Well, he did say it was only a theory....

[MK27]

Personally I don't see how it's realistic to think there is a hard limit to how fast we can go... Think about this one...
You are in a rocket ship heading along at half the speed of light. Inside your ship, control signals are flowing around as needed. Since electrical signals generally move at some significant fraction of the speed of light ( Wave propagation speed - Wikipedia, the free encyclopedia ). Does it not strike you as entirely possible that a signal travelling from the back of the ship to the front exceeds the speed of light by quite a significant margin?

Nope, that's why it's the theory of relativity. The speed of light emitted and encountered by a particle is constant regardless of the speed of the particle in relation to other particles. If an electron were emitting radiation, or stood to be affected by radiation from outside the ship, that radiation would be moving at c regardless of the relative motion of the ship.

Otherwise, the light from your headlights would be exceeding the speed of light as soon as you put the car in gear, and so would all the light striking from the front, relative to the surface it reflects off of. But that is not what happens, because the speed of light is constant everywhere regardless of scale, relative velocity of the particles involved, etc.

We see this effect all the time in 2 way radio communications. It's called the "Doppler Effect" where a signal from a source travelling toward you is shifted slightly higher in frequency

Exactly. The frequency of light changes, not it's velocity. That is why you have red-blue shift. There are astronomical bodied travelling at a significant percentage of the speed of light in relation to us, but the photons (light as particle) emitted by such bodies that reach us are still travelling at c, only the frequency of the light (as wave) is shifted to reflect the relative velocity of the source.

[Whoops, looks like a few other people already brought this up.]

[MK27]

But time dilation has been experimentally verified, with very real clocks and very real jets.

Recently it was also done with a normal car.

Einstein's time dilation apparent when obeying the speed limit

So all that time you spend commuting may actually add a few microseconds to your life.

[megafiddle]

Lot's of opportunities for mistakes in a measurement like that (the super C neutrinos).

I wonder how the gravitational field affects things? Or the motion of the earth? And if
everything like that was properly taken into account?

The theories of relativity incorporate the constancy of light speed. But the constancy is
a consequence of Maxwell's electromagnetic field equations.
The limit of C for objects is a consequence of relativity.

[EVOEx]

I've got one question about time dilation that has been on my mind for months now, but I never had the chance to ask anybody who knew more about physics than I do. I think now is a good time...
Yes, we have verified time slows down as you move faster: the clocks obviously ran slower. But at the same time, the speed of light is the same. So, if we stop measuring time by the atom decay the most exact clocks use right now (I believe) but start measuring the speed of time by the speed of light. For instance, make the light travel the distance it travels over in one second (in vacuum of course), and that is one second. Since the speed of light is constant for all observers, doesn't that completely change the idea of a relative time?
So could it not be that, yes, we measured relative time, but that was only true because of atoms decaying slower at faster speeds? It would probably still mean our senses would say time runs slower, but that's only because of slowing down chemical reactions, not the actual slowing down of time.

My question really is: what is wrong with this idea? If it was true, I think the consequences on physics would be too high and I can not believe for one second I am the first person to think of this - so my logic has to be flawed. Can anybody tell me where?

That article is misquoting Einstein's theories, specifically Einstein said nothing can accelerate from < c to past c. Particles can in fact be created at faster than the speed of light. As for those experiments, we would have to see what the parameters were exactly, the detectors in use, blah, blah, blah.

If you haven't seen the movie K-Pax, you should ;-).

[megafiddle]

I've got one question about time dilation that has been on my mind for months now, but I never had the chance to ask anybody who knew more about physics than I do. I think now is a good time...
Yes, we have verified time slows down as you move faster: the clocks obviously ran slower. But at the same time, the speed of light is the same. So, if we stop measuring time by the atom decay the most exact clocks use right now (I believe) but start measuring the speed of time by the speed of light. For instance, make the light travel the distance it travels over in one second (in vacuum of course), and that is one second. Since the speed of light is constant for all observers, doesn't that completely change the idea of a relative time?
So could it not be that, yes, we measured relative time, but that was only true because of atoms decaying slower at faster speeds? It would probably still mean our senses would say time runs slower, but that's only because of slowing down chemical reactions, not the actual slowing down of time.

My question really is: what is wrong with this idea? If it was true, I think the consequences on physics would be too high and I can not believe for one second I am the first person to think of this - so my logic has to be flawed. Can anybody tell me where?


If you haven't seen the movie K-Pax, you should ;-).

I am not a physicist, but perhaps this may help.

