What are the prospects of NASA simply sending a probe out using the "new" electron propulsion system to actually test out some of our theories in this area?  I, for one, would think that this would be as good an investment as some of the other programs that we are currently funding.

I agree.  Let's petition for a solar-powered ion-propulsion probe to see just how fast we can go and put many of our theories to the ultimate test.

  Just wondering as I was contemplating approaching the speed of light.  Would a probe moving this fast even be able to transmit data back to us?  I mean if the probe sent the data in the form of light back towards us wouldn't the data be still(?)? 

  Like this;  If a source ejects something at the speed of light, but is itself moving at the speed of light, wouldn't the ejected something then be stationary?  Maybe we would have to send out successive probes just to retrieve the information left behind by former probes.  It is fun to ponder such circumstances.

  In the unlikely(?) event of such a probe actually surpassing the speed of light (assuming that many of our current theories are wrong) would we then be unable to retrieve the data at all?  Or would we have to send out a reciever probe ahead of the one that we want to recieve the data from.  I feel a paradox coming on.  My...mind...is...too...small...ugh.

Okay, cool.  I guess that you have made yet another point as to why such a mission should be done.  We would then have confirmation or rejection of these non-problems.

According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

Nothing with mass can reach the speed of light.

I know, I was joking. But still, what is mass? Light sometimes behaves as a particle and sometimes as a wave, does that mean that it sometimes has mass? Some solar-powered probes are powered by light because the photons hit the probe, get reflected off and because of conservatoin of momentum, the probe gets moved by the photon. But that would mean that the photon has mass, so how is it moving at the speed of light?

We can never be absolutely certain what is right, but at the moment, scientists are saying that the speed of light is constant relative to the observer.

That is a fairly well held fact in physics today.  It has been tested many times.  For example, we wouldn't get the Doppler effect with light if that property wasn't true.

Why not? Sound's speed varies depending on who is observing it, and the Doppler effect still works with that. In fact, I'd have thought that the Doppler effect was dependent on the speed being variable. Obviously that's not true, because we can see the red-shift of all the stars that are moving away from us, but it's counter-intuitive.

Huh. I didn't know about the photon not having momentum but not having mass. It doesn't seem like it should work, because usually momentum is mass x velocity, but then, it doesn't seem like anything to do with light should work really.

I meant that the speed of sound is variable depending on who is observing it. If you are travelling away from the sound source, then the sound will be slower for you than for someone who is stationary.

And red-shift is just the Doppler effect affecting how you see stars. A star might be yellow, but as it is moving away from you then the Doppler effect causes it to appear redder. Red-shift.
It's a standard scientific definition thingy. Sorry if I lost you somewhere else.

Doppler: If my hypothetical near-light-speed spacecraft sent an audio message back to earth, the signal would travel back to earth at the speed of light, but Earth would (eventually) hear my voice sounding v-e-r-y  s-l-o-w, right?

Back to the ionic drive probe: the advantage of ionic drive is that it produces very high exit velocities (around 70,000 mi/sec as opposed to chemical rocketry's 20,000 (IIRC)), so it's a very efficient conversion of electricity to propulsion. The trouble is twofold: one, the stream is very rarified, so that actual acceleration produced is slow. This means that we can't expect to power the thing with the sun, because the sun's rays will fade long before the probe is done accelerating.

The other problem is that even with the high exhaust velocities, energy density is still an issue. You have to carry enough propellent and energy (nuclear is currently the best candidate; most long-range probes have been nuclear powered) to reach those high velocities.

This is why the energy balance is more interesting to me than the time dilation; it's what's holding us back in space.

A theory I read in a book somewhere: Travelling just over the speed of light in space will lead to time slowing down in the shuttle itself. That would probably look and feel amazing - And probably scary...?

I think you mean just under.  And this effect happens no matter what speed you are traveling.  It just happens to a greater extent when you are traveling faster.

When you walk to your kitchen to get a sandwhich that is sitting on a table, you are moving in time slower than that sandwhich is.  It isn't noticable, but it's happening.

mathsyperson wrote:

Yes, but you're assuming that light obeys the same laws of physics as everything else. According to the laws of physics, nothing can reach the speed of light, so light's broken that one already!

Someone correct me if I'm wrong, but I believe relativity implies that the speed of light is a barrier that cannot be crossed. Photons either travel at the speed of light, or their energy absorbed into matter. They have not broken the barrier.

Also, there is nothing to suggest that there are no particles that travel faster than the speed of light (tachyons). Relativity says if those particles exist, they cannot accelerate (slow down) to the speed of light.

Ricky wrote:

Also, there is nothing to suggest that there are no particles that travel faster than the speed of light (tachyons). Relativity says if those particles exist, they cannot accelerate (slow down) to the speed of light.

Such particles must have negative mass to be compatible with relativistic equations.  Not saying that it isn't possible, just that it makes no sense.

doesnt anti-matter, have negative mass?

numen wrote:

No, antimatter has opposite electric charge. As far as I know, negative mass has not been confirmed anywhere in physics.

ah yeh, i was thinking of dark matter :p which as you say, hasnt been confirmed to exist

i thought that darkmatter was supposed to explain why everything is accelerating away from eachover? i.e. negative force of gravity -> can only occur with a negative mass?

although, thats supposing that its due to a force of gravity at all...

Last edited by luca-deltodesco (2006-07-31 03:59:52)