how was speed of light measured?

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we cannot observe any event before light coming from that point reaches our eyes. so how can we measure the relative velocity between any reference frame & light? & how can we say this remains constant?

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You can measure round trip speeds.

There are implications of the constant speed of light — E&M giving you the wave equation, and special relativity. You can experimentally check the implications of constant lightspeed.

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let us suppose there are two particles travelling with speed of light toward each other. now one cant see the other till they collide. say there is a mirror behind the 1st 1 at a distance d from the pt of their meeting in a stationary reference frame. can the 1st 1 see the 2nd reflecting back to it & then measure the relative vel?

bt that cant also be possible for in the reference frame of the 1st d=0 & time taken by the 2nd particle to move the distance 2d is also 0.

how do we know the relative speed?

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Massive particles can't travel at c.

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no problem. we can still try to know what light would itself observe.

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not really, when you try to apply rules for one thing to another the naswer does not make any sense whatsoever.

like if i tried to play chess with the rules of monopoly it just wouldn't make sense.

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like if i tried to play chess with the rules of monopoly it just wouldn't make sense.

I accept your challenge and raise you Cluedo!

As you say particles can't travel at the speed of light, but what theoretical speed could a probe in space get to without disintegrating?

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it doesn't disintegrate at some magic speed, its just that even if you turned all the matter of the universe into energy and dumped it into the probes kinetic energy it still wouldn't be going the speed of light.

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Not only would it not disintegrate, but there would be no way to determine how fast its going. This is because there's no such thing as "absolute velocity," only velocity relative to other things. If you pour all the energy in the universe into accelerating your spaceship, you won't feel any different, and you won't be any closer to the speed of light. You turn your headlights on, the beams still recede away from you at C. Your velocity will, however, be C minus only a tiny fraction different from your original velocity.

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shouldnt the speed of light be zero wrt itself?

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There is no "wrt itself."

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There is no "wrt itself."

Well, rather, everything (in a valid frame of reference) is at rest wrt itself. Except that the reference frame of a photon is not a valid reference frame.

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Not only would it not disintegrate, but there would be no way to determine how fast its going. This is because there's no such thing as "absolute velocity," only velocity relative to other things. If you pour all the energy in the universe into accelerating your spaceship, you won't feel any different, and you won't be any closer to the speed of light. You turn your headlights on, the beams still recede away from you at C. Your velocity will, however, be C minus only a tiny fraction different from your original velocity.

At a high enough velocity wrt the CMBR any probe would disinegrate, would it not?

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Cosmic microwave background radiation... now microwaves they'd be electromagnetic waves right? What's their speed again?

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Why? Because the CMBR will be so blueshifted in the "forward" direction that the photons will have enough energy to bla bla bla? That's not velocity that's doing it, then, it's the fact that it's in the path of extremely high energy radiation. You could just as easily say "as long as it doesn't hit anything," which is the same as saying "being motionless won't cause you to disintegrate, as long as nothing moving at 0.9999C crashes into you." So build a huge wall at some arbitrarily large distance away, and fling your probe in its direction at a relative velocity of 0.9999999999C.

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Why? Because the CMBR will be so blueshifted in the "forward" direction that the photons will have enough energy to bla bla bla? That's not velocity that's doing it, then, it's the fact that it's in the path of extremely high energy radiation. You could just as easily say "as long as it doesn't hit anything," which is the same as saying "being motionless won't cause you to disintegrate, as long as nothing moving at 0.9999C crashes into you." So build a huge wall at some arbitrarily large distance away, and fling your probe in its direction at a relative velocity of 0.9999999999C.

Nice rant, but the probe still disintegrates due to the velocity wrt to the CMBR isotropy.

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Nice rant, but the probe still disintegrates due to the velocity wrt to the CMBR isotropy.

And how is that?

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And how is that?

The velocity with respect to the CMBR isotropy causes the blueshifting in the "forward" direction and the "extremely high energy radiation". I can understand your "relative" point of view, but saying the it's not the velocity doing it is somewhat misleading.

No matter what inertial frame you use, as light speed is approached, at some point any probe will disintegrate.

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Well that's where my giant wall comes in, to block it out in the forward direction. Would that not work?

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well it would be eroded(possibly quite fast) and start disintigrating as well.

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well it would be eroded(possibly quite fast) and start disintigrating as well.

No, the wall is at rest wrt the cmbr. The probe is approaching it at nearly C.

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well it would be eroded(possibly quite fast) and start disintigrating as well.

Exactly. The wall would have to be moving with it to shield it.

Maybe you could use a huge hollow sphere cooled to approach absolute zero, but eventually you run into the same problem.

Merged post follows:

Consecutive posts merged
No, the wall is at rest wrt the cmbr. The probe is approaching it at nearly C.

That would only block directly ahead. The probe would still intercept plenty of photons crossing it's path and your wall would be radiating as well.

Edited by J.C.MacSwell
Consecutive posts merged.
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So, you propose we wall off half the universe everytime we want to get somewhere? interesting....

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So, you propose we wall off half the universe everytime we want to get somewhere? interesting....

Pretty much something you can neither run nor hide from.

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Not proposing anything but a thought experiment. But sure, "wall off half the universe." I'm walled off from at least half the of it right now, actually, just from sitting on a planet.

Of course, there is still radiation from the "wall" itself, which could never be completely eliminated. But in any case the point stands. It's not your velocity that is the problem (inasmuch as there is no such thing as absolute velocity), but the fact that something else is approaching you at a very high relative velocity and thus radiating especially nasty photons at you. And that seems practically unavoidable in our own neighborhood once we ourselves are moving at very high velocities relative to the Earth (and, consequently, the cmbr).

Or is it? You're moving at arbitrarily high velocity relative to cmbr. An arbitary number of shield walls (at arbitrary distance from one another) is moving ahead and with you, each slightly slower than the last, so each is approaching both its adjacent walls at an arbitrarily small velocity. Problem solved?

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