# Perception and relativity

1. Dec 23, 2015

### udtsith

Is it appropriate to talk about objects that can travel 'near' the speed of light? E.g. compared to a hypothetical observer moving at 'near' zero speed (very close to zero temperature/energy) wouldn't we seem to be moving 'close' to the speed of light. E.g. isn't even traveling 3 meters/second close to traveling at the speed of light when compared to an object that is travelling a trillionth of a meter per second?

2. Dec 23, 2015

### Staff: Mentor

Yes, as long as you keep in mind that "speed" is relative.

No. First, you are talking about this hypothetical observer as though their speed was "near zero" in some absolute sense. That's not correct; speed is relative, as above.

Second, I'm not sure what sort of correlation you are envisioning between "speed" in the sense of relative speed in SR, and temperature/energy. They're different things.

No. If we assume that these "speeds" are both relative to the same reference frame, then an object traveling 3 meters/second, compared to an object traveling at a trillionth of a meter per second, will appear to be traveling at (3 - one trillionth) meters/second. For speeds this small compared to the speed of light, velocities can just be compared by simple subtraction.

3. Dec 23, 2015

### Staff: Mentor

In some fixed inertial frame, if $v_1/c<<1$ then that fact does not change even if we compare $v_1$ to some other $v_2$ such that $v_1/v_2>>1$

4. Dec 23, 2015

### udtsith

okay, thank you. I find it interesting on the possibility that an alien intelligence could evolve to process large time dilation effects. For example, an alien that evolved in an extreme gravity well or around a very fast blackhole (implausible but just for speculation). Suppose that alien lifted a flower at .1 meters per second . If that alien was moving 'close' to the speed of light that flower would rot in its hand before it reached its nose.

5. Dec 23, 2015

### alw34

yes, Even for objects with mass. Right now in a galaxy far, far,far away, an observer there would say YOU are moving faster than light right now, relative to them. That's because the universe is expanding. But here on earth, neither you nor I can measure such a thing....we appear stationary to each other if we are both sitting still.

Locally, say here on earth, nothing can move faster than light and only massles particles can move at the speed of light.

Perhaps you are thinking of a particle having little or no "temperature/energy' meaning virtually no zero point energy. But you don't need to set such a stringent parameter for 'zero speed'. Just pick an inertial [steady speed] reference frame moving with any observer, like yourself going straight down a highway at a fixed speed, and you have established 'zero speed' for relative observational measurements. Relative to you in that reference frame, signs are buzzing by at, say, 50mph. But the steering wheel in your vehicle moves at zero speed relative to you. On the other hand, relative to stationary observers at the highway, you are buzzing by them at 50mph....and so is your steering wheel. All three views are 'correct'. And relativity tells us, light still whizzes by each observer at exactly the same speed, 'c', no difference whatsoever.

6. Dec 23, 2015

### Staff: Mentor

No. The concept of "relative speed" is not well-defined for spatially separated objects in a curved spacetime. The "speed" that is faster than light in the case you describe is a coordinate speed only and does not correspond to any actual observation.

7. Dec 23, 2015

### alw34

While I understand exactly what you mean.... I doubt that aids the "starter".....
of course maybe my comment didn't either!!
.
Anyway, Wikipdia says it this way:

"...In cosmology, a Hubble volume, or Hubble sphere, is a spherical region of the Universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the Universe..."

https://en.wikipedia.org/wiki/Hubble_volume

8. Dec 23, 2015

### Staff: Mentor

Yes, that's why I posted in response to you; the term "speed" is being used in this thread in a specific sense, the sense of "relative speed" in SR, which is limited to the speed of light. Your post used "speed" in a different sense, which is only going to confuse things.

Yes, and that's why Wikipedia is not a reliable source.

It's true that you will find the term "speed" used in this sense even in textbooks on cosmology; but at least the textbooks take some care to point out that this "speed" is just a coordinate speed and is not something that anyone actually measures. It certainly is not the same as "speed" in the sense that term is being used in this thread.