest, the time it takes for light(at c) to travel to a distance of x and return would be 2x/c.

 

Now assume that you are moving at a velocity v. Now the time it would take for the light to travel a distance of x from you one way would be:

 

[math]T1 = \frac{x}{c+v}[/math]

 

and in the other:

 

[math]T2 = \frac{x}{c-v}[/math]

 

The round trip would take:

 

[math]\frac{x}{c+v}+\frac{x}{c-v}[/math]

 

which solves to

 

[math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

Which gives a different answer for different values of v for the round trip.

 

This is what the experiment mentioned was based on. It just failed to give any result other than one equal to 2x/c.

[Gozzer101]

Such an experiment was already done in the 19th century. It didn't use the Moon, but a set of mirrors. It got a null result. (it could not detect any motion of the Earth with respect to the universe. This same experiment has been repeated with increasing accuracy since then, with the same results. The conclusion this result led to was that there is no such thing as a reference frame that can be said to be "at rest with respect to the Universe". All Motion is Relative. Einstein's theory of Relativity uses this as one of its cornerstones.

 

Actually, this would not "cancel out the effect".

Assume you can measure your speed with respect to the universe with such a method. If you are at rest, the time it takes for light(at c) to travel to a distance of x and return would be 2x/c.

 

Now assume that you are moving at a velocity v. Now the time it would take for the light to travel a distance of x from you one way would be:

 

[math]T1 = \frac{x}{c+v}[/math]

 

and in the other:

 

[math]T2 = \frac{x}{c-v}[/math]

 

The round trip would take:

 

[math]\frac{x}{c+v}+\frac{x}{c-v}[/math]

 

which solves to

 

[math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

Which gives a different answer for different values of v for the round trip.

 

This is what the experiment mentioned was based on. It just failed to give any result other than one equal to 2x/c.

 

I agree with your equation: [math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

But if we consider the journey in the opposite direction then the equation is the same (if its both ways):

 

we get to the moon: [math]T1 = \frac{x}{c-v}[/math]

 

and back: [math]T2 = \frac{x}{c+v}[/math]

 

Which solves to exactly the same equation that you posted: [math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

If we consider only one way though then as your equations show they are different values:

 

One way to the moon in direction X: [math]T1 = \frac{x}{c+v}[/math]

 

One way to the moon opposite to X: [math]T2 = \frac{x}{c-v}[/math]

 

So as I'm looking at it a round trip in either direction is the same, as the equations are opposite and solve to the same, but a single trip then the equations are different. So have tests been done like this in one direction only, and so not using mirrors/reflectors, just using a beam and a detector.

 

Do we have any links to this? Or any extra info I could read up on to fully understand the concept.

 

Thanks

Edited January 10, 2012 by Gozzer101

[Janus]

I agree with your equation: [math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

But if we consider the journey in the opposite direction then the equation is the same (if its both ways):

 

we get to the moon: [math]T1 = \frac{x}{c-v}[/math]

 

and back: [math]T2 = \frac{x}{c+v}[/math]

 

Which solves to exactly the same equation that you posted: [math] \frac{2x}{c-\frac{v^2}{c}}[/math]

 

If we consider only one way though then as your equations show they are different values:

 

One way to the moon in direction X: [math]T1 = \frac{x}{c+v}[/math]

 

One way to the moon opposite to X: [math]T2 = \frac{x}{c-v}[/math]

 

So as I'm looking at it a round trip in either direction is the same, as the equations are opposite and solve to the same, but a single trip then the equations are different. So have tests been done like this in one direction only, and so not using mirrors/reflectors, just using a beam and a detector.

 

Do we have any links to this? Or any extra info I could read up on to fully understand the concept.

 

Thanks

 

You don't need to worry about the round trip in the other direction, what you do is compare one direction round trip to the round trip in a perpendicular direction. If you are moving in the direction of A, which remains a constant distance of x from you, the round trip time to x and back will be

 

[math] \frac{2x}{c-\frac{v^2}{c}}[/math] and depends on v

 

But the round trip to B, which is also a distance of x from you, but in a direction perpendicular to the direction of A, will have a round trip time of 2x/c, which is independent of v.

 

The experiment I mentioned worked by splitting a light beam and then sending the two halves in directions perpendicular to each other, where they were reflected back by mirrors. If the two halves the light beam took different times in there trips they would return out of phase with each other. As mentioned the experiment produced a null result even though it was sensitive enough to have picked up the Earth's orbital velocity around the Sun.

 

The whole point is that we live in a Relativistic universe, and in a relativistic universe, there is no test that you can do that will tell you your absolute velocity. In fact, the very concept of an absolute frame of reference from which you could measure your velocity has no meaning in a Relativistic Universe.

[Airbrush]

Our galaxy is moving around the center of the local group in an irregular orbit, pulled by other galaxies in different directions. It is moving relative to the center of gravity of the local group, 552 to 630 km/second.

 

http://en.wikipedia.org/wiki/Milky_way

Edited January 11, 2012 by Airbrush

[Tres Juicy]

Everyone knows we're going west.....

[michel123456]

Everyone knows we're going west.....

You mean civilizations go west.

[JustinW]

I thought the answer was yes. Which way are we going.........Yes. According to some we're falling. According to others the answer is out. According to others it's around in a circle counterclockwise to a clock that is upside down facing a wall inside of a ship traveling at a bazillion miles an hour through an expansive mush in an imaginary world where time is an entity. What's better than yes?

