Spacetime and Time Travel
This chapter raise most fundamental questions on the Theory of Relativity and its basic assumptions. Also it will provide detail explanation of Time travel in relative Spacetime.
To start this chapter of Spacetime and time travel, I will highlight our time measurement techniques once. We measure time through different types of clocks, and clocks are the devices to measure unit of time defined by us (S.I units). It might very precisely measure the unit of time depending on the technology but we know that it is just a human made device to measure the change of state of Existence. These clocks cannot measure the unit of time independently without being affected by the physical laws of existence and have to abide by these laws.
Spacetime terminology came into discussion when Albert Einstein gave the Special Theory of Relativity in 1905.
According to this theory Space and Time are one physical entity called Spacetime. Time can be referred as fourth dimension in which universe is existing in past but we have already passed through that time and universe is existing in future but we have not encountered it yet. He described that if an object is travelling at certain velocity then it can travel in time but the effect would be very small to observe. However if the speed can be increased near to speed of light then the time dilation effect will be more easy to observe, and when that object reaches upto the speed of light i.e. 300,000 kms/second, then the time would eventually stop for it.
For example if we moving on a road and passing few places behind hence they exist, but we have passed through that location so we cannot see them now. Similarly few places are yet to come on our way but we have not reached there to see them.
There are many paradoxes associated with Special Theory of Relativity which are already a matter of discussion worldwide so we are going to take one of them (Grandfather Paradox) and there could be few more paradoxes if we think deeply about time travel.
Grandfather paradox: This is one of the most famous paradox of time travel, it raises a contradiction that if someone ‘A’ goes back to past and kills own grandfather in his childhood then ‘A’ won’t exist anymore because his parents would not be born. If ‘A’ don’t exist then who killed the grandfather. There were several attempts to answer these time travel paradoxes such as parallel universe theory, time line protection hypothesis and many more. This parallel universe theory suggests that if someone changes the past then he will reach to a different parallel universe, or the changes made by him will create another parallel universe where he would not be existing.
There is a long list of paradoxes such as: Predestination Paradox, Bootstrap Paradox, Let’s Kill Hitler Paradox etc. (link has been provided in references to check these interesting paradoxes).
Basic assumption paradox: This paradox is a part of this research which has been added for further discussion. It is one of the fundamental paradox in the basic assumptions of this theory. Special theory of relativity is based on this basic postulate that the speed of light will remain constant in vacuum irrespective of speed of observer and source of light. The implication of this postulate was that speed-related measurements and duration were changed in order to accommodate this theory.
Let’s understand this from a hypothetical case: (See picture below)
If an object ‘A’ and an object ‘B’ (‘B’ part of light beam) are initially at same location and moving towards object ‘C’ (located at ‘X’ distance apart initially) with the velocity of 300,000 kms per second. And ‘C’ is also moving ahead in same direction of ‘A’ and ‘B’ with same velocity of 300,000 kms per second. In this case, ‘A’ won’t be able to reach ‘C’ because the relative velocity between them would be zero and ‘A’ would remain in rest with respect to ‘C’ with same distance between them ‘X’ after time t. However ‘B’ will still be able to reach ‘C’ with the velocity of 300,000 kms per second as it is a part of light beam. (Speed of ‘B’ will remain constant with respect to ‘C’ as per the basic postulate of special relativity theory).
Picture: Basic assumption on speed of light
The basic assumption of this theory is that the velocity of light will remain constant irrespective of velocity of observer and source of light. It raises a serious contradiction that why velocity of ‘B’ (part of light beam) remained constant with respect to ‘C’ moving with same velocity in same direction. In addition to that if ‘C’ would also be a part of another light beam, then under this condition, velocity between two light beams would relative or constant?
Hypothesis to remove this paradox: If we consider a hypothesis to remove this paradox by considering that speed of light is also relative between two objects depending on the speed of source and observer.
Any observer, who is travelling in the direction of light will observe time running slow for any event because the relative velocity of light will slow down with respect to that observer. It would seem to observer that time has been slowed down because he will be able to see the light waves coming slowly, enabling him to see the past event. It means he will be able to see the event later than it actually happened sometime ago.
However sound waves will take their own time to reach the position of observer because the speed of sound is very less compared to speed of light. So this kind of time travel won’t open any possibility to time travel in actual, but only makes possible to see past/future because observer’s speed is relative with respect to the speed of light.
In this case equation of relative time travel would change to:
Δt = Δt0 (1- v/v0)
Δt = the observing time duration (s)
Δt0 = the actual time duration on observer’s clock (s)
v = velocity of observer (m/s)
v0 = velocity of light in that medium (m/s)
Example: Lightning from sky clouds (See picture below)
In general we can listen that thunder sound is coming few seconds later than the actual lightning in the sky. We know that sound waves travel at approximately 343 metre per second and light waves travel at approximately 300,000 kms per second, so light waves reach to us much before than sound waves. Similar kind of phenomena could be observed with respect to light to see the past/future if we can travel near to speed of light.
