Gravity & Time Travel

[JakeEscherRoels]

Ok so bear with me here...

I've been listening to Carl Sagan lectures on audiobook while at work.

Black holes gravity is so strong it bends matter, light, space... the closer you get to the event horizon, the more intensely gravity pulls. So! Someone sending messages at regular intervals who is approaching a black hole to a fixed point. The messages would be received slower and slower as the black holes gravity pulls everything in harder. This means time is moving faster for the guy approaching the black hole.
Therefore? a stupid high level of Increase Gravity would affect TIME ITSELF! (Maybe? Right?)

I realize this is both:
1. Incredibly soft science fiction
2. Stupid high levels of increased gravity and there would be cheaper ways of doing it for sure.

[Dunadin777]

More or less, but this would be affecting the heavy-gravity person/entity in a way we consider less useful--it would make him appear to slow down to the rest of the world, provided they were somehow shielded from the relativistic distortion. Effectively, the gravity distortion you're talking about could at best be used as a very efficient suspended animation mechanism, and only useful if they could accurately turn it off.

[Purple Haze]

Originally Posted by JakeEscherRoels

Black holes gravity is so strong it bends matter, light, space... the closer you get to the event horizon, the more intensely gravity pulls. So! Someone sending messages at regular intervals who is approaching a black hole to a fixed point. The messages would be received slower and slower as the black holes gravity pulls everything in harder. This means time is moving faster for the guy approaching the black hole.
Therefore? a stupid high level of Increase Gravity would affect TIME ITSELF! (Maybe? Right?)

Wrong.

The messages would be received at the normal rate. The frequency on which the transmission is broadcast would get lower. This is gravitational red-shift.

[Agemegos]

Originally Posted by Purple Haze

Wrong.

The messages would be received at the normal rate.

Are you absolutely positive about that? Have you actually studied General Relativity?

The frequency on which the transmission is broadcast would get lower. This is gravitational red-shift.

Yes. And gravitational red-shift is a consequence of gravitational time dilation.

[Dunadin777]

Originally Posted by Brett

Are you absolutely positive about that? Have you actually studied General Relativity?

I think Haze is thinking of gravity with a minute effect on relativity. The frequency would be distorted by a much lower gravity well before the time between signals would be appreciably affected. But really, based on the OP's supposition, both would approach zero as the singularity source approached an escape velocity of c.

[Agemegos]

Originally Posted by Dunadin777

I think Haze is thinking of gravity with a minute effect on relativity. The frequency would be distorted by a much lower gravity well before the time between signals would be appreciably affected.

Are you sure about that? I have a feeling that gravitational frequency shifting is exactly the same effect as gravitational time dilation, and that they both become significant at the same point and for the same reason. The frequency shifting can be thought of as the effect of gravitational time dilation on successive peaks of the EM wave. That is, 1/frequency is effected the same as successive signals. But I never studied general relativity formally, and I might be wrong.

[Anaraxes]

Originally Posted by Brett

Are you absolutely positive about that?

Don't you have to ask "rate as measured by whom?"

[teviet]

Originally Posted by Brett

Are you sure about that? I have a feeling that gravitational frequency shifting is exactly the same effect as gravitational time dilation, and that they both become significant at the same point and for the same reason. The frequency shifting can be thought of as the effect of gravitational time dilation on successive peaks of the EM wave. That is, 1/frequency is effected the same as successive signals. But I never studied general relativity formally, and I might be wrong.

I have, and you're not. (I.e. you are right.)

If you're standing at the bottom of a deep gravity well, your clock ticks more slowly when compared with another clock standing at the top of the well. The atomic oscillations that determine the frequency of emitted light are also slowed. Of course you age more slowly and experience the passage of time more slowly, so the light looks perfectly normal to you (as it must).

If you're falling, you also have relative-velocity time dilation and Doppler shift to worry about, and the math starts to get more complicated.

The amount of dilation/frequency shift depends on both the strength of gravity and the height difference between you and the other guy. For ordinary gravities and heights, the effect is relatively small. Assuming uniform gravity g over a height h, the fractional change in time rates is g*h/c^2, where c is the speed of light.

That is, for a 1g field, it would have to extend around 1 lightyear before the difference in rates would be game-significant. In more ordinary units:

fractional rate change = 0.00000000000017*(gravity in g's)*(height difference in miles)

TeV

[teviet]

Originally Posted by Anaraxes

Don't you have to ask "rate as measured by whom?"

You do. The rates are measured by two clocks: one at the bottom of the gravity well and one at the top. (Assume they are stationary.)

The sender at the bottom sends pulses of 400 nanometre (violet) light every second. If the gravitational redshift factor is 1.5, then the receiver at the top receives pulses of 600 nanometre (orange) light every 1.5 seconds.

TeV

[Agemegos]

Originally Posted by Anaraxes

Don't you have to ask "rate as measured by whom?"

Yes, and for exactly the same reason that you have to ask "frequency as measured by whom?"