We measure light speed just as we would any velocity, with a measuring rod for distance
and a clock for time. The passengers in a very fast ship do not notice any change in their
clocks or themselves. Their thought processes, aging, metabolism, everything slows along
with the clocks. It's the relatively stationary observer on a planet that sees their clocks as
ticking away more slowly (along with everything else on the ship).

It is in measuring light speed that time dilation manifests itself.
Imagine a ship travelling past an observor on a planet. On board the ship are two parallel
mirrors facing each other. A photon bounces back and forth between the mirrors endlessly.
Also imagine that the whole thing is oriented so that the photon is traveling at right angles
to the direction of travel. This is the ship's clock. Each time the photon bounces off one of
the mirrors, that is one tick of the clock. Everything appears normal to the ship's passenger.

But what does the observer on the planet see? Because the clock is moving laterally while
the photon is moving vertically, the observor sees the photon travelling diagonally along a
zig-zag path. This is a longer path than what the passenger sees; he sees the photon moving
back and forth along the same vertical path. The planet bound observer sees the photon
travelling a longer path from mirror to mirror. At light's constant velocity, it must take longer
to get from mirror to mirror. The clock is ticking more slowly.

The passenger doesn't see or notice this slowing. Everything involving time, including time,
moves right along with the clock. This was Einstein's thought experiment.

So if we have a known velocity and distance, we can determine the clock rate. In this case
the distance itself is not known, but there are two obviously different distances. Therefore
there must be two different times. In other words an identical clock on the planet will tick
away at a different rate.

Einstein maintained that not only were Maxwell's equations true, but the principle of relativity
(Galileo) must also be true - that all laws of physics are true and the same in all reference
frames. The clocks cannot change or operate differently simply because they are moving.
Therefore time itself must have changed.

K-Pax - great movie

[MK27]

For instance, make the light travel the distance it travels over in one second (in vacuum of course), and that is one second. Since the speed of light is constant for all observers, doesn't that completely change the idea of a relative time?
So could it not be that, yes, we measured relative time, but that was only true because of atoms decaying slower at faster speeds?

My question really is: what is wrong with this idea?

There's nothing wrong with it, but it does not conflict with anything the way you may be thinking. As megafiddle implies, a clock using that method will give the exact same time as a clock using radioactive decay, because radioactive decay operates at the speed of light. You would in effect be measuring the same phenomenon.

The decay did not slow down, except relative to an observer. And there can be no one "objective scale" provided by an observer, since there could be any number of relative observers all providing different, arbitrary scales. The only non-relative objective scale would be that of the object itself, which is the same for all objects. The speed of light and the rate of decay are constant. The effects of relative velocity are relative (and, nb, velocity is always relative -- there is no "still fixed point" in the universe according to which the absolute velocity of an object might be defined*).

* there is a triangulated center, but your velocity relative to that has no special meaning.

[MK27]

K-Pax - great movie

Sounds up my alley. Have you seen The Man Who Fell to Earth or Primer? Both awesome.

The Man Who Fell to Earth (film) - Wikipedia, the free encyclopedia
***Primer (film) - Wikipedia, the free encyclopedia***

[Clairvoyant1332]

***Primer (film) - Wikipedia, the free encyclopedia***

Now there's a movie that takes at least two or three viewings to get.

[EVOEx]

I am not a physicist, but perhaps this may help.

[...]

Sorry for snipping your quote, I didn't want to fill an entire page with it again. Thanks for your help, guys. It makes sense, except that I don't understand one single part of it: if we would make a photon (or multiple, but let's stay with one for simplicity) bounce back and forth between mirrors, it would make sense, but when the mirrors would start moving (when they're inside a spaceship for instance), the photon wouldn't start moving along with it, would it? So the photon would simply be lost, from what I understand. To make it bounce back and forth indefinitely, would it not have to be sent out diagonally, matched up with the speed of the mirrors, which is exactly what an external observer would see?
What am I misunderstanding here?


Thanks a lot guys!

[Clairvoyant1332]

That's exactly it. From the perspective of an observer on the ground, the photon is moving diagonally. But from the perspective on a person on the ship, the photon is moving perpendicular to the mirrors. As a result, time must slow down for the person on the ship in order for the photon to take the expected amount of time to move between the mirrors relative to the person on the sip.

EDIT:

To expand on this, if it weren't for time dilation, the observer on the ship would know he was moving because the photon would appear to be moving slower. Part of special relativity states that all inertial (i.e. non-accelerating) frames of reference are equivalent. Without time dilation, that would not be the case.