 

 

 

 

 

By the way: that is my favorite face to use.

 

On second examination of the topic question, it seems I was referring to the movement of the universe and not our galaxy. But if the expansion observed is correct it could be said that our galaxy is moving away from all else at the same time all else is moving away from our galaxy. If this is the case then how could galaxies collide? Is it that galaxy clusters collide within and that it is the clusters that are moving apart? And to get back to the general area of the topic question, we are on a path to collide with the galaxy andromida. I think that was it's name. Also are we moving towards andromida or is andromida moving towards us?

 

 

Sorry for the missassumption.

Edited January 11, 2012 by JustinW

[Airbrush]

But if the expansion observed is correct it could be said that our galaxy is moving away from all else at the same time all else is moving away from our galaxy. If this is the case then how could galaxies collide? Is it that galaxy clusters collide within and that it is the clusters that are moving apart? And to get back to the general area of the topic question, we are on a path to collide with the galaxy andromida. I think that was it's name. Also are we moving towards andromida or is andromida moving towards us?

 

The only expansion in the universe is between superclusters of galaxies. Within a cluster, galaxies may collide or orbit their center of mass indefinitely. We and Andromeda are both moving towards the other. Superclusters (clusters of clusters) are by definition gravitationally bound.

 

We are members of the Virgo supercluster which is about 110 Million light years across.

 

http://en.wikipedia.org/wiki/Superclusters

Edited January 11, 2012 by Airbrush

[Airbrush]

The most absolute relative motion we can say is our motion relative to the center of mass of the Virgo supercluster. We move around the sun at S speed, the sun moves around the galaxy at G speed, the milky way moves around our Local Group at LG speed, and the Local Group moves around the center of mass of the Virgo supercluster at V speed. Depending on where each orbit is will add or subtract to the absolute relative motion of us, and our velacity is always changing.

[michel123456]

The most absolute relative motion we can say is our motion relative to the center of mass of the Virgo supercluster. We move around the sun at S speed, the sun moves around the galaxy at G speed, the milky way moves around our Local Group at LG speed, and the Local Group moves around the center of mass of the Virgo supercluster at V speed. Depending on where each orbit is will add or subtract to the absolute relative motion of us, and our velacity is always changing.

 

Shouldn't we feel some centrifugal force coming from all those orbits? for example caused by orbiting the Sun, and caused by the rotation of the solar system around the galaxy?

[Janus]

Shouldn't we feel some centrifugal force coming from all those orbits? for example caused by orbiting the Sun, and caused by the rotation of the solar system around the galaxy?

 

Let's see, the centripetal acceleration for the Earth orbiting the Sun is 0.006 m/s^2, or about 0.0006g. An increase of 0.0006g for a 75 kg person works out to be a difference in weight of 46 grams (1.6 oz). Now add in the fact that the centripetal acceleration caused by the Sun's gravity is 0.006 m/s^2, meaning it would cancel even the 46 gram difference. So you have would an unnoticeable difference that wouldn't exist in the first place.

[D H]

Shouldn't we feel some centrifugal force coming from all those orbits? for example caused by orbiting the Sun, and caused by the rotation of the solar system around the galaxy?

No. You can't feel gravity. What you feel as weight is the force of the ground pushing up on you. Take this force away and you will feel a bit queazy. There is very little variation in gravitational acceleration during a roller coaster ride. It is the extreme variations in the normal force during a roller coaster ride that gives you that sick to the stomach feeling. From a Newtonian perspective, you don't feel gravitation because it affects everything equally (acceleration due to gravity of a test mass is independent of mass). From a relativistic perspective, you don't feel gravitation because it is a fictitious force. There is something you do feel from those orbits. (You missed the biggest, which is the Earth and Moon orbiting one another.) What you missed are tidal forces.

 

The Earth as a whole accelerates toward the Moon, or the Sun, or the rest of the galaxy. So do you, but by a slightly different amount than does the Earth as whole. The difference between the acceleration of the Earth as a whole toward these other objects and your acceleration toward these other objects results in something that you can feel. These are tidal forces and they are small.

[DrRocket]

Such an experiment was already done in the 19th century. It didn't use the Moon, but a set of mirrors. It got a null result. (it could not detect any motion of the Earth with respect to the universe. This same experiment has been repeated with increasing accuracy since then, with the same results. The conclusion this result led to was that there is no such thing as a reference frame that can be said to be "at rest with respect to the Universe". All Motion is Relative. Einstein's theory of Relativity uses this as one of its cornerstones.

 

Relativity shows that there is no preferred reference frame with respect to the formulation of the laws of mechanics. But there is a reference frame with respect to which it can be reasonably said to be "at rest with respect to the universe". That (local) reference frame is the frame in which the cosmic background radiation (CMBR) is isotropic. The observed CMBR is red-shifted in one direction and blue-shifted in the opposite direction, revealing a motion relative to that reference frame.

 

This reference frame is of utility in cosmology and is commonly used in that discipline. Note that this frame is local and appies to the special theory of relativity. There are no global frames in general relativity in the presence of gravity.

 

Relative to this reference frame the galaxy is moving towards the Hydra constellation at about 300 km/sec.

 

http://hypertextbook...riciaKong.shtml

 

http://en.wikipedia....round_radiation

Edited January 15, 2012 by DrRocket