# Confirming what I have read.

• robert135

#### robert135

My background is in CS, not physics, but I dabble a bit.

I am hoping to confirm a few things I have read and think that I understand. I am hoping these are easy questions. I will try to make them yes or no questions.

1) If one travels close to the speed of light, then time slows down?

2) Einstein predicted that gravity is just a distortion of spacetime?

3) The inertia of an object is related to its mass?

4) The speed of light through an object is slower than the speed of light in a vacuum?

5) We have successfully accelerated particles damn near the speed of light? If so how close?

6) We have accelerated particles faster than the slower speed of light through objects?

Thanks, I am looking many of these up as well.

My background is in CS, not physics, but I dabble a bit.

I am hoping to confirm a few things I have read and think that I understand. I am hoping these are easy questions. I will try to make them yes or no questions.

1) If one travels close to the speed of light, then time slows down?

This is not true, but would be appear to be the case from the intertial frame you're accelerating away from. In other words, you would seem to redshift to them, but from your perspective, they would appear to "speed up"

2) Einstein predicted that gravity is just a distortion of spacetime?

I don't know what he predicted exactly, but the Stress-Energy Tensor describes gravity as a result of momentum, shear (stress), and density. In that sense, spacetime is what energy passes through, and the passage of that energy deforms spacetime from its initial configuration.

3) The inertia of an object is related to its mass?

Well, yes, one is a measurement of the other.

4) The speed of light through an object is slower than the speed of light in a vacuum?
Yes. The speed of light in vacuum is the upper limit of the speed of light, but not the ONLY speed it travels at in a given medium.

5) We have successfully accelerated particles damn near the speed of light? If so how close?

Within a fractional percentage of c in that medium: around 99.99% at the ALICE accelerator. It should be noted that is a little deceptive however. The energies at which particles, atoms, etc collide is a better measure. That's why you hear talk of "GeV or TeV" and not "99.89 or 99.99999 %" outside of the media.

6) We have accelerated particles faster than the slower speed of light through objects?

That would be impossible. You can change properties of a medium (other than vacuum) by doping it or other means, but the speed of light in a given homogeneous medium is the RULE. As for what happens when light "brakes" between mediums... do some reading on Čerenkov Radiation for an example.

robert135 said:
Thanks, I am looking many of these up as well.

Good questions, all of them are very good quetions.

6) We have accelerated particles faster than the slower speed of light through objects?

Yes, if I understood your question correctly.
Light is slower in media (water, glass) (about 0.7c)
So particles can travel faster then 0.7c in glass. Of course, they travel slower then c.
In such cases particles emit Cherenkov's radiation.

5) We have successfully accelerated particles damn near the speed of light? If so how close?
At the http://en.wikipedia.org/wiki/Lep" [Broken], 99.999,999,998,793 %.

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This is not true, but would be appear to be the case from the intertial frame you're accelerating away from. In other words, you would seem to redshift to them, but from your perspective, they would appear to "speed up"
I think you've got it wrong here, both of you would reciprocally observe the other one to slow down by the same factor if you were moving apart at constant speed (and both of you would see the other redshifted by the same factor)--it sounds like you're saying here that it's not reciprocal, that they see you slow down but you see them speed up? If so that's not correct, all relativistic effects must be reciprocal between different inertial observers or else the first postulate of relativity (saying the laws of physics work exactly the same in all inertial frames) would be violated...
Frame Dragger said:
That would be impossible. You can change properties of a medium (other than vacuum) by doping it or other means, but the speed of light in a given homogeneous medium is the RULE. As for what happens when light "brakes" between mediums... do some reading on Čerenkov Radiation for an example.
Not true, particles can travel through a medium faster than the speed of light in that medium (and light itself only appears slowed down because it's repeatedly being absorbed and reemitted, between absorption events it's traveling at the same speed it does in a vacuum--see ZapperZ's post #4 on the Physics Forum FAQ). Cherenkov radiation is a direct result of a particle traveling through a medium faster than light travels through that medium (see here for example), it's akin to the "sonic boom" that results when an object flies through air faster than the speed of sound.