Why Can't We Travel Faster Than Light?

ToraToraTora | 12:43 Wed 01st Nov 2023 | Science

144 Answers

....because we are already travelling through spacetime at the speed of light, there is only one speed in spacetime.

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ClareTG0ld

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// As you speed up mass increases ... //This isn't true, although is a common misconception. To be more precise, your kinetic energy increases. Mass stays the same throughout. No doubt E= mc^2 tricks people into thinking that if mass goes up then energy does, but that equation only holds when you are at rest; otherwise, the equation readsE^2 - p^2c^2 = m^2...

15:59 Wed 01st Nov 2023

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Khandro

^^ What's more, if you were driving a car at the speed of light, your headlamps would be ineffective. 😄

ClareTG0ld

I expect you're joking, Khandro, but it's a good opportunity anyway to address that idea:

// if you were driving a car at the speed of light, your headlamps would be ineffective. //

In fact, and for two reasons, this isn't true. The first is that you can't drive a car at the speed of light anyway, as I've explained at length in this and other threads on the topic.

But you can get arbitrarily close, I suppose, as near as dammit. Let's say, then, that you were driving your car at 299,792,457.9995 m/s (which is to say well over 99.99999999% of the speed of light). Would your headlights still work?

Yes! The same law that says you can't reach the speed of light also tells us that you will always measure light travelling at the exact same speed, no matter how arbitarily fast you're moving. What's more, somebody stupid enough to stand in front of you would also measure light leaving your headlights at the speed of light - 299,792,458 m/s, rather than almost twice that.

I won't prove this, but it follows quite wonderfully from the maths of relativity. In short, then, your headlights would work perfectly (assuming they could survive the stresses of such high speeds), no matte how fast you travelled.

Finally, it's worth noting that the same maths tells you that you could carry on accelerating -- at a constant rate -- and never reach (nor exceed, of course) the speed of light.

ClareTG0ld

NB Khandro, please don't let the above stop you from saying similar things in future! Just using the humour as an excuse to take the question seriously and hopefully share a few useful nuggets of physics :)

Old_Geezer

Yeah but by the time the light reflected off something back to your eye, you'd be on top of it !

Anyway you can't go as fast as light in a vacuum because light has been training since the dawn of time and so has perfected light speed.

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