Let’s suppose, initially there were three persons (A, B, C) located at middle point between sky and earth surface, at ‘X/2’ distance from earth surface. Person ‘A’ started travelling towards the sky and person ‘C’ started travelling towards earth surface while person ‘B’ remained at same position. The lightning ‘L’ occurred at an altitude of ‘X’ distance in the sky at time t1 (Time is progressing as t1 t2 t3). The ‘L’ will reach to ‘B’ at time t2 and to earth surface at time t3. The three persons will observe the same lightning event at difference timings. Person ‘A’ who travelled towards sky would see the future event with respect to stationary person ‘B’ because the light waves would take time to reach person B location at time t2. Person ‘C’ would see the past with respect person ‘B’ because light wave will reach to earth surface at time t3, later than person ‘B’ location.
Picture: Lightning from sky
However light travel at very high speed so this time difference would be so small that we will need special arrangements to measure it. The effect of seeing past/future of same event will be more visible, if those persons will be able to travel near to the speed of light.
According to General Theory of Relativity spacetime is dynamically curved due to the presence of matter. It causes time dilation depending upon the curvature of spacetime (Gravity). This time dilation effect has been observed through various experiments in the history.
The atomic clocks installed on GPS (Global Positioning System) satellite and ISS (International Space Station) shows different time compared to the clocks on earth. Basically there are two reasons for this time dilation, first is gravity (General theory of relativity) and second is the velocity of these systems (Special theory of relativity). These atomic clocks are one of the precise time measuring devices available as of now. As such there is no place in the space where gravity is absolutely zero but at very high altitude such as GPS satellite, the effect of gravity becomes very small which causes the clocks to measure time differently. The clock on earth measures the unit of time, relative to the gravity of earth, therefore the measurement of time would be different in other gravity zones. If a clock would be placed near the gravity of Sun, Jupiter etc. it will measure time differently depending upon the gravity. Hence these clocks on ISS and GPS satellites are being calibrated regularly in order to match the clocks on earth.
Let me tell you that discussion for universal time measurement has already begun. The focus of the discussion is to standardize the time more accurately. Although there are many discussions going on at different forums but it has not been fully concluded yet. Suggestions have been made that operating a clock in microgravity environment like International space station will provide much accuracy and stability for maintaining standard base of time unit. Several new technology based clocks are in development stage such as optical clocks etc.
Apart from local disturbances (thermal noise and vibration sensitivity), if a clock measures time differently due to effect of velocity and gravity then, should we consider it as actual time dilation or another local disturbance. To define a time unit which will be universally constant everywhere could be much more effective to measure Existence.
What if we consider a clock to be operate where it will be least affected by any physical laws (gravity, velocity, acceleration, waves etc.) and local disturbance (thermal noise and vibration sensitivity). Keeping a clock in quiescent space environment with least gravitational effect will provide higher level of accuracy in time measurement and it would measure the unit of ab. Time as universal constant without being affected by local disturbances.
To conclude this chapter, I will reiterate the same from where I started, we need to consider our time measurement techniques again. Should we consider time dilation shown on our clocks as actual time dilation?
The laws of existence affect all the physical entities in a different way. All the physical entities have a different effect of gravity on them depending on their nature. It is not necessary that the biological clock of living beings would be affected in same measurement scale as it affects the normal clocks. Rather than considering the time dilation effect in atomic clock (at International Space Station and GPS satellites) as actual time dilation, it could be considered as local disturbance due to velocity and gravity on that particular clock with particular time measurement technology.
If we pay close attention to the clocks from ancient times to recent ones, the people started measuring time by day and night of earth. The Earth rotates about it axis making it day and night but that duration is not very precise, so humans made hourglass clocks to further measure the time more accurately. Today’s highly sophisticated clocks measure time upto a high accuracy level of billionth of second. Ultimately, these clocks are just measuring day and night of earth which is eventually the change of state of Existence only.
Hence we can conclude from this chapter:
Actual time travel is not be possible, even if we will be able to travel near to the speed of light then we could only see past events and future events with respect to stationary observer. In this condition we won’t be able to hear anything because sound waves would take their own time to reach that location. In addition to that, time in general theory of relativity shows variation (time dilation effect) due to local physical conditions on clock. The local physical conditions of Existence change so drastically (such as point of initial singularity) that makes spacetime non-relevant at all, so spacetime is not very useful in terms of measuring the Entire Existence.