[anduril462]

Sorry for snipping your quote, I didn't want to fill an entire page with it again. Thanks for your help, guys. It makes sense, except that I don't understand one single part of it: if we would make a photon (or multiple, but let's stay with one for simplicity) bounce back and forth between mirrors, it would make sense, but when the mirrors would start moving (when they're inside a spaceship for instance), the photon wouldn't start moving along with it, would it? So the photon would simply be lost, from what I understand. To make it bounce back and forth indefinitely, would it not have to be sent out diagonally, matched up with the speed of the mirrors, which is exactly what an external observer would see?
What am I misunderstanding here?

Get a friend to stand on the sidewalk in front of your house. Grab a tennis ball and get in your car. Start driving down your street, you don't need to be going very fast. Pick up your tennis ball, and toss it straight up into the air (not so high that it hits the roof of your car). It should come down right back into your hand. You didn't have to toss it "forward" to counteract the motion of the car. Have your friend watch you do this as you drive past him/her. Within the frame of reference of your car, the tennis ball is not moving forward. You threw it straight up, and it came straight back down. The person outside your car, however, saw the tennis ball move diagonally (or more accurately in a parabola). The same applies for the photon.

[GReaper]

Yeah, but it's been said that a photon isn't affected by the surrounding particles' movement, meaning that if the ship accelerates, the photon won't!

[anduril462]

Yeah, but it's been said that a photon isn't affected by the surrounding particles' movement, meaning that if the ship accelerates, the photon won't!

The ship is accelerating, but the particles inside aren't accelerating with respect to the frame of reference of the ship.

[megafiddle]

Sounds up my alley. Have you seen The Man Who Fell to Earth or Primer? Both awesome.

The Man Who Fell to Earth (film) - Wikipedia, the free encyclopedia
***Primer (film) - Wikipedia, the free encyclopedia***

I've heard of The Man Who Fell to Earth but never saw it. Both sound good.

I put them on my "movies to see" list. Thanks.

[megafiddle]

...if we would make a photon (or multiple, but let's stay with one for simplicity) bounce back and forth between mirrors, it would make sense, but when the mirrors would start moving (when they're inside a spaceship for instance), the photon wouldn't start moving along with it, would it? So the photon would simply be lost, from what I understand. To make it bounce back and forth indefinitely, would it not have to be sent out diagonally, matched up with the speed of the mirrors, which is exactly what an external observer would see?

Good idea with the snipping.

I believe the photon would be left behind only if the ship were accelerating. The effects described
are according to special relativity, where only uniform non-accelerating motion is considered (it's
the special case of zero acceleration. General relativity deals with acceleration and gravity).

The more sensational effects of special relativity often overshadow the equally important effects
of the principle of relativity. Suppose you are sitting at your workbench adjusting your mirrors
so that the photon bounces vertically back and forth between them. You get everything perfect and
then load everything up in the ship and take off. After reaching cruising speed, you start up the photon.
The principle of relativity states that everything must occur exactly as it did while you were "at rest"
setting up the photon clock. If the photon had to be fired off vertically to keep it within the mirrors,
that will always be true as long as you are in uniform non-accelerating motion. Regardless of your
speed, you are in a reference frame that is physically no different than any other. And again, it must
be uniform non-accelerating motion. This is referred to as an inertial reference frame. It is only
the outside observer, moving relative to you, that sees the diagonal photon path.

(back to your original question)
The relativity of simultaneity is probably more relevant to your question. It states that observers
in different reference frames will judge the occurrence of two events differently. If you fire off two
strobes so that they appear simultaneous to you, they may not appear simultaneous to someone
else. They might even appear to occur in a different order.
The same principle applies to events separated in time also, the interval might appear different to
different observers (simultaneity is simply zero time interval difference).

One way to set up your time measurement would be to send a light pulse along a 1 lightsecond long
measuring rod. A strobe at the light pulse source would fire to indicate the start of the pulse's journey.
Another strobe would indicate the arrival of the pulse at the end of the lightsecond measuring rod. If
you were equidistant from the two strobes, you should see a 1 second interval between the strobes.
Anyone performing this measurement will arrive at the same duration for "one second", as long as
the apparatus was in their own reference frame. It would be a perfectly valid way to generate a time
interval of 1 second. The problem occurs when an observer of the measurement apparatus is moving
relative to it. Different observers will see different values for '1 second' from the very same device.
The arrival of the two strobe signals will change depending on your relative speed.

A common question comes up concerning relativity of simultaneity: "Why not just correct or take into
account the differences due to relative motion? We do that all the time with ordinary events like lightning
strikes. Even though we hear the sound of lightning some time after we see it, we know that the sound
and flash occurred simultaneously when we take into account our distance and the speed of sound and
speed of light."
And of course you do have to take all this into account when you interpret the measurement. But even
when you do, the time interval may still be different, due to relativistic effects.

And to make matters worse, everyone may not even agree on the length of the measuring rod.