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#### tanstaafl76

##### Guest
If the notion of speed is always relative since nothing in the universe is truly "fixed" in place, I'm confused as to why the speed of light is considered the max speed; If two photons are heading in the exact opposite direction from each other, isn't their speed, relative to each other, 2 x C?

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#### origin

##### Guest
tanstaafl76":1wxcqlkc said:
If the notion of speed is always relative since nothing in the universe is truly "fixed" in place, I'm confused as to why the speed of light is considered the max speed; If two photons are heading in the exact opposite direction from each other, isn't their speed, relative to each other, 2 x C?

The speed of light in a vaccum (c) always has the same value. That means if I were traveling at 180,000 miles per second and I measured the speed of light that was coming behind me you might expect the speed to be 180,000 mps - 186,000 mps (186,000 mps is approximately c) or 6000 mps but you won't it will measure at 186,000 mps. In the same scenario but with light coming towards you might expect the speed to measured at 180,000 mps + 186,000 mps or 366,000 mps but of course you would measure light at 186,000 mps or c.

Special relativity showed that nothing can exceed c.

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#### mythx

##### Guest
I guess what you're asking is that "If a photon could see, what would the world look like?" I'm not sure. I suppose a photon experiences no time, and distances are infinitely short. Wow, I don't know what it would see when looking at an oncoming photon.

I've often wondered if the constant we refer to as the speed of light is the true limit, and photon merely approach that speed. As though nothing could actually reach this constant, photons just come closest.

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#### origin

##### Guest
mythx":2je6t9qb said:
I've often wondered if the constant we refer to as the speed of light is the true limit, and photon merely approach that speed. As though nothing could actually reach this constant, photons just come closest.

This doesn't really make sense. A photon and light are the same thing. So the speed of light is the speed of a photon. So in essence you are saying does the speed of a photon only approach the speed of a photon.

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#### mythx

##### Guest
origin":2bzp7bat said:
mythx":2bzp7bat said:
I've often wondered if the constant we refer to as the speed of light is the true limit, and photon merely approach that speed. As though nothing could actually reach this constant, photons just come closest.

This doesn't really make sense. A photon and light are the same thing. So the speed of light is the speed of a photon. So in essence you are saying does the speed of a photon only approach the speed of a photon.

Please reread what I wrote, I'm comparing the speed of a photon to "the constant we refer to as the speed of light". Please take the time to read my words if you're going to accuse me of not making sense.

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#### DrRocket

##### Guest
tanstaafl76":1xogu0lu said:
If the notion of speed is always relative since nothing in the universe is truly "fixed" in place, I'm confused as to why the speed of light is considered the max speed; If two photons are heading in the exact opposite direction from each other, isn't their speed, relative to each other, 2 x C?

Special relativity tells you how to relate speed, distance and time in one reference frame to those quantities in another frame in uniform motion with respect to the first frame. In a single reference frame two photons approaching one another from opposite directions will have a closing velocity of 2c.

There are some problems with attaching a reference frame to a photon, due to singularities in the Lorentz transformation. But for purposes of argument suppose that we have two massive particles traveling in opposite directions relative to some observer, call it "A", and that they are traveling at nearly the speed of light. A will see their closing velocity as almost 2c. But from the perspective of an observer riding either of the two particles, the speed of the other particle and hence the observed closing velocity will be just a little less than c.

In special relativity velocities don't add vectorially in the fashion that you are used to from Newtonian mechanics.

If the math doesn't bother you this Wiki article is pretty good. http://en.wikipedia.org/wiki/Special_relativity

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#### tanstaafl76

##### Guest
DrRocket":zzacb1te said:
Special relativity tells you how to relate speed, distance and time in one reference frame to those quantities in another frame in uniform motion with respect to the first frame. In a single reference frame two photons approaching one another from opposite directions will have a closing velocity of 2c.

There are some problems with attaching a reference frame to a photon, due to singularities in the Lorentz transformation. But for purposes of argument suppose that we have two massive particles traveling in opposite directions relative to some observer, call it "A", and that they are traveling at nearly the speed of light. A will see their closing velocity as almost 2c. But from the perspective of an observer riding either of the two particles, the speed of the other particle and hence the observed closing velocity will be just a little less than c.

I would think from A's perspective, both massive particles would be observed as only traveling at c, whereas if you were "riding" one of the particles, the other would appear to be approaching at a rate of 2c.

DrRocket":zzacb1te said:
In special relativity velocities don't add vectorially in the fashion that you are used to from Newtonian mechanics.

If the math doesn't bother you this Wiki article is pretty good. http://en.wikipedia.org/wiki/Special_relativity

It doesn't bother me, I just tend not to understand it :lol: But I'll give it a shot, thanks for the link.

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#### DrRocket

##### Guest
tanstaafl76":38e31en6 said:
DrRocket":38e31en6 said:
There are some problems with attaching a reference frame to a photon, due to singularities in the Lorentz transformation. But for purposes of argument suppose that we have two massive particles traveling in opposite directions relative to some observer, call it "A", and that they are traveling at nearly the speed of light. A will see their closing velocity as almost 2c. But from the perspective of an observer riding either of the two particles, the speed of the other particle and hence the observed closing velocity will be just a little less than c.

I would think from A's perspective, both massive particles would be observed as only traveling at c, whereas if you were "riding" one of the particles, the other would appear to be approaching at a rate of 2c.

A will see both bodies traveling at nearly c in opposite directions, so their closing velocity (the rate of change of the distance between them) in A's reference frame will be nearly 2c.

But if you are riding on one of the particles the approaching particle, in your reference frame, will be traveling at only about c.

No one ever perceives anything with mass or carrying information as traveling faster than c.

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#### tanstaafl76

##### Guest
DrRocket":3e166kkd said:
No one ever perceives anything with mass or carrying information as traveling faster than c.

Not to get tangential but wouldn't quantum entanglement be an exception to that?

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#### origin

##### Guest
mythx":1e54dk14 said:
Please reread what I wrote, I'm comparing the speed of a photon to "the constant we refer to as the speed of light". Please take the time to read my words if you're going to accuse me of not making sense.

This does not help. The constant we refer to as the speed of light is the speed of light. Since a photon is light, (EM radiation) it travels at the speed of light.

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#### Mee_n_Mac

##### Guest
tanstaafl76":37cvl2n5 said:
DrRocket":37cvl2n5 said:
No one ever perceives anything with mass or carrying information as traveling faster than c.

Not to get tangential but wouldn't quantum entanglement be an exception to that?

While the determination of a particles quantum state does indeed seem to happen immediately nobody that I know has figured out how to encode information into such an entagled pair so for now, the rule is still true. No information can be passed at superluminal speeds.

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#### DrRocket

##### Guest
tanstaafl76":238t7nrx said:
DrRocket":238t7nrx said:
No one ever perceives anything with mass or carrying information as traveling faster than c.

Not to get tangential but wouldn't quantum entanglement be an exception to that?

No, it is not an exception. That is an important point. There is no means whereby information is actually transmitted by quantum entanglement. It does not violate special relativity. If it did that would be a really big deal. Because it would be such a big deal this issue have been evaluated very carefully and it is not possible to transmit information by this phenomena.

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