Cosmic Speed Limit

[Bomb#20]

I had electromagnetics and I understand Maxwell’s Equations.

Given C is not relative across all frames and motion is..

A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.

A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.

Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

CMBR dipole anisotropy

"From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."

[skepticalbip]

^^^
Indeed, that sorta works if we only make the assumption of a universally uniform temperature as a proxy for Newton's universal inertial reference frame. The near uniformity does seem to be real with only a bit of granularity.

[steve_bank]

I had electromagnetics and I understand Maxwell’s Equations.

Given C is not relative across all frames and motion is..

A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.

A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.

A spaceship in orbit around Earth and deaprts out into space. On board I can measure acceeration and calculate change in velocity. I turn off the engine and the ship is traveling at a constant speed.

I have a number for velocity but it is relative to the starting point. I get the same change in velocity regradless of the velocity of starting point.

From relativity there is no absolute frame, so it is impossible to know an absolute velocity of a starting point. Given that, on the ship how do I know what percent of C the ship is traveling at?

There is no such thing as absolute velocity in relativity. That is a Newtonian concept. Newtonian imagined a fixed universal reference frame. Relativity assumes that measurements in all reference frames are equally valid regardless of their relative motions with respect to each other. In relativity, the second, meter, kilogram, etc. measured in a reference frame in motion with respect to the observer are all dependent on the relative velocity between those two reference frames.

A Mach meter is relative to the ground. The aircraft has a velocity km/hour. Velocity is in an inertial frame. The Erath is moving around the sun and sun around the galaxy. I do not see how an avsolute velocity can be assigned.

The only solution I can see is that C as a limit applies only within an inertial frame. But that does not make sense either. Just thinking out loud.

[steve_bank]

I had electromagnetics and I understand Maxwell’s Equations.

Given C is not relative across all frames and motion is..

A jet has a Mach meter, %speed of sound. I want to build a %light speed meter for a space ship.

A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.

Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

CMBR dipole anisotropy

"From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."

But relativity says there can be no absolute frame.

This out of my depth, how can the CMBR be eater moving or stationary in terms of position.

[bigfield]

Actually we've built a %light speed meter with respect to the cosmic microwave background, and the CMB is light, and it registers about 0.12% c.

CMBR dipole anisotropy

"From the CMB data it is seen that the earth appears to be moving at 368±2 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB)."

But relativity says there can be no absolute frame.

This out of my depth, how can the CMBR be eater moving or stationary in terms of position.

It's not an absolute frame. It's just a useful reference frame for measuring the speeds of things at intergalactic scales.

When you look at the CMB, you're looking at light that's been travelling for over 13 billion years from points in space that are moving away from us very quickly.

Those distant points in space are moving away from us faster on one side than the other. Add all of those velocities up and you get a rest frame that differs from Earth's rest frame.

[Loren Pechtel]

I must nitpick: Mr. One Mug effectively says you can't accelerate anything to superluminal velocity. That does not, however, preclude the existence of particles whose only existence is at superluminal velocity. Useless for travel, not useless for communications.

After all, the same equation says you can't accelerate anything to lightspeed, yet we talk by radio all the time.

(Now, the paradoxes are another matter.)

[skepticalbip]

I must nitpick: Mr. One Mug effectively says you can't accelerate anything to superluminal velocity. That does not, however, preclude the existence of particles whose only existence is at superluminal velocity. Useless for travel, not useless for communications.

After all, the same equation says you can't accelerate anything to lightspeed, yet we talk by radio all the time.

(Now, the paradoxes are another matter.)

Not exactly. It is anything with rest mass (fermions) that can't be accelerated to lightspeed. We talk by radio using photons (EM radiation) which have no rest mass and can only exist at c... at least according to uncle Al and cousin Max.

ETA:
Now if you come up with a way of using tachyons (which may or may not exist) for communication it would earn you Nobel prizes in a couple different fields.

[bilby]

^^^
Indeed, that sorta works if we only make the assumption of a universally uniform temperature as a proxy for Newton's universal inertial reference frame. The near uniformity does seem to be real with only a bit of granularity.

But the CMB is nevertheless an arbitrary choice of reference frame. Why should we give it special status, while we don't give the same status to my personal reference frame? I am, after all, according to my observations, at the centre of the universe. I am the standard of normality against which everything is measured, and the fact that the CMB is moving at 368km.s^-1 is only of passing interest to me.

[bilby]

A mach meter measures the speed of the aircraft with respect to the air. If you are imagining your %light speed meter to be measuring the ship's speed with respect to light then it will always register zero regardless of how much or how long the ship has accelerated.


There is no such thing as absolute velocity in relativity. That is a Newtonian concept. Newtonian imagined a fixed universal reference frame. Relativity assumes that measurements in all reference frames are equally valid regardless of their relative motions with respect to each other. In relativity, the second, meter, kilogram, etc. measured in a reference frame in motion with respect to the observer are all dependent on the relative velocity between those two reference frames.

A Mach meter is relative to the ground. The aircraft has a velocity km/hour. Velocity is in an inertial frame. The Erath is moving around the sun and sun around the galaxy. I do not see how an avsolute velocity can be assigned.

The only solution I can see is that C as a limit applies only within an inertial frame. But that does not make sense either. Just thinking out loud.

c as a limit applies equally in EVERY inertial frame. That's the great and extraordinary finding that made Einstein famous.

Regardless of how you are moving, c is always a constant. No frames are preferred. It's bizarre; But it's the only way of thinking about reality that works.

Oh, and by the way, a Mach meter is relative to the air, not to the ground.

[skepticalbip]

c as a limit applies equally in EVERY inertial frame. That's the great and extraordinary finding that made Einstein famous.

Uncle Albert's unique step here is that he dared to accept the result of the many, many measurements that showed c was constant for any observer. No other scientist at the time dared to risk their reputation by contradicting Newton... that is if they even considered that Newton could have been wrong. The common interpretation was that either the measurements or set ups were wrong.